强相互作用极化费米气体中涡旋束缚态的研究

涡旋态的研究对理解冷原子体系超流特性有很重要的价值.文章在简要回顾涡旋态研究历史的基础上,介绍了文章作者近期在超冷极化费米气体中涡旋态的工作.通过应用平均场的方法,从理论上研究了强相互作用极化费米气中单个涡旋态的结构.发现在涡旋态核中,Andreev束缚态的填充可引起一种量子相变.这就提供了一种新颖的探测Andreev束缚态的方法,在冷原子物理中,可通过吸收成像方法来完成.进一步文章作者对涡旋态的核尺寸进行了研究,发现涡旋态的核尺寸随着体系极化程度的增加而变大.     

强相互作用极化费米气体中涡旋束缚态的研究

涡旋态的研究对理解冷原子体系超流特性有很重要的价值. 文章在简要回顾涡旋态研究历史的基础上,介绍了文章作者近期在超冷极化费米气体中涡旋态的工作.通过应用平均场的方法,从理论上研究了强相互作用极化费米气中单个涡旋态的结构.发现在涡旋态核中,Andreev 束缚态的填充可引起一种量子相变.这就提供了一种新颖的探测Andreev 束缚态的方法,在冷原子物理中,可通过吸收成像方法来完成.进一步文章作者对涡旋态的核尺寸进行了研究,发现涡旋态的核尺寸随着体系极化程度的增加而变大.     

Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases

We analyze the properties of a single impurity immersed in a Fermi sea. At positive energy and scattering lengths, we show that the system possesses a well-defined but metastable excitation, the repulsive polaron, and we calculate its energy, quasiparticle residue and effective mass. From a thermodynamic argument we obtain the number of particles in the dressing cloud, illustrating the repulsive character of the polaron. Identifying the important 2- and 3-body decay channels, we furthermore calculate the lifetime of the repulsive polaron. The stability conditions for the formation of fully spin polarized (ferromagnetic) domains are then examined for a binary mixture of atoms with a general mass ratio. Our results indicate that mass imbalance lowers the critical interaction strength for phase-separation, but that very short quasiparticle decay times will complicate the experimental observation of itinerant ferromagnetism. Finally, we present the spectral function of the impurity for various coupling strengths and momenta.     

Normal state of highly polarized Fermi gases: The bound state

We consider a highly polarized Fermi gas with a single ↓ atom within a Fermi sea of ↑ atoms. We extend a preceding many-body analysis to the case where a bound state is formed between the ↓ atom and an ↑ atom.     

Normal state of highly polarized Fermi gases: full many-body treatment

We consider a single atom within a Fermi sea of atoms. We elucidate by a full many-body analysis the quite mysterious agreement between Monte Carlo results and approximate calculations taking only into account single particle-hole excitations. It results from a nearly perfect destructive interference of the contributions of states with more than one particle-hole pair. This is linked to the remarkable efficiency of the expansion in powers of hole wave vectors, the lowest order leading to perfect interference. Going up to two particle-hole pairs gives an essentially perfect agreement with known exact results. Hence our treatment amounts to an exact solution of this problem.     

Normal state of highly polarized Fermi gases: Full many-body treatment

We present a full many-body analysis of the problem of a single ↓ atom resonantly interacting with a Fermi sea of ↑ atoms. A series of successive approximations permits us to clarify the quite mysterious agreement between Monte Carlo results and approximate calculations taking only into account single particle-hole excitations. We show that it results from a nearly perfect destructive interference of the contributions of states with more than one particle-hole pair. Our treatment provides, at the same time, an essentially exact solution to this problem.     

Cold attractive spin polarized Fermi lattice gases and the doped positive U Hubbard model

Experiments on polarized fermion gases performed by trapping ultracold atoms in optical lattices allow the study of an attractive Hubbard model for which the strength of the on-site interaction is tuned by means of a Feshbach resonance. Using a well-known particle-hole transformation we discuss how results obtained for this system can be reinterpreted in the context of a doped repulsive Hubbard model. In particular, we show that the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state corresponds to the striped state of the two-dimensional doped positive U Hubbard model. We then use the results of numerical studies of the striped state to relate the periodicity of the FFLO state to the spin polarization. We also comment on the relationship of the d(x(2)-y(2)) superconducting phase of the doped 2D repulsive Hubbard model to a d-wave spin density wave state for the attractive case.     

Degenerate Fermi gases of ytterbium

Evaporative cooling was performed to cool fermionic 173Yb atoms in a crossed optical dipole trap. The large elastic collision rate leads to efficient evaporation and we have successfully cooled the atoms to 0.37+/-0.06 of the Fermi temperature, that is to say, to a quantum degenerate regime. In this regime, a plunge of evaporation efficiency was observed as a result of Fermi degeneracy.     

Thermal slip of Fermi gases

A kinetic equation which describes the behavior of Fermi gases in a half-spatial problem of thermal slip has been deduced for the first time. For the problem, an analytic solution has been found and the function of distribution of particles flying to a wall has been obtained in explicite form. It has been analyzed how the slip equation and velocity depend on the parameter that is defined as the ratio of the chemical potential to the product of Boltzmann’s constant by the absolute temperature. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 11–17, July, 2006.     

Collision of one dimensional (1D) spin polarized Fermi gases in an optical lattice

In this work we analyze the dynamical behavior of the collision between two clouds of fermionic atoms with opposite spin polarization. By means of the time-evolving block decimation (TEBD) numerical method, we simulate the collision of two one-dimensional clouds in a lattice. There is a symmetry in the collision behaviour between the attractive and repulsive interactions. We analyze the pair formation dynamics in the collision region, providing a quantitative analysis of the pair formation mechanism in terms of a simple two-site model.     

Metastability in spin-polarized Fermi gases

We study the role of particle transport and evaporation on the phase separation of an ultracold, spin-polarized atomic Fermi gas. We show that the previously observed deformation of the superfluid paired core is a result of evaporative depolarization of the superfluid due to a combination of enhanced evaporation at the center of the trap and the inhibition of spin transport at the normal-superfluid phase boundary. These factors contribute to a nonequilibrium jump in the chemical potentials at the phase boundary. Once formed, the deformed state is highly metastable, persisting for times of up to 2?s.     

Driving particle current through narrow channels using a classical pump

We study a symmetric exclusion process in which the hopping rates at two chosen adjacent sites vary periodically in time and have a relative phase difference. This mimics a colloidal suspension subjected to external time-dependent modulation of the local chemical potential. The two special sites act as a classical pump by generating an oscillatory current with a nonzero dc value whose direction depends on the applied phase difference. We analyze various features in this model through simulations and obtain an expression for the dc current via a novel perturbative treatment.     

Collisions in zero temperature Fermi gases

We examine the collisional behavior of two-component Fermi gases released at zero temperature from a harmonic trap. Using a phase-space formalism to calculate the collision rate during expansion, we find that Pauli blocking plays only a minor role for momentum changing collisions. As a result, for a large scattering cross section, Pauli blocking will not prevent the gas from entering the collisionally hydrodynamic regime. In contrast to the bosonic case, hydrodynamic expansion at very low temperatures is therefore not evidence for fermionic superfluidity.     

Universal Fermi gases in mixed dimensions

We investigate a two-species Fermi gas in which one species is confined in a two-dimensional plane (2D) or one-dimensional line (1D) while the other is free in the three-dimensional space (3D). We discuss the realization of such a system with the interspecies interaction tuned to resonance. When the mass ratio is in the range 0.0351相似文献   

BCS-BEC crossover in mix-dimensional Fermi gases

We investigate a mix-dimensional Fermi-Fermi mixtures in which one species is confined in two-dimensional (2D) space while the other is free in three-dimensional space (3D). We determine the superfluid transition temperature T _c for the entire BCS-BEC crossover including the important effects of noncondensed pairs. We find that the transition temperature reduces while the imbalance of mass is increased or lattice spacing is reduced. In spin imbalance case, the stability of superfluid is sharply destroyed by increasing the polarization.     

I-theorem for Fermi and Bose gases

The work is concerned with the change of discrimination information during self-organization of Fermi and Bose gases. A new measure of the discrimination information which determines the degree of order of states in the open systems is presented. The I-theorem for a system with known effective Hamiltonian and a system whose Hamiltonian form is not defined is proved.     

Magnetic properties of degenerate atomic Fermi gases

Magnetic effects in a degenerate atomic Fermi gas, such as the exchange enhancement of the paramagnetic susceptibility and the existence of the phase transition to the ferromagnetic state with the spontaneous polarization of the atomic spins, are discussed. The propagation of spin waves in the atomic system is considered.     

费米超流气体的非线性Landau-Zener 隧穿

在平均场近似下,通过对相平面和不动点的分析, 研究了非线性两能级系统中费米超流气体的Landau-Zener 隧穿现象. 研究发现,费米子间的相互作用能够显著地影响量子隧穿. 当相互作用参数c小于临界值c^*时,在绝热极限下隧穿仍然满足量子绝热定理, 而大于这一临界值时,量子绝热定理不再满足. 最后通过和线性情况比较,得到了c*时隧穿率与扫描速率间满足的指数关系.