Bose–Einstein condensation (BEC) is a quantum phenomenon in which a macroscopic number of bosons occupy the lowest energy state and acquire coherence at low temperatures. In three-dimensional antiferromagnets, a magnetic-field-induced transition has been successfully described as a magnon BEC. For a strictly two-dimensional (2D) system, it is known that BEC cannot take place due to the presence of a finite density of states at zero energy. However, in a realistic quasi-2D magnet consisting of stacked magnetic layers, a small but finite interlayer coupling stabilizes marginal BEC but such that 2D physics is still expected to dominate. This 2D-limit BEC behaviour has been reported in a few materials but only at very high magnetic fields that are difficult to access. The honeycomb S = 1/2 Heisenberg antiferromagnet YbCl₃ exhibits a transition to a fully polarized state at a relatively low in-plane magnetic field. Here, we demonstrate the formation of a quantum critical 2D Bose gas at the transition field, which, with lowering the field, experiences a BEC marginally stabilized by an extremely small interlayer coupling. Our observations establish YbCl₃, previously a Kitaev quantum spin liquid material, as a realization of a quantum critical BEC in the 2D limit.

玻色-爱因斯坦凝聚（BEC）是一种量子现象，其中宏观数量的玻色子占据最低能态并在低温下获得相干性。在三维反铁磁体中，磁场引起的转变已成功地描述为磁振子 BEC。对于严格的二维 (2D) 系统，众所周知，由于零能量下存在有限态密度，BEC 无法发生。然而，在由堆叠磁性层组成的现实准二维磁体中，小但有限的层间耦合稳定了边际 BEC，但预计二维物理仍然占主导地位。这种二维极限 BEC 行为已在一些材料中得到报道，但仅限于难以接近的非常高的磁场。蜂窝状S  = 1/2 海森堡反铁磁体YbCl ₃在相对较低的面内磁场下表现出向完全极化状态的转变。在这里，我们演示了在过渡场处形成量子临界 2D 玻色气体，随着过渡场的降低，该气体会经历通过极小的层间耦合而略微稳定的 BEC。我们的观察结果表明，YbCl ₃（以前是一种基塔耶夫量子自旋液体材料）是二维极限下量子临界 BEC 的实现。