Two-dimensional ferrovalley materials

In analogy to ferroelectric materials with spontaneous charge polarization, or ferromagnetic materials with spontaneous spin polarization, ferrovalley materials with spontaneous valley polarization, have been proposed as part of valleytronics. As a new member of the ferroic family, the ferrovalley materials show the potential to break through the restriction of the external fields induced polarization states, becoming an exciting new frontier for the next-generation nonvolatile information storage. The nondegenerate valleys can originate from two intrinsic mechanisms, which are ferromagnetism and ferroelectricity in the two-dimensional hexagonal and orthorhombic lattices, respectively. Such ferrovalley systems could demonstrate distinctive properties, such as anomalous valley Hall effect and valley-selective linear dichroism. On account of these, ferrovalley materials provide an advantageous platform to realize the functional devices, which is of great importance in extending the practical applications of valleytronics. In this chapter, Section 1 will make a brief introduction to the valleytronics based on the traditional hexagonal crystallographic systems; Section 2 will review some external approach to induce valley polarization in these systems; Sections 3 and 4 will outline two kinds of ferrovalley materials; and Section 5 will summarize the conclusions and provide an outlook.