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TT: Fachverband Tiefe Temperaturen

TT 15: Superconductivity: Fe-based Superconductors - Other Materials and Theory

TT 15.1: Vortrag

Montag, 1. April 2019, 15:00–15:15, H23

Specific heat of RbEuFe₄As₄ - a magnetic superconductor — •Kristin Willa^1,2, Matthew Smiley², Roland Willa^2,3, Alex Koshelev², Jin-Ke Bao², Mercoury Kanatzidis², Ulrich Welp², and Wai-Kwong Kwok² — ¹Institute for Solid-State Physics, Karlsruhe Institute of Technology, Germany — ²Materials Science Division, Argonne National Laboratory, USA — ³Institute for Theoretical Condensed Matter Physics, Karlsruhe Institute of Technology, Germany

We report detailed nanocalorimetric measurements on the newly discovered magnetic superconductor RbEuFe₄As₄. We investigated the superconducting transition at T_c=37K and extracted the phase boundary for in and out of plane magnetic fields obtaining an anisotropy ratio of 1.8. Large superconducting fluctuations are observed as well as a vortex lattice melting transition identified as a step of 4-5% of the zero field jump in specific heat. The melting line is considerably below the upper-critical-field line which is in quantitative agreement with theoretical predictions. In small fields near the magnetic transition T_m = 14.9K, we resolved a cusp-like behavior 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.