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中国物理学会期刊

一维吸引相互作用单自旋翻转费米气体的淬火动力学及量子颤振性质

Quench dynamics and quantum flutter properties of one-dimensional attractive single-spin flipped Fermi gases

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  • 在量子多体系统中,杂质可引发丰富的非平衡动力学行为.基于一维吸引相互作用费米气体的严格可解模型,系统研究了该体系中杂质诱导的淬火动力学.通过建立高效计算多体关联函数的解析方法,重点考察了杂质与背景粒子间的关联行为以及杂质动量的时间演化.结果表明:弱吸引相互作用下,杂质初始动量低于或高于费米动量时,系统分别呈现出束缚态或气态主导的特征;强吸引相互作用下,无论杂质初始动量是否大于费米动量,系统在局域关联处都呈现束缚态主导的特征;杂质动量的演化表现出量子颤振行为,其周期对应的能量与自旋向下粒子的化学势相吻合,并已通过热力学与离散Bethe ansatz方程等多种方法予以验证.本工作为深入理解吸引相互作用下量子多体系统的非平衡动力学提供了理论依据.

    The nonequilibrium dynamics of impurities in quantum many-body systems constitute a frontier topic in ultracold-atom physics and are key to uncovering the microscopic mechanisms of polaron physics and collective excitation behavior. Compared with the extensively studied repulsively interacting systems, the dynamical behavior under attractive interactions remains insufficiently explored in a systematic manner, owing to the complex coupling mechanisms among bound states, gas states, and irregular states. We take the ground state of a one-dimensional ideal Fermi gas as the initial state, and add a spin-down impurity particle carrying momentum. The impurity has an attractive interaction with the spin-up fermions. We study the time-evolution behavior of the two-body correlation functions between particles of different spins and the impurity momentum. First, based on the exact solution from the Bethe ansatz (BA) , starting from the eigenstate wave functions, we analytically simplify the overlap integrals, the matrix elements of the one-body correlation function of spin-up particles, and the matrix elements of the two-body correlation functions between particles of different spins into finite-sum expressions of simple functions, thereby enabling efficient computations of the occupation probabilities of a large number of eigenstates and the long-time evolution of the correlation functions. Second, in the weakly attractive regime: when the total momentum is less than or equal to the Fermi momentum, the dynamics exhibits features of bound states. The two-body correlation function between particles of different spins shows a stable correlation peak and an outward-propagating correlation hole, and its evolution period agrees with the period corresponding to the energy difference between the two bound states with the highest occupation probabilities. Conversely, when the total momentum is greater than the Fermi momentum, due to the superposition of bound states, gas states, and irregular states, the system exhibits complex oscillatory behavior, alternating between bound-state-like and gas-state-like features. In the strongly attractive regime, regardless of the initial momentum, the system exhibits bound-state-dominated features locally, characterized by a localized correlation peak and Friedel-like oscillations induced by scattering interference. Finally, we rigorously characterize the phenomenon of quantum flutter, namely the periodic oscillation of the impurity momentum, and find that the quantum-flutter period is strictly determined by the chemical potential of the spin-down impurity particle \mu_\downarrow \rm c, namely \tau=2\pi/|\mu_\downarrow \rm c|. We obtain consistent results for the chemical potential of the spin-down particle via four approaches: For zero total momentum, it can be obtained by deriving the critical magnetic field and the critical chemical potential at the quantum phase-transition point from the thermodynamic BA equations, and it can also be derived from the discrete BA equations via the energy required to inject a single spin-down particle into the ground state of the ideal Fermi gas; when the total momentum equals the Fermi momentum, the energy difference between the gas-state magnon and the bound-state exciton equals this chemical potential; when the total momentum is greater than the Fermi momentum, by using the relation between the single-particle excitation energy and the dressed energy, we can derive that the energy difference between the bound-state exciton states and the gas-state magnon states, which have relatively high occupation probabilities and form pairwise correspondences, equals this chemical potential. This work investigates the evolution laws of quench dynamics and quantum flutter in systems with attractive interactions, which helps deepen the understanding of nonequilibrium-state properties in quantum many-body physics and provides theoretical support for related ultracold-atom experiments on impurity dynamics.

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