Specific heat measurements of the high-temperature magnetic superconductor RbEuFe₄As₄

We report specific heat measurements on the newly discovered magnetic superconductor RbEuFe₄As₄. We investigated the superconducting transition at T_c = 38K and extracted the phase boundary for in and out of plane magnetic fields and obtained an anisotropy ratio of 1.8. At small fields near the magnetic transition temperature T_m ~ 14.9K, we resolved a cusp-like behavior in the temperature dependence of the specific heat curve that shifts to lower temperatures for fields along the c-axis and a broad shoulder that shifts to higher temperatures for in-plane fields. We can reproduce our measured calorimetry data quantitatively by Monte-Carlo simulations of an anisotropic easy-plane 2D Heisenberg model that suggests that the cusp in specific heat is due to a BKT transition and the high temperature hump at higher fields marks a crossover from a paramagnetically disordered to an ordered state.