arXiv subjects

J. E. Thomas

Publications and source records attributed to J. E. Thomas.

At least 37 records · Page 2Linked to original sources

arXiv2013

Observation of conformal symmetry breaking and scale invariance in expanding Fermi gases

We precisely test scale invariance and local thermal equilibrium in the hydrodynamic expansion of a Fermi gas of atoms as a function of interaction strength. After release from an anisotropic optical trap, we observe that a resonantly interacting gas obeys scale-invariant hydrodynamics, where the mean square cloud size $\langle{\mathbf{r}}^2\rangle=\langle x^2+y^2+z^2\rangle$ expands ballistically (like a noninteracting gas) and the energy-averaged bulk viscosity is consistent with zero, $0.005(0.016)\,\hbar\,n$, with $n$ the density. In contrast, the aspect ratios of the cloud exhibit anisotropic "elliptic" flow with an energy-dependent shear viscosity. Tuning away from resonance, we observe conformal symmetry breaking, where $\langle{\mathbf{r}}^2\rangle$ deviates from ballistic flow.

cond-mat.quant-gas↗

arXiv2012

Optical Control of the Scattering Length and Effective Range for Magnetically Tunable Feshbach Resonances in Ultracold Gases

We describe two-field optical techniques to control interactions in Feshbach resonances for two-body scattering in ultra-cold gases. These techniques create a molecular dark state in the closed channel of a magnetically tunable Feshbach resonance, greatly suppressing optical scattering compared to single optical field methods. The dark-state method enables control of the effective range, by creating narrow features that modify the energy dependence of the scattering phase shift, as well as control of the elastic and inelastic parts of the zero-energy s-wave scattering amplitude. We determine the scattering length and the effective range from an effective range expansion, by calculating the momentum-dependent scattering phase shift from the two-body scattering state.

cond-mat.quant-gas↗

arXiv2012

Polarons in the radio-frequency spectrum of a quasi-two-dimensional Fermi gas

We measure radio-frequency spectra for a two-component mixture of a $^6$Li atomic Fermi gas in the quasi-two-dimensional regime. Near the Feshbach resonance, where the transverse Fermi energy is large compared to the confinement-induced dimer binding energies for the initial and final states, we find that the observed resonances do not correspond to transitions between confinement-induced dimers. The spectrum appears to be well-described by transitions between noninteracting polaron states in two dimensions.

cond-mat.quant-gas↗

arXiv2011

Optical control of Feshbach resonances in Fermi gases using molecular dark states

We propose a general method for optical control of magnetic Feshbach resonances in ultracold atomic gases with more than one molecular state in an energetically closed channel. Using two optical frequencies to couple two states in the closed channel, inelastic loss arising from spontaneous emission is greatly suppressed by destructive quantum interference at the two-photon resonance, i.e., dark-state formation, while the scattering length is widely tunable by varying the frequencies and/or intensities of the optical fields. This technique is of particular interest for a two-component atomic Fermi gas, which is stable near a Feshbach resonance.

cond-mat.quant-gas↗

arXiv2011

Searching for Perfect Fluids: Quantum Viscosity in a Universal Fermi Gas

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 the transport coefficients are universal functions of the 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 $\hbar\,n$ 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 is obtained from the damping rate of the radial breathing mode, while high temperature data is 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 the shear viscosity to the entropy density, which is compared that conjectured for a perfect fluid.

cond-mat.quant-gas↗

arXiv2011

Universal Quantum Viscosity in a Unitary Fermi Gas

A Fermi gas of atoms with resonant interactions is predicted to obey universal hydrodynamics, where the shear viscosity and other transport coefficients are universal functions of the density and temperature. At low temperatures, the viscosity has a universal quantum scale $\hbar n$ where $n$ is the density, while at high temperatures the natural scale is $p_T^3/\hbar^2$ where $p_T$ is the thermal momentum. We employ breathing mode damping to measure the shear viscosity at low temperature. At high temperature $T$, we employ anisotropic expansion of the cloud to find the viscosity, which exhibits precise $T^{3/2}$ scaling. In both experiments, universal hydrodynamic equations including friction and heating are used to extract the viscosity. We estimate the ratio of the shear viscosity to the entropy density and compare to that of a perfect fluid.

cond-mat.quant-gas↗

arXiv2009

Is an Ultra-Cold Strongly Interacting Fermi Gas a Perfect Fluid?

Fermi gases with magnetically tunable interactions provide a clean and controllable laboratory system for modeling interparticle interactions between fermions in nature. The s-wave scattering length, which is dominant a low temperature, is made to diverge by tuning near a collisional (Feshbach) resonance. In this regime, two-component Fermi gases are stable and strongly interacting, enabling tests of nonperturbative many-body theories in a variety of disciplines, from high temperature superconductors to neutron matter and quark-gluon plasmas. We have developed model-independent methods for measuring the entropy and energy of this model system, providing a benchmark for calculations of the thermodynamics. Our experiments on the expansion of rotating strongly interacting Fermi gases in the normal fluid regime reveal extremely low viscosity hydrodynamics. Combining the thermodynamic and hydrodynamic measurements enables an estimate of the ratio of the shear viscosity to the entropy density. A strongly interacting Fermi gas in the normal fluid regime is found to be a nearly perfect fluid, where the ratio of the viscosity to the entropy density is close to a universal minimum that has been conjectured by string theory methods.

cond-mat.quant-gas↗

arXiv2009

Controlling spin current in a trapped Fermi gas

We study fundamental features of spin current in a very weakly interacting Fermi gas of $^6$Li. By creating a spin current and then reversing its flow, we demonstrate control of the spin current. This reversal is predicted by a spin vector evolution equation in energy representation, which shows how the spin and energy of individual atoms become correlated in the nearly undamped regime of the experiments. The theory provides a simple physical description of the spin current and explains both the large amplitude and the slow temporal evolution of the data. Our results have applications in studying and controlling fundamental spin interactions and spin currents in ultra-cold gases.

cond-mat.other↗

arXiv2009

Inelastic Collisions of a Fermi Gas in the BEC-BCS Crossover

We report the measurement of inelastic three-body and two-body collisional decay rates for a two-component Fermi gas of $^6$Li, which are highly suppressed by the Pauli exclusion principle. Our measurements are made in the BEC-BCS crossover regime, near the two-body collisional (Feshbach) resonance. At high temperature (energy) the data shows a dominant three-body decay process, which is studied as a function of bias magnetic field. At low energy, the data shows a coexistence of two-body and three-body decay processes near and below the Feshbach resonance. Below resonance, the observed two-body inelastic decay can arise from molecule-atom and molecule-molecule collisions. We suggest that at and above resonance, an effective two-body decay rate arises from collisions between atoms and correlated (Cooper) pairs that can exist at sufficiently low temperature.

cond-mat.other↗

arXiv2008

Observation of Anomalous Spin Segregation in a Trapped Fermi Gas

We report the observation of spin segregation, i.e., separation of spin density profiles, in a trapped ultracold Fermi gas of $^6$Li with a magnetically tunable scattering length close to zero. The roles of the spin components are inverted when the sign of the scattering length is reversed. The observed density profiles are in qualitative agreement with the spin-wave theory applied previously to explain spin segregation in a Bose gas, but disagree in amplitude by two orders of magnitude. The observed atomic density profiles are far from equilibrium, yet they persist for $\simeq$ 5 seconds in a trap with an axial frequency of $\simeq$ 150 Hz. We attribute this long lifetime to Fermi statistics: The scattering amplitude is nonzero only for atoms in opposite states, and vanishes for atoms in the same state. By measuring the magnetic field at which spin segregation ceases, we precisely determine the zero crossing in the scattering length of $^6$Li as $527.5\pm0.2$ G.

cond-mat.other↗

arXiv2008

Virial theorem for confined universal Fermi gases

Optically-trapped two-component Fermi gases near a broad Feshbach resonance exhibit universal thermodynamics, where the properties of the gas are independent of the details of the two-body scattering interactions. We present a global proof that such a universal gas obeys the virial theorem for {\it any} trapping potential $U$ and any spin mixture, without assuming either the local density approximation or harmonic confinement. The total energy of the gas is given in scale invariant form by $E=<ε\partial U/\partialε>$, where $ε$ is an {\it arbitrary} energy scale in terms of which all length and energy scales that appear in the confining potential are written. This result enables model-independent energy measurement in traps that are anharmonic as well as anisotropic by observing only the cloud profile, and provides a consistency check for many-body calculations in the universal regime.

cond-mat.stat-mech↗

arXiv2007

Is a gas of strongly interacting atomic fermions a nearly perfect fluid?

We use all-optical methods to produce a highly-degenerate Fermi gas of spin-1/2 $^6$Li atoms. A magnetic field tunes the gas near a collisional (Feshbach) resonance, producing strong interactions between spin-up and spin-down atoms. This atomic gas is a paradigm for strong interactions in nature, and provides tests of current quantum many-body calculational methods for diverse systems, including very high temperature superconductors, nuclear matter in neutron stars, and the quark-gluon plasma of the Big Bang. We have measured properties of a breathing mode over a wide range of temperatures. At temperatures both below and well above the superfluid transition, the frequency of the mode is nearly constant and very close to the hydrodynamic value. However, explaining both the frequency and the damping rate in the normal collisional regime has not been achieved. Our measurements of the damping rate as a function of the energy of the gas are used to estimate an upper bound on the viscosity. Using our new measurements of the entropy of the gas, we estimate the ratio of the shear viscosity to the entropy density, and compare the result with the lower bound for quantum viscosity recently predicted using string theory methods.

cond-mat.other↗

arXiv2006

Sound propagation in a Fermi gas near a Feshbach resonance

Sound waves are observed and studied in an optically trapped degenerate Fermi gas of spin-up and spin-down atoms with magnetically tunable interactions. Measurements are made throughout the crossover region, from a weakly-interacting Fermi gas through the resonant Fermi superfluid regime to a Bose condensate of dimer molecules. The measured sound velocities test the equation of state and confirm the universal hypothesis.

cond-mat.other↗

arXiv2006

Measurement of the Entropy and Critical Temperature of a Strongly Interacting Fermi Gas

We report a model-independent measurement of the entropy, energy, and critical temperature of a degenerate, strongly interacting Fermi gas of atoms. The total energy is determined from the mean square cloud size in the strongly interacting regime, where the gas exhibits universal behavior. The entropy is measured by sweeping a bias magnetic field to adiabatically tune the gas from the strongly interacting regime to a weakly interacting regime, where the entropy is known from the cloud size after the sweep. The dependence of the entropy on the total energy quantitatively tests predictions of the finite-temperature thermodynamics.

cond-mat.other↗

arXiv2005

Virial Theorem and Universality in a Unitary 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 $^6$Li, 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.

cond-mat.other↗

arXiv2005

Two Transitions in the Damping of a Unitary Fermi Gas

We measure the temperature dependence of the radial breathing mode in an optically trapped, strongly-interacting Fermi gas of $^6$Li, 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.

cond-mat.soft↗

arXiv2005

Heat Capacity of a Strongly-Interacting Fermi Gas

We have measured the heat capacity of an optically-trapped, strongly-interacting Fermi gas of atoms. A precise input of energy to the gas is followed by single-parameter thermometry, which determines the empirical temperature parameter $\tilde{T}$ 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 temperature. The theory calibrates $\tilde{T}$, yields excellent agreement with the data, and predicts the onset of superfluidity at the observed transition point.

cond-mat.other↗

arXiv2004

Breakdown of Hydrodynamics in the Radial Breathing Mode of a Strongly-Interacting Fermi Gas

We measure the magnetic field dependence of the frequency and damping time for the radial breathing mode of an optically trapped, Fermi gas of $^6$Li atoms near a Feshbach resonance. The measurements address the apparent discrepancy between the results of Kinast et al., [Phys. Rev. Lett. {\bf 92}, 150402 (2004)] and those of Bartenstein et al., [Phys. Rev. Lett. {\bf 92}, 203201 (2004)]. Over the range of magnetic field from 770 G to 910 G, the measurements confirm the results of Kinast et al. Close to resonance, the measured frequencies are in excellent agreement with predictions for a unitary hydrodynamic gas. At a field of 925 G, the measured frequency begins to decrease below predictions. For fields near 1080 G, we observe a breakdown of hydrodynamic behavior, which is manifested by a sharp increase in frequency and damping rate. The observed breakdown is in qualitative agreement with the sharp transition observed by Bartenstein et al., at 910 G.

cond-mat.soft↗