Realization of metallic state in 1T-TaS2 with persisting long-range order of charge density wave

TL;DR

This study demonstrates that 1T-TaS2 can become metallic through surface alkali doping without losing its long-range charge density wave order, revealing new insights into its electronic state and doping effects.

Contribution

It shows that alkali doping induces metallization in 1T-TaS2 while preserving long-range CDW order, with in-gap excitations linked to reduced Coulomb energy.

Findings

Long-range CDW order persists after alkali doping.

Metallization is associated with in-gap excitations near the lower Hubbard band.

Reduced Coulomb energy explains the in-gap excitations.

Abstract

Metallization of 1T-TaS2 is generally initiated at the domain boundary of charge density wave (CDW), at the expense of its long-range order. However, we demonstrate in this study that the metallization of 1T-TaS2 can be also realized without breaking the long-range CDW order upon surface alkali doping. By using scanning tunneling microscopy, we find the long-range CDW order is always persisting, and the metallization is instead associated with additional in-gap excitations. Interestingly, the in-gap excitation is near the top of the lower Hubbard band, in contrast to a conventional electron-doped Mott insulator where it is beneath the upper Hubbard band. In combination with the numerical calculations, we suggest that the appearance of the in-gap excitations near the lower Hubbard band is mainly due to the effectively reduced on-site Coulomb energy by the adsorbed alkali ions.