Quantum critical Bose gas in the two-dimensional limit in the honeycomb antiferromagnet YbCl$_3$ under magnetic fields

TL;DR

This study demonstrates 2D-limit Bose-Einstein condensation in the honeycomb antiferromagnet YbCl3 under magnetic fields, revealing quantum fluctuations and renormalized interactions characteristic of 2D systems.

Contribution

It provides experimental evidence of 2D-limit BEC in a honeycomb antiferromagnet with extremely small interlayer coupling, highlighting unique quantum fluctuations and interaction effects.

Findings

Observation of BEC near the critical magnetic field Hs

Detection of 2D quantum fluctuations and highly mobile Bose gas

Identification of logarithmic renormalization of boson interactions

Abstract

BEC is a quantum phenomenon, where a macroscopic number of bosons occupy the lowest energy state and acquire coherence at low temperatures. It is realized not only in $^4$He and dilute atomic gases, but also in quantum magnets, where hardcore bosons, introduced by the Matsubara-Matsuda transformation of spins, condense. In 3D antiferromagnets, an XY-type long-range ordering (LRO) occurs near a magnetic-field-induced transition to a fully polarized state (FP) and has been successfully described as a BEC in the last few decades. An attractive extension of the BEC in 3D magnets is to make their 2D analogue. For a strictly 2D system, BEC cannot take place due to the presence of a finite density of states at zero energy, and a Berezinskii-Kosterlitz-Thouless (BKT) transition may instead emerge. In a realistic quasi-2D magnet consisting of stacked 2D magnets, a small but finite interlayer coupling stabilizes marginal LRO and BEC, but such that 2D physics, including BKT fluctuations, is still expected to dominate. A few systems were reported to show such 2D-limit BEC, but at very high magnetic fields that are difficult to access. The honeycomb $S$ = 1/2 Heisenberg antiferromagnet YbCl$_3$ with an intra-layer coupling $J\sim$ 5 K exhibits a transition to a FP state at a low in-plane magnetic field of $H_{\rm s}$ = 5.93 T. Here, we demonstrate that the LRO right below $H_{\rm s}$ is a BEC in the 2D-limit stabilized by an extremely small interlayer coupling $J_{\perp}$ of 10$^{-5}J$. At the quantum critical point Hs, we capture 2D-limit quantum fluctuations as the formation of a highly mobile, interacting 2D Bose gas in the dilute limit. A much-reduced effective boson-boson repulsion Ueff as compared with that of a prototypical 3D system indicates the presence of a logarithmic renormalization of interaction unique to 2D.