Multigap nodeless superconductivity in nickel chalcogenideTlNi2Se2

Abstract

Low-temperature thermal conductivity measurements were performed on single crystals of $\mathrm{TlNi}{}_{2}\mathrm{Se}{}_{2}$, a nickel chalcogenide heavy-electron superconductor with ${T}_{c}\ensuremath{\simeq}3.7$ K. In zero field, the residual electronic contribution at $T\ensuremath{\rightarrow}0$ K (${\ensuremath{\kappa}}_{0}/T$) was well separated from the total thermal conductivity, which is less than 0.45% of its normal-state value. Such a tiny residual ${\ensuremath{\kappa}}_{0}/T$ is unlikely contributed by the nodal quasiparticles. The nodeless gap structure is supported by the very weak field dependence of ${\ensuremath{\kappa}}_{0}(H)/T$ in low magnetic fields. In the whole field range, ${\ensuremath{\kappa}}_{0}(H)/T$ exhibits an $\mathsf{S}$-shaped curve, as in the case of nickel pnictides $\mathrm{BaNi}{}_{2}\mathrm{As}{}_{2}$ and $\mathrm{SrNi}{}_{2}\mathrm{P}{}_{2}$. This common feature of nickel-based superconductors can be explained by multiple nodeless superconducting gaps.