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arXiv:1311.0769 · New Journal of Physics

Bosonic thermoelectric transport and breakdown of universality

Adam Rançon, Cheng Chin, Kathryn Levin

Published Nov 26, 2014• 17 citations
arXivHTMLPDF

Abstract

We discuss the general principles of transport in normal phase atomic gases, comparing Bose and Fermi systems. Our study shows that two-dimensional bosonic transport is non-universal with respect to different dissipation mechanisms. Near the superfluid transition temperature T c , a striking similarity between the fermionic and bosonic transport emerges because super-conducting (fluid) fluctuation transport for Fermi gases is dominated by the bosonic, Cooper pair component. As in fluctuation theory, one finds that the Seebeck coefficient changes sign at T c and the Lorenz number approaches zero at T c . Our findings appear quantitatively consistent with recent Bose gas experiments.

DOI
10.1088/1367-2630/16/11/113072

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We discuss the general principles of transport in normal phase atomic gases, comparing Bose and Fermi systems. Our study shows that two-dimensional bosonic transport is non-universal with respect to different dissipation mechanisms. Near the superfluid transition temperature T c , a striking similarity between the fermionic and bosonic transport emerges because super-conducting (fluid) fluctuation transport for Fermi gases is dominated by the bosonic, Cooper pair component. As in fluctuation theory, one finds that the Seebeck coefficient changes sign at T c and the Lorenz number approaches zero at T c . Our findings appear quantitatively consistent with recent Bose gas experiments.
In condensed matter physics, transport measurements are essential not only for the characterization of materials, but also to discern between quantum phases and identify new ones. The extension of these measurements into atomic quantum gases is emerging and will expand the scope of quantum simulation and atomtronics. To push this frontier, we demonstrate an innovative approach to extract transport properties from the time-resolved redistribution of the particles and energy of a trapped atomic gas. Based on the two-dimensional (2D) Bose gas subject to weak three-body recombination we find clear evidence of both conductive and thermoelectric currents. We then identify the contributions to the currents from thermoelectric forces and determine the Seebeck coefficient (a.k.a. thermopower) and Lorenz number, both showing anomalous behavior in the fluctuation and superfluid regimes. Our results call for further exploration of the transport properties, particularly thermoelectric properties, of atomic quantum gases.
Fermionic superfluid junctions typically exhibit suppressed thermal and spin transport due to the presence of a pairing gap but allow coherent particle transport. While dissipation generally weakens coherent transport, it can also induce excitations that open other transport channels. In this work, we experimentally study a one-dimensional superfluid junction of strongly interacting fermions with local particle loss and observe dissipation-induced thermal and spin transport that appear to saturate at strong dissipation. Notably, in this regime, the measured thermal and spin conductances are comparable to the universal quantized conductance of one-dimensional ideal Fermi gas. Qualitatively similar behavior is observed for two dissipation mechanisms, either spin-imbalanced or pairwise losses. Our findings provide new insights into transport in interacting open quantum systems and suggest possibilities of dissipative control of spin and thermoelectric transport.
We study the emergence of dissipation in an atomic Josephson junction between weakly coupled superfluid Fermi gases. We find that vortex-induced phase slippage is the dominant microscopic source of dissipation across the Bose-Einstein condensate-Bardeen-Cooper-Schrieffer crossover. We explore different dynamical regimes by tuning the bias chemical potential between the two superfluid reservoirs. For small excitations, we observe dissipation and phase coherence to coexist, with a resistive current followed by well-defined Josephson oscillations. We link the junction transport properties to the phase-slippage mechanism, finding that vortex nucleation is primarily responsible for the observed trends of conductance and critical current. For large excitations, we observe the irreversible loss of coherence between the two superfluids, and transport cannot be described only within an uncorrelated phase-slip picture. Our findings open new directions for investigating the interplay between dissipative and superfluid transport in strongly correlated Fermi systems, and general concepts in out-of-equilibrium quantum systems.
Strongly interacting, dilute Fermi gases exhibit a scale-invariant, universal thermodynamic behavior. This is notoriously difficult to understand theoretically because of the absence of a small interaction parameter. Here, we present a systematic comparison of theoretical predictions from different quantum many-body theories with recent experimental data of Nascimbne et al (2010 Nature 463 1057). Our comparisons have no adjustable parameters, either theoretically or experimentally. All the model approximations seem to fluctuate rather than converge on the experimental data. It turns out that a simple Gaussian pair fluctuation theory gives the best quantitative agreement, except at the critical superfluid transition region. In the normal state, we also calculate the equation of state by using a quantum cluster expansion theory and explore in detail its applicability to low temperatures. Using the accurate experimental result for the thermodynamic function S ( T ), we determine the temperature T of a trapped Fermi gas at unitarity as a function of a non-interacting temperature T i , which can be obtained by an adiabatic sweep to the free gas limit. By analyzing the recent experimental data, we find a characteristic temperature ( T / T F ) 0 =0.19±0.02 or ( T i / T F ) 0 =0.16±0.02 in a harmonic trap, below which there are deviations from normal Fermi-liquid-like behavior that may be attributed to pairing effects. Here, T F is the Fermi temperature for a trapped ideal, non-interacting Fermi gas. Our thorough comparison may shed light on the further theoretical development of strongly interacting fermions.
The physics in two-dimensional (2D) systems is very different from what we observe in three-dimensional (3D) systems. Thermal fluctuations in 2D systems are enhanced, and they prevent the conventional Bose-Einstein condensation (BEC) at non-zero temperatures by destroying the long-range order. However, a phase transition to a superfluid phase is still expected to occur in a 2D system along with an emergence of a quasi-long-range order, explained by the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. Within the BKT mechanism, a universal jump of the superfluid density in a 2D Bosonic system was theoretically predicted by Nelson and Kosterlitz, and was first observed in 2D \textsuperscript{4}He films by Bishop and Reppy. Recent experiments in trapped ultracold 2D Bose gas systems have shown signatures of the BKT transition, and its superfluidity. However, the universal jump in the superfluid density was not observed in these systems. Here we report the first observation of the universal jump in the superfluid density using an optically trapped ultracold 2D Bose gas. The measured superfluid phase space density at the BKT transition agrees well with the predicted value within our measurement uncertainty. Additionally, we measure the phase fluctuations in our density profiles to show that the BKT transition occurs first, followed by the BEC transition.