Unscreened Polaron Ordering Against High Electron Density on Mechanically Cleaved Surfaces of WO<sub>3</sub> (001) Films.

Abstract

The 2D electron gas (2DEG) is a successful testbed for investigating both small and large polarons, governed by enhanced fundamental couplings through dimensional confinement. However, accessible polaronic states are typically limited to low carrier densities, as polarons collapse due to enforced electronic screening at high densities. This has limited exploration into polaron interactions, including bipolaron formation, inter-polaron correlations, and broader collective polaron behaviors. Here, an unscreened polaronic state resilient to high electron densities, formed on the mechanically cleaved surface of a WO₃ (001) film, is reported. Using angle-resolved photoemission spectroscopy (ARPES) and atomic-scale scanning tunneling microscopy/spectroscopy (STM/STS), characteristic signatures of polaron formation are observed in the spectra of a surface-confined metallic 2DEG with an electron density of 2.6 × 10¹⁴ cm^-2, generated by a lack of oxygen ions at the surface. Notably, these vacancies adhere to the c(2 × 2) surface reconstruction, which is stabilized by the cooperation of excess electrons and surrounding lattice distortion, i.e., the formation of a polaron and its ordering, as revealed by the first-principles calculations. It is anticipated that this ordered polaron phase at high electron densities will pave new pathways for investigating exotic phases related to correlated polaron states or collective polaron movements, like bipolaron superconductivity.