Even-integer quantum Hall effect in an oxide caused by a hidden Rashba effect

In the presence of a 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 demonstrate 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 there is 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 the ab initio band structures of Bi₂O₂Se thin films, we can ascribe the only even-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 the 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.