Spontaneous Valley Polarization Caused by Crystalline Symmetry Breaking in Nonmagnetic LaOMX2 Monolayers

In order to achieve valley polarization, breaking the time-reversal symmetry in two-dimensional hexagonal lattices with inversion asymmetry is the heart of current valleytronic research, which, however, has caused studies to stagnate due to the inevitable drawbacks. In this work, we go beyond the conventional paradigm and demonstrate the novel valley physics caused by lowering the crystalline symmetry instead of breaking the time reversal symmetry. In particular, we translate our concept into concrete nonmagnetic LaOMX2 monolayers with a tetragonal lattice, confirming that a spontaneous structure distortion can cause the long-sought, considerably large valley polarization. In detail, the physics of valley-orbital coupling, valley-orbital-layer coupling, valley-contrasting linear dichroism, and interlayer exciton valleytronics are discussed.

Automated summary

Breaking the time-reversal symmetry has been the focus of valleytronic research to achieve valley polarization, but it has also led to inevitable drawbacks. This study demonstrates a novel valley physics by lowering the crystalline symmetry instead of breaking the time reversal symmetry, showing that a spontaneous structure distortion can cause considerably large valley polarization. The study discusses the physics of valley-orbital coupling, valley-orbital-layer coupling, valley-contrasting linear dichroism, and interlayer exciton valleytronics.