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

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 $H_{\mathrm{s}}$ = 5.93 T. Here, we demonstrate that the LRO right below $H_{\mathrm{s}}$ 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.

arXiv comment24 pages, 12 figures

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• Magnetization process of a quasi-two-dimensional quantum magnet: Two-step symmetry restoration and dimensional reduction
• Anomalous enhancement of magnetism by nonmagnetic doping in the honeycomb-lattice antiferromagnet ErOCl
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• Ferroelectricity in a magnon Bose-Einstein condensate: Nonreciprocal superfluidity, exceptional points, and Majorana bosons
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