Coexistence of gapless excitations and commensurate charge-density wave in the 2H-transition metal dichalcogenides

TL;DR

This paper reveals that in 2H-transition metal dichalcogenides, a unique electronic structure allows for coexistence of gapless excitations with a commensurate charge-density wave, explained through a minimal effective model based on first-principles analysis.

Contribution

The study introduces a minimal effective model for CDW formation in 2H-TMDs that accounts for the coexistence of gapless excitations and a commensurate CDW, based on first-principles Wannier functions analysis.

Findings

The low-energy Ta '5d_{z^2}' bands are dominated by next-nearest neighbor hopping.

A decoupling of the three sublattices occurs in the CDW phase.

One sublattice remains undistorted, leading to ungapped bands at the Fermi surface.

Abstract

An unexpected feature common to 2H-transition metal dichalcogenides (2H-TMDs) is revealed with first-principles Wannier functions analysis of the electronic structure of the prototype 2H-TaSe2: The low-energy Ta \red{``$5d_{z^2}$''} bands governing the physics of charge-density wave (CDW) is dominated by hopping between next-nearest neighbors. With this motivation we develop a minimal effective model for the CDW formation, in which the unusual form of the hopping leads to an approximate decoupling of the three sublattices. In the CDW phase one sublattice remains undistorted, leaving the bands associated with it ungapped everywhere in the Fermi surface, resolving the long-standing puzzle of coexistence of gapless excitations and commensurate CDW in the 2H-TMDs.