Superconducting density of states of PtPb4

TL;DR

This study uses millikelvin STM to investigate the superconducting density of states in PtPb4, revealing a fully opened gap consistent with BCS theory, with inhomogeneous features and signs of superconductivity persisting above the critical temperature and field.

Contribution

First detailed STM measurements of PtPb4's superconducting density of states, showing inhomogeneous gaps and superconducting signatures above critical parameters, highlighting defect roles.

Findings

Superconducting gap matches BCS predictions.

Superconducting signatures observed above $T_c$ and $H_{c2}.

Structural defects may influence superconducting properties.

Abstract

PtPb$_4$ is a type II superconductor with a bulk critical temperature $T_{c}\approx 3 $K and an upper critical field of $H_{c2}=0.36 $T. PtPb$_4$ is related to non-superconducting PtSn$_4$, which presents nodal arc states at the surface. Here we measure the superconducting density of states of PtPb$_4$ using millikelvin Scanning Tunneling Microscopy (STM). We observe a fully opened superconducting gap of $\Delta=0.48$\ meV similar to expectations from Bardeen Cooper and Schrieffer (BCS) theory ($\Delta_0=1.76k_BT_{c}=0.49 $meV). Measurements under magnetic fields applied perpendicular to the surface show a spatially inhomogeneous gap structure, presenting superconducting signatures at fields as high as 1.5 T, significantly above $H_{c2}=0.36 $T. On some locations we find that the superconducting density of states does not vanish above $T_{c}$. We can find signatures of a superconducting gap up to 5K. We discuss possible reasons for the observation of superconducting properties above $T_{c}$ and $H_{c2}$, emphasizing the role played by structural defects.