Single full gap with mixed type-I and type-II superconductivity on surface of the type-II Dirac semimetal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>PdTe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> by point-contact spectroscopy

Abstract

We report our point-contact spectroscopy (PCS) study on the superconducting state of the type-II Dirac semimetal ${\mathrm{PdTe}}_{2}$ with a superconducting transition temperature ${T}_{\mathrm{c}}\ensuremath{\sim}1.65$ K. Both mechanical- and soft-PCS differential conductance curves at 0.3 K show a consistent double-peak structure and they can be perfectly fitted by a single $s$-wave gap based on the Blonder-Tinkham-Klapwijk model. The gap follows a typical Bardeen-Cooper-Schrieffer temperature behavior, yielding ${\mathrm{\ensuremath{\Delta}}}_{0}\ensuremath{\sim}0.29$ meV and $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=4.15$ in the strong-coupling regime. A sudden suppression of the superconducting gap in a magnetic field around $H{}_{\mathrm{c}1}\ensuremath{\sim}130$ Oe is observed for most point contacts on ${\mathrm{PdTe}}_{2}$, characteristic of a first-order transition for type-I superconductors in field. However, for other contacts, a smooth evolution of the PCS conductance persists up to $H{}_{\mathrm{c}2}\ensuremath{\sim}600$ Oe, signaling a local type-II superconductivity. The observed admixture of type-I and type-II superconductivity can possibly arise from an inhomogeneous electron mean free path on the surface of ${\mathrm{PdTe}}_{2}$ due to its topological surface states.