光晶格中相互作用费米原子的反常膨胀
Anomalous Expansion of Attractively Interacting Fermionic Atoms in an Optical Lattice
摘要：
强关联可以极大地改变量子多粒子系统的热力学。当系统自动进入强关联状态时，会出现特别有趣的行为，因为熵和强相互作用之间的相互作用会导致违反直觉的效应。一个众所周知的例子是所谓的Pomeranchuk效应，当液体3He被压缩向其结晶相时发生。在这里，我们报告了一种新的反常，等熵效应的吸引相互作用的费米原子的自旋混合物中的光晶格。当我们逐步增加原子间的吸引力时，我们观察到气体不是收缩，而是膨胀。这种膨胀是由晶格势中的电子对形成引起的两种效应的组合引起的：随着吸引力的增加，量子涨落受到抑制，这导致熵的主导作用，以及自旋自由度的逐渐丧失，这迫使气体激发额外的轨道自由度并膨胀到陷阱的外部区域以保持熵。我们观察到的出乎意料的热力学揭示了费米子哈伯德模型中配对的基本特征。
英文摘要：
Strong correlations can dramatically modify the thermodynamics of a quantum many-particle system. Especially intriguing behaviour can appear when the system adiabatically enters a strongly correlated regime, for the interplay between entropy and strong interactions can lead to counterintuitive effects. A well known example is the so-called Pomeranchuk effect, occurring when liquid 3He is adiabatically compressed towards its crystalline phase. Here, we report on a novel anomalous, isentropic effect in a spin mixture of attractively interacting fermionic atoms in an optical lattice. As we adiabatically increase the attraction between the atoms we observe that the gas, instead of contracting, anomalously expands. This expansion results from the combination of two effects induced by pair formation in a lattice potential: the suppression of quantum fluctuations as the attraction increases, which leads to a dominant role of entropy, and the progressive loss of the spin degree of freedom, which forces the gas to excite additional orbital degrees of freedom and expand to outer regions of the trap in order to maintain the entropy. The unexpected thermodynamics we observe reveal fundamentally distinctive features of pairing in the fermionic Hubbard model.
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