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.

在高磁场存在的情况下，由于提升带简并，量子霍尔系统通常同时具有偶数和奇数整数量子化态。选择性控制这些量子化状态具有挑战性，但对于理解奇异的基态和操纵自旋纹理至关重要。在这里，我们演示了 Bi₂O₂Se 薄膜中的量子霍尔效应。在高达 50 T 的磁场中，我们只观察到偶数整数量子霍尔态，但没有奇整数态的迹象。然而，当将外延 Bi₂O₂Se 薄膜的厚度减小到一个晶胞时，我们在 SrTiO₃ 上生长的 Janus（不对称）薄膜中观察到奇数和偶整数状态。通过基于 Bi₂O₂Se 薄膜的从头动力学带结构的 Rashba 双层模型，我们可以将较厚薄膜中唯一的偶整数态归因于隐藏的 Rasbha 效应，其中 [Bi₂O₂] ²⁺ 层的两个扇区的局部反转对称性断裂产生相反的 Rashba 自旋极化， 它们相互补偿。在 SrTiO₃ 上生长的单晶胞 Bi₂O₂Se 薄膜中，顶面和底部界面引入的不对称性诱导了净极场。由此产生的全局 Rashba 效果会消除在较厚薄膜的对称情况下存在的条带简并。