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

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.