Piezovalley effect and magnetovalley coupling in altermagnetic semiconductors studied by first-principles calculations

Clarifying the physical origin of valley polarization and exploring promising ferrovalley materials are conducive to the application of valley degrees of freedom in the field of information storage. Here, we explore two altermagnetic semiconductors (monolayers $\mathrm{N}{\mathrm{b}}_{2}\mathrm{S}{\mathrm{e}}_{2}\mathrm{O}$ and $\mathrm{N}{\mathrm{b}}_{2}\mathrm{SeTeO}$) with N\'eel temperature above room temperature based on first-principles calculations. We find that uniaxial strain induces valley polarization without spin-orbital coupling (SOC) in altermagnets owing to the piezovalley effect, while uniaxial compressive strain transforms the intrinsic ferrovalley semiconductor into a semimetal, half metal, and metal. Moreover, moderate biaxial compressive strain renders Janus monolayer $\mathrm{N}{\mathrm{b}}_{2}\mathrm{SeTeO}$ to robust Dirac-conelike band dispersion. The SOC and intrinsic in-plane magnetocrystalline anisotropy energy induce Dirac-conelike altermagnets to generate apparent valley polarization through magnetovalley coupling. In terms of SOC perturbation, we elucidate the physical mechanism behind in-plane magnetization-induced valley polarization and demonstrate that valley polarization is negatively correlated with the band gap. The present work reveals the physical origin of valley polarization in altermagnets and expands the application of ferrovalley at room temperature in valleytronics.