Two-dimensional Bose fluids: An atomic physics perspective

TL;DR

This paper reviews the unique properties of two-dimensional Bose fluids, focusing on their phase transitions, order, and superfluidity, highlighting recent theoretical and experimental advances in ultra-cold atomic gases confined to 2D.

Contribution

It provides a comprehensive introduction to 2D Bose fluids from an atomic physics perspective, summarizing recent progress and offering insights for researchers in related fields.

Findings

Thermal fluctuations destroy Bose-Einstein condensation in 2D at any non-zero temperature.

Superfluidity persists in 2D systems despite the absence of true long-range order.

Residual quasi-long-range order influences superfluid behavior in finite-size 2D systems.

Abstract

The properties of phase transitions and the types of order present in the low-temperature states of matter are fundamentally dependent on the dimensionality of physical systems. Generally, highly ordered states are more robust in higher dimensions, while thermal and quantum fluctuations, which favour disordered states, play a more important role in lower dimensions. The case of a two-dimensional (2d) Bose fluid is particularly fascinating because of its "marginal" behaviour. In an infinite uniform 2d fluid thermal fluctuations at any non-zero temperature are strong enough to destroy the fully ordered state associated with Bose--Einstein condensation, but are not strong enough to suppress superfluidity in an interacting system at low, but non-zero temperatures. Further, the presence of residual "quasi-long-range" order at low temperatures leads to an interesting interplay between superfluidity and condensation in all experimentally relevant finite-size systems. In these notes we give an introduction to the physics of 2d Bose fluids from an atomic physics perspective. Our goal is to summarize the recent progress in theoretical understanding and experimental investigation of ultra-cold atomic gases confined to 2d geometry, and we also hope to provide a useful introduction to these systems for researchers working on related topics in other fields of physics.