Non-centrosymmetric superconductor Th_4Be_{{33}}Pt_{{16}} and heavy-fermion U_4Be_{{33}}Pt_{{16}} cage compounds

Abstract

Unconventional superconductivity in non-centrosymmetric superconductors has attracted a considerable amount of attention. While several lanthanide-based materials have been reported previously, the number of actinide-based systems remains small. In this work, we present the discovery of a novel cubic complex non-centrosymmetric superconductor {text {Th}}_4{text {Be}}_{{33}}{text {Pt}}_{{16}} (I{bar{4}}3d space group). This intermetallic cage compound displays superconductivity below T_{text {c}} = 0.90 pm 0.04 K, as evidenced by specific heat and resistivity data. {text {Th}}_4{text {Be}}_{{33}}{text {Pt}}_{{16}} is a type-II superconductor, which has an upper critical field {text {H}}_{{text {c}}2} = 0.27 T and a moderate Sommerfeld coefficient gamma _{text {n}} = 16.3 pm 0.8 mJ {text {mol}}^{-1}_{text {Th}} {text {K}}^{-2}. A non-zero density of states at the Fermi level is evident from metallic behavior in the normal state, as well as from electronic band structure calculations. The isostructural {text {U}}_4{text {Be}}_{{33}}{text {Pt}}_{{16}} compound is a paramagnet with a moderately enhanced electronic mass, as indicated by the electronic specific heat coefficient gamma _{text {n}} = 200 mJ {text {mol}}^{-1}_{text {U}} {text {K}}^{-2} and Kadowaki–Woods ratio A/gamma ^2 = 1.1 times 10^{-5} upmu Omega cm {text {K}}^2{text {mol}}_{text {U}}^2 (mJ)^{-2}. Both {text {Th}}_4{text {Be}}_{{33}}{text {Pt}}_{{16}} and {text {U}}_4{text {Be}}_{{33}}{text {Pt}}_{{16}} are crystallographically complex, each hosting 212 atoms per unit cell.