Figure - Normalized relaxation time of the spin dipole mode of a highly-polarized Fermi gas as a function of the reduced temperature T/TF. TF is the local Fermi temperature at the center of the majority cloud. The solid curve is the low temperature limit. The dashed curve is the expression 0.08 . The inset shows the average ratio of the minority cloud size to the majority clouds size as a function of the reduced temperature T/TF.
The quality of transport is one of the most important properties distinguishing states of matter. Of great technical importance, electrons in condensed matter materials can flow as currents or supercurrents, or be localized in an insulator, or even switch their state of conductivity through controllable parameters like an applied magnetic field. It is the task of many-body physics to develop models that may explain the observed transport properties in a system. Dilute atomic gases cooled to quantum degeneracy provide ideal systems for testing many-body theories. In particular, Feshbach resonances in atomic Fermi gases allow experimental control over the strength of two-body interactions, giving access to the BEC-BCS crossover regime.
In this work we present measurements of spin transport in ultracold gases of fermionic 6Li in a mixture of two spin states at a Feshbach resonance. In particular, we study the spin dipole mode, where the two spin components are displaced from each other against a harmonic restoring force. We prepare a highly-imbalanced, or polaronic, spin mixture with a spin dipole excitation and observe strong, unitarity limited damping of the spin dipole mode. In gases with small spin imbalance, below the Pauli limit for superfluidity, we observe strongly damped spin flow even in the presence of a superfluid core. This indicates strong mutual friction between superfluid and polarized normal spins, possibly involving Andreev reflection at the superfluid-normal interface.
These measurements provide the first quantitative test of theoretical calculations of the spin transport properties of highly polarized Fermi gases.