Enhancement of the superconducting transition temperature due to multiband effect in the topological nodal-line semimetal Pb${}_{1-x}$Sn${}_x$TaSe${}_2$

Abstract

We report a systematic study of the normal-state and superconducting properties of single crystal Pb${}_{1-x}$Sn${}_x$TaSe${}_2$ $(0\le x\le 0.23)$. Sn doping enhances the superconducting temperature $T_c$ up to 5.1 K while also significantly increasing impurity scattering in the crystals. For $x=0$ and 0.018, the specific heat jump at $T_c$ exceeds the Bardeen-Cooper-Schrieffer (BCS) weak-coupling value of 1.43, indicating the realization of strong-coupling superconductivity in undoped and slightly Sn-doped PbTaSe${}_2$. Substituting Pb with more Sn lowers the specific heat jump at $T_c$ below the BCS value of 1.43, which cannot be explained by a single-gap model. Rather, the observed specific heat data of moderately Sn-doped PbTaSe${}_2$ ($x=0.08$ and 0.15) are reproduced by a two-gap model. Our density functional theory calculations suggest that three-dimensional Fermi pockets appear due to a reduction of the spin-orbit gap with Sn doping, and the multiband effect arising from these emergent Fermi pockets enhances the effective electron-phonon coupling strength, leading to the increase in $T_c$ of Pb${}_{1-x}$Sn${}_x$TaSe${}_2$.

arXiv comment7 pages, 4 figures

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