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Sobirey L, Biss H, Luick N, Bohlen M, Moritz H, Lompe T. Observing the Influence of Reduced Dimensionality on Fermionic Superfluids. PHYSICAL REVIEW LETTERS 2022; 129:083601. [PMID: 36053698 DOI: 10.1103/physrevlett.129.083601] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/07/2021] [Accepted: 06/28/2022] [Indexed: 06/15/2023]
Abstract
Understanding the origins of unconventional superconductivity has been a major focus of condensed matter physics for many decades. While many questions remain unanswered, experiments have found the highest critical temperatures in layered two-dimensional materials. However, to what extent the remarkable stability of these strongly correlated 2D superfluids is affected by their reduced dimensionality is still an open question. Here, we use dilute gases of ultracold fermionic atoms as a model system to directly observe the influence of dimensionality on the stability of strongly interacting fermionic superfluids. We find that the superfluid gap follows the same universal function of the interaction strength regardless of dimensionality, which suggests that there is no inherent difference in the stability of two- and three-dimensional fermionic superfluids. Finally, we compare our data to results from solid state systems and find a similar relation between the interaction strength and the gap for a wide range of two- and three-dimensional superconductors.
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Affiliation(s)
- Lennart Sobirey
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
| | - Hauke Biss
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg
| | - Niclas Luick
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg
| | - Markus Bohlen
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg
| | - Henning Moritz
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg
| | - Thomas Lompe
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg
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Maki J, Zhang S, Zhou F. Dynamics of Strongly Interacting Fermi Gases with Time-Dependent Interactions: Consequence of Conformal Symmetry. PHYSICAL REVIEW LETTERS 2022; 128:040401. [PMID: 35148156 DOI: 10.1103/physrevlett.128.040401] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/19/2021] [Accepted: 12/24/2021] [Indexed: 06/14/2023]
Abstract
In this Letter, we investigate the effects of a time-dependent, short-ranged interaction on the long-time expansion dynamics of Fermi gases. We show that the effects of the interaction on the dynamics is dictated by how it changes under a conformal transformation, and derive an explicit criterion for the relevancy of time-dependent interactions near both the strongly and noninteracting scale invariant limits. In addition, we show that it is possible to engineer interactions that give rise to nonexponential thermalization dynamics in trapped Fermi gases. To supplement the symmetry analysis, we perform hydrodynamic simulations to show that the moment of inertia of the trapped gas indeed follows a universal time dependence that is determined jointly by the conformal symmetry and time-dependent scattering length a(t). Our results should also be relevant to the dynamics of other systems that are nearly scale invariant and that are governed by a nonrelativistic conformal symmetry.
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Affiliation(s)
- Jeff Maki
- Department of Physics and HKU-UCAS Joint Institute for Theoretical and Computational Physics at Hong Kong, Guangdong-Hong Kong Joint Laboratory of Quantum Matter, The University of Hong Kong, Hong Kong, China
| | - Shizhong Zhang
- Department of Physics and HKU-UCAS Joint Institute for Theoretical and Computational Physics at Hong Kong, Guangdong-Hong Kong Joint Laboratory of Quantum Matter, The University of Hong Kong, Hong Kong, China
| | - Fei Zhou
- Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1, Canada
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Bohlen M, Sobirey L, Luick N, Biss H, Enss T, Lompe T, Moritz H. Sound Propagation and Quantum-Limited Damping in a Two-Dimensional Fermi Gas. PHYSICAL REVIEW LETTERS 2020; 124:240403. [PMID: 32639806 DOI: 10.1103/physrevlett.124.240403] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/09/2020] [Revised: 04/26/2020] [Accepted: 05/22/2020] [Indexed: 06/11/2023]
Abstract
Strongly interacting two-dimensional Fermi systems are one of the great remaining challenges in many-body physics due to the interplay of strong local correlations and enhanced long-range fluctuations. Here, we probe the thermodynamic and transport properties of a 2D Fermi gas across the BEC-BCS crossover by studying the propagation and damping of sound modes. We excite particle currents by imprinting a phase step onto homogeneous Fermi gases trapped in a box potential and extract the speed of sound from the frequency of the resulting density oscillations. We measure the speed of sound across the BEC-BCS crossover and compare the resulting dynamic measurement of the equation of state both to a static measurement based on recording density profiles and to quantum Monte Carlo calculations and find reasonable agreement between all three. We also measure the damping of the sound mode, which is determined by the shear and bulk viscosities as well as the thermal conductivity of the gas. We find that the damping is minimal in the strongly interacting regime and the diffusivity approaches the universal quantum bound ℏ/m of a perfect fluid.
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Affiliation(s)
- Markus Bohlen
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
- Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, 24 rue Lhomond, Paris 75005, France
| | - Lennart Sobirey
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
| | - Niclas Luick
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
| | - Hauke Biss
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
| | - Tilman Enss
- Intitut für Theoretische Physik, Universität Heidelberg, Philosophenweg 19, Heidelberg 69120, Germany
| | - Thomas Lompe
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
| | - Henning Moritz
- Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
- The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany
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Oscillatory-like expansion of a Fermionic superfluid. Sci Bull (Beijing) 2020; 65:7-11. [PMID: 36659071 DOI: 10.1016/j.scib.2019.10.022] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2019] [Revised: 09/25/2019] [Accepted: 10/14/2019] [Indexed: 01/21/2023]
Abstract
We study the expansion behaviors of a Fermionic superfluid in a cigar-shaped optical dipole trap for the whole BEC-BCS crossover and various temperatures. At low temperature (0.06(1)TF), the atom cloud undergoes an anisotropic hydrodynamic expansion over 30 ms, which behaves like oscillation in the horizontal plane. By analyzing the expansion dynamics according to the superfluid hydrodynamic equation, the effective polytropic index γ¯ of Equation-of-State (EoS) of Fermionic superfluid is extracted. The γ¯ values show a non-monotonic behavior over the BEC-BCS crossover, and have a good agreement with the theoretical results in the unitarity and BEC side. The normalized quasi-frequencies of the oscillatory expansion are measured, which drop significantly from the BEC side to the BCS side and reach a minimum value of 1.73 around 1/kFa=-0.25. Our work improves the understanding of the dynamic properties of strongly interacting Fermi gas.
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Yin XY, Hu H, Liu XJ. Few-Body Perspective of a Quantum Anomaly in Two-Dimensional Fermi Gases. PHYSICAL REVIEW LETTERS 2020; 124:013401. [PMID: 31976732 DOI: 10.1103/physrevlett.124.013401] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/23/2019] [Revised: 11/10/2019] [Indexed: 06/10/2023]
Abstract
A quantum anomaly manifests itself in the deviation of the breathing mode frequency from the scale invariant value of 2ω in two-dimensional harmonically trapped Fermi gases, where ω is the trapping frequency. Its recent experimental observation with cold atoms reveals an unexpected role played by the effective range of interactions, which requires a quantitative theoretical understanding. Here we provide accurate, benchmark results on a quantum anomaly from a few-body perspective. We consider the breathing mode of a few trapped interacting fermions in two dimensions up to six particles and present the mode frequency as a function of scattering length for a wide range of effective range. We show that the maximum quantum anomaly gradually reduces as the effective range increases while the maximum position shifts towards the weak-coupling limit. We extrapolate our few-body results to the many-body limit and find a good agreement with the experimental measurements. Our results may also be directly applicable to a few-fermion system prepared in microtraps and optical tweezers.
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Affiliation(s)
- X Y Yin
- Centre for Quantum and Optical Science, Swinburne University of Technology, Melbourne, Victoria 3122, Australia
| | - Hui Hu
- Centre for Quantum and Optical Science, Swinburne University of Technology, Melbourne, Victoria 3122, Australia
| | - Xia-Ji Liu
- Centre for Quantum and Optical Science, Swinburne University of Technology, Melbourne, Victoria 3122, Australia
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Enss T. Bulk Viscosity and Contact Correlations in Attractive Fermi Gases. PHYSICAL REVIEW LETTERS 2019; 123:205301. [PMID: 31809121 DOI: 10.1103/physrevlett.123.205301] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/30/2019] [Indexed: 06/10/2023]
Abstract
The bulk viscosity determines dissipation during hydrodynamic expansion. It vanishes in scale invariant fluids, while a nonzero value quantifies the deviation from scale invariance. For the dilute Fermi gas the bulk viscosity is given exactly by the correlation function of the contact density of local pairs. As a consequence, scale invariance is broken purely by pair fluctuations. These fluctuations give rise also to logarithmic terms in the bulk viscosity of the high-temperature nondegenerate gas. For the quantum degenerate regime I report numerical Luttinger-Ward results for the contact correlator and the dynamical bulk viscosity throughout the BEC-BCS crossover. The ratio of bulk to shear viscosity ζ/η is found to exceed the kinetic theory prediction in the quantum degenerate regime. Near the superfluid phase transition the bulk viscosity is enhanced by critical fluctuations and has observable effects on dissipative heating, expansion dynamics, and sound attenuation.
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Affiliation(s)
- Tilman Enss
- Institut für Theoretische Physik, Universität Heidelberg, D-69120 Heidelberg, Germany
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Murthy PA, Defenu N, Bayha L, Holten M, Preiss PM, Enss T, Jochim S. Quantum scale anomaly and spatial coherence in a 2D Fermi superfluid. Science 2019; 365:268-272. [PMID: 31320537 DOI: 10.1126/science.aau4402] [Citation(s) in RCA: 28] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2018] [Accepted: 06/18/2019] [Indexed: 11/02/2022]
Abstract
Quantum anomalies are violations of classical scaling symmetries caused by divergences that appear in the quantization of certain classical theories. Although they play a prominent role in the quantum field theoretical description of many-body systems, their influence on experimental observables is difficult to discern. In this study, we discovered a distinctive manifestation of a quantum anomaly in the momentum-space dynamics of a two-dimensional (2D) Fermi superfluid of ultracold atoms. The measured pair momentum distributions of the superfluid during a breathing mode cycle exhibit a scaling violation in the strongly interacting regime. We found that the power-law exponents that characterize long-range phase correlations in the system are modified by the quantum anomaly, emphasizing the influence of this effect on the critical properties of 2D superfluids.
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Affiliation(s)
- Puneet A Murthy
- Physics Institute, Heidelberg University, Heidelberg, Germany.
| | - Nicolò Defenu
- Institute for Theoretical Physics, Heidelberg University, Heidelberg, Germany.
| | - Luca Bayha
- Physics Institute, Heidelberg University, Heidelberg, Germany
| | - Marvin Holten
- Physics Institute, Heidelberg University, Heidelberg, Germany
| | | | - Tilman Enss
- Institute for Theoretical Physics, Heidelberg University, Heidelberg, Germany
| | - Selim Jochim
- Physics Institute, Heidelberg University, Heidelberg, Germany
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