Probing valley quantum oscillations via the spin Seebeck effect in transition metal dichalcogenide/ferromagnet hybrids

TL;DR

The paper develops a theoretical framework for spin Seebeck–driven spin transport in a monolayer TMDC placed on a ferromagnetic insulator under a perpendicular magnetic field. By combining spin–valley coupling with a valley-asymmetric Landau level spectrum and interfacial magnon exchange, it predicts the generation of a valley-polarized spin current, I_v^z = I_{s,K} − I_{s,K'}, with pronounced quantum oscillations as μ and B vary. The analysis yields distinct conduction- and valence-band behaviors, including robust I_v in the conduction band due to the n=0 LL in K versus n=1 in K', and a valence-band regime where large proximity exchange splits valley-spin sectors, both producing observable oscillatory signatures. An experimental detection scheme via the inverse spin Hall effect in a Pt electrode is proposed, highlighting practical routes to map the predicted oscillations and realize valley control with purely spin excitations. Overall, the work presents a thermally driven, magnon-mediated avenue to couple spin, valley, and Landau quantization in TMDC-based hybrids, with implications for valleytronics integration in spintronic platforms.

Abstract

We theoretically investigate spin-valley-locked tunneling transport in a transition-metal dichalcogenide/ferromagnetic-insulator heterostructure under a perpendicular magnetic field, driven by the spin Seebeck effect. We demonstrate that spin-valley coupling together with the magnetic-field-induced valley-asymmetric Landau-level structure enables the generation of a valley-polarized spin current from valley-selective spin excitation. We compare the spin current and the valley-polarized spin current in the conduction and valence bands and clarify their distinct microscopic origins. We predict pronounced quantum oscillations of the valley-polarized spin current, providing a clear experimental signature of quantized valley states.

Figures (7)

• Figure 1: Schematic of the TMDC/ferromagnetic insulator heterostructure. A vertical temperature gradient $\delta T$ across the interface drives valley-polarized spin-tunneling transport via the spin Seebeck effect. An external magnetic field $\mathbf{B}$ is applied at a tilt angle $\theta$ relative to the surface normal ($z$-axis).
• Figure 2: The energy spectrum of the TMDC. The curves represent the energy bands without a magnetic field, and the lines represent the Landau levels in the presence of a perpendicular magnetic field.
• Figure 3: The density plots of the dimensionless spin current generated by the SSE for the $K$ and $K'$ valleys in the conduction band [subfigures (a)-(b)] and the valence band [subfigures (d)-(e)] and the corresponding valley-polarized spin current in the conduction band [subfigure (c)] and the valence band [subfigure (f)] as functions of the chemical potential $\mu$ and magnetic field $B$. Simulation parameters: $k_BT=1$ meV, $J_0S_0=20$ meV, and $\Gamma=2$ meV.
• Figure 4: The Landau level (LL) spectrum in both the conduction band [subfigures (a)-(d)] and the valence band [subfigures (e)-(h)] for both valleys and under two different spin-splitting strengths $J_0S_0=20$ meV and $J_0S_0=0$ meV. For the conduction band: (a) the spin-up LL indices are $n=0,1,2,\cdots$ and in (b) the spin-up LL indices are $n=1,2,\cdots$ with the absence of the $n=0$ LL. When the spin-splitting strength is $J_0S_0=0$ meV in (c) both the spin-up and spin-down LL indices are $n=0,1,2,\cdots$ and in (d) both the spin-up and spin-down LL indices are $n=1,2,\cdots$ with the absence of the $n=0$ LL. For the valence band: (e) the spin-up indices are $n=-1,-2,-3,\cdots$ and in (f) the spin-down LL indices are $n=0,-1,-2,-3,\cdots$. When the spin-splitting strength is $J_0S_0=0$ meV, in (g) the spin-up indices are $n=-1,-2,-3,\cdots$ and in (h) the spin-down LL indices are $n=0,-1,-2,\cdots$. Except for $J_0S_0$, other simulation parameters are the same as Fig. \ref{['density']}.
• Figure 5: The spin currents for each valley and the valley-polarized spin current in the conduction band as a function of chemical potential $\mu$ for magnetic fields $B=20, 30, 40$ T. (a) Spin current from the $K$ valley, $I_{s,K}$ when $J_0S_0=20$ meV. (b) Spin current from the $K'$ valley, $I_{s,K'}$ when $J_0S_0=20$ meV. (c) The resulting valley-polarized spin current, $I_v$ when $J_0S_0=20$ meV. Orange shading highlights the range of significant valley polarization. (d)-(f) are the corresponding spin-current behavior when $J_0S_0=0$ meV. A detailed comparison is discussed in the text. Simulation parameters: $k_BT=1$ meV and $\Gamma=2$ meV.