超冷费米气体研究的新进展

近几年,人们在超冷费米气体的实验和理论研究方面取得了一系列激动人心的进展,比如分子玻色—爱因斯坦凝聚的成功实现和BCS-BEC渡越过程的大量研究。在本文中,我们对超冷费米气体中的这些进展进行了介绍,并讨论了今后几年的发展趋势。     

梯形近似方法在超冷费米气体中的应用

梯形近似方法是研究低密度、短程力系统的主要方法,在典型的实验条件下,超冷费米气体满足梯形近似方法的使用条件。本文我们首先对超冷费米气体进行简要介绍,接着介绍了多体物理中的梯形近似方法在其中的应用,希望有助于学生对梯形近似方法的理解和掌握。     

超冷~6Li费米气体的密度涨落和亚泊松分布

本文实验测量了无相互作用下6Li超冷费米原子气体密度分布的空间噪声涨落.在量子简并条件下,研究了理想费米气体的空间原子噪声涨落和量子简并度之间的关系,在实验上研究了泡利排斥对量子简并费米气体密度涨落的有效抑制,实现了低温费米量子气体的亚泊松分布测量.本文发展的原子密度噪声测量方法和测量结果在强相关多体系统的温度测量和观测不可压缩量子相的相变方面具有较大的应用前景.     

超冷费米气体的膨胀动力学研究新进展

多体系统的非平衡动力学演化是当前物理学中最具挑战性的问题之一.超冷量子费米原子气体具有较强的可控性,是研究多体非平衡动力学的理想系统,可以用来模拟和理解大爆炸后的早期宇宙、重离子碰撞中产生的夸克-胶子以及核物理等动力学.一般多体系统演化是非常复杂的,往往需要利用对称性来研究.利用Feshbach共振可以制备标度不变的费米原子气体:无相互作用和幺正费米量子气体.当远离平衡态时,可利用普适的指数和函数来刻画,其动力学可以通过对系统的时空演化进行标度变换来识别.本文主要介绍近年来强相互作用超冷费米气体的膨胀动力学研究进展,包括原子气体的各向异性展开、标度动力学和Efimovian膨胀动力学.     

超流费米气体相滑移时的密度分布

当前在冷原子和玻色爱因斯坦凝聚(BEC)领域的一个重要问题是在Feshbach共振附近的冷费米气体如何从BEC态演变到BCS(Bardeen Schrieffer Cooper)态.本文进一步研究在Feshbach共振附近超流态的相滑移现象.通过具体的数值计算，给出了费米气体在相滑移时的粒子数密度的分布，并对不同温度下的相滑移的大小进行了分析.结果表明，相滑移现象可以作为实验上判断系统是否处于超流态的一个可行的判据. 关键词： 超流费米气体 相滑移 Feshbach共振     

光腔中的超冷原子气体

近年来,光腔和冷原子气体的耦合系统受到了越来越多的关注。本文简要综述了近年来该领域在理论和实验方面的一些进展,重点关注其中的超辐射相变,并围绕光腔–原子耦合这一特征,分别介绍了超冷玻色气体和费米气体中的新奇量子相和量子相变。这些研究工作展示了该系统在非平衡态物理、多体系统的量子模拟、人造规范势和人造自旋–轨道耦合等方向的价值和意义。     

超冷费米原子气体的超流动性

最近美国MIT的Ketterle W教授和他的同事们对超冷费米原子气体具有超流动性作出了实验论证，他们观察到在锂-6原子气体形成玻色-爱因斯坦凝结时会出现涡流运动，涡流呈现出持久的无摩擦的流动特性．Ketterle研究组用激光束将冷冻的原子固定在各自的位置上，然后再分离出若干激光光束来激发出涡流．通常玻色原子与费米原子在低温下的量子行为是很不相同的．     

费米原子对的玻色-爱因斯坦凝聚(续)

费米原子组成的分子BEC之发现被认为是冷量子气体研究的一个里程碑．除本身的意义外，它还被作为向对凝聚超流体研究进军的桥头堡。     

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.     

玻色气体的磁性

物质磁性一直是凝聚态物理研究的重要课题.以往对磁性的探索主要是以费米子(局域或巡游的电子)为研究对象.由于传统的玻色系统液氦没有自旋,不表现磁性,玻色系统的磁性很少被关注.碱金属原子气体玻色-爱因斯坦凝聚的实现,在开辟了冷原子物理研究领域的同时,也打开了研究玻色系统磁性的大门.这是因为碱金属原子通常具有超精细结构,是旋量玻色气体,能够展示磁性.文章通过对比费米气体的相关结果,介绍了旋量玻色气体磁性的研究概况和最新进展,特别是铁磁性玻色气体的磁性相变以及在低温下铁磁性凝聚体的动力学特征.     

Density and spin response of a strongly interacting Fermi gas in the attractive and quasirepulsive regime

Recent experimental advances in ultracold Fermi gases allow for exploring response functions under different dynamical conditions. In particular, the issue of obtaining a "quasirepulsive" regime starting from a Fermi gas with an attractive interparticle interaction while avoiding the formation of the two-body bound state is currently debated. Here, we provide a calculation of the density and spin response for a wide range of temperature and coupling both in the attractive and quasirepulsive regime, whereby the system is assumed to evolve nonadiabatically toward the "upper branch" of the Fermi gas. A comparison is made with the available experimental data for these two quantities.     

Ultracold Atomic Fermi–Bose Mixtures in Bichromatic Optical Dipole Traps: A Novel Route to Study Fermion Superfluidity

The study of low density, ultracold atomic Fermi gases is a promising avenue to understand fermion superfluidity from first principles. One technique currently used to bring Fermi gases in the degenerate regime is sympathetic cooling through a reservoir made of an ultracold Bose gas. We discuss a proposal for trapping and cooling of two-species Fermi–Bose mixtures into optical dipole traps made from combinations of laser beams having two different wavelengths. In these bichromatic traps it is possible, by a proper choice of the relative laser powers, to selectively trap the two species in such a way that fermions experience a stronger confinement than bosons. As a consequence, a deep Fermi degeneracy can be reached having at the same time a softer degenerate regime for the Bose gas. This leads to an increase in the sympathetic cooling efficiency and allows for higher precision thermometry of the Fermi–Bose mixture.     

量子气体中的输运行为

输运测量是了解物质性质的一个重要手段.本文简单介绍最近在量子气体中实现的输运实验及其主要结论,包括在类似于介观物理器件中的Landauer输运和强相互作用费米气体中的自旋输运行为.我们着重讨论自旋动力学的特殊性以及其由于全同粒子相互作用所导致的特殊自旋扩散流的形式.     

Fermi gas of atoms

A rapidly developing field, experimental physics of ultracold gases of Fermi atoms, is briefly reviewed. The contribution of this field to fundamental physics is shown along with connection to other fields which explore systems of Fermi particles. The basic parameters of atomic Fermi gas are described together with its unique properties and advantages and disadvantages in comparison to other Fermi systems. The prospects of this field and its short history are considered. Research groups working in this field are listed.     

Temperature dependence of the universal contact parameter in a unitary Fermi gas

The contact I, introduced by Tan, has emerged as a key parameter characterizing universal properties of strongly interacting Fermi gases. For ultracold Fermi gases near a Feshbach resonance, the contact depends upon two quantities: the interaction parameter 1/(k(F)a), where k(F) is the Fermi wave vector and a is the s-wave scattering length, and the temperature T/T(F), where T(F) is the Fermi temperature. We present the first measurements of the temperature dependence of the contact in a unitary Fermi gas using Bragg spectroscopy. The contact is seen to follow the predicted decay with temperature and shows how pair-correlations at high momentum persist well above the superfluid transition temperature.     

“Slow” wave of zero-sound in degenerate Fermi gas

The dynamic Hartree-Fock theory with point-like interaction is used to calculate the speed of sound and damping factor of a zero-sound wave propagating in a degenerate Fermi gas. This wave propagates slower than Fermi velocity. It is shown, that if the interaction is weak and density is small, then the damping of such a wave can be small. A possibility of discovering such waves in ultracold Fermi gases is discussed.     

On the role of the uncertainty principle in superconductivity and superfluidity

We discuss the general interplay between the uncertainty principle and the onset of dissipationless transport phenomena such as superconductivity and superfluidity. We argue that these phenomena are possible because of the robustness of many-body quantum states with respect to the external environment, which is directly related to the uncertainty principle as applied to coordinates and momenta of the carriers. In the case of superconductors, this implies relationships between macroscopic quantities such as critical temperature and critical magnetic field, and microscopic quantities such as the amount of spatial squeezing of a Cooper pair and its correlation time. In the case of ultracold atomic Fermi gases, this should be paralleled by a connection between the critical temperature for the onset of superfluidity and the corresponding critical velocity. Tests of this conjecture are finally sketched with particular regard to the understanding of the behaviour of superconductors under external pressures or mesoscopic superconductors, and the possibility to mimic these effects in ultracold atomic Fermi gases using Feshbach resonances and atomic squeezed states.     

Critical temperature and thermodynamics of attractive fermions at unitarity

The unitarity regime of the BCS-BEC crossover can be realized by diluting a system of two-component lattice fermions with an on-site attractive interaction. We perform a systematic-error-free finite-temperature simulation of this system by diagrammatic determinant Monte Carlo method. The critical temperature in units of Fermi energy is found to be T(C)/epsilonF=0.152(7). We also report the behavior of the thermodynamic functions, and discuss the issues of thermometry of ultracold Fermi gases.