The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
J E Thomas - One of the best experts on this subject based on the ideXlab platform.
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shear viscosity of a unitary Fermi Gas near the superfluid phase transition
Physical Review Letters, 2015Co-Authors: James Joseph, Ethan Elliott, J E ThomasAbstract:Measurements of a Fermi Gas below the superfluid transition temperature provide the first determination of the local shear viscosity, revealing features that were hidden in previous trap-averaged measurements.
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thermodynamic measurements in a strongly interacting Fermi Gas
Journal of Low Temperature Physics, 2009Co-Authors: Le Luo, J E ThomasAbstract: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 6Li 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 regime is obtained from the fundamental thermodynamic relation T=∂ E/∂ S by parameterizing the E(S) data using two different power laws that are joined with continuous E and T at a certain entropy S c, where the fit is optimized. We observe a significant change in the scaling of E with S above and below S c. Taking the fitted value of S c as an estimate of the critical entropy for a superfluid-normal fluid phase transition in the strongly interacting Fermi Gas, we estimate the critical parameters. Our E(S) data are also used to experimentally calibrate the endpoint temperatures obtained for adiabatic sweeps of the magnetic field between the ideal and strongly interacting regimes. This enables the first experimental calibration of the temperature scale used in experiments on Fermionic pair condensation, where the ideal Fermi Gas temperature is measured before sweeping the magnetic field to the strongly interacting regime. Our calibration shows that the ideal Gas temperature measured for the onset of pair condensation corresponds closely to the critical temperature T c estimated in the strongly interacting regime from the fits to our E(S) data. We also calibrate the empirical temperature employed in studies of the heat capacity and obtain nearly the same T c. We determine the ground state energy by three different methods, using sound velocity measurements, by extrapolating E(S) to S=0 and by measuring the ratio of the cloud sizes in the strongly and weakly interacting regimes. The results are in very good agreement with recent predictions. Finally, using universal thermodynamic relations, we estimate the chemical potential and heat capacity of the trapped Gas from the E(S) data.
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measurement of sound velocity in a Fermi Gas near a feshbach resonance
Physical Review Letters, 2007Co-Authors: James Joseph, B Clancy, J Kinast, A Turlapov, J E ThomasAbstract:Sound waves are excited 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 theories of hydrodynamic wave propagation and predictions of the equation of state. At resonance, the sound velocity exhibits universal scaling with the Fermi velocity, to within 1.8% over a factor of 30 in density.
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heat capacity of a strongly interacting Fermi Gas
Science, 2005Co-Authors: J Kinast, A Turlapov, J E Thomas, Qijin Chen, Jelena Stajic, K LevinAbstract: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.
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evidence for superfluidity in a resonantly interacting Fermi Gas
Physical Review Letters, 2004Co-Authors: J Kinast, Michael E. Gehm, S. L. Hemmer, A Turlapov, J E ThomasAbstract:We observe collective oscillations of a trapped, degenerate Fermi Gas of 6Li atoms at a magnetic field just above a Feshbach resonance, where the two-body physics does not support a bound state. The Gas exhibits a radial breathing mode at a frequency of 2837(05) Hz, in excellent agreement with the frequency of nu(H) identical with sqrt[10nu(x)nu(y)/3]=2830(20) Hz predicted for a hydrodynamic Fermi Gas with unitarity-limited interactions. The measured damping times and frequencies are inconsistent with predictions for both the collisionless mean field regime and for collisional hydrodynamics. These observations provide the first evidence for superfluid hydrodynamics in a resonantly interacting Fermi Gas.
Gabriel Wlazlowski - One of the best experts on this subject based on the ideXlab platform.
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perfect fluid behavior of a dilute Fermi Gas near unitary
Physical Review A, 2015Co-Authors: Gabriel Wlazlowski, Wei Quan, Aurel BulgacAbstract:We present an ab initio calculation of the shear viscosity as a function of interaction strength in a two-component unpolarized Fermi Gas near the unitary limit, within a finite temperature quantum Monte Carlo (QMC) framework and using the Kubo linear-response formalism. The shear viscosity decreases as we tune the interaction strength 1/ak_F from the Bardeen-Cooper-Schrieffer side of the Feshbach resonance towards Bose-Einstein condensation limit and it acquires the smallest value for 1/ak_F approx 0.4, with a minimum value of (eta/s)_min approx 0.2 hbar/k_B, which is about twice as small as the value reported for experiments in quark-gluon plasma (eta/s)_QGP lesssim 0.4 hbar/k_B. The Fermi Gas near unitarity thus emerges as the most "perfect fluid" observed so far in nature. The clouds of dilute Fermi Gas near unitarity exhibit the unusual attribute that, for the sizes realized so far in the laboratory or larger (less than 10^9 atoms), can sustain quantum turbulence below the critical temperature, but at the same time the classical turbulence is suppressed in the normal phase.
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shear viscosity of a unitary Fermi Gas
Physical Review Letters, 2012Co-Authors: Gabriel Wlazlowski, Piotr Magierski, Joaquin E DrutAbstract:We present the first ab initio determination of the shear viscosity eta of the Unitary Fermi Gas, based on finite temperature quantum Monte Carlo calculations and the Kubo linear-response formalism. We determine the temperature dependence of the shear viscosity to entropy density ratio eta/s. The minimum of eta/s appears to be located above the critical temperature for the superfluid-to-normal phase transition with the most probable value being eta/s approx 0.2 hbar/kB, which almost saturates the Kovtun-Son-Starinets universal value hbar/(4 pi kB).
Wolfgang Ketterle - One of the best experts on this subject based on the ideXlab platform.
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pauli paramagnetism of an ideal Fermi Gas
Physical Review A, 2013Co-Authors: Tout T Wang, Timur M Rvachov, Jaehoon Choi, Wolfgang KetterleAbstract:We show how to use trapped ultracold atoms to measure the magnetic susceptibility of a two-component Fermi Gas. The method is illustrated for a non-interacting Gas of $^6$Li, using the tunability of interactions around a wide Feshbach resonances. The susceptibility versus effective magnetic field is directly obtained from the inhomogeneous density profile of the trapped atomic cloud. The wings of the cloud realize the high field limit where the polarization approaches 100%, which is not accessible for an electron Gas.
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correlations and pair formation in a repulsively interacting Fermi Gas
Physical Review Letters, 2012Co-Authors: Christian Sanner, Aviv Keshet, Wujie Huang, Jonathon I Gillen, Wolfgang KetterleAbstract:A degenerate Fermi Gas is rapidly quenched into the regime of strong effective repulsion near a Feshbach resonance. The spin fluctuations are monitored using speckle imaging and, contrary to several theoretical predictions, the samples remain in the paramagnetic phase for an arbitrarily large scattering length. Over a wide range of interaction strengths a rapid decay into bound pairs is observed over times on the order of $10\ensuremath{\hbar}/{E}_{F}$, preventing the study of equilibrium phases of strongly repulsive Fermions. Our work suggests that a Fermi Gas with strong short-range repulsive interactions does not undergo a ferromagnetic phase transition.
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suppression of density fluctuations in a quantum degenerate Fermi Gas
Physical Review Letters, 2010Co-Authors: Christian Sanner, Yongil Shin, Aviv Keshet, Ralf Gommers, Wujie Huang, Wolfgang KetterleAbstract:We study density profiles of an ideal Fermi Gas and observe Pauli suppression of density fluctuations (atom shot noise) for cold clouds deep in the quantum degenerate regime. Strong suppression is observed for probe volumes containing more than 10 000 atoms. Measuring the level of suppression provides sensitive thermometry at low temperatures. After this method of sensitive noise measurements has been validated with an ideal Fermi Gas, it can now be applied to characterize phase transitions in strongly correlated many-body systems.
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itinerant ferromagnetism in a Fermi Gas of ultracold atoms
Science, 2009Co-Authors: Yeryoung Lee, Jaehoon Choi, Caleb A Christensen, Tony Hyun Kim, J H Thywissen, David E Pritchard, Wolfgang KetterleAbstract:Can a Gas of spin-up and spin-down Fermions become ferromagnetic because of repulsive interactions? We addressed this question, for which there is not yet a definitive theoretical answer, in an experiment with an ultracold two-component Fermi Gas. The observation of nonmonotonic behavior of lifetime, kinetic energy, and size for increasing repulsive interactions provides strong evidence for a phase transition to a ferromagnetic state. Our observations imply that itinerant ferromagnetism of delocalized Fermions is possible without lattice and band structure, and our data validate the most basic model for ferromagnetism introduced by Stoner.
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phase diagram of a two component Fermi Gas with resonant interactions
Nature, 2008Co-Authors: Yongil Shin, Andre Schirotzek, Christian H. Schunck, Wolfgang KetterleAbstract:A major controversy has surrounded the stability of superfluidity in spin-polarized Fermi Gas systems with resonant interactions when the 'up' and 'down' spin components are imbalanced. This problem is explored for a Fermi Gas of 6Li atoms, using tomographic techniques to map out the superfluid phases as the temperature and density imbalance are varied. Evidence is found for various types of phase transitions, enabling quantitative tests of theoretical calculations on the stability of resonant superfluidity. The pairing of Fermions lies at the heart of superconductivity and superfluidity. The stability of these pairs determines the robustness of the superfluid state, and the quest for superconductors with high critical temperature equates to a search for systems with strong pairing mechanisms. Ultracold atomic Fermi Gases present a highly controllable model system for studying strongly interacting Fermions1. Tunable interactions (through Feshbach collisional resonances) and the control of population or mass imbalance among the spin components provide unique opportunities to investigate the stability of pairing2,3,4—and possibly to search for exotic forms of superfluidity5,6. A major controversy has surrounded the stability of superfluidity against an imbalance between the two spin components when the Fermions interact resonantly (that is, at unitarity). Here we present the phase diagram of a spin-polarized Fermi Gas of 6Li atoms at unitarity, experimentally mapping out the superfluid phases versus temperature and density imbalance. Using tomographic techniques, we reveal spatial discontinuities in the spin polarization; this is the signature of a first-order superfluid-to-normal phase transition, and disappears at a tricritical point where the nature of the phase transition changes from first-order to second-order. At zero temperature, there is a quantum phase transition from a fully paired superfluid to a partially polarized normal Gas. These observations and the implementation of an in situ ideal Gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity.
Thomas Schafer - One of the best experts on this subject based on the ideXlab platform.
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hydrodynamic fluctuations and the minimum shear viscosity of the dilute Fermi Gas at unitarity
Physical Review A, 2013Co-Authors: Clifford Chafin, Thomas SchaferAbstract:We study hydrodynamic fluctuations in a nonrelativistic fluid. We show that in three dimensions, fluctuations lead to a minimum in the shear viscosity to entropy density ratio $\ensuremath{\eta}/s$ as a function of the temperature. The minimum provides a bound on $\ensuremath{\eta}/s$ which is independent of the conjectured bound in string theory, $\ensuremath{\eta}/s\ensuremath{\ge}\ensuremath{\hbar}/(4\ensuremath{\pi}{k}_{B})$, where $s$ is the entropy density. For the dilute Fermi Gas at unitarity, we find $\ensuremath{\eta}/s\ensuremath{\gtrsim}0.2\ensuremath{\hbar}$. This bound is not universal---it depends on the thermodynamic properties of the unitary Fermi Gas and on empirical information about the range of validity of hydrodynamics. We also find that the viscous relaxation time of a hydrodynamic mode with frequency $\ensuremath{\omega}$ diverges as $1/\sqrt{\ensuremath{\omega}}$, and that the shear viscosity in two dimensions diverges as $\mathrm{ln}(1/\ensuremath{\omega})$.
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hydrodynamic fluctuations and the minimum shear viscosity of the dilute Fermi Gas at unitarity
Physical Review A, 2013Co-Authors: Clifford Chafin, Thomas SchaferAbstract:We study hydrodynamic fluctuations in a non-relativistic fluid. We show that in three dimensions fluctuations lead to a minimum in the shear viscosity to entropy density ratio $\eta/s$ as a function of the temperature. The minimum provides a bound on $\eta/s$ which is independent of the conjectured bound in string theory, $\eta/s \geq \hbar/(4\pi k_B)$, where $s$ is the entropy density. For the dilute Fermi Gas at unitarity we find $\eta/s\gsim 0.2\hbar$. This bound is not universal -- it depends on thermodynamic properties of the unitary Fermi Gas, and on empirical information about the range of validity of hydrodynamics. We also find that the viscous relaxation time of a hydrodynamic mode with frequency $\omega$ diverges as $1/\sqrt{\omega}$, and that the shear viscosity in two dimensions diverges as $\log(1/ \omega)$.
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shear viscosity and damping of collective modes in a two dimensional Fermi Gas
Physical Review A, 2012Co-Authors: Thomas SchaferAbstract:The shear viscosity of a two-dimensional Fermi Gas interacting via a short-range potential with scattering length ${a}_{2\mathrm{d}}$ in kinetic theory is computed. It is found that classical kinetic theory predicts that the shear viscosity to entropy density ratio of a strongly interacting two-dimensional Gas is comparable to that of the three-dimensional unitary Gas. Results are applied to the damping of collective modes of a trapped Fermi Gas, and compared to experimental data recently obtained by Vogt et al. [Phys. Rev. Lett. 108, 070404 (2012)].
Alessandro Tanzini - One of the best experts on this subject based on the ideXlab platform.
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seiberg witten theory as a Fermi Gas
Letters in Mathematical Physics, 2017Co-Authors: Giulio Bonelli, Alba Grassi, Alessandro TanziniAbstract:We explore a new connection between Seiberg–Witten theory and quantum statistical systems by relating the dual partition function of SU(2) Super Yang–Mills theory in a self-dual $$\Omega $$ background to the spectral determinant of an ideal Fermi Gas. We show that the spectrum of this Gas is encoded in the zeroes of the Painleve $$\mathrm{III}_3$$ $$\tau $$ function. In addition, we find that the Nekrasov partition function on this background can be expressed as an O(2) matrix model. Our construction arises as a four-dimensional limit of a recently proposed conjecture relating topological strings and spectral theory. In this limit, we provide a mathematical proof of the conjecture for the local $${\mathbb P}^1 \times {\mathbb P}^1$$ geometry.
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seiberg witten theory as a Fermi Gas
arXiv: High Energy Physics - Theory, 2016Co-Authors: Giulio Bonelli, Alba Grassi, Alessandro TanziniAbstract:We explore a new connection between Seiberg-Witten theory and quantum statistical systems by relating the dual partition function of SU(2) Super Yang-Mills theory in a self-dual Omega-background to the spectral determinant of an ideal Fermi Gas. We show that the spectrum of this Gas is encoded in the zeroes of the Painleve III tau function. In addition we find that the Nekrasov partition function on this background can be expressed as an O(2) matrix model. Our construction arises as a four-dimensional limit of a recently proposed conjecture relating topological strings and spectral theory. In this limit, we provide a mathematical proof of the conjecture for the local P1xP1 geometry.