Zero-bias anomaly in nanoscale hole-doped Mott insulators on a triangular silicon surface

Abstract

Adsorption of 1/3 monolayer of Sn on a heavily-doped p-type Si(111) substrate results in the formation of a hole-doped Mott insulator, with electronic properties that are remarkably similar to those of the high-Tc copper oxide compounds. In this work, we show that the maximum hole-density of this system increases with decreasing domain size as the area of the Mott insulating domains approaches the nanoscale regime. Concomitantly, scanning tunneling spectroscopy data at 4.4 K reveal an increasingly prominent zero bias anomaly (ZBA). We consider two different scenarios as potential mechanisms for this ZBA: chiral 2 − 2 + i wave superconductivity and a dynamical Coulomb blockade (DCB) effect. The latter arises due to the formation of a resistive depletion layer between the nano-domains and the substrate. Both models fit the tunneling spectra with weaker ZBAs, while the DCB model clearly fits better to spectra recorded at higher temperatures or from the smallest domains with the strongest ZBA. Consistently, STS spectra from the lightly-doped substrates display oscillatory behavior that can be attributed to conventional Coulomb staircase behavior, which becomes stronger for smaller sized domains. We conclude that the ZBA is predominantly due to a DCB effect, while a superconducting instability is absent or a minor contributing factor.

Experimental observation of pseudogap in a modulation-doped Mott insulator: Sn/Si(111)-($\sqrt{3}\times \sqrt{3}$)-R30$^\textrm{o}$

“…A spatially uniform energy gap of approximately 20 meV, irrespective of the existence of Sn vacancy, is identified and symmetric with respect to E F . The results are consistent with the previous findings, except that no apparent van Hove singularity is observable below E F in our study [33,34]. The robustness of the small energy gap against the Sn vacancy hints at its delocalized nature.…”

“…In order to corroborate this opinion, we have explored the doping evolution of PG by taking advantage of the √ 3-Sn nanodomain size-dependent hole doping level, albeit unknown cause [33]. Here the √ 3-Sn nanodomains are isolated and laterally encircled by the neighboring Sn/Si(111)-(2…”

“…To date, however, only limited experiments were performed, especially leaving the electronic structure of √ 3-Sn in underdoped regions poorly understood. It remains a puzzle whether the opened energy gap at the Fermi level (E F ) from the tunneling experiment [33] relates to superconductivity, and if no why the Coulomb gap increases in magnitude with increasing doping concentration. Here we resolve this issue by combining a molecular beam epitaxy system (MBE) with cryogenic scanning tunneling microscopy (STM).…”

“…It should be noted that such PG feature has been alternatively explained as a dynamical Coulomb blockade effect [33]. Given that several different Si(111) substrates with varying doping levels have been explored and the resistive depletion layer between the √ 3-Sn domains and the substrates has been largely modified, the PG feature should profoundly change with the varying substrates in the Coulomb blockade model.…”

Design and realization of topological Dirac fermions on a triangular lattice

“…Noteworthy, the indenene LDOS exhibits a second spectral depression that is always pinned at zero bias, irrespective of local fluctuations. Various mechanisms have been suggested as the origin of such a zero-bias anomaly (ZBA) which, however, depend on the specific probing details [22][23][24] . It is therefore not considered as an intrinsic feature of the electronic structure.…”