Superconductivity in noncentrosymmetric BaAl₄ derived structures

Abstract

Ternary intermetallics Ep-T-X, crystallizing in ordered variants of the BaAl4 structure type, have been investigated systematically with respect to their formation and stability. For this, a comprehensive overview of the BaAl4 derivative structure types including group-subgroup relations was established. Special emphasis was laid on compounds where inversion symmetry is missing in the respective crystal structures and where superconductivity is observed at low temperatures. EpTX3 compounds crystallize in the noncentrosymmetric BaNiSn3 structure type (space-group I4mm; a≈0.4 and c≈1 nm), an ordered ternary derivative of BaAl4. Superconductivity below 3 K was found for seven members of this series, as evidenced from heat capacity and electrical resistivity measurements. Although the Rashba-like spin-orbit coupling in noncentrosymmetric systems can enable a mixture of spin-singlet and spin-triplet pairs in the superconducting condensate, the experimental data basically indicate a predominant s-wave superconducting state in all of these compounds. For this family of compounds, fully relativistic density functional theory (DFT) calculations of the electronic structure and phonon properties were done. Despite the different size of spin-orbit coupling depending on the actual choice of elements for Ep, T, and X that result in different spin-orbit splittings of the Fermi surfaces, the experimental observation of a prevalent spin-singlet pairing in the superconducting phases of the EpTX3 compounds is supported.