講演題目： Quantum critical 2D Bose gas in the honeycomb antiferromagnet YbCl3
講師: Dr. Yosuke Matsumoto (Max Planck Institute for Solid State Research)
日時: 2025年12月17日 17:00-
場所: 理学部2号館 2-211 講義室
要旨:
Bose-Einstein condensation (BEC) is a quantum phenomenon, where a
macroscopic number of bosons occupy the lowest energy state and acquire
coherence at low temperatures. It manifests itself not only in superfluid
4He and dilute atomic gases but also in quantum magnets[1]. In three-
dimensional (3D) antiferromagnets, an XY-type long-range ordering (LRO)
near a magnetic-field-induced transition to a fully polarized state (FP)
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 two-
dimensional (2D) analogue. In a strictly 2D system, BEC cannot take
place at a finite temperature due to the presence of a finite density of
states at zero energy, but instead, a Berezinskii-Kosterlitz-Thouless (
BKT) transition may 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 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.
YbCl3 is a pseudo-spin 1/2 honeycomb Heisenberg antiferromagnet with
intra-layer coupling of J ~ 5 K and exhibits a transition to a FP state
at an in-plane saturation field Hs = 5.93 T [2-4]. Here, we demonstrate
that the LRO right below Hs is a BEC but close to the 2D-limit,
marginally stabilized by an extremely small interlayer coupling J⊥ of
the order of 10-5J [5]. 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 compared with that of a prototypical 3D system
indicates the presence of a logarithmic renormalization of interaction,
which is unique to 2D. Thus, the old candidate for a Kitaev quantum spin
liquid, YbCl3, is now established as an ideal arena for a quantum
critical BEC in the 2D limit.
[1] V. Zapf, M. Jaime, and C. D. Batista, Rev. Mod. Phys. 86, 563 (2014).
[2] G. Sala et al., Phys. Rev. B 100, 180406(R) (2019).
[3] J. Xing et al., Phys. Rev. B 102, 014427 (2020).
[4] G. Sala et al., Nat Commun 12, 171 (2021).
[5] Y. Matsumoto et al., Nat. Phys. 20, 1131–1138 (2024).