Superconducting properties of pt-type and bct-type YRh4B4

Abstract YRh 4 B 4 , which exhibits high- T c superconductivity in the RRh 4 B 4 system, has two types of crystal structures, CeCo 4 B 4 type (pt-type) and LuRu 4 B 4 type (bct-type). Both types of structure have similar T c (pt-type; T c = 10.6 K, bct-type; T c = 9.8 K), but the μ 0 H c 2 of both compounds is very different, pt-type; μ 0 H c 2 = 2.50 T and bct-type; μ 0 H c 2 = 9.85 T. From our results, one can see that the difference in T c between pt-YRh 4 B 4 and bct-YRh 4 B 4 is mainly caused from the difference of the density of state at Fermi energy. The high- T c in pt-YRh 4 B 4 is derived from the high Debye temperature caused by vibrations of boron. In contrast, in bct-YRh 4 B 4 it is caused by a stronger electron–phonon coupling than in pt-YRh 4 B 4 . Regarding the symmetry of superconductivity, it is suggested that pt-YRh 4 B 4 is a clean limit superconductor and has basically a fully gapped superconducting gap, but is slightly anisotropic from the results of C el ∝ exp ( − Δ / k B T ) below T c and Δ γ ( H ) ∝ H 0.65 . However, bct-YRh 4 B 4 is a dirty limit superconductor and has an anisotropic superconducting gap from the results of C el ∝ T 3 below T c and Δ γ ( H ) ∝ H .