Quantum loops in the 1T transition metal dichalcogenides

TL;DR

This paper introduces a quantum loop model to describe bonding in 1T transition metal dichalcogenides, predicting phases and impurity effects that align with observed distorted structures and suggesting potential for a loop liquid phase.

Contribution

It develops a minimal quantum loop model specific to 1T transition metal dichalcogenides, capturing their unique orbital-driven bonding and phase behavior.

Findings

Identifies phases resembling 1T' and trimerized structures.

Predicts long-range textures from impurities like Ti/Cr defects.

Proposes the emergence of a loop liquid phase in these materials.

Abstract

Loop arrangements and their quantum superpositions describe several interesting many-particle states. We propose that they also describe bonding in a class of transition metal dichalcogenides. We present an effective quantum loop model for monolayers with 1T structure and a d$^2$ valence electron configuration: materials of the form MX$_2$ (M = Mo, W and X=S, Se, Te) and AM$'$Y$_2$ (A = Li, Na; M$'$ = V, Nb and Y = O, S, Se). Their t$_{2g}$ orbitals exhibit strongly directional overlaps between neighbouring atoms, favouring the formation of valence bonds. A transition metal atom forms two valence bonds, each with one of its neighbours. When connected, these bonds form loops that cover the triangular lattice. We construct a minimal Rokhsar-Kivelson-like model with resonance processes that cut and reconnect loops that run in proximity. The resulting dynamics is more constrained than in traditional quantum dimer models, with a `bending' constraint that arises from orbital structure. In the resulting phase diagram, we find phases that resemble distorted phases seen in materials, viz., the 1T$'$ and trimerized phases. As a testable prediction, we propose that a single d$^1$ or d$^3$ impurity will terminate a loop and give rise to a long-ranged texture. For example, a Ti/Cr defect in LiVO$_2$ will produce one or more domain walls that propagate outward from the impurity. We discuss the possibility of a loop liquid phase that can emerge in these materials.