Interlayer coupling driven phase evolution in hyperbolic $1T$-TaS$_2$

TL;DR

This study investigates how interlayer coupling influences phase transitions in $1T$-TaS$_2$, revealing a three-dimensional percolation process and hyperbolic optical behavior through spectroscopic ellipsometry.

Contribution

It uncovers the role of interlayer interactions in phase evolution and identifies $1T$-TaS$_2$ as a tunable hyperbolic medium, advancing understanding of its complex phase transitions.

Findings

Interlayer coupling drives a 3D percolation transition.

$1T$-TaS$_2$ exhibits natural hyperbolic behavior in the visible range.

An intermediate phase emerges during heating, affecting domain morphology.

Abstract

Understanding how microscopic interactions control macroscopic phase transitions is central to quantum materials, where charge density waves (CDWs), Mott states, and superconductivity often compete. In $1T$-TaS$_2$, this competition is tied to a sequence of CDW phases and a hysteretic metal-insulator transition, but details of the transition, especially the role of interlayer coupling, remain unresolved. In this work, spectroscopic ellipsometry is used to determine the uniaxial dielectric response of bulk $1T$-TaS$_2$ from room temperature down to the commensurate insulating state. The room-temperature data reveal natural type-II hyperbolic behavior in the visible range, with negative in-plane and positive out-of-plane permittivity. Temperature-dependent ellipsometry combined with anisotropic Bruggeman effective medium analysis shows that the metallic domains responsible for percolation evolve from disc-like to needle-like shapes, and that, upon heating, an additional intermediate phase emerges. These results identify the transition in $1T$-TaS$_2$ as a three-dimensional, interlayer-driven percolation process and establish this material as a natural, tunable hyperbolic medium.