Luciuk C, Smale S, Böttcher F, Sharum H, Olsen B A, Trotzky S, Enss T, Thywissen J H
Department of Physics, University of Toronto, Ontario M5S 1A7, Canada.
5. Physikalisches Institut and Centre for Integrated Quantum Science and Technology, Universität Stuttgart, D-70569 Stuttgart, Germany.
Phys Rev Lett. 2017 Mar 31;118(13):130405. doi: 10.1103/PhysRevLett.118.130405.
We measure the transport properties of two-dimensional ultracold Fermi gases during transverse demagnetization in a magnetic field gradient. Using a phase-coherent spin-echo sequence, we are able to distinguish bare spin diffusion from the Leggett-Rice effect, in which demagnetization is slowed by the precession of a spin current around the local magnetization. When the two-dimensional scattering length is tuned to be comparable to the inverse Fermi wave vector k_{F}^{-1}, we find that the bare transverse spin diffusivity reaches a minimum of 1.7(6)ℏ/m, where m is the bare particle mass. The rate of demagnetization is also reflected in the growth rate of the s-wave contact, observed using time-resolved spectroscopy. The contact rises to 0.28(3)k_{F}^{2} per particle, which quantifies how scaling symmetry is broken by near-resonant interactions, unlike in unitary three-dimensional systems. Our observations support the conjecture that, in systems with strong scattering, the local relaxation rate is bounded from above by k_{B}T/ℏ.
我们测量了二维超冷费米气体在磁场梯度中的横向退磁过程中的输运性质。利用相位相干自旋回波序列，我们能够区分裸自旋扩散与莱格特-赖斯效应，在莱格特-赖斯效应中，自旋电流围绕局部磁化强度的进动会减缓退磁过程。当二维散射长度被调整到与费米波矢的倒数(k_{F}^{-1})相当时，我们发现裸横向自旋扩散率达到最小值(1.7(6)\hbar/m)，其中(m)是裸粒子质量。退磁速率也反映在利用时间分辨光谱观测到的(s)波接触的增长率中。接触量上升到每个粒子(0.28(3)k_{F}^{2})，这量化了近共振相互作用如何打破标度对称性，这与幺正三维系统不同。我们的观测结果支持了这样一个猜想，即在具有强散射的系统中，局部弛豫率上限为(k_{B}T/\hbar)。
