Experimental demonstration of a two-band superconducting state for lead using scanning tunneling spectroscopy

TL;DR

This study provides experimental evidence of a two-band superconducting state in lead using scanning tunneling spectroscopy, revealing two distinct gaps and their dependence on crystal orientation and surface modifications.

Contribution

First direct experimental demonstration of lead's two-band superconductivity through detailed tunneling spectroscopy analysis.

Findings

Resolved two superconducting gaps with 150 μeV difference

Observed orientation-dependent coherence peak intensities

Confirmed different orbital characters of the two bands

Abstract

The type I superconductor lead (Pb) has been theoretically predicted to be a two-band superconductor. We use scanning tunneling spectroscopy (STS) to resolve two superconducting gaps with an energy difference of 150$\,\mu$eV. Tunneling into Pb(111), Pb(110) and Pb(100) crystals reveals a strong dependence of the two coherence peak intensities on the crystal orientation. We show that this is the result of a selective tunneling into the two bands at the energy of the two coherence peaks. This is further sustained by the observation of signatures of the Fermi sheets in differential conductance maps around subsurface defects. A modification of the density of states of the two bands by adatoms on the surface confirms the different orbital character of each to the two sub-bands.