Spin and Optical Fingerprints of Pressure-Driven Double Topological Phase Transition in Monoclinic BaIn2As2

Abstract Distinguishing topological from trivial phases is central to condensed matter physics, yet resolving distinct nontrivial regimes that share the same Z2 classification remains challenging. Here, using first-principles calculations, we show that monoclinic BaIn2As2 undergoes two successive pressure-driven topological phase transitions within the same P2/m structural phase, evolving from a topological insulator at ambient pressure to a trivial insulator at 16 GPa and then to a high-pressure topological semimetal at 26 GPa. The two nontrivial phases share the same Z2 = 1 classification but exhibit distinct surface spectra and opposite low-energy optical anisotropy. We establish a two-level identification scheme in which spin Hall conductivity provides a fingerprint for separating nontrivial phases from the trivial phase, with possible inverse spin Hall electrical readout, whereas monoclinic symmetry-enabled in-plane reflectance anisotropy resolves the two nontrivial phases. We further propose a diamond-anvil-cell platform combining yttrium-iron-garnet-based spin pumping with polarization-resolved infrared reflectance.