Superfluid behaviour of a two-dimensional Bose gas

TL;DR

This paper investigates superfluidity in a two-dimensional Bose gas, demonstrating a transition to superfluid behavior with minimal heating below a critical obstacle velocity, advancing understanding of 2D quantum fluids.

Contribution

It provides the first direct observation of superfluidity in a 2D ultracold Bose gas using a moving obstacle, highlighting the role of degeneracy and critical velocity.

Findings

Superfluid response varies with local degeneracy.

No significant heating observed below critical velocity.

Superfluid behavior confirmed in a 2D Bose gas.

Abstract

Two-dimensional (2D) systems play a special role in many-body physics. Because of thermal fluctuations, they cannot undergo a conventional phase transition associated to the breaking of a continuous symmetry. Nevertheless they may exhibit a phase transition to a state with quasi-long range order via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. A paradigm example is the 2D Bose fluid, such as a liquid helium film, which cannot Bose-condense at non-zero temperature although it becomes superfluid above a critical phase space density. Ultracold atomic gases constitute versatile systems in which the 2D quasi-long range coherence and the microscopic nature of the BKT transition were recently explored. However, a direct observation of superfluidity in terms of frictionless flow is still missing for these systems. Here we probe the superfluidity of a 2D trapped Bose gas with a moving obstacle formed by a micron-sized laser beam. We find a dramatic variation of the response of the fluid, depending on its degree of degeneracy at the obstacle location. In particular we do not observe any significant heating in the central, highly degenerate region if the velocity of the obstacle is below a critical value.