Ultracold strongly coupled gas: A near-ideal liquid

Abstract

Feshbach resonances of trapped ultracold alkali atoms allow to vary the atomic scattering length a. At very large values of a the system enters an universal strongly coupled regime in which its properties-the ground state energy, pressure etc.-become independent of a. We discuss transport properties of such systems. In particular, the universality arguments imply that the shear viscosity of ultracold Fermi atoms at the Feschbach resonance is proportional to the particle number density n, and the Plank constanth: η =hnαη, where αη is a universal constant. Using Heisenberg uncertainty principle and Einstein's relation between diffusion and viscosity we argue that the viscosity has the lower bound given by αη ≤ (6π) −1 . We relate the damping of low-frequency density oscillations of ultracold optically trapped 6 Li atoms to viscosity and find that the value of the coefficient αη is about 0.3. We also show that such a small viscosity can not be explained by kinetic theory based on binary scattering. We conclude that the system of ultracold atoms near the Feshbach resonance is a near-ideal liquid.

BCS/BEC crossover in quark matter and evolution of its static and dynamic properties from the atomic unitary gas to color superconductivity

“…It is also noteworthy that the bulk viscosity vanishes in the unitary limit [70]. These suggest that the intermediate of the BCS/BEC crossover is rather liquid-like as conjectured in [68].…”

“…(2) Shear viscosity takes almost minimum near the unitary limit in the present model calculation. It is close to the minimum bounds [11,68]. It is also noteworthy that the bulk viscosity vanishes in the unitary limit [70].…”

“…It is also worth mentioning that the hydrodynamic properties of the unitary Fermi gas have recently attracted considerable attention both experimentally [62,63,64,65,66,67] and theoretically [68,69,70]. The unitary Fermi gas is the intermediate of the BCS/BEC crossover, where the s-wave scattering length diverges.…”

Initial state fluctuations and final state correlations in relativistic heavy-ion collisions

“…Since then the quark gluon plasma at temperatures up to 2-3 T c seems to be a rather strongly coupled liquid instead of the anticipated dilute gas of partons. The realization of the nearly perfect fluid dynamic behaviour of the QGP has triggered connections to other systems with similar properties in condensed matter physics [22] and was the basis for exploiting the AdS/CFT correspondance [23].…”

Short-Lived Lattice Quasiparticles for Strongly Interacting Fluids

“…More precisely, the mean-free path becomes smaller than the interaction range, and, eventually, the De Broglie length. Remarkable examples in point are found mostly in the framework of quantum fluids, namely quark-gluon plasmas [2], electrons in graphene [3] and Bose-Einstein condensates in optical lattices [4]. Note that these extreme and exotic states of matter cover a breathtaking range of densities and temperatures, from one particle/fm 3 and trillions Kelvin degrees in ultra hot-dense quark-gluon plasmas, to about 10 12 particles/cm 3 and tens of nanokelvins in ultra-cold Bose-Einstein condensates.…”