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。     

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。     

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     

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。     

超冷费米原子气的状态方程

理解强相互作用粒子系统的行为是当今凝聚态物理学家所面临的挑战之一.中子星是一类典型的量子多体系统.当中子间的相互作用达到量子力学允许的最大的有效值时,其中所有的物质将共享同样的热力学特性;这样的状态被称为单一性极限(unitary limit).许多理论物理学家认为,6Li超冷费米原子气是研究原子间强相互作用的极佳对象.     

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．     

用于冷原子的高精度磁场锁定系统

磁场调控的Feshbach共振是调控原子间相互作用最常用的基本工具,减小磁场起伏,对于提高超冷原子散射共振的稳定性有着重要意义.本文通过一套分流磁场锁定系统,实现了百高斯磁场下相对不确定度为10^-6量级的磁场锁定,相较于未经锁定时,低频电流噪声得到45 dB以上的抑制.利用本文的锁定方法,~6Li原子团Rabi振荡相干时间提高了9.6倍,有效延长了超冷原子系统的相干时间.同时根据原子的Raman损耗谱标定了磁场均方根噪声,通过选择无相互作用的528 Gs (1 Gs=10^-4 T)处进行检测,磁场均方根噪声抑制到1.2 mGs,相较于未经锁定时,磁场均方根噪声降低16倍,磁场锁定相对不确定度为2.27×10^-6.这样的磁场锁定系统,可以为超冷原子气体提供精确稳定的背景磁场,对延长量子存储寿命、精确调控原子散射、开展凝聚态物理模拟等超冷量子气体实验有重要意义.     

费米6Li冷原子装载动力学的研究

我们用简单的系统实现了6 Li费米原子的冷却及俘获,研究了磁光阱中冷原子的装载动力学.由于其质量小和反冲动量大等特点,装载、冷却及俘获过程和其他的碱金属原子具有极大的不同.我们测量了装载率、单体碰撞损失率和原子数与温度的关系.基于气体动能和原子碰撞散射的理论计算来解释实验的测量结果.实现的长寿命6Li冷原子对将来研究超冷费米原子的量子简并及其强相互作用具有重要的意义.     

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。     

Virial expansion for a strongly correlated Fermi system and its application to ultracold atomic Fermi gases

A strongly correlated Fermi system plays a fundamental role in very different areas of physics, from neutron stars, quark–gluon plasmas, to high temperature superconductors. Despite the broad applicability, it is notoriously difficult to be understood theoretically because of the absence of a small interaction parameter. Recent achievements of ultracold trapped Fermi atoms near a Feshbach resonance have ushered in enormous changes. The unprecedented control of interaction, geometry and purity in these novel systems has led to many exciting experimental results, which are to be urgently understood at both low and finite temperatures. Here we review the latest developments of virial expansion for a strongly correlated Fermi gas and their applications on ultracold trapped Fermi atoms. We show remarkable, quantitative agreements between virial predictions and various recent experimental measurements at about the Fermi degenerate temperature. For equations of state, we discuss a practical way of determining high-order virial coefficients and use it to calculate accurately the long-sought third-order virial coefficient, which is now verified firmly in experiments at ENS and MIT. We discuss also virial expansion of a new many-body parameter—Tan’s contact. We then turn to less widely discussed issues of dynamical properties. For dynamic structure factors, the virial prediction agrees well with the measurement at the Swinburne University of Technology. For single-particle spectral functions, we show that the expansion up to the second order accounts for the main feature of momentum-resolved rf-spectroscopy for a resonantly interacting Fermi gas, as recently reported by JILA. In the near future, more practical applications with virial expansion are possible, owing to the ever-growing power in computation.     

Electric-field-modified Feshbach resonances in ultracold atom–molecule collision

We present a detailed analysis of near zero-energy Feshbach resonances in ultracold collisions of atom and molecule,taking the He–PH system as an example, subject to superimposed electric and magnetic static fields. We find that the electric field can induce Feshbach resonance which cannot occur when only a magnetic field is applied, through couplings of the adjacent rotational states of different parities. We show that the electric field can shift the position of the magnetic Feshbach resonance, and change the amplitude of resonance significantly. Finally, we demonstrate that, for narrow magnetic Feshbach resonance as in most cases of ultracold atom–molecule collision, the electric field may be used to modulate the resonance, because the width of resonance in electric field scale is relatively larger than that in magnetic field scale.     

Energy Spectrum of a Degenerate Fermi Gas at the BEC-BCS Crossover

A theoretical study of the BCS-BEC crossover is presented. Starting from a two-channel Boson-Fermion resonance model, the BCS-Bogoiubov mean-field method and the Green＇s function method are adopted. The result shows that we can end up with a BCS-type theory but with a composite order parameter. Calculation shows that the Bose condensate of BCS Cooper pairs is proportional to the molecular BEC of Bose molecules. The resonance superfluid phase is indicated by the energy spectrum with an obvious interpretation of the transition mechanism.     

Fulde–Ferrell–Larkin–Ovchinnikov states in equally populated Fermi gases in a two-dimensional moving optical lattice

We study the possibility of stabilizing a Fulde–Ferrell–Larkin–Ovchinnikov(FFLO) state in an equally populated two-component Fermi gas trapped in a moving two-dimensional optical lattice. For a system with nearly half filling, we find that a finite pairing momentum perpendicular to the moving direction can be spontaneously induced for a proper choice of lattice velocity. As a result, the total pairing momentum is tilted towards the nesting vector to take advantage of the significant enhancement of the density of states.     

Polarons in alkaline-earth-like atoms with multiple background Fermi surfaces

We study the impurity problem in a Fermi gas of ¹⁷³Yb atoms near an orbital Feshbach resonance (OFR), where a single moving particle in the ³P₀ state interacts with two background Fermi seas of particles in different nuclear states of the ground ¹S₀ manifold. By employing wave function ansatz to molecule and polaron states, we investigate various properties of the molecule, the attractive polaron, and the repulsive polaron states. In comparison to the case where only one Fermi sea is populated, we find that the presence of an additional Fermi sea acts as an energy shift between the two channels of the OFR. In addition, quantum fluctuations near the Fermi level can also induce sizable effects to various properties of the attractive and repulsive polarons.     

Pseudogap phenomena in ultracold atomic Fermi gases

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high Tc superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum sinmlator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high Tc superconductivity. One obstacle to the ultimate understand- ing of high Tc superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics comnnmity, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fuctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high Tc superconductivity.     

Radio-frequency spectroscopy of weakly bound molecules in ultracold Fermi gas

We create weakly bound Feshbach molecules in ultracold Fermi gas40K by sweeping a magnetic field across a broad Feshbach resonance point 202.2 G with a rate of 20 ms/G and perform the dissociation process using radio-frequency(RF) technology. From RF spectroscopy, we obtain the binding energy of the weakly bound molecules in the vicinity of Feshbach resonance. Our measurement also shows that the number of atoms generated from the dissociation process is different at various magnetic fields with the same RF amplitude, which gives us a deeper understanding of weakly bound Feshbach molecules.     

Boson-like quantum dynamics of association in ultracold Fermi gases

We study the collective association dynamics of a cold Fermi gas of 2N atoms in M atomic modes into a single molecular bosonic mode. When the atomic translational motion is slow compared to the atom-molecule conversion rate, the many-body fermionic problem for 2^M amplitudes is effectively reduced to a dynamical system of min{N, M} + 1 amplitudes, making the solution no more complex than the solution of a two-mode Bose-Einstein condensate and allowing realistic calculations with up to 10⁴ particles. The many-body dynamics is shown to be formally similar to the dynamics of the bosonic system under the mapping of boson particles to fermion holes, producing collective enhancement effects due to many-particle constructive interference.