Orbital-doublet-driven even spin Chern insulators

Quantum spin Hall insulators hosting edge spin currents hold great potential for low-power spintronic devices. In this work, we present a universal approach to achieve a high and near-quantized spin Hall conductance plateau within a sizable bulk gap. Using a nonmagnetic four-band model Hamiltonian, we demonstrate that an even spin Chern (ESC) insulator can be accessed by tuning the sign of spin-orbit coupling (SOC) within a crystal symmetry-enforced orbital doublet. With the assistance of a high spin Chern number of C_S=2 and spin U(1) quasi-symmetry, this orbital-doublet-driven ESC phase is endowed with the near-double-quantized spin Hall conductance. We identify 12 crystallographic point groups supporting such a sign-tunable SOC. Furthermore, we apply our theory to realistic examples, and show the phase transition from a trivial insulator governed by positive SOC in RuI_3 monolayer to an ESC insulator dominated by negative SOC in RuBr_3 monolayer. This orbital-doublet-driven ESC insulator, RuBr_3, showcases nontrivial characteristics including helical edge states, near-double-quantized spin Hall conductance, and robust corner states. Our work provides new pathways in the pursuit of the long-sought quantum spin Hall insulators.