Intermediate coupled superconductivity in yttrium intermetallics

Non-magnetic YIn₃, LaIn₃ and LuIn₃ with a superconducting transition temperature T_c of 0.78, 0.71 and 0.24 K were investigated for superconductivity. Similarly, rare-earth compound LaSn₃ has been reported to exhibit superconductivity around 6.25 K, whereas the non-magnetic YSn₃ 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 YSn₃ compared to YIn₃ compound. It is observed from the ground state properties that the slight difference in the energy band structures of YIn₃, YIn₂Sn and YSn₃ gives rise to various complex Fermi surfaces which are multiply connected and exhibit vast differences. The Fermi level lies on a sharp peak in YSn₃ which has a higher density of states N(E_F), whereas Fermi level lies on the shoulder of a sharp peak in YIn₃. The electron localization function (ELF) and difference charge density maps clearly illustrate the difference in the nature of bonding; the Ysbnd Sn bonds are clearly more ionic (due to larger bond length) than Ysbnd 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 YX₃ intermetallics. A softening of the lowermost acoustic modes is observed in YIn₃, whereas all the modes in YSn₃ are observed to have positive frequencies which imply its greater stability. Since λ_el-ph < 1, both YIn₃ and YSn₃ 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 Ysbnd In and Insbnd In bonds in YIn₃ may be the cause of lower T_c and filamentary nature of superconductivity. Insertion of Sn-atom in the YIn₃ 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 YIn₂Sn may exhibit intermediate superconducting transition temperature.