Origin of the normal-state shallow zero-bias dip in STS

Ascertain the microscopic mechanism responsible for the shallow approximately 2 meV dip at the Fermi level observed in scanning tunneling spectroscopy of both trigonal (H) and octahedral (T) polymorph layers of 4Hb-TaSSe in the normal state, determining whether it originates from Coulomb blockade in the tunneling junction or from phonon-assisted inelastic tunneling.

Background

In the normal state (above the superconducting critical temperature), both H- and T-polymorph layers of 4Hb-TaSSe exhibit a shallow dip in the differential conductance around the Fermi level. The authors note that this feature is tip-dependent and commonly observed in tunneling spectra on transition metal dichalcogenide metals. They explicitly state that its origin is not entirely clear, and mention two prevailing hypotheses in the literature: Coulomb blockade phenomena in the tunneling junction and phonon-assisted inelastic tunneling.

Clarifying the origin of this spectroscopic feature is important for interpreting STM/STS data in layered TMD superconductors and for distinguishing intrinsic electronic properties from artifacts of the tunneling junction or inelastic processes.