Spin-Valley Switch Effects in 2D Materials

• Spin-Valley Switch Effect is a phenomenon where external controls reversibly manipulate coupled spin and valley states in 2D materials.
• It underpins diverse implementations, from superconducting hybrids to ferroelectric spin valves and ultrafast optical protocols, with significant device applications.
• Experimental realizations leverage band structure selectivity, spin-orbit coupling, and Berry curvature to achieve high contrast switching in transport and excitation channels.

Spin-valley switch effect denotes a class of phenomena in which an external control parameter reversibly changes the spin and valley character of excitations or transport channels. In the published literature, the expression is used for switching between pure elastic cotunneling and pure crossed Andreev reflection in superconducting hybrids (Majidi et al., 2014), switching valley excitation from one valley to another within tens of femtoseconds by a coherent optical protocol (Rana et al., 2023), switching between low- and high-resistance states through ferroelectric-domain-controlled valley-dependent spin matching in a two-dimensional valley spin valve (Huang et al., 2024), reversing electrically generated out-of-plane spin polarization in monolayer WSe$_2$ by changing current polarity (Hung et al., 2019), and continuously switching a silicon qubit between a spin mode and a valley mode by tuning the valley splitting with a static electric field (Bourdet et al., 2018). This suggests that the term identifies a broader operational principle: selective control over states whose accessibility is fixed by coupled spin and valley quantum numbers.

1. Conceptual basis in spin-valley coupling and valley pseudospin

Two-dimensional materials such as graphene and monolayer transition-metal dichalcogenides possess two inequivalent valleys, $\mathbf{K}$ and $\mathbf{K}'$, so that the valley degree of freedom can be treated as a pseudospin (Rana et al., 2023). In monolayer transition-metal dichalcogenides, strong spin-orbit coupling and broken inversion symmetry produce spin-valley coupling, so that the two nonequivalent valleys are associated with distinct spin states (Ominato et al., 2020). A standard low-energy description is

$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$

with $\tau=\pm$ for valley and $s=\pm$ for spin (Ominato et al., 2020).

In p-doped monolayer WSe$_2$, the valence band has substantially large energy splitting, about $450$ meV, and opposite spins are locked to their respective sub-band in each $K$ valley, enabling electrically accessible out-of-plane spin polarization through the valley Hall effect (Hung et al., 2019). In monolayer TMDs interfaced with two-dimensional ferromagnetic semiconductors, direct coupling of valley states to spin-polarized states of the ferromagnet can produce a valley-selective gap opening due to spin-momentum locking, yielding halfmetallicity and electrically switchable valley polarization (Zhang et al., 2021).

Taken together, these results indicate that spin-valley switching does not require a single microscopic definition. What is common is that the active transport or excitation channel is selected by spin-valley locking, valley-selective hybridization, or valley-dependent availability of states.

2. Microscopic mechanisms

A recurring mechanism is state selectivity imposed by the band structure. In the hole-doped MoS$_2$ superconducting spin valve, Andreev reflection is suppressed for a range of the chemical potential because an incoming electron from a unique spin/valley subband would require a reflected hole of opposite spin and valley, and such a hole state does not exist in the left ferromagnetic region for $\mathbf{K}$0 (Majidi et al., 2014). In buckled honeycomb antiferromagnet/superconductor junctions, the joint action of perpendicular electric field and antiferromagnetic exchange field tunes the effective spin-valley-dependent bandgap,

$\mathbf{K}$1

so that only one spin-valley sector remains conducting at subgap energies and the system behaves as a spin-valley half-metal (Lu et al., 4 Sep 2025).

A second mechanism is matching and mismatching of valley-dependent spin polarization across a junction or domain wall. In the non-volatile valley spin valve based on ferroelectric 1T''-MoS$\mathbf{K}$2, the giant resistance change occurs because transmission depends strongly on matching or mismatching the valley-dependent spin polarizations in two domains with the same or opposite ferroelectric polarization orientations when the chemical potential lies within the spin-split valleys (Huang et al., 2024).

A third mechanism is Hall-type deflection driven by Berry curvature. In monolayer TMDs, Berry curvature has opposite signs at the two valleys, so a valley-polarized spin current produces a transverse spin-current Hall effect and transverse spin accumulation (Ominato et al., 2020). In p-doped WSe$\mathbf{K}$3, an in-plane electric field generates a valley Hall effect and, because of spin-valley locking, an out-of-plane spin current (Hung et al., 2019). In bilayer VS$\mathbf{K}$4, the Berry curvature is both valley-contrasting and layer-locked, underpinning a switchable anomalous valley Hall response (Shen et al., 27 Jan 2026). In monolayer VPSe$\mathbf{K}$5, breaking $\mathbf{K}$6 symmetry by electric field, Janus structure, or ferroelectric substrate produces switchable valley polarization and anomalous valley Hall effect (Sun et al., 25 Feb 2025).

The Berry-curvature route is not universal. In buckled 2D hexagonal tunnel junctions, the electric spin Hall effect and electric valley Hall effect arise from a perpendicular-electric-field-induced backreflection phase in the junction spacer and are explicitly independent of Berry curvature (Zeng, 2 Oct 2025). Silicon nanostructures provide yet another mechanism: valley blockade suppresses tunneling between states of opposite valley index, and tunneling is allowed only between states with identical valley index, enabling gate- and bias-controlled spin-valley transport (Crippa et al., 2015).

3. Superconducting and Andreev-switch realizations

The MoS$\mathbf{K}$7 ferromagnetic/superconducting/ferromagnetic structure provides a canonical superconducting realization. Using scattering formalism, the proposed device exhibits pure elastic electron cotunneling in the parallel configuration and pure crossed Andreev reflection in the low-energy regime of the antiparallel configuration, without fixing of a unique parameter, by reversing the magnetization in the right ferromagnetic region (Majidi et al., 2014). The nonlocal charge current in the right ferromagnetic region is fully valley- and spin-polarized, and the type of polarization can be changed by reversing the magnetization direction (Majidi et al., 2014). The corresponding conductances are

$\mathbf{K}$8

$\mathbf{K}$9

where $\mathbf{K}'$0 and $\mathbf{K}'$1 are the transmission amplitudes for CT and CAR (Majidi et al., 2014). The strong spin-orbit interaction $\mathbf{K}'$2 and the topological term $\mathbf{K}'$3 enhance the charge conductance of CT and CAR and make them present for long lengths of the superconducting region; the thermal conductance is linear in temperature at low temperatures and increases exponentially at higher temperatures, with a maximum at $\mathbf{K}'$4 that moves toward larger exchange fields as temperature increases (Majidi et al., 2014).

A closely related switch was later formulated in buckled honeycomb AF/S and AF/S/AF junctions. There, electric field and antiferromagnetic exchange field produce a spin-valley polarized half-metallic phase, local Andreev reflection can be eliminated, and pure CAR can be generated without local AR and elastic cotunneling over a wide range of electric field (Lu et al., 4 Sep 2025). By adjusting the electric field, the device switches between pure CAR and pure EC, so that the spin-valley character of the nonlocal current is selected electrically rather than by magnetic reversal (Lu et al., 4 Sep 2025). This version emphasizes that the spin-valley switch effect can be used as an electrical measurement of crossed Andreev reflection.

4. Electrical, magnetic, and ferroelectric control in two-dimensional valves

Several device proposals convert spin-valley selectivity into an ON/OFF resistance contrast or a switchable Hall response. In ferroelectric 1T''-MoS$\mathbf{K}'$5, switching between the uniformly polarized state and the state with oppositely polarized domains separated by a domain wall results in a resistance change of as high as $\mathbf{K}'$6, because transmission is strongly dependent on matching or mismatching the valley-dependent spin polarizations in the two domains (Huang et al., 2024). In bilayer VS$\mathbf{K}'$7, interlayer sliding breaks inversion symmetry and induces switchable out-of-plane ferroelectric polarization coexisting with interlayer antiferromagnetism; the spin-orbit-coupled valley polarization can be reversibly switched either by ferroelectric polarization reversal or by a magnetic-field-induced spin-flip transition, and electric and magnetic switching are functionally equivalent in modulating valley, layer, and spin indices (Shen et al., 27 Jan 2026). In TMD/2D-ferromagnet heterojunctions such as MoTe$\mathbf{K}'$8/CoCl$\mathbf{K}'$9 and CoCl$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$0/MoTe$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$1/CoCl$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$2, direct hybridization produces valley-selective gap opening, halfmetallicity, electrically switchable valley polarization, and spin/valley filter and valve effects (Zhang et al., 2021).

Platform	Control parameter	Switched response
1T''-MoS$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$3	Uniform polarization vs oppositely polarized domains with a domain wall	Resistance change of as high as $$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$4 (Huang et al., 2024)
Bilayer VS$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$5	Ferroelectric polarization reversal or magnetic-field-induced spin-flip transition	Reversibly switched valley polarization and switchable Hall response (Shen et al., 27 Jan 2026)
MoTe$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$6/CoCl$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$7, CoCl$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$8/MoTe$$H_{\mathrm{eff}} = \hbar v \left( \tau k_x \sigma^x + k_y \sigma^y \right) + \frac{\Delta}{2} \sigma^z - \tau s \lambda \frac{\sigma^z - 1}{2},$$9/CoCl$\tau=\pm$0	Electric field; parallel or antiparallel magnetization	Electrically switchable valley polarization, spin/valley filter and valve effects (Zhang et al., 2021)

Experimental work on monolayer WSe$\tau=\pm$1 established an electrical route to the same general idea. A nonlocal spin valve built on a lateral graphene channel detected an out-of-plane spin current generated in p-doped WSe$\tau=\pm$2, and changing the current direction reversed the spin polarization at the interface; with the ferromagnetic probe magnetization switchable between $\tau=\pm$3 and $\tau=\pm$4, four distinct states were identified (Hung et al., 2019). A related tunneling structure showed that a pure valley current induced by the valley Hall effect in a WSe$\tau=\pm$5 monolayer exerts an out-of-plane damping-like spin torque on an overlaid Fe or CoFe layer, with torque efficiency tunable through gating (Sousa et al., 2022). These results place the switch effect in direct contact with spin-orbit torque and memory-device architectures.

5. Ultrafast, thermal, photonic, and quantum-information variants

The spin-valley switch effect is not limited to dc electronic transport. An all-optical coherent-control protocol using three time-separated, linearly polarized, few-cycle laser pulses switches valley excitation from $\tau=\pm$6 to $\tau=\pm$7 on the fly within tens of femtoseconds, a timescale faster than any valley decoherence time, and is applicable to both gapped and gapless two-dimensional materials, with monolayer graphene and MoS$\tau=\pm$8 used as test systems (Rana et al., 2023). The key control variable is the carrier-envelope phase of each pulse, and the protocol is robust against dephasing times, wavelengths, and time delays (Rana et al., 2023).

Thermally driven switching appears in TMDC/ferromagnetic-insulator heterostructures under perpendicular magnetic field. There, the spin Seebeck effect drives spin-valley-locked tunneling transport, the magnetic field produces a valley-asymmetric Landau-level structure, and a valley-polarized spin current emerges from valley-selective spin excitation (Hu et al., 6 Feb 2026). By tuning the chemical potential, the sign and magnitude of the valley-polarized spin current can be switched; pronounced quantum oscillations provide a clear experimental signature of quantized valley states (Hu et al., 6 Feb 2026).

Photonic implementations extend the same logic to topological light transport. In a coupled nonlinear ring-resonator lattice, optical Kerr nonlinearity and cross-mode modulation generate a spin-dependent staggered sublattice potential, enabling a quantum spin-valley Hall effect of light and optically reconfigurable manipulation of both spin and valley degrees of freedom (Yang et al., 2021). Switching between two pump configurations changes the sign of the effective mass term and produces topological edge states with definite spin and valley indices (Yang et al., 2021).

Silicon quantum devices provide a qubit-scale realization. In a silicon “corner dot,” spin-orbit coupling mixes $\tau=\pm$9 and $s=\pm$0, and a static electric field acting on the valley splitting $s=\pm$1 continuously switches the qubit between a spin mode and a valley mode (Bourdet et al., 2018). Near the anti-crossing condition $s=\pm$2, electrical addressability is maximal, whereas in the spin mode the qubit is more robust against relaxation and decoherence (Bourdet et al., 2018). In silicon nano field-effect devices, valley blockade and spin-valley Kondo physics realize a gate- and bias-controlled spin-valley switch based on valley index conservation (Crippa et al., 2015).

6. Topological forms, experimental signatures, and conceptual boundaries

Topological versions of the effect make the switched quantity an edge mode rather than a bulk carrier population. In zigzag nanoribbons of silicene- or germanene-like honeycomb lattices with intrinsic spin-orbit coupling, side potentials can generate the quantum spin-valley Hall effect, valley-polarized quantum spin Hall effect, and spin-polarized quantum anomalous Hall effect, and spin-valley polarized insulating states can be used to realize a perfect spin-valley switch (Lu et al., 4 Sep 2025). In altermagnets, a gate-tunable sublattice-staggered potential electrically interconverts helical and chiral topological phases: helical spin-valley-momentum-locked edge states are characterized by a composite spin-valley Chern number $s=\pm$3, a chiral quantum anomalous Hall phase has $s=\pm$4, and reversing the potential switches the transmitted spin-valley polarization (Chen et al., 6 Mar 2026).

Several experimental and theoretical signatures recur across platforms. Nonlocal conductance changes sign when the device switches between EC-dominated and CAR-dominated transport in superconducting junctions (Lu et al., 4 Sep 2025). Nonlocal voltage changes sign with current direction and ferromagnetic detector orientation in WSe$s=\pm$5/graphene spin valves, while a nonmagnetic control device shows no signal (Hung et al., 2019). The coexistence of spin Hall and valley Hall effects generates an additional longitudinal neutral current that is both spin and valley polarized, allows control of spin density by tuning the magnitude of the valley Hall effect, and can suppress the Hanle effect in Hall-bar geometries (Zhang et al., 2018). This provides one objective caution against an overly narrow definition: some spin-valley switch effects are detected through resistance changes, some through nonlocal spin or valley accumulation, some through topological edge transport, and some through qubit-mode conversion.

The literature therefore uses the same expression for related but non-identical mechanisms. Some realizations rely on Berry curvature and Hall responses (Hung et al., 2019), some on superconducting selection rules (Majidi et al., 2014), some on direct hybridization and valley-selective gap opening (Zhang et al., 2021), some on electric-field-induced backreflection phase independent of Berry curvature (Zeng, 2 Oct 2025), and some on symmetry breaking in altermagnets or ferroelectrics (Chen et al., 6 Mar 2026). The shared content is the reversible selection of transport, excitation, or topological channels by coupled spin and valley quantum numbers.