Interlayer Coupling Driven Phase Evolution in Hyperbolic 1T-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 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 reveals that the metallic domains driving percolation evolves from disc-like to needle-like, 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.