Jonas Erhardt (PhD thesis): From Bulk to Boundary: Topological Classification and Spectroscopy of the Quantum Spin Hall Insulator Indenene

In 2005, Kane and Mele revolutionized solid-state physics by introducing the time-reversal symmetric quantum spin Hall insulator (QSHI), a novel insulating phase whose band order cannot be adiabatically deformed to match the atomic limit of trivial insulators without closing its band gap [1, 2], resulting in metallic spin-momentum locked edge modes connected in Kramers pairs and protected by time-reversal symmetry [3, 4], which makes QSHIs highly attractive for spintronic and quantum technologies and sparked extensive material searches [5–9]; however, many models including Kane-Mele inefficiently utilize spin–orbit coupling, motivating approaches like bismuthene on SiC with large band gaps but limited by small domain sizes, while the more recent indenene system—a triangular indium monolayer on SiC—retains strong SOC, matches substrate periodicity, and offers scalability [10–14]; this thesis provides a full topological classification of indenene, including experimental strategies and stabilization via graphene capping, demonstrates high-quality growth with micron-scale domains and a ~120 meV gap, and identifies orbital angular momentum (OAM) staggering as a key topological signature confirmed via CD-ARPES and STS, alongside chiral OAM textures and Rashba-related band splitting [15–22]; edge-state measurements reveal non-monotonic dispersion with multiple Kramers pairs and TRS-allowed inter-pair backscattering leading to partial localization, though one channel remains protected [23–34]; graphene intercalation preserves properties under ambient conditions, enabling ex-situ studies, and further exploration suggests additional QSHI candidates and even higher-order topological phases in related systems, establishing indenene as a benchmark platform for triangular lattice topological materials [35–37].