Even-integer Quantum Hall Effect in an Oxide Caused by Hidden Rashba Effect

TL;DR

This study reveals that a hidden Rashba effect in Bi2O2Se thin films suppresses odd-integer quantum Hall states in thicker films, but reducing the thickness to one unit cell introduces asymmetry that enables both even- and odd-integer states.

Contribution

The paper demonstrates how the hidden Rashba effect explains the absence of odd-integer quantum Hall states in thicker films and how asymmetry in ultra-thin films induces both even- and odd-integer states.

Findings

Thicker Bi2O2Se films show only even-integer quantum Hall states.

Ultra-thin (one unit cell) Bi2O2Se films exhibit both even- and odd-integer states.

Hidden Rashba effect causes the suppression of odd-integer states in thicker films.

Abstract

In the presence of high magnetic field, quantum Hall systems usually host both even- and odd-integer quantized states because of lifted band degeneracies. Selective control of these quantized states is challenging but essential to understand the exotic ground states and manipulate the spin textures. Here, we study the quantum Hall effect in Bi2O2Se thin films. In magnetic fields as high as 50 T, we observe only even-integer quantum Hall states, but no sign of odd-integer states. However, when reducing the thickness of the epitaxial Bi2O2Se film to one unit cell, we observe both odd- and even-integer states in this Janus (asymmetric) film grown on SrTiO3. By means of a Rashba bilayer model based on ab initio band structures of Bi2O2Se thin films, we can ascribe the absence of odd-integer states in thicker films to the hidden Rasbha effect, where the local inversion symmetry breaking in two sectors of the [Bi2O2]2+ layer yields opposite Rashba spin polarizations, which compensate with each other. In the one unit cell Bi2O2Se film grown on SrTiO3, the asymmetry introduced by top surface and bottom interface induces a net polar field. The resulting global Rashba effect lifts the band degeneracies present in the symmetric case of thicker films.