Quantum oscillations of valley current driven by microwave irradiation in transition-metal dichalcogenide/ferromagnet hybrids

Abstract

We theoretically study spin and valley transport in a transition-metal dichalcogenide(TMDC)/ferromagnet heterostructure under a perpendicular magnetic field. We find that microwave-driven spin pumping induces a valley-selective spin excitation, a direct consequence of the valley-asymmetric Landau levels in the TMDC conduction band. This process generates a pure valley current which, as our central finding, exhibits pronounced quantum oscillations as a function of chemical potential. These oscillations provide a definitive experimental signature of the quantized valley states and establish another pathway to interface spintronics and valleytronics.

Figures (3)

• Figure 1: (a) Schematic of the proposed system. A monolayer of transition-metal dichalcogenide (TMDC) is placed on a ferromagnetic insulator (FI). Microwave irradiation induces ferromagnetic resonance in the FI, generating a pure spin current ($I_s$) at the interface via spin pumping. A perpendicular magnetic field ($B$) is applied to the entire heterostructure. (b) Electronic band structure of the TMDC in the absence of a magnetic field, showing the characteristic spin-valley locking at the $K$ and $K'$ points. (c) In the presence of a magnetic field, the energy bands quantize into discrete Landau levels. In the conduction band, there exists a valley-selective energy splitting between the lowest Landau levels of the $K$ and $K'$ valleys as the key mechanism enabling the generation of a pure valley current from the pumped spin current.
• Figure 2: Valley transport in a WSe$_2$/FI heterostructure as a function of chemical potential $\mu$ for magnetic fields $B=$ 5 to 20 T. (a) spin current from the $K$ valley, $I_{s,K}$, (b) spin current from the $K'$ valley, $I_{s,K'}$, and (c) the resulting pure valley current, $I_v$. Simulation parameters: $k_B T=1$ meV, $J_0S_0=20$ meV, $\Gamma=2$ meV.
• Figure 3: Valley transport in a MoS$_2$/FI heterostructure. Panels and simulation parameters are the same as in Fig. \ref{['WSe2']}. The quantum oscillations in the valley current (c) are less pronounced than in WSe$_2$ due to the smaller spin-orbit coupling in MoS$_2$.