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DANSHITA Ippei

    Department of Science　Associate Professor
Last Updated :2024/05/15

Researcher Information

J-Global ID

Research Interests

  • Quantum phase transition   Quantum simulation   Optical lattice   Superfluid   Ultracold gas   

Research Areas

  • Natural sciences / Semiconductors, optical and atomic physics / Theory of ultracold gases

Association Memberships

  • 日本物理学会   

Published Papers

  • Ryui Kaneko; Masaya Kunimi; Ippei Danshita
    Physical Review A 109 (1) L011301-1 - L011301-6 2024/01 [Refereed]
  • Masaya Kunimi; Ippei Danshita
    Physical Review A American Physical Society (APS) 108 (6) 2469-9926 2023/12 [Refereed]
  • Naoto Tsuji; Ippei Danshita; Shunji Tsuchiya
    Encyclopedia of Condensed Matter Physics Elsevier 1 174 - 186 2023/10 [Refereed][Invited]
  • Ryui Kaneko; Ippei Danshita
    Physical Review A 2023/08 [Refereed]
  • Mathias Mikkelsen; Ippei Danshita
    PHYSICAL REVIEW A AMER PHYSICAL SOC 107 (4) 043313  2469-9926 2023/04 [Refereed]
     
    It is well established that the noise correlations measured by time-of-flight imaging in cold-atom experiments, which correspond to the density-density correlations in the momentum space of trapped atomic gases, can probe the spin structure factor deep in the Mott-insulating regime of SU(2) Hubbard models. We explicitly derive the mathematical relation between the noise correlations and the spin structure factor in the strong-interaction limit of SU(N) Hubbard models at any integer filling rho. By calculating the ground states of one-dimensional SU(N) Fermi-Hubbard models for 2 N 6 with use of the density-matrix renormalization-group method, we confirm the relation numerically in the regime of strong interactions U >> t, where U and t denote the on-site interaction and the hopping energy. We show that the deviation between the actual noise correlations and those obtained from the spin structure factor scales as approximately (t/U )2 for rho = 1 at intermediate and large lattice sizes on the basis of numeric and semianalytic arguments.
  • Daichi Kagamihara; Ryui Kaneko; Shion Yamashika; Kota Sugiyama; Ryosuke Yoshii; Shunji Tsuchiya; Ippei Danshita
    Physical Review A American Physical Society (APS) 107 (3) 033305  2469-9926 2023/03 [Refereed]
  • Shimpei Goto; Ryui Kaneko; Ippei Danshita
    Physical Review B American Physical Society (APS) 107 (2) 024307  2469-9950 2023/01 [Refereed]
  • Mathias Mikkelsen; Ryui Kaneko; Daichi Kagamihara; Ippei Danshita
    Physical Review A American Physical Society (APS) 106 (4) 2469-9926 2022/10 [Refereed]
  • Shiono Asai; Shimpei Goto; Ippei Danshita
    Progress of Theoretical and Experimental Physics Oxford University Press (OUP) 2022 (3) 2022/03 [Refereed]
     
    Abstract We analyze the dynamics of the infinite-dimensional Bose–Hubbard model with spatially inhomogeneous dissipation in the hardcore boson limit by solving the Lindblad master equation with use of the Gutzwiller variational method. We consider dissipation processes that correspond to inelastic light scattering in the case of Bose gases in optical lattices. We assume that the dissipation is applied to half of the lattice sites in a spatially alternating manner. We focus on steady states at which the system arrives after long time evolution. We find that when the average particle density is varied, the steady state exhibits a transition between a state in which the sites without dissipation are vacuum and one containing a finite number of particles at those sites. We associate the transition with the notion that the sites with dissipation tend towards a local state at infinite temperature.
  • Yoshihiro Machida; Ippei Danshita; Daisuke Yamamoto; Kenichi Kasamatsu
    Physical Review A American Physical Society (APS) 105 (3) 2469-9926 2022/03 [Refereed]
  • Ryui Kaneko; Ippei Danshita
    Communications Physics Springer Science and Business Media {LLC} 5 (1) 2399-3650 2022/03 [Refereed]
  • Kazuma Nagao; Yosuke Takasu; Yoshiro Takahashi; Ippei Danshita
    Physical Review Research American Physical Society (APS) 3 (4) 043091-1 - 043091-18 2021/11 [Refereed]
  • Masaya Kunimi; Ippei Danshita
    Physical Review A American Physical Society (APS) 104 (4) 043322-1 - 043322-9 2469-9926 2021/10 [Refereed]
  • Shimpei Goto; Ryui Kaneko; Ippei Danshita
    Physical Review B American Physical Society (APS) 104 (4) 045133-1 - 045133-9 2469-9950 2021/07 [Refereed]
  • Ryui Kaneko; Yoshihide Douda; Shimpei Goto; Ippei Danshita
    Journal of the Physical Society of Japan Physical Society of Japan 90 (7) 073001-1 - 073001-4 0031-9015 2021/07 [Refereed]
  • Masaya Kunimi; Kazuma Nagao; Shimpei Goto; Ippei Danshita
    Physical Review Research American Physical Society (APS) 3 (1) 013060-1 - 013060-15 2021/01 [Refereed]
  • Shimpei Goto; Ippei Danshita
    Physical Review Research American Physical Society (APS) 2 (4) 043236  2020/11 [Refereed]
  • Shimpei Goto; Ippei Danshita
    Physical Review A American Physical Society (APS) 102 (3) 033316  2469-9926 2020/09 [Refereed]
  • Yosuke Takasu; Tomoya Yagami; Hiroto Asaka; Yoshiaki Fukushima; Kazuma Nagao; Shimpei Goto; Ippei Danshita; Yoshiro Takahashi
    Science Advances American Association for the Advancement of Science (AAAS) 6 (40) eaba9255 - eaba9255 2020/09 [Refereed]
     
    An optical lattice quantum simulator is an ideal experimental platform to investigate nonequilibrium dynamics of a quantum many-body system, which is, in general, hard to simulate with classical computers. Here, we use our quantum simulator of the Bose-Hubbard model to study dynamics far from equilibrium after a quantum quench. We successfully confirm the energy conservation law in the one- and three-dimensional systems and extract the propagation velocity of the single-particle correlation in the one- and two-dimensional systems. We corroborate the validity of our quantum simulator through quantitative comparisons between the experiments and the exact numerical calculations in one dimension. In the computationally hard cases of two or three dimensions, by using the quantum-simulation results as references, we examine the performance of a numerical method, namely, the truncated Wigner approximation, revealing its usefulness and limitation. This work constitutes an exemplary case for the usage of analog quantum simulators.
  • Yusuke Ozaki; Kazuma Nagao; Ippei Danshita; Kenichi Kasamatsu
    Physical Review Research American Physical Society (APS) 2 (3) 2020/08 [Refereed]
  • Daisuke Yamamoto; Takeshi Fukuhara; Ippei Danshita
    Communications Physics Springer Science and Business Media LLC 3 (1) 2020/03 [Refereed]
     
    AbstractQuantum antiferromagnets with geometrical frustration exhibit rich many-body physics but are hard to simulate by means of classical computers. Although quantum-simulation studies for analyzing such systems are thus desirable, they are still limited to high-temperature regions, where interesting quantum effects are smeared out. Here we propose a feasible protocol to perform analog quantum simulation of frustrated antiferromagnetism with strong quantum fluctuations by using ultracold Bose gases in optical lattices at negative absolute temperatures. Specifically, we show from numerical simulations that the time evolution of a negative-temperature state subjected to a slow sweep of the hopping energy simulates quantum phase transitions of a frustrated Bose–Hubbard model with sign-inverted hoppings. Moreover, we quantitatively predict the phase boundary between the frustrated superfluid and Mott-insulator phases for triangular lattices with hopping anisotropy, which serves as a benchmark for quantum simulation.
  • Masaya Kunimi; Ippei Danshita
    Physical Review A American Physical Society ({APS}) 100 (6) 063617  2469-9926 2019/12 [Refereed]
     
    We investigate the stability of nonequilibrium steady states of Bose-Einstein condensates with a local one-body loss in the presence of double potential barriers. We construct an exactly solvable mean-field model, in which the local loss and the potential barriers take the form of a delta function. Using the exact solutions of our model, we show that there are parameter regions in which two steady-state solutions are dynamically stable, i.e., the model exhibits bistability. We also find that unidirectional hysteresis phenomena appear when the local-loss rate is varied in some parameter region.
  • Shimpei Goto; Ippei Danshita
    Physical Review Letters American Physical Society ({APS}) 123 (14) 0031-9007 2019/10 [Refereed]
  • Daisuke Yamamoto; Giacomo Marmorini; Masahiro Tabata; Kazuki Sakakura; Ippei Danshita
    Physical Review B American Physical Society ({APS}) 100 (14) 2469-9950 2019/10 [Refereed]
  • Masaya Kunimi; Ippei Danshita
    Physical Review A 99 (4) 043613-1 - 043613-9 2019/04 [Refereed]
  • Kazuma Nagao; Masaya Kunimi; Yosuke Takasu; Yoshiro Takahashi; Ippei Danshita
    Physical Review A American Physical Society ({APS}) 99 (2) 2469-9926 2019/02 [Refereed]
     
    We analyze the time evolution of the Bose-Hubbard model after a sudden quantum quench to a weakly interacting regime. Specifically, motivated by a recent experiment at Kyoto University, we numerically simulate redistribution of the kinetic and on-site interaction energies at an early time, which was observed in nonequilibrium dynamics of ultracold Bose gases in a cubic optical lattice starting with a singly occupied Mott-insulator state. In order to compute the short-time dynamics corresponding to the experimental situation, we apply the truncated-Wigner approximation to the Bose-Hubbard model on a cubic lattice. We show that our semiclassical approach quantitatively reproduces the fast redistribution dynamics. We further analyze spatial spreading of density-density correlations at equal time in the Bose-Hubbard model on a square lattice with a large filling factor. When the system is initially prepared in a coherent state, we find that a propagation velocity of the correlation wave packet in the correlation function strongly depends on the final interaction strength, and it is bounded by twice the maximum group velocity of the elementary excitations. In contrast, when the system is initially in a Mott-insulator state, the propagation velocity of the wave packet is approximately independent of the final interaction strength.
  • Shimpei Goto; Ippei Danshita
    Physical Review B American Physical Society ({APS}) 99 (5) 2469-9950 2019/02 [Refereed]
  • Goto Shimpei; Danshita Ippei; Ono Koki; Amano Yoshiki; Higomoto Toshiya; Takahashi Yoshiro
    Meeting Abstracts of the Physical Society of Japan The Physical Society of Japan 74.2 520 - 520 2019
  • Nagao, K.; Takahashi, Y.; Danshita, I.
    Physical Review A American Physical Society 97 (4) 2469-9934 2018/04 [Refereed]
     
    We study the Higgs mode of superfluid Bose gases in a three-dimensional optical lattice, which emerges near the quantum phase transition to the Mott insulator at commensurate fillings. Specifically, we consider responses of the Higgs mode to temporal modulations of the onsite interaction and the hopping energy. In order to calculate the response functions including the effects of quantum and thermal fluctuations, we map the Bose-Hubbard model onto an effective pseudospin-1 model and use a perturbative expansion based on the imaginary-time Green's function theory. We also include the effects of an inhomogeneous trapping potential by means of a local density approximation. We find that the response function for the hopping modulation is equal to that for the interaction modulation within our approximation. At the unit filling rate and in the absence of a trapping potential, we show that the Higgs mode can exist as a sharp resonance peak in the dynamical susceptibilities at typical temperatures. However, the resonance peak is significantly broadened due to the trapping potential when the modulations are applied globally to the entire system. We suggest that the Higgs mode can be detected as a sharp resonance peak by partial modulations around the trap center.
  • Yoshida, T.; Danshita, I.; Peters, R.; Kawakami, N.
    Physical Review Letters AMER PHYSICAL SOC 121 (2) 0031-9007 2018 [Refereed]
     
    One of the most challenging problems in correlated topological systems is a realization of the reduction of topological classification, but very few experimental platforms have been proposed so far. We here demonstrate that ultracold dipolar fermions (e.g., Er-167, Dy-161, and Cr-53) loaded in an optical lattice of twol-eg ladder geometry can be the first promising test bed for the reduction Z -> Z(4), where solid evidence for the reduction is available thanks to their high controllability. We further give a detailed account of how to experimentally access this phenomenon; around the edges, the destruction of one-particle gapless excitations can be observed by the local radio frequency spectroscopy, while that of gapless spin excitations can be observed by a time-dependent spin expectation value of a superposed state of the ground state and the first excited state. We clarify that even when the reduction occurs, a gapless edge mode is recovered around a dislocation, which can be another piece of evidence for the reduction.
  • Takafumi Tomita; Shuta Nakajima; Ippei Danshita; Yosuke Takasu; Yoshiro Takahashi
    Science Advances American Association for the Advancement of Science ({AAAS}) 3 (12) 2375-2548 2017/12 [Refereed]
     
    Dissipation is ubiquitous in nature and plays a crucial role in quantum systems such as causing decoherence of quantum states. Recently, much attention has been paid to an intriguing possibility of dissipation as an efficient tool for the preparation and manipulation of quantum states. We report the realization of successful demonstration of a novel role of dissipation in a quantum phase transition using cold atoms. We realize an engineered dissipative Bose-Hubbard system by introducing a controllable strength of two-body inelastic collision via photoassociation for ultracold bosons in a three-dimensional optical lattice. In the dynamics subjected to a slow ramp-down of the optical lattice, we find that strong on-site dissipation favors the Mott insulating state: The melting of the Mott insulator is delayed, and the growth of the phase coherence is suppressed. The controllability of the dissipation is highlighted by quenching the dissipation, providing a novel method for investigating a quantum many-body state and its nonequilibrium dynamics.
  • Ippei Danshita; Masanori Hanada; Masaki Tezuka
    Progress of Theoretical and Experimental Physics Oxford University Press ({OUP}) 2017 (8) 2050-3911 2017/08 [Refereed]
     
    We suggest that the holographic principle, combined with recent technological advances in atomic, molecular, and optical physics, can lead to experimental studies of quantum gravity. As a specific example, we consider the Sachdev-Ye-Kitaev (SYK) model, which consists of spin-polarized fermions with an all-to-all complex random two-body hopping and has been conjectured to be dual to a certain quantum-gravitational system. Achieving low-temperature states of the SYK model is interpreted as a realization of a stringy black hole, provided that the holographic duality is true. We introduce a variant of the SYK model, in which the random two-body hopping is real. This model is equivalent to the original SYK model in the large-N limit. We show that this model can be created in principle by confining ultracold fermionic atoms into optical lattices and coupling two atoms with molecular states via photo-association lasers. This development serves as an important first step towards an experimental realization of such systems dual to quantum black holes. We also show how to measure out-of-time-order correlation functions of the SYK model, which allow for identifying the maximally chaotic property of the black hole.
  • Yamamoto Daisuke; Ueda Hiroshi; Danshita Ippei; Marmorini Giacomo; Momoi Tsutomo; Shimokawa Tokuro
    Meeting Abstracts of the Physical Society of Japan The Physical Society of Japan 72 1276 - 1276 2017 

    三角格子反強磁性XXZ模型の飽和磁場近傍での希薄マグノンのBose凝縮と、その結果として現れる磁気秩序に関して議論する。スピン空間で等方的なHeisenberg模型の場合、一般に高磁場相は2:1状態あるいはV状態と呼ばれる共面的な磁気構造をとると信じられている。本研究ではこの等方的な場合を含むスピンSのXXZ模型一般における高磁場相の磁気秩序に関して、熱力学極限における対称性の破れの観点から大規模厳密対角化の結果を用いた詳細な議論を行う。

  • Danshita Ippei; Tsuchiya Shunji
    Meeting Abstracts of the Physical Society of Japan The Physical Society of Japan 72 (0) 610 - 610 2017 

    光格子中のBose気体の超流動相を考える。整数充填率かつMott転移近傍では超流動秩序変数の振幅の振動に対応するHiggsモードが現れる。我々の以前の研究で、Ginzburg-Landau理論を用いて、Higgsモードに対する障壁ポテンシャルの効果を考え、Higgs束縛状態という新奇な振幅モードが現れることを示した。本研究では、Gutzwiller変分波動関数を用いてこのHiggs束縛状態を解析する。

  • Yamamoto, D.; Ueda, H.; Danshita, I.; Marmorini, G.; Momoi, T.; Shimokawa, T.
    Physical Review B 96 (1) 2469-9950 2017 [Refereed]
  • Danshita, I.; Tsuchiya, S.
    Physical Review A AMER PHYSICAL SOC 96 (4) 2469-9926 2017 [Refereed]
     
    We study effects of a potential barrier on collective modes of superfluid Bose gases in optical lattices. We assume that the barrier is created by local suppression of the hopping amplitude. When the system is in a close vicinity of the Mott transition at commensurate fillings, where an approximate particle-hole symmetry emerges, there exist bound states of Higgs amplitude mode that are localized around the barrier. By applying the Gutzwiller mean-field approximation to the Bose-Hubbard model, we analyze properties of normal modes of the system with a special focus on the Higgs bound states. We show that when the system moves away from the Mott transition point, the Higgs bound states turn into quasibound states due to inevitable breaking of the particle-hole symmetry. We use a stabilization method to compute the resonance energy and linewidth of the quasibound states. We compare the results obtained by the Gutzwiller approach with those by the Ginzburg-Landau theory. We find that the Higgs bound states survive even in a parameter region far from the Mott transition, where the Ginzburg-Landau theory fails.
  • Kunimi, M.; Danshita, I.
    Physical Review A AMER PHYSICAL SOC 95 (3) 2469-9926 2017 [Refereed]
     
    We study the decay of superflow via thermally activated phase slips in one-dimensional Bose gases in a shallow optical lattice. By using the Kramers formula, we numerically calculate the nucleation rate of a thermally activated phase slip for various values of the filling factor and flow velocity in the absence of a harmonic trapping potential. Within the local density approximation, we derive a formula connecting the phase-slip nucleation rate with the damping rate of a dipole oscillation of the Bose gas in the presence of a harmonic trap. We use the derived formula to directly compare our theory with the recent experiment done by the LENS group [L. Tanzi et al., Sci. Rep. 6, 25965 (2016)]. From the comparison, the observed damping of dipole oscillations in a weakly correlated and small velocity regime is attributed dominantly to thermally activated phase slips rather than quantum phase slips.
  • Goto, S.; Danshita, I.
    Physical Review A AMER PHYSICAL SOC 96 (6) 2469-9926 2017 [Refereed]
     
    Recently, a cooling scheme for ultracold atoms in a bilayer optical lattice has been proposed (A. Kantian et al., arXiv: 1609.03579). In their scheme, the energy offset between the two layers is increased dynamically such that the entropy of one layer is transferred to the other layer. Using the full-Hilbert-space approach, we compute cooling dynamics subjected to the scheme in order to show that their scheme fails to cool down two-component fermions. We develop an alternative cooling scheme for two-component fermions, in which the spin-exchange interaction of one layer is significantly reduced. Using both full-Hilbert-space and matrix-product-state approaches, we find that our scheme can decrease the temperature of the other layer by roughly half.
  • Giacomo Marmorini; Daisuke Yamamoto; Ippei Danshita
    PHYSICAL REVIEW B AMER PHYSICAL SOC 93 (22) 224402-1 - 224402-8 2469-9950 2016/06 [Refereed]
     
    The quantum triangular XXZ model has recently enjoyed a wealth of new theoretical results, especially in relation to the modeling of the Ba3CoSb2O9 compound. In particular, it has been understood that in a longitudinal magnetic field the umbrella (cone) phase, classically stable in all the easy-plane region of the ground-state phase diagram, is considerably reduced by the effect of quantum fluctuations. We provide more quantitative information for this phenomenon at arbitrary value of the site spin S, by employing the dilute Bose gas expansion, valid in the high-field regime; our results improve the available estimates of the 1/S expansion. We quantify the extent to which a higher spin suppresses the effect of quantum fluctuations. Besides, we show how in three-dimensional layered systems a relatively small antiferromagnetic interlayer coupling has a similar consequence of bringing back the umbrella phase in some part of the phase diagram.
  • Kazuma Nagao; Ippei Danshita
    PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS OXFORD UNIV PRESS INC 2016 (6) 063I01-1 - 063I01-16 2050-3911 2016/06 [Refereed]
     
    We study collective modes of superfluid Bose gases in optical lattices at commensurate fillings. We focus on the vicinity of the quantum phase transition to the Mott insulator, where there exists the Higgs amplitude mode in addition to the Nambu-Goldstone phase mode associated with the spontaneous U(1) symmetry breaking. We analyze finite-temperature effects on the damping of the collective modes by using an effective spin-1 model and field-theoretical methods based on the finite-temperature Green's function. We calculate the damping rates up to 1-loop order and evaluate them analytically and numerically. We show that the damping rate of the Higgs mode increases with increasing temperature but it remains underdamped up to the typical temperature achieved in experiments. Moreover, we find that the Nambu-Goldstone mode attenuates via a Landau damping process resulting from interactions with the Higgs mode and it can be overdamped at the typical temperature in a certain parameter region.
  • Takumi Ohta; Shu Tanaka; Ippei Danshita; Keisuke Totsuka
    PHYSICAL REVIEW B AMER PHYSICAL SOC 93 (16) 165423-1 - 165423-15 2469-9950 2016/04 [Refereed]
     
    We study the ground-state phase diagram and dynamics of the one-dimensional cluster model with several competing interactions. Paying particular attention to the relation between the entanglement spectrum (ES) and the bulk topological (winding) number, we first map out the ground-state phases of the model and determine the universality classes of the transitions from the exact solution. We then investigate the dynamical properties during interaction sweeps through the critical points of topological phase transitions. When the sweep speed is slow, the correlation functions and the entanglement entropy exhibit spatially periodic structures. On top of this, the levels in the ES oscillate temporally during the dynamics. By explicitly calculating the above quantities for excited states, we attribute these behaviors to the Bogoliubov quasiparticles generated near the critical points. We also show that the ES reflects the strength of the Majorana correlation even for the excited states.
  • Daisuke Yamamoto; Giacomo Marmorini; Ippei Danshita
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN PHYSICAL SOC JAPAN 85 (2) 024706-1 - 024706-6 0031-9015 2016/03 [Refereed]
     
    We study the magnetization process of the spin-1/2 antiferromagnetic Heisenberg model on a layered triangular lattice by means of a numerical cluster mean-field method with a scaling scheme (CMF+S). It has been known that antiferromagnetic spins on a two-dimensional (2D) triangular lattice with quantum fluctuations exhibit a one-third magnetization plateau in the magnetization curve under magnetic field. We demonstrate that the CMF+S quantitatively reproduces the magnetization curve including the stabilization of the plateau. We also discuss the effects of a finite interlayer coupling, which is unavoidable in real quasi-2D materials. It has been recently argued for a model of the layered-triangular-lattice compound Ba3CoSb2O9 that such interlayer coupling can induce an additional first-order transition at a strong field. We present the detailed CMF+S results for the magnetization and susceptibility curves of the fundamental Heisenberg Hamiltonian in the presence of magnetic field and weak antiferromagnetic interlayer coupling. The extra first-order transition appears as a quite small jump in the magnetization curve and a divergence in the susceptibility at a strong magnetic field similar to 0.712 of the saturation field.
  • Kunimi Masaya; Danshita Ippei
    Meeting Abstracts of the Physical Society of Japan The Physical Society of Japan 71 411 - 411 2016 

    一次元の浅い光格子中に閉じ込められたボース気体の熱的活性化によるphase slip rateをGross-Pitaevskii方程式とBogoliubov方程式を用いて計算した。最近行われた光格子中のdipole振動の実験との比較を行った結果を報告する。

  • Tezuka Masaki; Danshita Ippei; Hanada Masanori
    Meeting Abstracts of the Physical Society of Japan The Physical Society of Japan 71 440 - 440 2016 

    N個のフェルミオン準位の任意の4個がランダムに相互作用するSachdev-Ye-Kitaev模型を冷却気体系で実現する提案のため、この模型を研究する背景を述べ、結合原子・分子混合模型で複数の分子状態と原子状態の結合を制御することで、この模型が実現されることを示す。

  • Danshita Ippei; Tezuka Masaki; Hanada Masanori
    Meeting Abstracts of the Physical Society of Japan The Physical Society of Japan 71 441 - 441 2016 

    Sachdev-Ye-Kitaev（SYK）模型を光格子中の冷却気体系で実現する方法を提案する。SYK模型は全てのサイト間をランダムにつなぐ二体ホッピング項から成るが、光会合過程を介して分子状態を中間状態として取り入れることでそのような二体ホッピングが導入できる。SYK模型の定常状態として実現されるブラックホール的な状態の特徴的な性質を同定するために、時間非順序型相関関数の観測方法も提案する。

  • Takeru Nakayama; Ippei Danshita; Tetsuro Nikuni; Shunji Tsuchiya
    PHYSICAL REVIEW A AMER PHYSICAL SOC 92 (4) 043610-1 - 043610-19 1050-2947 2015/10 [Refereed]
     
    We study collective modes of superfluid Bose gases in optical lattices combined with potential barriers. We assume that the system is in the vicinity of the quantum phase transition to a Mott insulator at a commensurate filling, where emergent particle-hole symmetry gives rise to two types of collective mode, namely a gapless Nambu-Goldstone (NG) phase mode and a gapful Higgs amplitude mode. We consider two kinds of potential barrier: One does not break the particle-hole symmetry while the other does. In the presence of the former barrier, we find Higgs bound states that have binding energies lower than the bulk Higgs gap and are localized around the barrier. We analyze tunneling properties of the NG mode incident to both barriers to show that the latter barrier couples the Higgs bound states with the NG mode, leading to Fano resonance mediated by the bound states. Thanks to the universality of the underlying field theory, it is expected that Higgs bound states may be present also in other condensed-matter systems with a particle-hole symmetry and spontaneous breaking of a continuous symmetry, such as quantum dimer antiferromagnets, superconductors, and charge-density-wave materials.
  • Takumi Ohta; Shu Tanaka; Ippei Danshita; Keisuke Totsuka
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN PHYSICAL SOC JAPAN 84 (6) 063001-1 - 063001-4 0031-9015 2015/06 [Refereed]
     
    We numerically study quantum phase transitions and dynamical properties in the one-dimensional cluster model with several interactions by using the time-evolving block decimation method for infinite systems and the exact diagonalization. First, boundaries among several quantum phases of the model are determined from energy gap and each phase is characterized by order parameters and the entanglement spectrum (ES). We confirm that in the model with open boundary condition the degeneracy of the lowest levels in the ES corresponds to that of the ground states. Then, using the time-dependent Bogoliubov transformation with open boundary condition, we investigate dynamical properties during an interaction sweep through the critical point which separates two topological phases involving four-fold degeneracy in the ground state. After a slow sweep across the critical point, we observe spatially periodic structures in the string correlation functions and the entanglement entropy. It is shown that the periodicities stem from the Bogoliubov quasiparticles generated near the critical point.
  • Nagao Kazuma; Danshita Ippei
    Meeting Abstracts of the Physical Society of Japan The Physical Society of Japan (JPS) 70 571 - 571 2189-079X 2015
  • Daisuke Yamamoto; Giacomo Marmorini; Ippei Danshita
    PHYSICAL REVIEW LETTERS AMER PHYSICAL SOC 114 (2) 027201-1 - 027201-5 0031-9007 2015/01 [Refereed]
     
    Magnetization processes of spin-1/2 layered triangular-lattice antiferromagnets (TLAFs) under a magnetic field H are studied by means of a numerical cluster mean-field method with a scaling scheme. We find that small antiferromagnetic couplings between the layers give rise to several types of extra quantum phase transitions among different high-field coplanar phases. Especially, a field-induced first-order transition is found to occur at H approximate to 0.7H(s), where H-s is the saturation field, as another common quantum effect of ideal TLAFs in addition to the well-established one-third plateau. Our microscopic model calculation with appropriate parameters shows excellent agreement with experiments on Ba3CoSb2O9 [T. Susuki et al., Phys. Rev. Lett. 110, 267201 (2013)]. Given this fact, we suggest that the Co2+-based compounds may allow for quantum simulations of intriguing properties of this simple frustrated model, such as quantum criticality and supersolid states.
  • Ippei Danshita; Daisuke Yamamoto; Yasuyuki Kato
    PHYSICAL REVIEW A AMER PHYSICAL SOC 91 (1) 013630-1 - 013630-17 1050-2947 2015/01 [Refereed]
     
    We study superfluid (SF) states of strongly interacting Bose-Bose mixtures with equal mass and intracomponent interaction in optical lattices both in the presence and absence of a barrier potential. We show that the SF order parameters obey the two-component nonlinear Schrodinger equation (NLSE) with not only cubic but also quintic nonlinearity in the vicinity of the first-order transitions to the Mott insulators with even fillings. In the case of no barrier potential, we analyze solitary-wave solutions of the cubic-quintic NLSE. When the SF state changes from a ground state to a metastable one, a standard dark solitary wave turns into a bubblelike dark solitary wave, which has a nonvanishing density dip and no pi-phase kink even in the case of a standing solitary wave. It is shown that the former and latter solitary waves are dynamically unstable against an out-of-phase fluctuation and an in-phase fluctuation, respectively, and the dynamical instabilities are weakened when one approaches the transition point. We find that the size and the inertial mass of the solitary waves diverge at the first-order transition point. We suggest that the divergence of the inertial mass may be detected through measurement of the relation between the velocity and the phase jump of the solitary wave. In the presence of a barrier potential, we reveal that when the barrier strength exceeds a certain critical value, the SF state that was metastable without the barrier is destabilized towards complete disjunction of the SF. The presence of the critical barrier strength indicates that the strong barrier potential qualitatively changes the criticality near the metastability limit of the SF state. We derive critical behaviors of the density, the compressibility, and the critical current near the metastability limit induced by the barrier. It is also found that the relation between the supercurrent and the phase jump across the barrier exhibits a peculiar behavior, owing to the nontopological nature of the bubblelike solitary wave.
  • J. B. Reeves; B. Gadway; T. Bergeman; I. Danshita; D. Schneble
    NEW JOURNAL OF PHYSICS IOP PUBLISHING LTD 16 (6) 065011-1 - 065011-8 1367-2630 2014/06 [Refereed]
     
    We investigate the interplay of disorder and interactions in the accelerated transport of a Bose-Einstein condensate through an incommensurate optical lattice. We show that interactions can effectively cancel the damping of Bloch oscillations (BOs) due to the disordered potential and we provide a simple model to qualitatively capture this screening effect. We find that the characteristic interaction energy, above which interactions and disorder cooperate to enhance, rather than reduce, the damping of BOs, coincides with the average disorder depth. This is consistent with results of a mean-field simulation.
  • Daisuke Yamamoto; Giacomo Marmorini; Ippei Danshita
    PHYSICAL REVIEW LETTERS AMER PHYSICAL SOC 112 (25) 0031-9007 2014/06 [Refereed]
  • Ippei Danshita; Rafael Hipolito; Vadim Oganesyan; Anatoli Polkovnikov
    PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS OXFORD UNIV PRESS INC 2014 (4) 043I03-1 - 043I03-8 2050-3911 2014/04 [Refereed]
     
    We propose an experimental scheme for studying the emergence of effectively classical nonlinear revivals-the Fermi-Pasta-Ulam (FPU) phenomenon-in interacting quantum many-body systems, e.g. ultracold atoms. Specifically, we suggest and analyze a setup of one-dimensional Bose gases confined into an optical lattice. The strength of quantum fluctuations is controlled by tuning the number of atoms per lattice sites (filling factor). By simulating the real-time dynamics of the Bose-Hubbard model by means of the exact numerical method of time-evolving block decimation, we investigate the effects of quantum fluctuations on the FPU recurrence and show that strong quantum fluctuations cause significant damping of the FPU oscillation.
  • Ippei Danshita
    JOURNAL OF LOW TEMPERATURE PHYSICS SPRINGER/PLENUM PUBLISHERS 175 (1-2) 222 - 228 0022-2291 2014/04 [Refereed]
     
    We discuss transport of trapped one-dimensional superfluids through a single impurity potential, in connection with quantum phase-slip nucleation rate I". We specifically consider damping of dipole oscillations induced by sudden displacement of the trapping potential, which has been investigated in previous experiments. Applying the time-evolving block decimation method to the 1D Bose-Hubbard model with an impurity potential in the hardcore limit, we calculate the dynamics of dipole oscillations and extract the damping rate from the oscillations. We show that there is a broad parameter region in which the damping rate G of the oscillation obeys the formula GaeI"/vaev (2K-2) with the Tomonaga-Luttinger parameter K, regardless of whether the impurity potential is repulsive or attractive. We find that in that parameter region the damping rate is almost symmetric with respect to the change of the sign of the impurity strength.
  • Daisuke Yamamoto; Takeshi Ozaki; Carlos A. R. Sa de Melo; Ippei Danshita
    JOURNAL OF LOW TEMPERATURE PHYSICS SPRINGER/PLENUM PUBLISHERS 175 (1-2) 258 - 264 0022-2291 2014/04 [Refereed]
     
    We study a first-order phase transition between superfluid and Mott insulator phases in binary Bose mixtures loaded into a hypercubic optical lattice. The system is described by a two-component Bose-Hubbard model. Considering the difference between the two kinds of bosons in the intra-component interaction strength, we discuss the metastability of the system and the hysteresis associated with the first-order superfluid-Mott insulator transition. It is found that the sweeping of hopping amplitude induces a conventional hysteresis-loop behavior. We also find an anomalous hysteresis behavior when the chemical potential is varied. In the anomalous hysteresis, the phase transition occurs in a unidirectional way and a hysteresis loop does not form.
  • Daisuke Yamamoto; Giacomo Marmorini; Ippei Danshita
    PHYSICAL REVIEW LETTERS AMER PHYSICAL SOC 112 (12) 0031-9007 2014/03 [Refereed]
     
    The triangular lattice of S = 1/2 spins with XXZ anisotropy is a ubiquitous model for various frustrated systems in different contexts. We determine the quantum phase diagram of the model in the plane of the anisotropy parameter and the magnetic field by means of a large-size cluster mean-field method with a scaling scheme. We find that quantum fluctuations break up the nontrivial continuous degeneracy into two first-order phase transitions. In between the two transition boundaries, the degeneracy-lifting results in the emergence of a new coplanar phase not predicted in the classical counterpart of the model. We suggest that the quantum phase transition to the nonclassical coplanar state can be observed in triangular-lattice antiferromagnets with large easy-plane anisotropy or in the corresponding optical-lattice systems.
  • Yasuyuki Kato; Daisuke Yamamoto; Ippei Danshita
    PHYSICAL REVIEW LETTERS AMER PHYSICAL SOC 112 (5) 0031-9007 2014/02 [Refereed]
     
    Quantum criticality near a tricritical point is studied in the two-component Bose-Hubbard model on square lattices. The existence of a quantum tricritical point on a boundary of a superfluid-insulator transition is confirmed by quantum Monte Carlo simulations. Moreover, we analytically derive the quantum tricritical behaviors on the basis of an effective field theory. We find two significant features of the quantum tricriticality that are its characteristic chemical potential dependence of the superfluid transition temperature and a strong density fluctuation. We suggest that these features are directly observable in existing experimental setups of Bose-Bose mixtures in optical lattices.
  • Daisuke Yamamoto; Takeshi Ozaki; Carlos A. R. Sa de Melo; Ippei Danshita
    PHYSICAL REVIEW A AMER PHYSICAL SOC 88 (3) 1050-2947 2013/09 [Refereed]
     
    We study the first-order quantum phase transitions of Bose gases in optical lattices. A special emphasis is placed on an anomalous hysteresis behavior, in which the phase transition occurs in a unidirectional way and a hysteresis loop does not form. We first revisit the hardcore Bose-Hubbard model with dipole-dipole interactions on a triangular lattice to analyze accurately the ground-state phase diagram and the hysteresis using the cluster mean-field theory combined with cluster-size scaling. Details of the anomalous hysteresis are presented. We next consider the two-component and spin-1 Bose-Hubbard models on a hypercubic lattice and show that the anomalous hysteresis can emerge in these systems as well. In particular, for the former model, we discuss the experimental feasibility of the first-order transitions and the associated hysteresis. We also explain an underlying mechanism of the anomalous hysteresis by means of the Ginzburg-Landau theory. From the given cases, we conclude that the anomalous hysteresis is a ubiquitous phenomenon of systems with a phase region of lobe shape that is surrounded by the first-order boundary.
  • Daisuke Yamamoto; Ippei Danshita
    PHYSICAL REVIEW B AMER PHYSICAL SOC 88 (1) 1098-0121 2013/07 [Refereed]
     
    We study the magnetic phase diagram and hysteresis behavior of weakly coupled spin dimers on a triangular lattice using the cluster mean-field method with cluster-size scaling. We find that the magnetization curve has plateaus at 1/3 and 2/3 of the total magnetization, in which local singlet and triplet states form a superlattice pattern. Moreover, if increasing (decreasing) the magnetic field from the 1/3 (2/3) plateau, the Bose-Einstein condensation (BEC) of triplons occurs on the superlattice background, leading to the transition into "magnon supersolid" phase. We also find that the first-order transition between these solid states and the standard magnon BEC state exhibits an anomalous hysteresis upon cycling the magnetic field; the transition can occur only from solid to BEC, and the system cannot return to the initial solid state in the reverse process.
  • Ippei Danshita
    PHYSICAL REVIEW LETTERS AMER PHYSICAL SOC 111 (2) 0031-9007 2013/07 [Refereed]
     
    We study superflow decay via quantum phase slips in trapped one-dimensional (1D) quantum gases through dipole oscillations induced by sudden displacement of the trapping potential. We find the relation between the damping rate of the dipole oscillation G and the phase-slip nucleation rate Gamma as G proportional to Gamma/v, where v is the flow velocity. This relation allows us to show that damping of 1D Bose gases in optical lattices, which has been extensively studied in experiment, is due to quantum phase slips. It is also found that the damping rate versus the flow velocity obeys the scaling formula for an impurity potential even in the absence of an explicit impurity. We suggest that the damping rate at a finite temperature exhibits a universal crossover behavior upon changing the flow velocity.
  • Ippei Danshita; L. Mathey
    Physical Review A - Atomic, Molecular, and Optical Physics 87 (2) 1050-2947 2013/02 [Refereed]
     
    We study the quantum phases of one-dimensional Bose-Fermi mixtures in optical lattices. Assuming repulsive interparticle interactions, equal mass, and unit total filling, we calculate the ground-state phase diagram by means of both the Tomonaga-Luttinger liquid theory and time-evolving block decimation method. We demonstrate the existence of a counterflow superfluid phase of polaron pairs, which are composite particles consisting of two fermions and two bosonic holes, in a broad range of the parameter space. We find that this phase naturally emerges in 174Yb-173Yb mixtures, realized in recent experiments, at low temperatures. © 2013 American Physical Society.
  • Daisuke Yamamoto; Akiko Masaki; Ippei Danshita
    PHYSICAL REVIEW B AMER PHYSICAL SOC 86 (5) 1098-0121 2012/08 [Refereed]
     
    We study the ground-state phase diagrams of hardcore bosons with long-range interactions on a square lattice using the linear spin-wave theory and a cluster mean-field method. Specifically, we consider the two types of long-range interaction: One consists only of the nearest-and next-nearest-neighbor interactions, and the other is the dipole-dipole interaction that decays with the interparticle distance r as similar to r(-3). It is known from previous analyses by quantum Monte Carlo methods that a checkerboard supersolid (CSS) is absent in the ground-state phase diagram of the former case while it is present in the latter. In the former, we find that quantum fluctuations around mean-field solutions are enhanced by the direct competition between the checkerboard and striped solid orders and that they destabilize the CSS phase. On the other hand, the emergence of the CSS phase in the latter case can be attributed to the absence of such a competition with other solid orders. We also show that the cluster mean-field method allows for the determination of phase boundaries in a precise quantitative manner when scaling with respect to the cluster size is taken into account. It is found that the phase transition between the superfluid and the solid (or CSS) is of the first order in the vicinity of the particle-hole symmetric line.
  • Takuya Saito; Ippei Danshita; Takeshi Ozaki; Tetsuro Nikuni
    PHYSICAL REVIEW A AMER PHYSICAL SOC 86 (2) 1050-2947 2012/08 [Refereed]
     
    We study the stability of superflow of Bose gases in optical lattices by analyzing the Bose-Hubbard model within the Gutzwiller mean-field approximation. We calculate the excitation spectra of the homogeneous Bose-Hubbard model at unit filling to determine the critical momenta for the Landau and dynamical instabilities. These two critical momenta are shown to approach each other when the on-site interaction increases towards the Mott transition point. In order to make a direct connection with realistic experiments, we next take into account a parabolic trapping potential and compute the real-time dynamics of dipole oscillations induced by suddenly displacing the trap center. We consider the following two cases: standard soft-core bosons, whose interparticle interactions include the on-site one only; and hard-core bosons with long-range dipole-dipole interactions. For both cases, we show that the dipole oscillation is significantly damped when the maximum local momentum exceeds a certain threshold, which quantitatively agrees with the critical momentum for the dynamical instability in the homogeneous system. In the case of dipolar hard-core bosons, the dynamical instability of dipole oscillations leads to the formation of checkerboard density waves in the superfluid phase near the boundary to the supersolid phase.
  • Ippei Danshita; Anatoli Polkovnikov
    PHYSICAL REVIEW A AMER PHYSICAL SOC 85 (2) 1050-2947 2012/02 [Refereed]
     
    We study the decay of superflow of a one-dimensional (1D) superfluid in the presence of a periodic potential. In 1D, superflow at zero temperature can decay via quantum nucleation of phase slips even when the flow velocity is much smaller than the critical velocity predicted by mean-field theories. Applying the instanton method to the O(2) quantum rotor model, we calculate the nucleation rate of quantum phase slips Gamma. When the flow momentum p is small, we find that the nucleation rate per unit length increases algebraically with p as Gamma/L proportional to p(2K-2), where L is the system size and K is the Tomonaga-Luttinger parameter. Based on the relation between the nucleation rate and the quantum superfluid-insulator transition, we present a unified explanation on the scaling formulas of the nucleation rate for periodic, disorder, and single-barrier potentials. Using the time-evolving block decimation method, we compute the exact quantum dynamics of the superflow decay in the 1D Bose-Hubbard model at unit filling. From the numerical analyses, we show that the scaling formula is valid for the case of the Bose-Hubbard model, which can quantitatively describe Bose gases in optical lattices.
  • Daisuke Yamamoto; Ippei Danshita; Carlos A. R. Sa de Melo
    PHYSICAL REVIEW A AMER PHYSICAL SOC 85 (2) 1050-2947 2012/02 [Refereed]
     
    We study phase transitions and hysteresis in a system of dipolar bosons loaded into triangular optical lattices at zero temperature. We find that the quantum melting transition from supersolid to superfluid phase is first order, in contrast with the previous report. We also find that due to strong quantum fluctuations the supersolid ( or solid)-superfluid transition can exhibit an anomalous hysteretic behavior, in which the curve of density versus chemical potential does not form a standard loop structure. Furthermore, we show that the transition occurs unidirectionally along the anomalous hysteresis curve.
  • Daisuke Yamamoto; Ippei Danshita; Carlos A R Sá De Melo
    Journal of Physics: Conference Series Institute of Physics Publishing 400 (1) 012083  1742-6596 2012 [Refereed]
     
    We investigate the hysteresis characteristics in a system of dipolar Bose gases loaded into a triangular lattice. We perform a large-size cluster mean-field analysis for the corresponding dipolar Bose-Hubbard model in the hard-core boson limit and show that in varying the chemical potential the system can exhibit a hysteretic behavior without forming the standard "hysteresis-loop" structure. In this anomalous hysteresis, the quantum melting transition from the supersolid (or solid) to superfluid state can occur, but the reverse process (quantum solidification) is impossible. © Published under licence by IOP Publishing Ltd.
  • Ippei Danshita; Anatoli Polkovnikov
    PHYSICAL REVIEW A AMER PHYSICAL SOC 84 (6) 1050-2947 2011/12 [Refereed]
     
    We study the quantum phase transition between the superfluid and the Mott insulator in the one-dimensional (1D) Bose-Hubbard model. Using the time-evolving block-decimation method, we numerically calculate the tunneling splitting of two macroscopically distinct states with different winding numbers. From the scaling of the tunneling splitting with respect to the system size, we determine the critical point of the superfluid-to-Mott-insulator transition for arbitrary integer filling factors. We find that the critical values versus the filling factor in 1D, 2D, and 3D are well approximated by a simple analytical function. We also discuss the condition for determining the transition point from a perspective of the instanton method.
  • Anzi Hu; L. Mathey; Eite Tiesinga; Ippei Danshita; Carl J. Williams; Charles W. Clark
    PHYSICAL REVIEW A AMER PHYSICAL SOC 84 (4) 1050-2947 2011/10 [Refereed]
     
    We suggest an experimentally feasible procedure to observe counterflow and paired superfluidity in ultracold atom systems. We study the time evolution of one-dimensional mixtures of bosonic atoms in an optical lattice following an abrupt displacement of an additional weak confining potential. We find that the dynamic responses of the paired superfluid phase for attractive interspecies interactions and the counterflow superfluid phase for repulsive interactions are qualitatively distinct and reflect the quasi-long-range order that characterizes these phases. These findings suggest a clear experimental procedure for their detection, and give an intuitive insight into their dynamics.
  • Daisuke Yamamoto; Ippei Danshita
    Journal of Physics: Conference Series Institute of Physics Publishing 273 (1) 012020  1742-6596 2011 [Refereed]
     
    We investigate the stability of superflow of bosons with isotropic dipole-dipole interactions in a two-dimensional optical lattice. We perform linear stability analyses for the dipolar Bose-Hubbard model in the hardcore boson limit, and show that the superflow can exist in a supersolid phase unless the velocity exceeds a certain critical value and that the critical value is remarkably smaller than that in the standard superfluid phase. Additionally, it is found that there exists a parameter range in which the SS phases are stabilized by a finite superflow. We also discuss the influence of quantum fluctuations on these results within the cluster mean-field approximation. © Published under licence by IOP Publishing Ltd.
  • Ippei Danshita; Anatoli Polkovnikov
    PHYSICAL REVIEW B AMER PHYSICAL SOC 82 (9) 1098-0121 2010/09 [Refereed]
     
    We study the quantum dynamics of supercurrents of one-dimensional Bose gases in a ring optical lattice to verify instanton methods applied to coherent macroscopic quantum tunneling (MQT). We directly simulate the real-time quantum dynamics of supercurrents, where a coherent oscillation between two macroscopically distinct current states occurs due to MQT. The tunneling rate extracted from the coherent oscillation is compared with that given by the instanton method. We find that the instanton method is quantitatively accurate when the effective Planck's constant is sufficiently small. We also find phase slips associated with the oscillations.
  • Ippei Danshita; Daisuke Yamamoto
    Physical Review A - Atomic, Molecular, and Optical Physics 82 (1) 1050-2947 2010/07 [Refereed]
     
    We study superfluidity of supersolid phases of dipolar Bose gases in two-dimensional optical lattices. We perform linear stability analyses for the corresponding dipolar Bose-Hubbard model in the hard-core boson limit to show that a supersolid can have stable superflow until the flow velocity reaches a certain critical value. The critical velocity for the supersolid is found to be significantly smaller than that for a conventional superfluid phase. We propose that the critical velocity can be used as a signature to identify the superfluidity of the supersolid phase in experiment. © 2010 The American Physical Society.
  • Ippei Danshita; Daisuke Yamamoto
    PHYSICAL REVIEW A AMER PHYSICAL SOC 82 (1) 2469-9926 2010/07 [Refereed]
     
    We study superfluidity of supersolid phases of dipolar Bose gases in two-dimensional optical lattices. We perform linear stability analyses for the corresponding dipolar Bose-Hubbard model in the hard-core boson limit to show that a supersolid can have stable superflow until the flow velocity reaches a certain critical value. The critical velocity for the supersolid is found to be significantly smaller than that for a conventional superfluid phase. We propose that the critical velocity can be used as a signature to identify the superfluidity of the supersolid phase in experiment.
  • Yoshihiro Yunomae; Daisuke Yamamoto; Ippei Danshita; Nobuhiko Yokoshi; Shunji Tsuchiya
    Physical Review A - Atomic, Molecular, and Optical Physics 80 (6) 1050-2947 2009/12 [Refereed]
     
    We study the stability of superfluid Fermi gases in deep optical lattices in the BCS-Bose-Einstein condensation (BEC) crossover at zero temperature. Within the tight-binding attractive Hubbard model, we calculate the spectrum of the low-energy Anderson-Bogoliubov (AB) mode as well as the single-particle excitations in the presence of superfluid flow in order to determine the critical velocities. To obtain the spectrum of the AB mode, we calculate the density response function in the generalized random-phase approximation applying the Green's function formalism developed by Côté and Griffin to the Hubbard model. We find that the spectrum of the AB mode is separated from the particle-hole continuum having the characteristic rotonlike minimum at short wavelength due to the strong charge-density-wave fluctuations. The energy of the rotonlike minimum decreases with increasing the lattice velocity and it reaches zero at the critical velocity which is smaller than the pair-breaking velocity. This indicates that the superfluid state is energetically unstable due to the spontaneous emission of the short-wavelength rotonlike excitations of the AB mode instead due to pair breaking. We determine the critical velocities as functions of the interaction strength across the BCS-BEC crossover regime. © 2009 The American Physical Society.
  • Yoshihiro Yunomae; Daisuke Yamamoto; Ippei Danshita; Nobuhiko Yokoshi; Shunji Tsuchiya
    PHYSICAL REVIEW A AMER PHYSICAL SOC 80 (6) 2469-9926 2009/12 [Refereed]
     
    We study the stability of superfluid Fermi gases in deep optical lattices in the BCS-Bose-Einstein condensation (BEC) crossover at zero temperature. Within the tight-binding attractive Hubbard model, we calculate the spectrum of the low-energy Anderson-Bogoliubov (AB) mode as well as the single-particle excitations in the presence of superfluid flow in order to determine the critical velocities. To obtain the spectrum of the AB mode, we calculate the density response function in the generalized random-phase approximation applying the Green's function formalism developed by Cote and Griffin to the Hubbard model. We find that the spectrum of the AB mode is separated from the particle-hole continuum having the characteristic rotonlike minimum at short wavelength due to the strong charge-density-wave fluctuations. The energy of the rotonlike minimum decreases with increasing the lattice velocity and it reaches zero at the critical velocity which is smaller than the pair-breaking velocity. This indicates that the superfluid state is energetically unstable due to the spontaneous emission of the short-wavelength rotonlike excitations of the AB mode instead due to pair breaking. We determine the critical velocities as functions of the interaction strength across the BCS-BEC crossover regime.
  • R. V. Mishmash; I. Danshita; Charles W. Clark; L. D. Carr
    Physical Review A - Atomic, Molecular, and Optical Physics 80 (5) 1050-2947 2009/11 [Refereed]
     
    We present a fully quantum many-body treatment of dark solitons formed by ultracold bosonic atoms in one-dimensional optical lattices. Using time-evolving block decimation to simulate the single-band Bose-Hubbard Hamiltonian, we consider the quantum dynamics of density and phase engineered dark solitons as well as the quantum evolution of mean-field dark solitons injected into the quantum model. The former approach directly models how one may create quantum entangled dark solitons in experiment. While we have already presented results regarding the latter approach elsewhere, we expand upon those results in this work. In both cases, quantum fluctuations cause the dark soliton to fill in and may induce an inelasticity in soliton-soliton collisions. Comparisons are made to the Bogoliubov theory which predicts depletion into an anomalous mode that fills in the soliton. Our many-body treatment allows us to go beyond the Bogoliubov approximation and calculate explicitly the dynamics of the system's natural orbitals. © 2009 The American Physical Society.
  • Ippei Danshita; Carlos A. R. Sa de Melo
    PHYSICAL REVIEW LETTERS AMER PHYSICAL SOC 103 (22) 0031-9007 2009/11 [Refereed]
     
    We perform a stability analysis of superfluid (SF) and supersolid (SS) phases of polarized dipolar bosons in two-dimensional optical lattices at high filling factors and zero temperature, and obtain the phase boundaries between SF, checkerboard SS (CSS), striped SS (SSS), and collapse. We show that the phase diagram can be explored through the application of an external field and the tuning of its direction with respect to the optical lattice plane. In particular, we find a transition between the CSS and SSS phases.
  • R. V. Mishmash; I. Danshita; Charles W. Clark; L. D. Carr
    PHYSICAL REVIEW A AMER PHYSICAL SOC 80 (5) 2469-9926 2009/11 [Refereed]
     
    We present a fully quantum many-body treatment of dark solitons formed by ultracold bosonic atoms in one-dimensional optical lattices. Using time-evolving block decimation to simulate the single-band Bose-Hubbard Hamiltonian, we consider the quantum dynamics of density and phase engineered dark solitons as well as the quantum evolution of mean-field dark solitons injected into the quantum model. The former approach directly models how one may create quantum entangled dark solitons in experiment. While we have already presented results regarding the latter approach elsewhere [R. V. Mishmash and L. D. Carr, Phys. Rev. Lett. 103, 140403 (2009)], we expand upon those results in this work. In both cases, quantum fluctuations cause the dark soliton to fill in and may induce an inelasticity in soliton-soliton collisions. Comparisons are made to the Bogoliubov theory which predicts depletion into an anomalous mode that fills in the soliton. Our many-body treatment allows us to go beyond the Bogoliubov approximation and calculate explicitly the dynamics of the system's natural orbitals.
  • Anzi Hu; L. Mathey; Ippei Danshita; Eite Tiesinga; Carl J. Williams; Charles W. Clark
    Physical Review A - Atomic, Molecular, and Optical Physics 80 (2) 1050-2947 2009/08 [Refereed]
     
    We study the quantum phases of mixtures of ultracold bosonic atoms held in an optical lattice that confines motion or hopping to one spatial dimension. The phases are found by using the Tomonaga-Luttinger liquid theory as well as the numerical method of time-evolving block decimation (TEBD). We consider a binary mixture of equal density with repulsive intraspecies interactions and either repulsive or attractive interspecies interaction. For a homogeneous system, we find paired and counterflow superfluid phases at different filling and hopping energies. We also predict parameter regions in which these types of superfluid order coexist with charge-density wave order. We show that the Tomonaga-Luttinger liquid theory and the TEBD qualitatively agree on the location of the phase boundary to superfluidity. We then describe how these phases are modified and can be detected when an additional harmonic trap is present. In particular, we show how experimentally measurable quantities, such as time-of-flight images and the structure factor, can be used to distinguish the quantum phases. Finally, we suggest applying a Feshbach ramp to detect the paired superfluid state and a π/2 pulse followed by Bragg spectroscopy to detect the counterflow superfluid phase. © 2009 The American Physical Society.
  • Anzi Hu; L. Mathey; Ippei Danshita; Eite Tiesinga; Carl J. Williams; Charles W. Clark
    PHYSICAL REVIEW A AMER PHYSICAL SOC 80 (2) 2469-9926 2009/08 [Refereed]
     
    We study the quantum phases of mixtures of ultracold bosonic atoms held in an optical lattice that confines motion or hopping to one spatial dimension. The phases are found by using the Tomonaga-Luttinger liquid theory as well as the numerical method of time-evolving block decimation (TEBD). We consider a binary mixture of equal density with repulsive intraspecies interactions and either repulsive or attractive interspecies interaction. For a homogeneous system, we find paired and counterflow superfluid phases at different filling and hopping energies. We also predict parameter regions in which these types of superfluid order coexist with charge-density wave order. We show that the Tomonaga-Luttinger liquid theory and the TEBD qualitatively agree on the location of the phase boundary to superfluidity. We then describe how these phases are modified and can be detected when an additional harmonic trap is present. In particular, we show how experimentally measurable quantities, such as time-of-flight images and the structure factor, can be used to distinguish the quantum phases. Finally, we suggest applying a Feshbach ramp to detect the paired superfluid state and a pi/2 pulse followed by Bragg spectroscopy to detect the counterflow superfluid phase.
  • Ippei Danshita; Pascal Naidon
    PHYSICAL REVIEW A AMER PHYSICAL SOC 79 (4) 1050-2947 2009/04 [Refereed]
     
    We determine the ground-state properties of a gas of interacting bosonic atoms in a one-dimensional optical lattice. The system is modeled by the Bose-Hubbard Hamiltonian. We show how to apply the time-evolving block decimation method to systems with periodic boundary conditions and employ it as a reference to find the ground state of the Bose-Hubbard model. Results are compared with recently proposed approximate methods, such as Hartree-Fock-Bogoliubov (HFB) theories generalized for strong interactions and the variational Bijl-Dingle-Jastrow method. We find that all HFB methods do not bring any improvement to the Bogoliubov theory and therefore provide correct results only in the weakly interacting limit, where the system is deeply in the superfluid regime. On the other hand, the variational Bijl-Dingle-Jastrow method is applicable for much stronger interactions but is essentially limited to the superfluid regime as it reproduces the superfluid-Mott-insulator transition only qualitatively.
  • Y. Yunomae; I. Danshita; D. Yamamoto; N. Yokoshi; S. Tsuchiya
    Journal of Physics: Conference Series 150 (3) 1742-6588 2009 [Refereed]
     
    We study the stability of superfluid flow of two-component Fermi gases in one-dimensional optical lattices. We find that the density fluctuation mode, the so-called Anderson-Bogoliubov mode, has the roton-like structure as seen in superfluid 4He. With increasing supercurrent, one of the roton-like minima reaches zero before the pair breaking occurs. This means that the instability of the superfluid Fermi gas is due to the spontaneous emission of the roton-like excitations of the Anderson-Bogoliubov mode instead of due to the Cooper pair breaking. We calculate the critical velocity determined by the roton-like structure for one-dimensional optical lattices. © 2009 IOP Publishing Ltd.
  • Ippei Danshita; Charles W. Clark
    PHYSICAL REVIEW LETTERS AMER PHYSICAL SOC 102 (3) 0031-9007 2009/01 [Refereed]
     
    We study the dynamics of strongly correlated one-dimensional Bose gases in a combined harmonic and optical lattice potential subjected to sudden displacement of the confining potential. Using the time-evolving block decimation method, we perform a first-principles quantum many-body simulation of the experiment of Fertig et al. [Phys. Rev. Lett. 94, 120403 (2005)] across different values of the lattice depth ranging from the superfluid to the Mott insulator regimes. We find good quantitative agreement with this experiment: the damping of the dipole oscillations is significant even for shallow lattices, and the motion becomes overdamped with increasing lattice depth as observed. We show that the transition to overdamping is attributed to the decay of superfluid flow accelerated by quantum fluctuations, which occurs well before the emergence of Mott insulator domains.
  • L. Mathey; Ippei Danshita; Charles W. Clark
    PHYSICAL REVIEW A AMER PHYSICAL SOC 79 (1) 1050-2947 2009/01 [Refereed]
     
    We identify a one-dimensional supersolid phase in a binary mixture of near-hard-core bosons with weak, local interspecies repulsion. We find realistic conditions under which such a phase, defined here as the coexistence of quasisuperfluidity and quasi-charge-density-wave order, can be produced and observed in finite ultracold atom systems in a harmonic trap. Our analysis is based on Luttinger liquid theory supported by numerical calculations using the time-evolving block decimation method. Clear experimental signatures of these two orders can be found, respectively, in time-of-flight interference patterns and the structure factor S(k) derived from density correlations.
  • Ryoko Ichihara; Ippei Danshita; Tetsuro Nikuni
    PHYSICAL REVIEW A AMER PHYSICAL SOC 78 (6) 1050-2947 2008/12 [Refereed]
     
    We study stability of the first excited state of quasi-one-dimensional Bose-Einstein condensates in a double-well potential, which is called "pi state." The density notch in the pi state can be regarded as a standing dark soliton. From the excitation spectrum, we determine the critical barrier height, above which the pi state is dynamically unstable. We find that the critical barrier height decreases monotonically as the number of condensate atoms increases. We also simulate the dynamics of the pi state by solving the time-dependent Gross-Pitaevskii equation. We show that due to the dynamical instability the dark soliton starts to move away from the trap center and exhibits a large-amplitude oscillation.
  • Ippei Danshita; Carlos A. R. Sa de Melo; Charles W. Clark
    PHYSICAL REVIEW A AMER PHYSICAL SOC 77 (6) 1050-2947 2008/06 
    We study the quantum phases of bosons confined to a combined potential of a one-dimensional double-well optical lattice and a parabolic trap (two-legged ladders). We apply the time-evolving block decimation method to the corresponding ladders described by a two-legged Bose-Hubbard model. In the absence of a parabolic trap, the system of bosons in the double-well optical lattice exhibits a reentrant quantum phase transition between Mott insulator and superfluid phases at unit filling as the tilt of the double wells is increased. We show that this reentrant phase transition still occurs in the presence of a parabolic trap, and we suggest that it can be detected experimentally by measuring matter-wave interference patterns.
  • I. Danshita; J. E. Williams; C. A. R. Sa de Melo; C. W. Clark
    LASER PHYSICS MAIK NAUKA/INTERPERIODICA/SPRINGER 18 (3) 318 - 321 1054-660X 2008/03 [Refereed]
     
    We study the superfluid-to-Mott insulator transition of bosons in a two-legged ladder optical lattice of a type accessible in current experiments on double-well optical lattices. The zero-temperature phase diagram is mapped out, with a focus on its dependence upon interchain hopping and the tilt between double wells. We find that the unit-filling Mott phase exhibits a nonmonotonic behavior as a function of the tilt parameter, producing a reentrant phase transition between the Mott insulator and superfluid phases.
  • R. Ichihara; I. Danshita; K. Egawa; T. Nikuni
    LASER PHYSICS MAIK NAUKA/INTERPERIODICA/SPRINGER 18 (3) 314 - 317 1054-660X 2008/03 
    We investigate the stability of the first excited state, the so-called "pi-state," of Bose-Einstein condensates in a double-well potential. From the condition of complex excitation energies, we determine the critical barrier height, above which the pi-state is dynamically unstable. We find that the critical barrier height decreases monotonically as the number of condensate atoms increases. We also simulate the dynamics of the pi-state by solving the time-dependent Gross-Pitaevskii equation. Our simulation results show that the pi-state in the dynamically unstable region exhibits distinctively different behavior from that in the dynamically stable region.
  • I. Danshita; J. E. Williams; C. A. R. Sa de Melo; C. W. Clark
    PHYSICAL REVIEW A AMER PHYSICAL SOC 76 (4) 1050-2947 2007/10 [Refereed]
     
    We study the superfluid and insulating phases of bosons in double-well optical lattices, and focus on the specific example of a two-legged ladder, which is currently accessible in experiments. We obtain the zero-temperature phase diagram using both mean-field and time-evolving block decimation techniques. We find that the mean-field approach describes the correct phase boundaries only when the intrachain hopping is sufficiently small in comparison to the on-site repulsion. We show the dependence of the phase diagram on the interchain hopping or tilt between double wells. We find that the Mott-insulator phase at unit filling exhibits a nonmonotonic behavior as a function of the tilt parameter, producing a reentrant phase transition between Mott insulator and superfluid phases. Finally, we determine the critical point separating the insulating and superfluid phases at commensurate fillings, where the Berezinskii-Kosterlitz-Thouless transition occurs.
  • Ippei Danshita; Shunji Tsuchiya
    JOURNAL OF LOW TEMPERATURE PHYSICS SPRINGER/PLENUM PUBLISHERS 148 (3-4) 337 - 343 0022-2291 2007/08 [Refereed]
     
    We study elementary excitations of Bose-Einstein condensates in a one-dimensional periodic potential and discuss the stability of superfluid flow based on the Kronig-Penney model. We analytically solve the Bogoliubov equations and calculate the excitation spectrum. The Landau and dynamical instabilities occur in the first condensate band when the superfluid velocity exceeds certain critical values as in a sinusoidal potential. It is found that the onset of the Landau instability coincides with the point where the perfect transmission of low-energy excitations is forbidden, while the dynamical instability occurs when the effective mass is negative. The condensate band has a swallow-tail structure when the periodic potential is shallow compared to the mean-field energy. We find that the upper side of a swallow-tail is dynamically unstable although the excitation spectrum has a linear dispersion reflecting the positive effective mass.
  • Ippei Danshita; Shunji Tsuchiya
    PHYSICAL REVIEW A AMER PHYSICAL SOC 76 (1) 1050-2947 2007/07 [Refereed]
     
    In their recent paper [Phys. Rev. A 71, 033622 (2005)], Seaman studied Bloch states of the condensate wave function in a Kronig-Penney potential and calculated the band structure. They argued that the effective mass is always positive when a swallowtail energy loop is present in the band structure. In this Comment, we reexamine their argument by actually calculating the effective mass. It is found that there exists a region where the effective mass is negative even when a swallowtail is present. Based on this fact, we discuss the interpretation of swallowtails in terms of superfluidity.
  • Ippei Danshita; Shunji Tsuchiya
    PHYSICAL REVIEW A AMER PHYSICAL SOC 75 (3) 1050-2947 2007/03 [Refereed]
     
    We study the stability of Bose-Einstein condensates with superfluid currents in a one-dimensional periodic potential. By using the Kronig-Penney model, the condensate and Bogoliubov bands are analytically calculated and the stability of condensates in a periodic potential is discussed. The Landau and dynamical instabilities occur in a Kronig-Penney potential when the quasimomentum of the condensate exceeds certain critical values as in a sinusoidal potential. It is found that the onsets of the Landau and dynamical instabilities coincide with the point where the perfect transmission of low energy excitations through each potential barrier is forbidden. The Landau instability is caused by the excitations with small q and the dynamical instability is caused by the excitations with q=pi/a at their onsets, where q is the quasimomentum of excitation and a is the lattice constant. A swallow-tail energy loop appears at the edge of the first condensate band when the mean-field energy is sufficiently larger than the strength of the periodic potential. We find that the upper portion of the swallow-tail is always dynamically unstable, but the second Bogoliubov band has a phonon spectrum reflecting the positive effective mass.
  • K. Iigaya; S. Konabe; I. Danshita; T. Nikuni
    LASER PHYSICS MAIK NAUKA/INTERPERIODICA/SPRINGER 17 (2) 215 - 220 1054-660X 2007/02 
    We investigate the Landau damping of Bogoliubov excitations in a dilute Bose gas moving in an optical lattice at a finite temperature. Using a 1D tight-binding model, we explicitly obtain the Landau damping rate, the sign of which determines the stability of the condensate. We find that the sign changes at a certain velocity, which is exactly the same as the critical velocity determined by the Landau criterion of superfluidity. This coincidence reveals the microscopic mechanism of the Landau instability.
  • Kiyohito Iigaya; Satoru Konabe; Ippei Danshita; Tetsuro Nikuni
    PHYSICAL REVIEW A AMERICAN PHYSICAL SOC 74 (5) 1050-2947 2006/11 [Refereed]
     
    We investigate Landau damping of Bogoliubov excitations in a dilute Bose gas moving in an optical lattice at finite temperatures. Using a one-dimensional tight-binding model, we explicitly obtain the Landau damping rate, the sign of which determines the stability of the condensate. We find that the sign changes at a certain condensate velocity, which is exactly the same as the critical velocity determined by the Landau criterion of superfluidity. This coincidence of the critical velocities reveals the microscopic mechanism of the Landau instability. This instability mechanism shows that a thermal cloud plays a crucial role in the breakdown of superfluids, since the thermal cloud is a vital source of Landau damping. We also examine the possibility of simultaneous disappearance of all damping processes.
  • Ippei Danshita; Nobuhiko Yokoshi; Susumu Kurihara
    New Journal of Physics 8 1367-2630 2006/03 [Refereed]
     
    We consider the tunnelling of phonon excitations across a potential barrier spatially separating two condensates with different macroscopic phases. We analyse the relation between the phase difference φ of the two condensates and the transmission coefficient T by solving the Bogoliubov equations. It is found that T strongly depends on φ, and that the perfect transmission of low-energy excitations disappears when the phase difference reaches the critical value which gives the maximum supercurrent of the condensate. We also discuss the feasibility of observing the phase differences in experiments. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
  • ボース系超流動-絶縁体-超流動接合におけるフォノントンネリングの位相差依存性
    段下一平; 余越伸彦; 栗原 進
    New Journal of Physics 8 44  2006
  • I. Danshita; S. Kurihara; S. Tsuchiya
    AIP Conference Proceedings 850 41 - 42 0094-243X 2006 
    We investigate the excitation spectrum of a Bose-Einstein condensate in a Kronig-Penney potential. We solve the Bogoliubov equations analytically and obtain the band structure of the excitation spectrum. We find that the excitation spectrum is gapless and linear at low energies. This property is found to be attributed to the anomalous tunneling behavior of low energy excitations, which has been predicted by Kagan et al. © 2006 American Institute of Physics.
  • Kyota Egawa; Ippei Danshita; Nobuhiko Yokoshi; Susumu Kurihara
    AIP Conference Proceedings 850 43 - 44 0094-243X 2006 [Refereed]
     
    We investigate collective excitations of Bose-Einstein condensates in double-well traps. We numerically solve the Bogoliubov-de Gennes equations with a double-well trap and show that a crossover from the dipole mode to the Josephson plasma oscillation occurs in the lowest excitation. It is found that the anomalous tunneling property of the low energy excitations dominates the crossover. © 2006 American Institute of Physics.
  • 二重井戸中のボース・アインシュタイン凝縮体の集団励起
    段下一平; 江川恭太; 余越伸彦; 栗原 進
    Journal of the Physical Society of Japan 74 (12) 3179 - 3185 2005/12
  • Ippei Danshita; Kyota Egawa; Nobuhiko Yokoshi; Susumu Kurihara
    Journal of the Physical Society of Japan 74 (12) 3179 - 3185 0031-9015 2005/12 [Refereed]
     
    We investigate collective excitations of Bose-Einstein condensates at absolute zero in a double-well trap. We solve the Bogoliubov equations with a double-well trap, and show that the crossover from the dipole mode to the Josephson plasma mode occurs in the lowest energy excitation. It is found that the anomalous tunneling property of low energy excitations is crucial to the crossover. ©2005 The Physical Society of Japan.
  • Danshita, I; S Kurihara; S Tsuchiya
    PHYSICAL REVIEW A AMERICAN PHYSICAL SOC 72 (5) 1050-2947 2005/11 [Refereed]
     
    With use of the Kronig-Penney model, we study the excitation spectrum of a Bose-Einstein condensate in a one-dimensional periodic potential. We solve the Bogoliubov equations analytically and obtain the band structure of the excitation spectrum for arbitrary values of the lattice depth. We find that the excitation spectrum is gapless and linear at low energies, and that it is due to the anomalous tunneling of low-energy excitations, predicted by Kagan et al.
  • I. Danshita; N. Yokoshi; S. Kurihara
    Laser Physics 15 (2) 371 - 375 1054-660X 2005/02 [Refereed]
     
    We investigate the quasiresonant tunneling of phonon excitations across a potential barrier spatially separating two condensates with different macroscopic phases. We confirm that the quasiresonant tunneling occurs through the quasibound state. The quasibound state is induced by a potential well created by the condensate density. We analyze the relation between the phase difference Φ of two condensates and the transmission coefficient T by solving the Bogoliubov-de Gennes equations. As a result, the transmission coefficient T is found to depend strongly on Φ because of the Φ dependence of the quasibound state. Copyright © 2005 by MAIK "Nauka/Interperiodica" (Russia).
  • ボース・アインシュタイン凝縮体間のフォノントンネリングの位相差依存性
    段下一平; 余越伸彦; 栗原 進
    Journal of Low Temperature Physics 134 (1/2) 737 - 742 2004/01
  • Ippei Danshita; Nobuhiko Yokoshi; Susumu Kurihara
    Journal of Low Temperature Physics 134 (1-2) 737 - 742 0022-2291 2004/01 [Refereed]
     
    We consider the tunneling of phonon excitations across a potential barrier spatially separating two condensates, which have different phases. We analyze the relation between the phase difference of two condensates and the coefficient of tunneling transmission by solving the Bogoliubov-de Gennes equations. As a result, the transmission coefficient T depends strongly on because of the -dependent quasi-bound state. © 2004 Plenum Publishing Corporation.

MISC

Research Grants & Projects

  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2021/04 -2024/03 
    Author : 高須 洋介; 段下 一平
     
    本研究では、光格子中のボース凝縮またはフェルミ縮退領域まで冷却されたイッテルビウム(Yb)原子を用いて、制御された非平衡開放量子多体系の量子ダイナミクスを実験的に研究することで、これまでそれほど探究されなかった「制御された開放量子多体系」というフロンティアを開拓することを目的としている。孤立量子系で行ってきた、研究代表者の高須が遂行する実験的研究と研究分担者の段下が遂行する理論的研究の一致を開放量子系でも目指す。また散逸が誘起する、エンタングルメントの体積則・面積則の転移などの新たな物理現象の探索を行う。2021年度は、研究代表者の高須は、光格子中に捕獲されたボース原子およびフェルミ原子を用いた非平衡ダイナミクスの実験的測定を行った。また、研究分担者の段下は、ボソン冷却原子系、フェルミオン冷却原子系だけでなく、スピン系を含めた。新たな理論的な実験提案を含む様々な理論的研究を行った。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2018/06 -2023/03 
    Author : 高橋 義朗; 高須 洋介; 段下 一平
     
    光格子中に導入された極低温原子気体を用いて、特に、非標準的な光格子をデザインすることによって初めて可能となる特異な多重軌道の自由度と、2電子系原子のみが特別に有する高スピン対称性SU(N)に着目し、その二つの自由度が織りなすユニークな新量子物性を開拓することを目的とする。 この研究目的に向けて、1)すでに実験的な観測に成功した局在不純物スピンによる量子スピン輸送の研究について、新たに準安定状態に局在した不純物スピンの振舞いを直接観測することに成功した。また、大きな質量比を持つ原子混合系の研究においてYb,Er,Liの3種類の超低温混合原子系を実現し、特に、Yb原子との共同冷却によりErのボース・アインシュタイン凝縮を生成することに成功した。2）SU(N)スピン系１次元等方光格子における最低温度をポメランチュク冷却効果により実現した結果をさらに発展させて、スピンインバランスを導入した系についてもスピン相関を系統的に観測することに成功した。また、昨年度においてその基本的な振る舞いの観測に成功した、散逸によって駆動された負温度量子磁性について、特にスピン・チャージ分離を制御しながら系統的な測定を行い学術論文としてほぼまとめることができた。3)これまでに見出したYb原子リドベルク高軌道励起を、光ピンセットアレー中の単一Yb原子に対して、新しく準安定状態を経由したロスを極小にした方法での励起に成功した。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2018/04 -2021/03 
    Author : Danshita Ippei
     
    It is conjectured that investigating the temperature dependence of the quantum Lyapunov exponent, which characterizes the chaotic nature of quantum dynamics, is useful for judging whether or not a quantum many-body system is a holographically dual to a quantum gravity system. In this study, we have improved the numerical efficiency of methods based on the use of matrix product states for computing quantum many-body systems at finite temperatures. Moreover, we have proposed a way for creating quantum-degenerate gases at negative absolute temperatures with a geometric frustration, which may be used for realizing a Fermi gas on a flat band that is conjectured to be dual to a quantum gravity system.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2013/05 -2018/03 
    Author : TAKAHASHI YOSHIRO
     
    By using ultracold ytterbium atoms in an optical lattice which is a periodic potential, we could perform quantum simulation of quantum many-body system and contributed better understanding of quantum condensed matter. In particular, we achieved the following attainments like realization of topological Thouless pump, enhanced anti-ferromagnetic spin correlation in a SU(4) system, revealing the non-trivial effect of dissipation on the superfluid-Mott insulator transition, realization of Lieb optical lattice and observation of novel behavior of atoms in a flat band, revealing a new experimental platform suitable for the study of Kondo effect, development of a new method of quantum gas microscope, and the advanced theories related with the above topics.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2013/04 -2016/03 
    Author : Danshita Ippei
     
    We have studied the effects of impurities on properties of ultracold quantum gases, such as quantum phase transitions. Our main focus has been placed on single-component and two-component Bose gases, which are precisely controllable in experiments. We have successfully resolved some long-standing problems and found novel phenomena. Specifically, we have proposed a way to experimentally quantify the breakdown of superfluid flow due to quantum phase slips in a system of one-dimensional single-component Bose gases. As for two-component Bose gases in optical lattices, we have found surface critical phenomena caused by impurity potential. Moreover, we consider a relativistic superfluid of single-component Bose gases in optical lattices to predict the existence of a novel elementary excitation, namely the Higgs bound state.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2010 -2010 
    Author : DANSHITA Ippei
     
    The Fermi-Pasta-Ulam recurrence has been a foundation of the nonlinear physics in the sense that the pursuit of the understanding of the FPU recurrence led to the discovery of "solitons" and the development of chaos theory. Nevertheless, experimental observation of the FPU recurrence is very rare. In this work, we proposed that the FPU recurrence can be actually observed in experiments with one-dimensional Bose gases confined into an optical lattice. Moreover, we showed that strong quantum fluctuations cause significant damping of the FPU oscillation.
  • 日本学術振興会：科学研究費助成事業
    Date (from‐to) : 2005 -2007 
    Author : 段下 一平
     
    ごく最近、NISTの実験グループによって「二重井戸光格子」という斬新な光格子が実現され注目を集めている。二重井戸光格子とは単位胞が二重井戸ポテンシャルの二次元光格子で、波長の異なる二つの二次元光格子を重ね合わせて作られる。申請者は、その二次元平面と垂直な方向にも独立な周期ポテンシャルがあり、三次元空間で見ると二本脚梯子型ポテンシャルが二次元的に並んでいるとみなすことができることを指摘した。SrCu_2O_3などの銅酸化物やピロリン酸バナジル(VO)_2P_2O_7といった化合物もこのような二本脚梯子型の結晶構造を持つが、二重井戸光格子に閉じ込められているのはBose粒子であるという点でこの系は全く新しい未開拓な系であった。さらに、鎖内(鎖間)ホッピングt_‖(t_⊥)、on-site相互作用U、二重井戸の傾斜λ、サイトあたりの粒子数νといったを自在に調節できるという利点からも、新奇な現象の発見が期待されていた。 申請者は二本脚梯子の場合に焦点をあて、二重井戸光格子中のBose気体における絶対零度での超流動・絶縁体転移を理論的に調べた。Time-evolving block decimation法でBose-Hubbardモデルを数値的に解くことにより超流動・絶縁体転移の相境界を特定し、相境界が二重井戸に特有のパラメータである鎖間ホッピングt_⊥と傾斜λに顕著に依存することを明らかにした。特にν=1の場合に、二重井戸を傾けていくことによって、絶縁体から超流動に転移し、そしてまた絶縁体に戻るというリエントラント相転移が起こるという筋書きを示した。このリエントラント相転移が起こる理由が、λ=Uの点で二重井戸の両サイトに粒子が一つずついる状態と片方のサイトだけに粒子が二ついる状態が共鳴することにより、系が超流動相を指向するためであることを指摘した。

Media Coverage

  • 二次元での量子シミュレーションの性能を検証する新手法を確立 量子シミュレータの開発に貢献する研究成果
    Date : 2022/03
    Writer: Other than myself
    Publisher, broadcasting station: 近畿大学
    Program, newspaper magazine: NEWS RELEASE
    https://newscast.jp/news/6201240 Internet
  • 超低温の原子の気体が液滴となる新たな形成機構を解明 気体と液体の両方の特徴をもつ、物質の新しい状態
    Date : 2022/03
    Writer: Other than myself
    Publisher, broadcasting station: 近畿大学
    Program, newspaper magazine: NEWS RELEASE
    https://newscast.jp/news/1705688 Internet
  • 近大など、二次元での量子シミュレーションの性能を検証する新手法を確立
    Date : 2022/03
    Writer: Other than myself
    Publisher, broadcasting station: マイナビニュース
    https://news.mynavi.jp/techplus/article/20220323-2301346/ Internet
  • 約-270℃まで冷却した原子は気体と液体の特徴を持つことが判明、近大が確認
    Date : 2022/03
    Writer: Other than myself
    Publisher, broadcasting station: マイナビニュース
    https://news.mynavi.jp/techplus/article/20220304-2285228/ Internet
  • 冷却原子を用いた量子多体ダイナミクスの量子シミュレーション ―非局所相関の伝搬の観測とエネルギー保存則の検証に成功―
    Date : 2020/10
    Writer: Other than myself
    Publisher, broadcasting station: 近畿大学
    Program, newspaper magazine: NEWS RELEASE
    https://newscast.jp/news/3222759 Internet
  • エネルギー保存則 確認 京大・近大 量子計算 精度高まる
    Date : 2020/10
    Writer: Other than myself
    Program, newspaper magazine: 日刊工業新聞
    ページ27 Paper
  • 量子多体系の非局所相関とエネルギー再分配にて実験結果と理論計算が一致
    Date : 2020/10
    Writer: Other than myself
    Publisher, broadcasting station: マイナビニュース
    https://news.mynavi.jp/techplus/article/20201007-1382055/ Internet
  • 負の絶対温度を持つ気体 物性物理学の難題解明に貢献
    Date : 2020/04
    Writer: Other than myself
    Program, newspaper magazine: 科学新聞
    第3773号 ページ4 Paper
  • フラストレートした量子磁性体の量子シミュレーション方法を提唱 –負の絶対温度をもつ気体の有効利用–
    Date : 2020/03
    Writer: Other than myself
    Publisher, broadcasting station: 近畿大学
    Program, newspaper magazine: NEWS RELEASE
    https://www.kindai.ac.jp/news-pr/news-release/2020/03/019641.html Internet
  • 「近藤効果」の厳密な計算機シミュレーションに成功 「量子多体系」に関するさまざまな物理現象の謎の解明に期待
    Date : 2019/10
    Writer: Other than myself
    Publisher, broadcasting station: 近畿大学
    Program, newspaper magazine: NEWS RELEASE
    https://newscast.jp/news/863714 Internet
  • 見られていると絶縁体が安定化する －観測による量子多体状態の制御技術を確立－
    Date : 2017/12
    Writer: Other than myself
    Publisher, broadcasting station: 京都大学
    Program, newspaper magazine: 最新の研究成果を知る
    https://www.kyoto-u.ac.jp/ja/research-news/2017-12-25-1 Pr
  • 絶縁体は見られると安定する - 京大、量子多体状態の制御技術を確立
    Date : 2017/12
    Writer: Other than myself
    Publisher, broadcasting station: マイナビニュース
    https://news.mynavi.jp/techplus/article/20171227-563702/ Internet
  • Deeper understanding of quantum fluctuations in 'frustrated' layered magnetic crystals
    Date : 2015/03
    Writer: Other than myself
    Publisher, broadcasting station: PHYS.ORG
    https://phys.org/news/2015-03-deeper-quantum-fluctuations-frustrated-layered.html Internet
  • Unraveling a quantum phase transition
    Date : 2014/03
    Writer: Other than myself
    Program, newspaper magazine: nano werk
    https://www.nanowerk.com/news2/newsid=34857.php Internet
  • Simulations reveal exotic quantum phase transitions in optically trapped superfluid atoms
    Date : 2014/03
    Writer: Other than myself
    Publisher, broadcasting station: PHYS.ORG
    https://phys.org/news/2014-03-simulations-reveal-exotic-quantum-phase.html Internet

Academic Contribution

  • Associate Editor, Journal of the Physical Society of Japan
    Date (from-to) :2019/04/01-Today
    Role: Peer review
    Type: Peer review etc
    Organizer, responsible person: Ippei Danshita
  • Quantum Simulation of Novel Phenomena with Ultracold Atoms and Molecules 2024
    Date (from-to) :2024/04/22-2024/04/25
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Ippei Danshita (Chair), Tin-Lun Ho, Gyu-Boong Jo, Keisuke Totsuka
  • Quantum Simulation of Novel Phenomena with Ultracold Atoms and Molecules 2021
    Date (from-to) :2021/12/13-2021/12/16
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Ippei Danshita, Tin-Lun Ho, Gyu-Boong Jo (Chair)
  • The Fourth Kyoto-Beijing-Tokyo Workshop on Ultracold Atomic Gases
    Date (from-to) :2019/09/30-2019/10/03
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Ippei Danshita (Chair), Shunsuke Furukawa, Masatoshi Sato, Keisuke Totsuka, Masahito Ueda, Hui Zhai
  • Quantum Simulation of Novel Phenomena with Ultracold Atoms 2019
    Date (from-to) :2019/05/06-2019/05/07
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Ippei Danshita, Tin-Lun Ho, Gyu-Boong Jo (Chair)
  • Novel Quantum States in Condensed Matter 2017
    Date (from-to) :2017/10/23-2017/11/24
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Sumio Ishihara, Hirokazu Tsunetsugu, Masao Ogata, Yukitoshi Motome, Takami Tohyama, Shuichi Murakami, Yoichi Yanase, Masatoshi Sato (Chair), Keisuke Totsuka, Ippei Danshita, Manfred Sigrist, Leon Balents
  • Quantum Thermodynamics: Thermalization and Fluctuations
    Date (from-to) :2017/09/27-2017/09/30
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Ippei Danshita, Keisuke Fujii, Hisao Hayakawa, Eiki Iyoda, Keiji Saito, Takahiro Sagawa (Chair), Hal Tasaki, Yoshiro Takahashi
  • 多自由度と相関効果が生み出す超伝導の新潮流〜BCSからBECまで〜
    Date (from-to) :2015/06/09-2015/06/10
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: 池田 浩章（代表）、 有田 亮太郎、大野 義章、黒木 和彦、紺谷 浩、段下 一平、遠山 貴己、松田 祐司、柳瀬 陽一
  • Novel Quantum States in Condensed Matter 2014
    Date (from-to) :2014/11/04-2014/12/05
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Hikaru Kawamura, Leon Balents, Ippei Danshita, Norio Kawakami, Yoshio Kuramoto, Frederic Mila, Naoto Nagaosa, Masao Ogata, Takami Tohyama (Chair), Keisuke Totsuka, Hirokazu Tsunetsugu
  • Higgs Modes in Condensed Matter and Quantum Gases
    Date (from-to) :2014/06/23-2014/06/25
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Ippei Danshita (Chair), Yusuke Kato, Norio Kawakami, Muneto Nitta, Masaki Oshikawa, Yoshiro Takahashi, Keisuke Totsuka, Shunji Tsuchiya, Naoto Tsuji
  • Ultracold Gases: Superfluidity and Strong Correlations
    Date (from-to) :2012/01/11-2012/01/13
    Role: Planning etc
    Type: Academic society etc
    Organizer, responsible person: Masahiko Okumura, Ippei Danshita, Shunji Tsuchiya, Keita Kobayashi, Atsushi Yamamoto, Shohei Watabe, Yoji Ohashi, Masahiko Machida (Chair)

Other link

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