[论文解读] Imbalanced Fermi Gases

K. B. Gubbels, H. T. C. Stoof|arXiv (Cornell University)|May 2, 2012

Cold Atom Physics and Bose-Einstein Condensates被引用 1

一句话总结

本综述研究了具有自旋和质量不平衡的强关联冷原子费米气体，重点探讨了相分离、无能隙Sarma超流以及超固态等超流相。通过平均场理论、图解方法和重整化群技术，该研究为描述超冷原子气体中的实验结果提供了定量框架，推动了对不同极化度和相互作用强度下相图的理解。

ABSTRACT

We consider imbalanced Fermi gases with strong attractive interactions, for which Cooper-pair formation plays an important role. The two-component mixtures consist either of identical fermionic atoms in two different hyperfine states, or of two different atomic species both occupying only a single hyperfine state. In both cases, the number of atoms for each component is allowed to be different, which leads to a spin imbalance, or spin polarization. Two different atomic species also lead to a mass imbalance. Imbalanced Fermi gases are relevant to condensed-matter physics, nuclear physics and astroparticle physics. They have been studied intensively in recent years, following their experimental realization in ultracold atomic Fermi gases. The experimental control in such a system allows for a systematic study of the equation of state and the phase diagram as a function of temperature, spin polarization and interaction strength. In this review, we discuss the progress in understanding strongly-interacting imbalanced Fermi gases, where a main goal is to describe the results of the highly controlled experiments. We start by discussing Feshbach resonances, after which we treat the imbalanced Fermi gas in mean-field theory to give an introduction to the relevant physics. We encounter several unusual superfluid phases, including phase separation, gapless Sarma superfluidity, and supersolidity. To obtain a more quantitative description of the experiments, we review also more sophisticated techniques, such as diagrammatic methods and the renormalization-group theory. We end the review by discussing two theoretical approaches to treat the inhomogeneous imbalanced Fermi gas, namely the Landau-Ginzburg theory and the Bogoliubov-de Gennes approach.

研究动机与目标

• 理解在实验调控下强关联非平衡费米气体的相图。
• 解释相分离和无能隙Sarma超流等奇异超流相的出现机制。
• 建立理论模型与超冷原子费米气体中实验观测之间的桥梁。
• 发展用于描述自旋和质量非平衡体系的定量理论工具。
• 利用朗道-金兹堡理论和Bogoliubov-de Gennes方法应对非均匀性带来的挑战。

提出的方法

• 应用平均场理论模拟非平衡费米系统中的库珀对形成。
• 利用Feshbach共振调节超冷原子气体中的相互作用强度。
• 采用图解技术实现对平均场理论的超越。
• 应用重整化群理论以捕捉标度不变行为。
• 构建朗道-金兹堡理论以描述非均匀超流相。
• 实施Bogoliubov-de Gennes方法以描述空间非均匀的超流序参量。

实验结果

研究问题

• RQ1 自旋不平衡如何影响超冷费米气体中库珀对的形成与稳定性？
• RQ2 在何种条件下，非平衡费米系统中会涌现出无能隙Sarma超流或相分离？
• RQ3 Feshbach共振如何实现对相互作用强度和极化的实验调控？
• RQ4 哪些理论框架能准确描述非平衡费米气体中的状态方程与相变行为？
• RQ5 阱中系统的非均匀性如何影响超流序参量的空间结构？

主要发现

• 由于费米面不匹配，强非平衡费米气体中出现相分离，导致正常相与超流相共存。
• 当费米面不匹配超过超流能隙时，无能隙Sarma超流得以稳定，其激发谱具有有限能量。
• 在某些参数区域，竞争序参量的存在使超固态成为可能的基态。
• 图解法与重整化群方法为平均场预测提供了定量修正，显著提升了与实验数据的一致性。
• 朗道-金兹堡理论与Bogoliubov-de Gennes方法成功描述了阱中非平衡费米气体的空间非均匀性。
• Feshbach共振实现了对相互作用强度的精确调控，使超冷原子气体成为研究非平衡超流的理想平台。