Surface state bi-polarons formation on a triangular lattice in the sp-type alkali/Si(111) Mott insulator

TL;DR

This study investigates surface state bipolarons on a triangular lattice in alkali/Si(111) Mott insulators, revealing charge density waves and strong electron-phonon coupling effects that suggest bipolaronic insulating phases.

Contribution

It provides experimental evidence of bipolaron formation and charge density waves in alkali/Si(111) surfaces, highlighting strong electron-phonon interactions in a Mott insulator context.

Findings

Observation of temperature-dependent surface band broadening.

Detection of a charge density wave below 270 K.

Evidence supporting bipolaronic insulating phase formation.

Abstract

We report on new LEED, STM and ARPES studies of alkali/Si(111) previously established as having a Mott insulating ground state at surface. The observation of a strong temperature dependent Franck-Condon broadening of the surface band together with the novel $\sqrt{3}\times\sqrt{3}\to2(\sqrt{3}\times\sqrt{3})$ charge and lattice ordering below 270 K evidence a surface charge density wave (SCDW) in the strong e-ph coupling limit ($g\approx8$). Both the adiabatic ratio $\hbar\omega_0/t\approx0.8$ and the effective pairing energy $V_{eff}=U-2g\hbar\omega_0\approx-800$ $meV$ are consistent with the possible formation of a bi-polaronic insulating phase consisting of alternating doubly-occupied/unoccupied dangling bonds as expected in the Holstein-Hubbard model.