A quantum critical Bose gas of magnons in the quasi-two-dimensional antiferromagnet YbCl₃ under magnetic fields

Bose–Einstein condensation (BEC) is a quantum phenomenon where a macroscopic number of bosons occupy the lowest energy state. In three-dimensional antiferromagnets, a magnetic-field-induced transition is described as magnon BEC. In strictly two-dimensional (2D) systems, BEC is impossible due to a finite density of states at zero energy. However, quasi-2D magnets with stacked layers and weak interlayer coupling can exhibit marginal BEC. This study demonstrates the formation of a quantum critical 2D Bose gas in YbCl₃ at the transition field, showcasing BEC marginally stabilized by extremely weak interlayer coupling. This establishes YbCl₃ as a realization of a quantum critical BEC in the 2D limit.

Yosuke Matsumoto, Simon Schnierer, Jan A. N. Bruin, Jürgen Nuss, Pascal Reiss, George Jackeli, Kentaro Kitagawa, Hidenori Takagi