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arXiv:1107.2024 · New Journal of Physics

Critical spin transport in Bose gases

R Kittinaradorn, R. A. Duine, Henk T. C. Stoof

Published May 4, 2012• 4 citations
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Abstract

We consider spin transport in a two-component atomic Bose gas in three dimensions, at temperatures just above the critical temperature for Bose-Einstein condensation. In these systems the spin conductivity is determined by spin drag, i.e., frictional drag between the two spin components due to interactions. We find that in the critical region the temperature dependence of the spin conductivity deviates qualitatively from the Boltzmann result and is fully determined by the critical exponents of the phase transition. We discuss the size of the critical region where these results may be observed experimentally.

DOI
10.1088/1367-2630/14/5/055007

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We consider spin transport in a two-component atomic Bose gas in three dimensions, at temperatures just above the critical temperature for Bose-Einstein condensation. In these systems the spin conductivity is determined by spin drag, i.e., frictional drag between the two spin components due to interactions. We find that in the critical region the temperature dependence of the spin conductivity deviates qualitatively from the Boltzmann result and is fully determined by the critical exponents of the phase transition. We discuss the size of the critical region where these results may be observed experimentally.
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In this Thesis, we show that in a rotating two-component Bose mixture, the spin drag between the two different spin species shows a Hall effect. This spin drag Hall effect can be observed experimentally by studying the out-of-phase dipole mode of the mixture. We determine the damping of this mode due to spin drag as a function of temperature. We find that due to Bose stimulation there is a strong enhancement of the damping for temperatures close to the critical temperature for Bose-Einstein condensation. We also show the difference between spin drag in Bose gases, Fermi gases and Bose-Fermi mixtures. Furthermore, we investigate coupled spin and heat transport in Bose gases, and calculate the associated coefficients. Finally, we calculate the power spectrum resulting from spin current fluctuations in a Bose gas.
We consider coupled spin and heat transport in a two-component atomic Bose gas in the noncondensed state. We find that the transport coefficients show a temperature dependence reflecting the bosonic enhancement of scattering and discuss experimental signatures of the spin-heat coupling in spin accumulation, spin separation, and total dissipation. Close to the critical temperature for Bose-Einstein condensation, we find that the spin-heat coupling is strongly reduced, which is also reflected in the spin caloritronics figure of merit that determines the thermodynamic efficiency of spin-heat conversion.
Recent experiments [Jo, Science 325, 1521 (2009)] have presented evidence of ferromagnetic correlations in a two-component ultracold Fermi gas with strong repulsive interactions. Motivated by these experiments we consider spin drag, i.e., frictional drag due to scattering of particles with opposite spin, in such systems. We show that when the ferromagnetic state is approached from the normal side, the spin drag relaxation rate is strongly enhanced near the critical point. We also determine the temperature dependence of the spin diffusion constant. In a trapped gas the spin drag relaxation rate determines the damping of the spin dipole mode, which therefore provides a precursor signal of the ferromagnetic phase transition that may be used to experimentally determine the proximity to the ferromagnetic phase.
Spin drag, the friction between interacting atoms of opposite spin, is the single relaxation mechanism of spin currents in cold atomic gases. Experiments are ongoing and one of the quantities of interest is the spin drag relaxation rate in a two-component atomic Fermi gas. Motivated by these experiments, we use the Kubo formalism to obtain an expression for the spin drag conductivity. The sunset approximation to the selfenergy is considered and a T 2 dependence of the relaxation rate is found. The effect of the BCS phase transition is studied by taking into account pair correlations with a ladder summation. Preliminary results indicate a sudden increase of the spin drag relaxation rate close to but above the critical temperature.