Two-dimensional valleytronic semiconductor with spontaneous spin and valley polarization in single-layer Cr2Se3

Two-dimensional (2D) valleytronic materials, exhibiting valley-contrasting physics, are both fundamentally intriguing and practically appealing to be used in nanoscale devices. In this work, through first-principles calculations and model analysis, we identify single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$ as a promising 2D valleytronic semiconductor. We reveal that single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$ is a ferromagnetic semiconductor and harbors valley features. In contrast to most previously reported similar systems suffering from a natural in-plane magnetization, it favors the robust out-of-plane magnetization. This, combined with the broken inversion symmetry and strong spin-orbit coupling strength, renders that a sizable valley polarization occurs spontaneously in single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$, thus facilitating the observation of intriguing anomalous valley Hall effect. More remarkably, by employing strain and ferroelectric substrate, the on-off switching of the valley polarization is realized in single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$. The underlying physics is discussed in detail. Our work provides a tantalizing candidate for realizing and manipulating the valley and spin physics.