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。     

BCS-BEC渡越过程超流费米气体在线性变化势中的集体激发

基于超流流体力学方程组和标度理论,数值研究了Bardeen-Cooper-Schrieffer (BCS)-Bose-Einstein Condensate (BEC)渡越过程中超流费米气体在线性减小谐振子势下的集体激发.详细研究了包括谐振子势频率线性减小的速度,减小到的幅度以及势阱各向异性参数对集体激发的影响.通过频谱分析和数据拟合,分别研究了各向同性势和各向异性势下在最小振幅激发下所对应的模式,且数值计算得到的频率值在整个BCS-BEC渡越过程中与前期的理论结果符合得很好.该工作可以为后面超流费米气体集体激发的实验提供有益的理论参考     

BCS-BEC渡越过程中超流费米气体Efimovian膨胀的研究

基于耦合流体力学方程组,分别研究了各向同性和各向异性轴对称势阱下Bardeen-CooperSchrieffer–Bose-Einstein condensate(BCS-BEC)渡越过程中超流费米气体的Efimovian膨胀.当费米气体处于幺正极限,体系标度不变,气体膨胀尺寸展现出一系列平台结构,气体的演化由对数周期函数描述;当费米气体处于非幺正极限超流区域,体系标度不变性破缺,气体的演化偏离对数周期变化;另外我们还发现对于各向异性的费米气体,即使处于幺正极限,谐振子势的各向异性也会导致体系标度不变性破缺.     

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.     

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。     

超流费米气体中两维孤子的动力学

我们利用解析和数值的方法,研究从Bardeen-Cooper-Schrieffer(BCS)超流到玻色-爱因斯坦凝聚(BEC)渡越的过程里超流费米气体中两维(2D)孤子的形成和演化.基于超流流体力学方程,在准二维和长波近似下,推导描述弱非线性激发带正色散项的Kadomtsev-Petviashvili方程;给出整个BCS-BEC渡越的2D孤子解,以及数值求解孤子在囚禁势中的演化.数值结果显示由于Snake(横向)不稳定性,大振幅的暗孤子会衰变为大量涡旋-反涡旋对,并且这个不稳定性在不同超流区域不同.     

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．     

费米气体在调制缺陷晶格中的超流特性

本文研究了费米气体在调制缺陷的一维环形光晶格中的超流特性．在流体动力学模型和两模近似下,整个系统的动力学性质可以等效为单摆模型的哈密顿量,从而得到系统发生超流相变的临界条件ρc ．系统在BEC-BCS整个渡越区中,临界原子数密度将使得系统存在缺陷导致阻尼的正常态之外的另一种状态,即平面波连续穿过缺陷的超流态．系统的超流特性依赖于缺陷的强度、类型以及原子间的相互作用,并且由于缺陷与原子间相互作用的耦合,系统在BCS端更容易维持超流态。     

超冷费米原子气——40K

1995年 ,在美国科罗拉多州“实验室天体物理联合研究所”(简称ＪＩＬＡ)的科学家 ,对87Ｒｂ原子气 ,首次实现了玻色 -爱因斯坦凝聚 (ＢＥＣ)之后 ,ＢＥＣ实验又陆续在7Ｌｉ、2 3Ｎａ和1Ｈ等系统中获得成功 .在ＢＥＣ凝聚体中 ,超冷原子被锁定到单一量子态 ,从而使量子力学法则在宏观世界中得到了展示 .然而 ,上述进展仅仅是量子凝聚故事的一半 .如所周知 ,中性碱金属原子还有另外一半 ,即费米原子———6 Ｌｉ和4 0 Ｋ等 .它们在极低温条件下的行为将会怎样呢 ?最近 ,同是来自美国科罗拉多ＪＩＬＡ的物理学家Ｄ .Ｓ .Ｊｉｎ和她的研究生Ｂ .…     

强磁场中超冷费米气体的相对论效应

由单粒子的弱相对论能谱及泊松公式,导出强磁场中费米气体的热力学势函数.在此基础上,运用热力学关系求解低温条件下系统的统计特征量的解析式,分析相对论效应对统计性质的影响机理.研究表明,磁场愈强,相对论效应愈明显.相对论效应引发的单调项与相应的振荡项的振幅相比,对总能,单调项远大于振幅;对化学势及磁矩,单调项与振幅几乎同一量级. 关键词： 强磁场 费米气体 相对论效应     

Collective modes in asymmetric ultracold Fermi systems

We derive the long wavelength effective action for the collective modes in systems of fermions interacting via a short-range s-wave attraction, featuring unequal chemical potentials for the two fermionic species (asymmetric systems). As a consequence of the attractive interaction, fermions form a condensate that spontaneously breaks the U(1) symmetry associated with total number conservation. Therefore at sufficiently small temperatures and asymmetries, the system is a superfluid. We reproduce previous results for the stability conditions of the system as a function of the four-fermion coupling and asymmetry. We obtain these results analyzing the coefficients of the low energy effective Lagrangian of the modes describing fluctuations in the magnitude (Higgs mode) and in the phase (Nambu-Goldstone, or Anderson-Bogoliubov, mode) of the difermion condensate. We find that for certain values of parameters, the mass of the Higgs mode decreases with increasing mismatch between the chemical potentials of the two populations, if we keep the scattering length and the gap parameter constant. Furthermore, we find that the energy cost for creating a position dependent fluctuation of the condensate is constant in the gapped region and increases in the gapless region. These two features may lead to experimentally detectable effects. As an example, we argue that if the superfluid is put in rotation, the square of the radius of the outer core of a vortex should sharply increase on increasing the asymmetry, when we pass through the relevant region in the gapless superfluid phase. Finally, by gauging the global U(1) symmetry, we relate the coefficients of the effective Lagrangian of the Nambu-Goldstone mode with the screening masses of the gauge field.     

An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems

Recent developments in the study of ultracold Rydberg gases demand an adwanced level of experimental sophistication, in which high atomic and optical densities must be combined with excellent control of external fields and sensitive Rydberg atom detection. We describe a tailored experimental system used to produce and study Rydberg-interacting atoms excited from dense ultracold atomic gases. The experiment has been optimized for fast duty cycles using a high flux cold atom source and a three beam optical dipole trap. The latter enables tuning of the atomic density and temperature over several orders of magnitude, all the way to the Bose--Einstein condensation transition. An elec- trode structure surrounding the atoms allows for precise control over electric fields and single-particle sensitive field ionization detection of Rydberg atoms. We review two experiments which highlight the influence of strong Rydberg---Rydberg interactions on different many-body systems. First, the Rydberg blockade effect is used to pre-structure an atomic gas prior to its spontaneous evolution into an ultracold plasma. Second, hybrid states of photons and atoms called dark-state polaritons are studied. By looking at the statistical distribution of Rydberg excited atoms we reveal correlations between dark-state polaritons. These experiments will ultimately provide a deeper understanding of many-body phenomena in strongly-interacting regimes, including the study of strongly-coupled plasmas and interfaces between atoms and light at the quantum level.     

Evolution of the vortex state in the BCS-BEC crossover of a quasi two-dimensional superfluid Fermi gas

We use the path-integral formalism to investigate the vortex properties of a quasi-two dimensional(2D) Fermi superfluid system trapped in an optical lattice potential.Within the framework of mean-field theory,the cooper pair density,the atom number density,and the vortex core size are calculated from weakly interacting BCS regime to strongly coupled while weakly interacting BEC regime.Numerical results show that the atoms gradually penetrate into the vortex core as the system evolves from BEC to BCS regime.Meanwhile,the presence of the optical lattice allows us to analyze the vortex properties in the crossover from three-dimensional(3D) to 2D case.Furthermore,using a simple re-normalization procedure,we find that the two-body bound state exists only when the interaction is stronger than a critical one denoted by G_c which is obtained as a function of the lattice potential's parameter.Finally,we investigate the vortex core size and find that it grows with increasing interaction strength.In particular,by analyzing the behavior of the vortex core size in both BCS and BEC regimes,we find that the vortex core size behaves quite differently for positive and negative chemical potentials.     

单一杂质半导体费米能级的普适公式

利用电中性条件，导出了掺单一杂质半导体费米能级的谱适公式，在具体应用时可作相应简化。     

p‐Wave Superfluid Phases of Fermi Molecules in a Bilayer Lattice Array

The superfluid $\mathbf{p}=p_x+{\mathit{ip}}_y$ phases in an ultracold gas of dipolar Fermi molecules lying in two parallel square lattices in 2D are investigated. As shown by a two‐body study, dipole moments oriented in opposite directions in each layer are the key ingredients in our mean‐field analysis from which unconventional superfluidity is predicted. The $T=0$ phase diagram summarizes our findings: stable and metastable superfluid phases appear as a function of both, the dipole–dipole interaction coupling parameter and filling factor. A first‐order phase transition, and thus a mixture of superfluid phases at different densities, is revealed from the coexistence curves in the metastable region. The model predicts that these superfluid phases can be observed experimentally at 10 nK in molecules of NaK confined in optical lattices of size $a=532$ nm. Other routes to reach higher temperatures require the use of subwavelength confinement technique .     

The shear viscosity of a two component Fermi gas with unequal population at low temperatures

The shear viscosity of spin-polarized Fermi gas at low temperatures is calculated by using the Green’s functions method. In the BEC limit where a Feshbach resonance gives rise to tightly bound dimer molecules, the contributions of the interactions between dimer-atom and imer-dimer take into account to the viscous relaxation rate.     

高技术用混合气体节制冷循环

高技术用混合气体节制冷循环吴沛宜，许名尧（西安交通大学动力系西安７１００４９）关键词：技术，气体，制冷循环。ＴＨＥＴＨＲＯＴＴＬＩＮＧＲＥＦＲＩＧＥＲＡＴＩＯＮＣＹＣＬＥＳＵＳＩＮＧＭＩＸＩＮＧＧＡＳＥＳＵＳＥＤＩＮＨＩＧＨＴＥＣＨＮＩＱＵＥ￥ＷｕＰ...