Superconducting phase diagram of the magnetically ordered superconductor RbEuFe₄As₄ in pulsed fields up to 86 T

The superconducting phase diagram of single-crystal RbEuFe₄As₄ (T_c ~ 36.5 K), which has long-range Eu magnetic order at T_m ~ 15 K, has been measured in pulsed magnetic fields up to 86 T using a proximity diode oscillator technique. Upon decreasing temperature, the anisotropy H_c2^ab/H_c2^c approaches unity at approximately 5 K, with pronounced curvature observed in H_c2-line, especially in the in-plane H_c2^ab(T). A two-band theoretical model for a Fermi surface in the form of warped cylinders yields high Maki parameters for both H // ab and H // c, indicating significant Pauli paramagnetic limiting effects for both orientations. However, no upturn at low temperature / high field characteristic of a transition into an FFLO state is observed.