A brief review on μSR studies of unconventional Fe- and Cr-based superconductors

Muon spin relaxation/rotation ( μ SR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remain a puzzle. Very recently double layered Fe-based super- conductors having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D structure have drawn considerable attention. Here we present a brief review of the characteristics of a few selected Fe- and Cr-based superconducting materials and highlight some of the major outstanding problems, with an emphasis on the superconducting pairing symmetries of these materials. We focus on μ SR studies of the newly discovered superconductors A Ca 2 Fe 4 As 4 F 2 ( A = K, Rb, and Cs), ThFeAsN, and A 2 Cr 3 As 3 ( A = K, Cs), which were used to determine the superconducting gap structures, the presence of spin fluctuations, and to search for time reversal symmetry breaking in the superconducting states. We also briefly discuss the results of μ SR investigations of the superconductivity in hole and electron doped BaFe 2 As 2 .