Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
MIT-Harvard Center for Ultracold Atoms, Cambridge, MA 02139, USA.
Science. 2020 Dec 4;370(6521):1222-1226. doi: 10.1126/science.aaz5756.
Transport of strongly interacting fermions is crucial for the properties of modern materials, nuclear fission, the merging of neutron stars, and the expansion of the early Universe. Here, we observe a universal quantum limit of diffusivity in a homogeneous, strongly interacting atomic Fermi gas by studying sound propagation and its attenuation through the coupled transport of momentum and heat. In the normal state, the sound diffusivity D monotonically decreases upon lowering the temperature, in contrast to the diverging behavior of weakly interacting Fermi liquids. Below the superfluid transition temperature, D attains a universal value set by the ratio of Planck's constant and the particle mass. Our findings inform theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons, and quarks.
强相互作用费米子的输运对现代材料的性质、核裂变、中子星的合并以及早期宇宙的膨胀至关重要。在这里，我们通过研究通过动量和热的耦合输运来观察均匀、强相互作用的原子费米气体中扩散率的普遍量子限制。在正常状态下，随着温度的降低，声扩散率 D 单调减小，与弱相互作用费米液体的发散行为相反。在超导转变温度以下，D 达到由普朗克常数与粒子质量之比决定的普适值。我们的发现为费米子输运的理论提供了信息，对电子、中子和夸克的流体动力学流动具有重要意义。
