Electron-phonon superconductivity in C-doped topological nodal-line semimetal Zr$_5$Pt$_3$: A muon spin rotation and relaxation ($μ$SR) study

Abstract

In the present work we demonstrate that C-doped Zr 5 Pt 3 is an electron-phonon superconductor (with critical temperature T C = 3.7 K) with a nonsymmorphic topological Dirac nodal-line semimetal state, which we report here for the first time. The superconducting properties of Zr 5 Pt 3 C 0.5 have been investigated by means of magnetization and muon spin rotation and relaxation (µSR) measurements. We find that at low temperatures the depolarization rate is almost constant and can be well described by a single-band s−wave model with a superconducting gap of 2∆(0)/k B T C = 3.84, close to the value of BCS theory. From transverse field µSR analysis we estimate the London penetration depth λ L = 469 nm, superconducting carrier density n s = 2×10 26 m −3 , and effective mass m * = 1.584 m e . Zero field µSR confirms the absence of any spontaneous magnetic moment in the superconducting ground state. To gain additional insights into the electronic ground state of C-doped Zr 5 Pt 3 , we have also performed first-principles calculations within the framework of density functional theory (DFT). The observed homogenous electronic character of the Fermi surface as well as the mutual decrease of T C and density of states at the Fermi level are consistent with the experimental findings. However, the band structure reveals the presence of robust, gapless fourfold-degenarate nodal lines protected by 6 3 screw rotations and glide mirror planes. Therefore, Zr 5 Pt 3 represents a novel, unprecedented condensed matter system to investigate the intricate interplay between superconductivity and topology.