Cryomagnetic scanning tunneling spectroscopy study of multi-gap spectra in superconducting 2H-NbSe2

Abstract

Scanning tunneling spectroscopy was performed on single crystals of superconducting 2H-NbSe2, at 300 mK and in a magnetic field, up to 5 T, applied parallel to the ab-plane. This novel field geometry allows the quasiparticle density-of-states spectrum to be measured under finite superfluid momentum, while avoiding contributions from the vortex-core bound states. At zero field, we observed a fully-gapped conductance spectrum with both gap-edge peaks and sub-gap kinks. These spectral features show a systematic evolution with the applied field: the kinks close in while the peaks move apart in low fields, and the zero-bias conductance has a two-sloped behavior over the entire field range, though dipping anomalously at 0.7 T. Our data was analyzed with recent theoretical models for quasiparticle tunneling into a current-carrying superconductor, and yielded distinct evidence for multiple superconducting gaps coming from various Fermi-surface sheets of different topologies, as well as possible implications on the origin of the coexisting charge-density-wave order.