Superconducting density of states of PtPb4

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 $\mathrm{\Delta }=0.48$\ meV similar to expectations from Bardeen Cooper and Schrieffer (BCS) theory (${\mathrm{\Delta }}_0=1.76k_B{T}_c=0.49$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.