Quantum Spin Hall Insulators Topological States Open Up as Researchers Solve QSHIs and Their Adjustable Transition to Excitonic Phases.

Zhongdong Han, Yiyu Xia, and Cornell University colleagues announced a quantum materials science breakthrough with National Institute for Materials Science's Kenji Watanabe and Takashi Taniguchi. By using twisted bilayer tungsten selenide to realise and regulate Quantum Spin Hall Insulators QSHIs, the group established a unique, periodic topological phase transition between QSHIs and Excitonic Insulators (EIs). The findings provide fresh insights into the electrical behaviour and fermiology of and provide a flexible platform for exploring uncommon quantum states.

Quantum Spin Hall Insulator Definition

The typical topological states of matter are QSHIs. The symmetry-protected topological states have an insulating bulk surrounded by pairs of helical edge states in two dimensions. Unlike typical insulators, QSHIs exhibit substantial edge conductivity. Spin conservation protects the QSHIs in the system under study because to the material's strong Ising spin-orbit coupling (SOC). In contrast to topological insulators protected only by time-reversal symmetry, Quantum Spin Hall Insulators (QSHIs) are Z topological invariant due to this protection mechanism. Importantly, these QSHIs remain strong in a perpendicular magnetic field. The topological phase transition between QSHIs and EIs has long intrigued theoretical researchers, but finding materials that can constantly access it has hampered experimental access. Earlier research on InAs/GaSb quantum wells and monolayers showed the coexistence of these states, but no topological phase transition was proven.

Moiré Materials Landau Level Realisation

The implementation and tunability of Quantum Spin Hall Insulators (QSHIs) in this work depend on band structure engineering. The material's enormous valley g-factor and tiny moiré bandwidth allow researchers to construct tunable electron-like and hole-like Landau levels (LLs) in opposite valleys with a perpendicular magnetic field. Once Landau levels are full, QSHIs appear. The experiment begins by polarising all holes into a valley with a strong field to generate a “vacuum state”. When field is reduced, hole- and electron-LLs are successively filled (half-band-filling) under charge neutrality. LL filling factors that totally fill electron- and hole-LLs produce QSHIs. Multiple Helical Edge States Observed The revelation that the system can sustain several conducting channels is significant. Team resolved up to four pairs of helical edge states in Quantum Spin Hall Insulators QSHIs. Each pair of fully filled electron- and hole-LLs contributes two counterpropagating helical edge states. The spin-valley locking in makes these edge states spin-polarized. Quantised conductance comes from each helical edge state pair. Quantum Spin Hall Insulators QSHIs have quantised longitudinal resistance and almost little Hall resistance in Hall bar geometry testing. The longitudinal resistance of a QSHI state with two helical edge states approaches the quantised value. Helical edge states are more susceptible to back scattering than chiral edge states, hence their quantisation is usually weaker than that of quantum Hall states. The researchers confirmed these conducting channels using nonlocal transport measures. The observed nonlocal resistance's reasonable agreement with Landauer-Büttiker quantised values supported helical edge state transport. Phase decoherence at counterpropagating channel equilibration contacts causes apparent quantised resistance, even if helical edge states are not dissipative.

Correlated State Oscillation