Intermediate coupled superconductivity in yttrium intermetallics

Abstract Non-magnetic YIn 3 , LaIn 3 and LuIn 3 with a superconducting transition temperature T c of 0.78, 0.71 and 0.24 K were investigated for superconductivity. Similarly, rare-earth compound LaSn 3 has been reported to exhibit superconductivity around 6.25 K, whereas the non-magnetic YSn 3 is a superconductor with T c of 7 K. The substitution of 13th group In-atoms by 14th group Sn-atoms is seen to enhance T c by nearly one order, although the lattice parameters increase by ∼1.0% in YSn 3 compared to YIn 3 compound. It is observed from the ground state properties that the slight difference in the energy band structures of YIn 3 , YIn 2 Sn and YSn 3 gives rise to various complex Fermi surfaces which are multiply connected and exhibit vast differences. The Fermi level lies on a sharp peak in YSn 3 which has a higher density of states N(E F ), whereas Fermi level lies on the shoulder of a sharp peak in YIn 3 . The electron localization function (ELF) and difference charge density maps clearly illustrate the difference in the nature of bonding; the Y Sn bonds are clearly more ionic (due to larger bond length) than Y In bonds. These results are consistent with the Bader charges which show loss of charges from Y-atoms and a gain of charges by In/Sn atoms. The dynamical properties also clearly illustrate the difference in the nature of bonds in YX 3 intermetallics. A softening of the lowermost acoustic modes is observed in YIn 3 , whereas all the modes in YSn 3 are observed to have positive frequencies which imply its greater stability. Since λ el-ph 3 and YSn 3 compounds exhibit type I superconductivity according to BCS theory. However, the smaller N(E F ) obtained from the density of states (DOS); the electron-phonon coupling constant λ el-ph obtained from the temperature dependent specific heat as well as the instability in phonon modes due to stronger Y In and In In bonds in YIn 3 may be the cause of lower T c and filamentary nature of superconductivity. Insertion of Sn-atom in the YIn 3 lattice further consolidates the superconducting nature due to increase in N(E F ) and γ (electronic component of specific heat), along with lowering of the frequency of imaginary modes from 5.6 THz to 1.5–0.6 THz. Thus T c is directly related to the valence electron concentration and ternary YIn 2 Sn may exhibit intermediate superconducting transition temperature.