Strain-engineering spin-valley locking effect in altermagnetic monolayer with multipiezo properties

Recently, altermagnetism (AM) in condensed matter systems has attracted much attention due to the physical properties arising from alternating spins in both real space and reciprocal space. In our work, we propose a stable monolayer Janus Nb2SeTeO with altermagnetic ground state and spin-valley locking (SVL) effect. The monolayer Janus Nb2SeTeO exhibits a multipiezo effect with a large out-of-plane piezoelectricity and piezovalley effect with large valley polarization. The piezovalley effect is induced by uniaxial strain effect in different directions, which contributes the anomalous valley Hall effect (AVHE) observed in AM systems. Moreover, compressive uniaxial strain could induce quantum anomalous Hall effect (QAHE) in the AM system, where the dissipationless topological edge states' chirality could be manipulated by the direction of uniaxial strain. These manifest topological phase transitions could be realized via piezovalley effect in the AM system. Furthermore, the AM quantum spin Hall effect (QSHE) could be induced by biaxial strain effect, which contributes quantized spin Hall conductance. Our work reveals that strain-engineering technique could serve as an important method to tune dissipationless edge states in monolayer Janus Nb2SeTeO. By designing the SVL effect, topological physics could be emerged in AM systems such as AVHE, QAHE, and QSHE.