Chin J K, Miller D E, Liu Y, Stan C, Setiawan W, Sanner C, Xu K, Ketterle W
Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Nature. 2006 Oct 26;443(7114):961-4. doi: 10.1038/nature05224.
The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. By using cold atomic gases, various models of condensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the copper oxides. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported but have not yet addressed superfluid behaviour. Here we report the observation of distinct interference peaks when a condensate of fermionic atom pairs is released from an optical lattice, implying long-range order (a property of a superfluid). Conceptually, this means that s-wave pairing and coherence of fermion pairs have now been established in a lattice potential, in which the transport of atoms occurs by quantum mechanical tunnelling and not by simple propagation. These observations were made for interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly as a result of a transition from superfluid to insulator. Such strongly interacting fermions in an optical lattice can be used to study a new class of hamiltonians with interband and atom-molecule couplings.
对处于周期性势场中的超流费米子对的研究，对于理解晶体材料中的超导性具有重要意义。通过使用冷原子气体，可以在高度可控的环境中研究各种凝聚态物质模型。光学晶格中的弱相互作用排斥性费米子在低温下可能会发生d波配对，这是铜氧化物中高温超导性的一种可能机制。晶格势场还可能大幅提高s波超流性的临界温度。近期在体原子气体方面的实验进展包括对费米子对凝聚态和高温超流性的观测。有关光学晶格中费米子和玻色子束缚对的实验已有报道，但尚未涉及超流行为。在此，我们报告了从光学晶格中释放出费米子原子对凝聚态时观察到明显的干涉峰，这意味着存在长程序（超流体的一种特性）。从概念上讲，这意味着在晶格势场中已确立了费米子对的s波配对和相干性，其中原子的输运是通过量子力学隧穿而非简单的传播实现的。这些观测是针对费什巴赫共振两侧的相互作用进行的。对于更大的晶格深度，相干性以可逆方式丧失，这可能是由于从超流体向绝缘体的转变所致。光学晶格中这种强相互作用的费米子可用于研究一类具有带间和原子 - 分子耦合的新型哈密顿量。
