Quasi-two-dimensional Bose-Einstein condensation of lattice bosons in spin-1/2 XXZ ferromagnet K${}_2$CuF${}_4$

Abstract

K${}_2$CuF${}_4$ is magnetically described as a spin-1/2, quasi-two-dimensional (2D), square-lattice XXZ ferromagnet with weak easy-plane anisotropy. The magnetic ordering for an applied magnetic field $H$ parallel to the $c$ axis is equivalent to the Bose\,--\,Einstein condensation (BEC) of lattice bosons, as discussed by Matsubara and Matsuda [Prog. Theor. Phys. \textbf{16}, 569 (1956)]. Magnetization and specific heat measurements were performed to obtain the temperature vs magnetic field phase diagram for $H\parallel c$. The phase boundary between polarized and ordered phases was found to be expressed by the power law $H_{\mathrm{c}}(T)-H_{\mathrm{c}}(0)\propto T^{\varphi }$ with exponent $\varphi \approx 1.0$ in a wide temperature range, in agreement with the theory of quasi-2D BEC.

5 pages, 4 figures, accepted for publication in Physics. Rev. B