Realization of a Wigner-Mott insulator in 6R-TaS$_2$ superconductor

TL;DR

This study provides direct microscopic evidence of a Wigner-Mott insulator in 6R-TaS$_2$, demonstrating how Coulomb interactions induce charge order and Mott localization at fractional fillings, advancing understanding of strongly correlated electron systems.

Contribution

First experimental observation of a Wigner-Mott insulator in 6R-TaS$_2$, showing the interplay of Coulomb interactions and charge order at fractional filling.

Findings

Observation of a $rac{1}{3}$-filled charge-ordered superstructure with a Mott gap.

Induction of charge order and insulating phases via electron injection.

Stability of the Wigner-Mott phase due to cooperative Coulomb effects.

Abstract

Wigner-Mott insulating states represent a paradigmatic manifestation of strong electronic correlations, in which long-range Coulomb interactions drive spontaneous charge ordering and enable Mott localization at fractional electronic fillings. Such states have been theoretically proposed to arise from the cooperative interplay between onsite and inter-site Coulomb interactions. However, experimental realizations of the simultaneous microscopic observation of interaction-driven charge order and genuine Mott localization, which are the defining hallmarks of a Wigner-Mott insulator, have remained elusive. Here we report the observation of a Wigner-Mott insulating state in 6R-TaS$_2$ using scanning tunneling microscopy. By locally injecting electrons into the depleted 1T layer, we induce distinct Star-of-David charge-ordered superstructures and realize a cascade of insulating phases. In particular, a $\sqrt{3}\times \sqrt{3}$ charge-ordered superstructure at one-third filling hosts a robust Mott gap despite fractional filling. The spontaneous relaxation from excited states back to the ground state demonstrates that this Wigner-Mott phase is stabilized by the cooperative effects of onsite and inter-site Coulomb interactions. Our results provide direct microscopic evidence for a Wigner-Mott mechanism and establish 6R-TaS$_2$ as a platform for the controlled realization and investigation of Wigner-Mott insulating states.