Valley Acoustoelectric Effect

We report on the novel valley acoustoelectric effect, which can arise in a 2D material, like a transition metal dichalcogenide monolayer, residing on a piezoelectric substrate. The essence of this effect lies in the emergence of a drag electric current (and a spin current) due to a propagating surface acoustic wave. This current consists of three contributions, one independent of the valley index and proportional to the acoustic wave vector, the other arising due to the trigonal warping of the electron dispersion, and the third one is due to the Berry phase, which Bloch electrons acquire traveling along the crystal. As a result, there appear components of the current orthogonal to the acoustic wave vector. Further, we build an angular pattern, encompassing nontrivial topological properties of the acoustoelectric current, and suggest a way to run and measure the conventional diffusive, warping, and acoustoelectric valley Hall currents independently. We develop a theory, which opens a way to manipulate valley transport by acoustic methods, expanding the applicability of valleytronic effects on acoustoelectronic devices.

Figure 1

System schematic. (a) 2D material (${\mathrm{MoS}}_2$) exposed to a surface acoustic wave (SAW) with the wave vector $\mathbf{k}$. The sample lies on a layer of dielectric on a piezoelectric substrate. Two interdigital transducers (IDTs) generate and detect the SAWs. (b) The first Brillouin zone of ${\mathrm{MoS}}_2$ with the schematic illustration of warping.

Figure 2

(a) The band structures of ${\mathrm{MoS}}_2$ under the optically induced imbalance of the valley populations. Yellow shaded regions indicate the filled states. The arrows signify the directions of spin in each valley. (b) Relative magnitudes of the AE warping (dashed blue curve) and valley Hall (solid red curve) components of the current density as functions of the relaxation time at $n=5\times {10}^{12}{\mathrm{cm}}^{-2}$.

Figure 3

Angular patterns of the $x$ and $y$ components of the diffusive (a),(d), warping (b),(e), and Hall (c),(f) current density in arbitrary units. The warping and Hall current petals are depicted smaller than the conventional diffusive current petal to show that their magnitudes are smaller. Yellow shading marks the areas of negative current (directed opposite to the $x$ or $y$ axis). Red dots indicate the angles ($\pi /2$, $3\pi /2$, 0, and $\pi$), at which only the unconventional current flows along the $x$ or $y$ direction.