Universal Spin Transport in a Strongly Interacting Fermi Gas (1101.0780v1)

Abstract: Transport of fermions is central in many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators, and electron spin, rather than charge, is being explored as a new carrier of information [1]. Neutrino transport energizes supernova explosions following the collapse of a dying star [2], and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe [3]. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics [4, 5]. It has been established that even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity [3, 6] when interactions are tuned to a scattering resonance. However, here we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coeffcient, the spin diffusivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin diffusivity approaches a minimum value set by the ratio of the reduced Planck's constant to the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state [7-9].

• The paper quantifies spin drag and diffusivity in ultracold 6Li using a Feshbach resonance to tune interactions to the unitarity limit.
• The paper demonstrates that spin transport exhibits universal temperature scalings, with diffusivity approaching a quantum limit near the Fermi temperature.
• The paper rules out a metastable ferromagnetic state in repulsive Fermi gases, as shown by the increasing spin diffusivity with positive scattering lengths.

Universal Spin Transport in a Strongly Interacting Fermi Gas: Insights and Implications

This paper by Ariel Sommer and collaborators investigates the transport properties of spin in a strongly interacting Fermi gas, specifically within the context of ultracold atomic gases near a Feshbach resonance. The focus on spin rather than charge transport stems from its relevance to spintronics, a field that seeks to exploit the spin of electrons to develop new types of electronic devices.

Key Findings and Methodology

The experimental setup involves a balanced mixture of two spin states of ultracold ^6Li fermionic atoms confined in a trap. By employing Feshbach resonances, the authors are able to tune interactions to the unitarity limit, where they observe universal behaviors in spin transport properties. In this regime, the system represents a nearly perfect fluid with minimal viscosity, an ideal setting to study non-equilibrium dynamics.

1. Spin Drag and Diffusivity: The researchers quantitatively examine spin transport by measuring spin drag (ζ_sd) and diffusivity (D_s) as functions of temperature. They determine the spin drag coefficient from the damping rate of spin currents—a measure of how momentum is transferred between particles of opposite spin. The spin diffusivity characterizes how efficiently spin currents even out spatial gradients in spin density. Notably, they observe that as temperature approaches the Fermi temperature, D_s reaches a quantum-limited value described by ħ/m.
2. Universal Scalings: Interestingly, the results reveal universal scalings with temperature. At high temperatures, spin transport properties exhibit power-law dependencies relatable to their scattering mechanics. Specifically, the spin drag coefficient shows a T^-1/2 dependence, while spin diffusivity increases following a T^3/2 scaling, consistent with theoretical predictions for Fermi gases in this regime.
3. Exclusion of Metastable Ferromagnetic State: Contrary to some theoretical expectations, the observations appear to exclude a metastable ferromagnetic state in repulsive Fermi gases, as evidenced by the increase in spin diffusivity with positive scattering lengths. The lack of a phase with stopped diffusion suggests that ferromagnetism in this context might not be stable or attainable within the experimental parameters investigated.

Implications and Future Directions

The findings contribute significantly to the understanding of spin transport in strongly interacting Fermi systems. The establishment of a quantum limit to spin diffusivity has implications for both theoretical and practical pursuits in condensed matter physics and spintronics. Practically, engineering materials or interactions that can reach such limits might lead to efficient spin-based devices.

The absence of a detected ferromagnetic phase prompts further theoretical and experimental scrutiny into the nature of spin interactions at different interaction strengths and the potential stabilization of such states. Furthermore, the observed behaviors question the existence and characteristics of a pseudo-gap or Fermi liquid state in these systems, meriting additional investigations, particularly at temperatures approaching superfluid conditions.

The study opens pathways for exploring non-equilibrium phenomena in similar strongly correlated systems, and its methodologies can be adapted for probing other exotic states of matter. As ultracold atomic systems continue to serve as versatile models for complex quantum phenomena, insights from such research will likely influence developments across various domains of quantum physics.