Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

Abstract

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.