Discovery of unconventional charge density wave at the surface of K0.9Mo6O17

TL;DR

This study reveals an unusual surface charge density wave in K0.9Mo6O17, characterized by a high transition temperature and a strong coupling regime, likely driven by electronic interactions rather than phonons.

Contribution

It uncovers an unconventional surface CDW with a higher transition temperature and stronger coupling than the bulk, mediated by electronic interactions.

Findings

Surface CDW transition temperature is 220 K.

Surface CDW gap is ten times larger than bulk.

No signatures of electron-phonon coupling detected.

Abstract

We use Angle Resolved Photoemission Spectroscopy (ARPES), Raman spectroscopy, Low Energy Electron Diffraction (LEED) and x-ray scattering to reveal an unusual electronically mediated charge density wave (CDW) in K0.9Mo6O17. Not only does K0.9Mo6O17 lack signatures of electron-phonon coupling, but it also hosts an extraordinary surface CDW, with TS CDW =220 K nearly twice that of the bulk CDW, TB CDW =115 K. While the bulk CDW has a BCS-like gap of 12 meV, the surface gap is ten times larger and well in the strong coupling regime. Strong coupling behavior combined with the absence of signatures of strong electron-phonon coupling indicates that the CDW is likely mediated by electronic interactions enhanced by low dimensionality.