Interlayer coupling driven phase evolution in hyperbolic 1 <i>T</i> -TaS2 revealed by spectroscopic ellipsometry

Understanding how microscopic interactions control macroscopic phase transitions is central to quantum materials, where charge density waves (CDW), Mott-states, and superconductivity often compete. In 1T-TaS2, 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-TaS2 from room temperature down to the commensurate insulating state. The room-temperature data reveal a natural 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 indicates that the metallic domains driving percolation evolves from disk-like to needlelike, and that during heating, an additional intermediate phase shows up. Our results identify the transition in 1T-TaS2 as a three-dimensional, interlayer-driven percolation process and establish this material as a natural, tunable hyperbolic medium.