Experimental manifestation of topological lifshitz transition by observing thickness-dependent shift of plasma frequency in topological semimetals

Abstract

Topological semimetals (TSMs), which host massless Dirac fermions, offer a promising platform to explore quantum phenomena. Topological Lifshitz transitions (LTs) are ubiquitous, with applications spanning ferromagnetism, superconductors, topological materials, and even high-energy physics. In this study, we observe a topological LT in epitaxial thin films of Bi0.96Sb0.04 without external excitations, where the TSM characteristics are retained throughout the transition. The LT is revealed by a thickness-dependent shift in plasma frequency, measured through terahertz (THz) optical conductivity spectroscopy. Only in ultrathin two-dimensional (2D) films (thickness <= 10 nm) does the emitted THz wave show a clear difference between left- and right-circularly polarized light, a circular photogalvanic effect that is a hallmark of Weyl semimetals. Such behavior signals a topological phase transition caused by inversion symmetry breaking in films with thickness <= 10 nm, thereby providing experimental confirmation of the LT in TSMs. The optical conductivity shows a similar critical change at a thickness of 10 nm, which matches the thickness-dependent trend in electrical conductivity measured in simple planar devices. Finally, a strong correlation is observed between the plasma frequency and carrier density in this 2D regime, consistent with prior theoretical predictions.