Avoided level crossing at the magnetic field induced topological phase transition due to spin-orbital mixing

TL;DR

This study reveals that in a topological crystalline insulator, an avoided crossing occurs at the topological phase transition due to spin-orbital mixing, preventing the expected gap closure at the critical magnetic field.

Contribution

The paper demonstrates the presence of an avoided crossing caused by spin-orbital mixing at the topological phase transition in a 3D topological insulator, challenging the expectation of gap closure.

Findings

Avoided crossing observed at the critical magnetic field.

No gap closure occurs at the topological phase transition.

Anticrossing is linked to orbital parity and spin mixing.

Abstract

In 3D topological insulators, an effective closure of the bulk energy gap with increasing magnetic field expected at a critical point can yield a band crossing at a gapless Dirac node. Using high-field magnetooptical Landau level spectroscopy on the topological crystalline insulator Pb1-xSnxSe, we demonstrate that such a gap closure does not occur, and an avoided crossing is observed as the magnetic field is swept through the critical field. We attribute this anticrossing to orbital parity and spin mixing of the N=0 levels. Concurrently, we observe no gap closure at the topological phase transition versus temperature suggesting that the anticrossing is a generic property of topological phase transitions.