Martin Zwierlein Abstract

Martin Zwierlein
Massachusetts Institute of Technology

Abstract:

Fixed impurities often play a dominant role for electron transport in materials at low temperature. What is the fate of a mobile impurity immersed in a Fermi sea? We study this question in a highly imbalanced spin mixture of fermionic atoms. Feshbach resonances allow to freely tune the interactions between spin states. A single spin down atom immersed in a Fermi sea of spin up atoms can do one of two things: For strong attraction, it can form a molecule with exactly one spin up partner, but for weaker interaction it will spread its attraction and surround itself with a collection of majority atoms. This spin down atom "dressed" with a spin up cloud constitutes the Fermi polaron. We have observed a striking spectroscopic signature of this quasi-particle for various interaction strengths, which allows us to directly measure the polaron energy and infer the quasi-particle residue Z. The polarons are weakly interacting, and can thus be identified with the quasi-particles of Landau's Fermi liquid theory. The spin transport of spin down atoms moving through the spin up gas at resonance displays universal properties, with a diffusion constant set by h/m, Planck's quantum divided by the atomic mass.