Transverse Demagnetization Dynamics of a Unitary Fermi Gas

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Abstract

Understanding the quantum dynamics of strongly interacting fermions is a challenge raised by diverse forms of matter, including high-temperature superconductors, neutron stars, and quark-gluon plasmas. An appealing benchmark is offered by cold atomic gases in the unitary limit of strong interactions, where the system is both scale-invariant and known to obey universal thermodynamics in equilibrium. Here we study the dynamics of a transversely magnetized unitary Fermi gas in an inhomogeneous magnetic field. We find that demagnetization is caused by diffusive spin transport with a diffusion constant that saturates at low temperatures to the conjectured quantum-mechanical lower bound ~hbar/m, where m is the particle mass. The development of pair correlations is observed by measuring Tan's contact parameter.

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Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas

Ariel T. Sommer, Lawrence Cheuk, Mark Ku … (2011)

We have observed the superfluid phase transition in a strongly interacting Fermi gas via high-precision measurements of the local compressibility, density and pressure down to near-zero entropy. Our data completely determine the universal thermodynamics of strongly interacting fermions without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity. In particular, the heat capacity displays a characteristic lambda-like feature at the critical temperature of \(T_c/T_F = 0.167(13)\). This is the first clear thermodynamic signature of the superfluid transition in a spin-balanced atomic Fermi gas. Our measurements provide a benchmark for many-body theories on strongly interacting fermions, relevant for problems ranging from high-temperature superconductivity to the equation of state of neutron stars.

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Universal Spin Transport in a Strongly Interacting Fermi Gas

Ariel Sommer, Martin W Zwierlein, Giacomo Roati … (2011)

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].

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Molecular Probe of Pairing in the BEC-BCS Crossover

G. B. Partridge, K. Strecker, R. I. Kamar … (2005)

We have used optical molecular spectroscopy to probe the many-body state of paired \(^6\)Li atoms near a broad Feshbach resonance. The optical probe projects pairs of atoms onto a vibrational level of an excited molecule. The rate of excitation enables a precise measurement of the closed-channel contribution to the paired state. This contribution is found to be quite small, supporting the concept of universality for the description of broad Feshbach resonances. The dynamics of the excitation provide clear evidence for pairing across the BEC-BCS crossover, and into the weakly interacting BCS regime.