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AIP conference proceedings

Universal Thermodynamics of a Strongly Interacting Fermi Gas

John E. Thomas, Joseph M. Kinast, Andrei V. Turlapov

Published Jan 1, 2006• 2 citations
Publisher

Abstract

We study the properties of an optically‐trapped, Fermi gas of 6Li atoms, near the center of a broad Feshbach resonance, where strong interactions are observed. Strongly interacting Fermi gases exhibit universal behavior, and are of interest as models of exotic systems ranging from high temperature superconductors to neutron stars and quark‐gluon plasmas. We measure the frequency and damping rate of the radial breathing mode of the trapped gas and consider quantum viscosity as a damping mechanism. We also demonstrate that the virial theorem holds and measure the heat capacity. In the experiments, energy is precisely added to the gas and an empirical temperature is determined from the spatial profiles of the cloud. Transitions are observed in both the damping rate and the heat capacity, measured as functions of the empirical temperature. Recent theory, using a pseudogap formalism, enables the first temperature calibration, and shows that the observed transition temperatures are close to that predicted for the onset of superfluidity in a strongly attractive Fermi gas.

DOI
10.1063/1.2354611

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We measure the shear viscosity in a two-component Fermi gas of atoms, tuned to a broad s-wave collisional (Feshbach) resonance. At resonance, the atoms strongly interact and exhibit universal behavior, where the equilibrium thermodynamic properties and transport coefficients are universal functions of density n and temperature T. We present a new calibration of the temperature as a function of global energy, which is directly measured from the cloud profiles. Using the calibration, the trap-averaged shear viscosity in units of ℏn is determined as a function of the reduced temperature at the trap center, from nearly the ground state to the unitary two-body regime. Low-temperature data are obtained from the damping rate of the radial breathing mode, whereas high-temperature data are obtained from hydrodynamic expansion measurements. We also show that the best fit to the high-temperature expansion data is obtained for a vanishing bulk viscosity. The measured trap-averaged entropy per particle and shear viscosity are used to estimate the ratio of shear viscosity to entropy density, which is compared with that conjectured for a perfect fluid.

•Aug 17, 2016•42 citations
We study the properties of an optically‐trapped, Fermi gas of 6Li atoms, near the center of a broad Feshbach resonance, where strong interactions are observed. Strongly interacting Fermi gases exhibit universal behavior, and are of interest as models of exotic systems ranging from high temperature superconductors to neutron stars and quark‐gluon plasmas. We measure the frequency and damping rate of the radial breathing mode of the trapped gas and consider quantum viscosity as a damping mechanism. We also demonstrate that the virial theorem holds and measure the heat capacity. In the experiments, energy is precisely added to the gas and an empirical temperature is determined from the spatial profiles of the cloud. Transitions are observed in both the damping rate and the heat capacity, measured as functions of the empirical temperature. Recent theory, using a pseudogap formalism, enables the first temperature calibration, and shows that the observed transition temperatures are close to that predicted for the onset of superfluidity in a strongly attractive Fermi gas.
Unitary Fermi gases, where the scattering length is large compared to the interparticle spacing, can have universal properties, which are independent of the details of the interparticle interactions when the range of the scattering potential is negligible. We prepare an optically trapped, unitary Fermi gas of 6Li, tuned just above the center of a broad Feshbach resonance. In agreement with the universal hypothesis, we observe that this strongly interacting many-body system obeys the virial theorem for an ideal gas over a wide range of temperatures. Based on this result, we suggest a simple volume thermometry method for unitary gases. We also show that the observed breathing mode frequency, which is close to the unitary hydrodynamic value over a wide range of temperature, is consistent with a universal hydrodynamic gas with nearly isentropic dynamics.
We have measured the heat capacity of an optically trapped, strongly interacting Fermi gas of atoms. A precise addition of energy to the gas is followed by single-parameter thermometry, which determines the empirical temperature parameter of the gas cloud. Our measurements reveal a clear transition in the heat capacity. The energy and the spatial profile of the gas are computed using a theory of the crossover from Fermi to Bose superfluids at finite temperatures. The theory calibrates the empirical temperature parameter, yields excellent agreement with the data, and predicts the onset of superfluidity at the observed transition point.
transition, critical parameters Abstract Strongly interacting Fermi gases provide a clean and controllable laboratory system for modeling strong interparticle interactions between fermions in nature, from high temperature superconductors to neutron matter and quark-gluon plasmas. Model-independent thermodynamic measurements, which do not require theoretical models for calibrations, are very important for exploring this important system experimentally, as they enable direct tests of predictions based on the best current non-perturbative many-body theories. At Duke University, we use all-optical methods to produce a strongly interacting Fermi gas of spin-1/2-up and spin-1/2-down 6 Li atoms that is magnetically tuned near a collisional (Feshbach) resonance. We conduct a series of measurements on the thermodynamic properties of this unique quantum gas, including the energy E, entropy S, and sound velocity c. Our model-independent measurements of E and S enable a precision study of the finite temperature thermodynamics. The E(S) data are directly compared to several recent predictions. The temperature in both the superfluid and normal fluid
We measure the temperature dependence of the radial breathing mode in an optically trapped, strongly-interacting Fermi gas of 6^66Li, just above the center of a broad Feshbach resonance. The frequency remains close to the unitary hydrodynamic value, while the damping rate reveals transitions at two well-separated temperatures, consistent with the existence of atom pairs above a superfluid transition.
We measure the shear viscosity in a two-component Fermi gas of atoms, tuned to a broad s-wave collisional (Feshbach) resonance. At resonance, the atoms strongly interact and exhibit universal behavior, where the equilibrium thermodynamic properties and transport coefficients are universal functions of density n and temperature T . We present a new calibration of the temperature as a function of global energy, which is directly measured from the cloud profiles. Using the calibration, the trap-averaged shear viscosity in units of ℏ n is determined as a function of the reduced temperature at the trap center, from nearly the ground state to the unitary two-body regime. Low-temperature data are obtained from the damping rate of the radial breathing mode, whereas high-temperature data are obtained from hydrodynamic expansion measurements. We also show that the best fit to the high-temperature expansion data is obtained for a vanishing bulk viscosity. The measured trap-averaged entropy per particle and shear viscosity are used to estimate the ratio of shear viscosity to entropy density, which is compared with that conjectured for a perfect fluid.
We measure the shear viscosity in a two-component Fermi gas of atoms, tuned to a broad s-wave collisional (Feshbach) resonance. At resonance, the atoms strongly interact and exhibit universal behavior, where the equilibrium thermodynamic properties and transport coefficients are universal functions of density n and temperature T. We present a new calibration of the temperature as a function of global energy, which is directly measured from the cloud profiles. Using the calibration, the trap-averaged shear viscosity in units of ℏn is determined as a function of the reduced temperature at the trap center, from nearly the ground state to the unitary two-body regime. Low-temperature data are obtained from the damping rate of the radial breathing mode, whereas high-temperature data are obtained from hydrodynamic expansion measurements. We also show that the best fit to the high-temperature expansion data is obtained for a vanishing bulk viscosity. The measured trap-averaged entropy per particle and shear viscosity are used to estimate the ratio of shear viscosity to entropy density, which is compared with that conjectured for a perfect fluid.