Observation of first and second sound in a BKT superfluid

Abstract

Superfluidity in its various forms has fascinated scientists since the observation of frictionless flow in liquid helium II. In three spatial dimensions (3D), it is conceptually associated with the emergence of long-range order (LRO) at a critical temperature $T_{\text{c}}$. One of its hallmarks, predicted by the highly successful two-fluid model and observed in both liquid helium and ultracold atomic gases, is the existence of two kinds of sound excitations, the first and second sound. In 2D systems, thermal fluctuations preclude LRO, but superfluidity nevertheless emerges at a nonzero $T_{\text{c}}$ via the infinite-order Berezinskii-Kosterlitz-Thouless (BKT) transition, which is associated with a universal jump in the superfluid density $n_{\text{s}}$ without any discontinuities in the fluid's thermodynamic properties. BKT superfluids are also predicted to support two sounds, but the observation of this has remained elusive. Here we observe first and second sound in a homogeneous 2D atomic Bose gas, and from the two temperature-dependent sound speeds extract its superfluid density. Our results agree with BKT theory, including the prediction for the universal superfluid-density jump.

6 pages, 3 figures

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• Observation of an Efimov spectrum in an atomic system
• Emergence of coherence in a uniform quasi-two-dimensional Bose gas
• Observation of scale invariance and universality in two-dimensional Bose gases
• Emergence of a Turbulent Cascade in a Quantum Gas
• Superfluid behaviour of a two-dimensional Bose gas
• Observation of the Berezinskii-Kosterlitz-Thouless Phase Transition in an Ultracold Fermi Gas
• Strongly Interacting Two-Dimensional Bose Gases
• Connecting Berezinskii-Kosterlitz-Thouless and BEC phase transitions by tuning interactions in a trapped gas
• Efficient Production of Large 39K Bose-Einstein Condensates

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