Concepts of ferrovalley material and anomalous valley Hall effect

Abstract

Valleytronics rooted in the valley degree of freedom is of both theoretical and technological importance as it offers additional opportunities for information storage, as well as electronic, magnetic and optical switches. In analogy to ferroelectric materials with spontaneous charge polarization, or ferromagnetic materials with spontaneous spin polarization, here we introduce a new member of ferroic family, that is, a ferrovalley material with spontaneous valley polarization. Combining a two-band k·p model with first-principles calculations, we show that 2H-VSe2 monolayer, where the spin–orbit coupling coexists with the intrinsic exchange interaction of transition-metal d electrons, is such a room-temperature ferrovalley material. We further predict that such system could demonstrate many distinctive properties, for example, chirality-dependent optical band gap and, more interestingly, anomalous valley Hall effect. On account of the latter, functional devices based on ferrovalley materials, such as valley-based nonvolatile random access memory and valley filter, are contemplated for valleytronic applications.

Electrically tunable polarizer based on 2D orthorhombic ferrovalley materials

“…The spontaneous in-plane polarization (P s ) estimated in monolayer GeSe can reach to 3.5×10 -10 C/m, which is well consistent with previous works [39][40][41]. [12], the spontaneous valley polarization is induced by ferromagnetism. Here, in monolayer GeSe, it couples to the primary ferroelectricity, indicating an entirely new mechanism for ferrovalley physics.…”

“…What's more, linearly polarized optical selectivity for valleys in such new ferrovalley materials gives rise to an additional relationship between optical field and valley index, rather than the valley-selective circular dichroism reported in hexagonal monolayers. As we know, Berry curvature drives an anomalous transverse velocity for carries around valley [19,43], which directly causes the existence of 10 valley Hall effect [6,10,18] and the more exciting anomalous valley Hall effect in ferrovalley VSe 2 [12].…”

“…The lattice constant b (a), as well as atomic coordinates, are optimized for each case. As shown in figure 7, when the internal generation of a mechanical strain, resulting 12 from an applied electric field, varies from the compressive to the tensile one, the band gap of monolayer GeSe increases linearly. However, it locates in the Vy valley with the y-polarized light selective excitation.…”

Electrical control of the anomalous valley Hall effect in antiferrovalley bilayers

“…14 Following the arrival of atomically thin two-dimensional (2D) electronic systems, 15,16 valleytronics, [17][18][19] which is the cousin of spintronics, has opened up additional opportunities for functional devices rooted in the valley degree of freedom. [20][21][22][23][24] Very recently, the concept of ferrovalley material, a system with spontaneous valley polarization, is proposed, 25 implying the practical use of valley index, as a new type of binary state, in a nonvolatile way similar to the spin of electrons. Owing to the existence of the anomalous valley Hall effect (AVHE), additional charge accumulations make the valley polarization an electrically measurable physical quantity in such intrinsically polarized materials.…”

“…However, the valley polarization originates from the inherent exchange interaction in previous study. 25 The energy-intensive magnetic way is thus necessary for the proposed ferrovalley monolayers to control its polarity, and then the sign of the Hall voltage in the AVHE. In this context, tuning the AVHE through advantageous electric methods is highly desirable and of great importance for the research on all-electric valleytronics.…”