Field-induced reentrant insulator state of a gap-closed topological insulator (Bi1−xSbx) in quantum-limit states

In the extreme quantum-limit states under high magnetic fields, enhanced electronic correlation effects can stabilize anomalous quantum states. Using band-tuning with a magnetic field, we realized a spin-polarized quantum-limit state in the field-induced semimetallic phase of a topological insulator ${\mathrm{Bi}}_{1\text{\ensuremath{-}}x}{\mathrm{Sb}}_{x}$. Further increase in the field injects more electrons and holes to this state and results in an unexpected reentrant insulator state in this topological semimetallic state. A single-particle picture cannot explain this reentrant insulator state, reminiscent of phase transitions due to many-body effects. Estimates of the binding energy and spacing of electron-hole pairs and the thermal de Broglie wavelength indicate that ${\mathrm{Bi}}_{1\text{\ensuremath{-}}x}{\mathrm{Sb}}_{x}$ may host the excitonic insulator phase in this extreme environment.