Perfect Spin-Valley Switch

• Perfect spin-valley switch is a phenomenon where one spin-valley channel remains conductive while all others are suppressed by external controls.
• It employs selective gap engineering, topological channel selection, and symmetry reversal in materials like silicene, MoS₂, and related 2D systems.
• The mechanism underpins applications in spintronics and valleytronics, enabling nonvolatile and ultrafast control in advanced device architectures.

A perfect spin-valley switch is a transport or state-selection regime in which a single spin-valley channel is left conducting or occupied while the complementary channels are suppressed, or in which the selected channel is fully inverted by reversing an external control parameter. In silicene, perfect spin and valley filtering are explicitly defined by $P_s=\pm1$ and $P_v=\pm1$, and the switch polarity can be inverted by reversing the perpendicular electric field or the circular polarization of off-resonant light (Mohammadi et al., 2015). Closely related realizations have been formulated in oxide ferroelectric heterostructures, MoS$_2$ superconducting spin valves, buckled-honeycomb AF/S/AF junctions, borophene $n$-$p$-$n$ devices, chiral graphene multilayers, and 2D altermagnets (Yamauchi et al., 2015, Majidi et al., 2014, Lu et al., 4 Sep 2025, Bidgoli et al., 2023, Wang et al., 2023, Chen et al., 6 Mar 2026).

1. Definitions and observables

Valley pseudospin denotes the binary valley degree of freedom associated with inequivalent extrema such as $K$ and $K'$. In the language used for two-dimensional materials, electrons possess an additional quantum attribute, the valley pseudospin, labelled as $K$ and $K'$, analogous to spin up and spin down (Rana et al., 2023). In monolayer MoS$P_v=\pm1$0, inversion asymmetry and time-reversal symmetry produce spin-valley coupling in the valence band, so that at $P_v=\pm1$1 the top of the valence band is spin-$P_v=\pm1$2, while at $P_v=\pm1$3 it is spin-$P_v=\pm1$4 (Heshmati et al., 2016).

In ballistic Dirac systems, the natural observables are spin- and valley-resolved transmission probabilities and the corresponding Landauer conductances. For silicene barriers, the zero-temperature conductance of a channel $P_v=\pm1$5 is written as

$P_v=\pm1$6

with $P_v=\pm1$7 the sample width (Mohammadi et al., 2015). Perfect spin filtering corresponds to $P_v=\pm1$8, and perfect valley filtering corresponds to $P_v=\pm1$9 (Mohammadi et al., 2015).

In superconducting hybrids, the same idea appears in a different guise. Instead of selecting a single normal-state channel, the switch can enforce pure elastic cotunneling (CT) or pure crossed Andreev reflection (CAR), with the nonlocal current fully valley- and spin-polarized in the right ferromagnetic or antiferromagnetic lead (Majidi et al., 2014, Lu et al., 4 Sep 2025). Taken together, these results suggest that “perfect” refers not to a single device architecture but to complete channel selectivity in the combined spin-valley sector.

2. Microscopic switching mechanisms

A common mechanism is selective gap engineering in valley- and spin-resolved Dirac cones. In silicene under a perpendicular electric field and off-resonant circularly polarized light, the effective mass is

$_2$0

so that the channel gap is

$_2$1

The switch is obtained by tuning $_2$2 and $_2$3 so that one channel remains propagating at $_2$4 while the others are driven into evanescent or forbidden regimes (Mohammadi et al., 2015).

The same logic appears in borophene in a more explicitly kinematic form. A necessary ingredient is that for one choice of $_2$5 the central-region wavevector $_2$6 be real, while for the other choices it is imaginary; for sufficiently long barriers, the unwanted channels then decay exponentially (Bidgoli et al., 2023). In buckled-honeycomb AF regions proximitized by antiferromagnetism, the field-controlled gap

$_2$7

can be closed for exactly one $_2$8 pair while the remaining three channels stay gapped, producing a spin-valley-polarized half-metal (Lu et al., 4 Sep 2025).

A second mechanism is topological channel selection. In silicene topological barriers, electrons allowed to transport at transition points must obey zero-Chern number, which is equivalent to zero mass and zero-Berry curvature, while electrons with non-zero Chern number are perfectly suppressed (Prarokijjak et al., 2017). In 2D altermagnets, a gate-tunable sublattice-staggered potential selectively gaps one valley and converts a helical spin-valley-momentum-locked phase with $_2$9 into a chiral quantum anomalous Hall phase with $n$0, reversing the transmitted spin-valley polarization (Chen et al., 6 Mar 2026).

A third mechanism is symmetry reversal in nonvolatile media. In BiAlO$n$1/BiIrO$n$2, ferroelectric off-centering changes the sign of $n$3, $n$4, $n$5, and $n$6, inverts the valley-Zeeman term $n$7, and therefore reverses the valley-contrasting spin polarization (Yamauchi et al., 2015).

3. Photo-induced silicene as the canonical ballistic switch

The most explicit ballistic formulation is the silicene strip exposed to off-resonant circularly polarized light and a perpendicular electric field. Near valley $n$8 and spin $n$9, the effective Hamiltonian is

$p$0

with $p$1 and $p$2 for right/left circular polarization (Mohammadi et al., 2015). Diagonalization gives

$p$3

so the switch is controlled directly by the channel-resolved mass.

The barrier problem is formulated for a strip of width $p$4 with $p$5, sandwiched between pristine leads. Perfect spin filtering requires that for one spin species $p$6 becomes purely imaginary, while for the other spin $p$7 is real. In the simplest case $p$8, this condition reduces to

$p$9

so that one spin is open in both valleys and the other is gapped out (Mohammadi et al., 2015).

Perfect valley filtering requires a complementary arrangement: both spin channels in one valley remain open, whereas in the other valley both spin channels are suppressed. A convenient choice is to tune $n$0 and $n$1 so that

$n$2

for both $n$3 at $n$4, while simultaneously

$n$5

for both spins at $n$6; then only the $n$7 valley carries current (Mohammadi et al., 2015).

The switch polarity is analytically reversible. Reversing the perpendicular field gives

$n$8

while reversing the light helicity gives

$n$9

For $K$0, $K$1, $K$2, $K$3, $K$4, and $K$5, perfect spin-up filtering occurs once $K$6; for $K$7 and $K$8, $K$9 over a wide range of $K'$0, while reversing $K'$1 yields $K'$2 (Mohammadi et al., 2015).

A related silicene variant places off-resonant light only on the second ferromagnetic region of an FNF junction. There, fully spin-polarized current can be realized in certain ranges of light intensity, perfect valley polarization can be achieved even when the staggered electric field is much smaller than the exchange field, and 100% tunneling magnetoresistance can be induced by circularly polarized light independently of barrier height in the normal region; the sign of the TMR reverses when the light helicity is changed (Ho et al., 2016).

4. Superconducting implementations

In hole-doped MoS$K'$3 F/S/F spin valves, the spin-valley switch appears as a configuration-controlled conversion between pure CT and pure CAR. For the chemical-potential window

$K'$4

no local Andreev reflection channel is available in the left ferromagnet, so $K'$5. In the parallel configuration only CT is allowed, whereas in the antiparallel configuration only CAR is allowed; the only transmitted mode in the parallel state is $K'$6, while in the antiparallel state it is $K'$7, so reversing the right magnetization flips both spin and valley polarizations of the nonlocal current (Majidi et al., 2014).

An electrically controlled counterpart has been formulated for AF/S and AF/S/AF junctions in generic buckled honeycomb systems such as silicene, germanene, and stanene. By varying $K'$8, one closes the gap of exactly one $K'$9 cone and keeps the other three channels gapped, thereby creating a spin-valley-polarized half-metal. Perfect CAR occurs when the right AF lead supports only the hole band of the time-reversed spin-valley channel and no electron band at the excitation energy; perfect EC occurs when the right lead supports only the electron band of the same channel (Lu et al., 4 Sep 2025).

The numerical example is explicit. For $K$0, $K$1, $K$2, $K$3, $K$4, $K$5, and $K$6, one finds that $K$7 gives $K$8, $K$9, $K'$0, $K'$1, while $K'$2 gives $K'$3, $K'$4, $K'$5, $K'$6 (Lu et al., 4 Sep 2025). The local conductance $K'$7 then jumps from $K'$8 to $K'$9, and the nonlocal conductance changes sign as the device moves between EC and CAR windows (Lu et al., 4 Sep 2025).

5. Topological and edge-state realizations

Topological implementations replace bulk-mode filtering by edge-mode selection. In 2D altermagnets, the low-energy Hamiltonian combines Dirac kinetics, altermagnetic exchange, SOC-induced valley-spin mass, and a gate-tunable sublattice potential. For $P_v=\pm1$00, the system is in a helical spin-valley-momentum-locked phase with $P_v=\pm1$01; for $P_v=\pm1$02, it enters a chiral quantum anomalous Hall phase with $P_v=\pm1$03. Reversing the gate potential switches which valley remains conducting and therefore flips the transmitted spin-valley polarization from $P_v=\pm1$04 to $P_v=\pm1$05 in a quantized way (Chen et al., 6 Mar 2026).

Engineered edge separation provides another route. In a quantum anomalous Hall insulator on a honeycomb lattice, staggered sublattice potential on strips along the sample edges pushes the down-spin edge states into inner boundaries while up-spin states remain on the outer boundaries, producing spatially separated chiral states with perfect spin polarization. An additional boundary potential yields a valley filter immune to short-range and smooth long-range scatterers (Liu et al., 2016). In zigzag silicene, germanene, or stanene ribbons with side potentials, the phase boundaries are fixed by $P_v=\pm1$06; depending on $P_v=\pm1$07, $P_v=\pm1$08, and ribbon width, the system exhibits quantum spin-valley Hall, valley-polarized quantum spin Hall, spin-polarized quantum anomalous Hall, and spin-valley polarized insulating states that can be used to realize a perfect spin-valley switch (Lu et al., 4 Sep 2025).

Silicene topological-barrier junctions make the topological criterion fully explicit. If one tunes the barrier parameters so that

$P_v=\pm1$09

for one species while all other $P_v=\pm1$10, then $P_v=\pm1$11 by Klein transmission and the remaining channels are exponentially suppressed in thick barriers. At low temperature and small bias, the selected current becomes

$P_v=\pm1$12

with giant magnetoresistance dips appearing when the parallel and antiparallel barrier configurations access different topological sectors (Prarokijjak et al., 2017).

6. Nonvolatile electrical and ultrafast control

In oxide heterostructures, the perfect spin-valley switch can be nonvolatile. In BiAlO$P_v=\pm1$13/BiIrO$P_v=\pm1$14 grown along $P_v=\pm1$15, the low-energy electrons are described by a Dirac-like Hamiltonian with a valley-Zeeman term proportional to ferroelectric polarization. At $P_v=\pm1$16, the out-of-plane spin polarization is

$P_v=\pm1$17

and $P_v=\pm1$18, so ferroelectric reversal inverts the valley-contrasting spin polarization perfectly (Yamauchi et al., 2015). The reported figures are $P_v=\pm1$19, maximum $P_v=\pm1$20, valley polarization tunable from $P_v=\pm1$21 to $P_v=\pm1$22, coercive field $P_v=\pm1$23, and ferroelectric switching time $P_v=\pm1$24 (Yamauchi et al., 2015).

Electrical isospin switching has also been proposed in chiral graphene multilayers encapsulated by TMDs. In aligned devices, reversing the perpendicular displacement field $P_v=\pm1$25 at fixed chemical potential flips the valley index and, because valley flip implies spin flip, yields perfect spin-plus-valley reversal. In anti-aligned devices, reversing $P_v=\pm1$26 flips valley while leaving spin fixed. In aligned devices one may also hold $P_v=\pm1$27 fixed and sweep $P_v=\pm1$28 to flip spin alone while preserving valley (Wang et al., 2023). The exchange gap of the fully spin-valley polarized phase is $P_v=\pm1$29, a representative gate-limited switching time is $P_v=\pm1$30, and the estimated switching energy is $P_v=\pm1$31 (Wang et al., 2023).

Ultrafast coherent control has been demonstrated directly for valley pseudospin. A three-pulse, all-optical protocol in graphene and MoS$P_v=\pm1$32 uses non-overlapping, CEP-stabilized, linearly polarized pulses to drive the sequence $P_v=\pm1$33-polarized $P_v=\pm1$34 balanced $P_v=\pm1$35 $P_v=\pm1$36-polarized within $P_v=\pm1$37, faster than typical valley decoherence times (Rana et al., 2023). The same work states that a possible scheme would interleave CEP-controlled linear pulses with few-cycle circular pulses to manipulate the combined four-state manifold $P_v=\pm1$38, but detailed simulation of that combined protocol remains a future direction (Rana et al., 2023).

7. Robustness, operating windows, and limitations

Perfect switching is generally a regime condition rather than a generic property of a material. In photo-irradiated silicene, the off-resonance condition $P_v=\pm1$39 ensures no real photon absorption; finite temperature smears the Fermi edge over $P_v=\pm1$40 at $P_v=\pm1$41, but the filtering remains nearly perfect as long as the induced gaps exceed a few $P_v=\pm1$42; imperfect interfaces or edge roughness may reduce overall conductance but do not lift the spin/valley selectivity so long as intervalley scattering is weak (Mohammadi et al., 2015). In 8-pmmn borophene, full spin and valley polarizations require the barrier length to exceed a specific value, $P_v=\pm1$43, so that the unwanted channel is exponentially suppressed (Bidgoli et al., 2023).

Disorder sensitivity depends strongly on the protection mechanism. In altermagnets, helical spin-valley-momentum-locked edge states remain quantized under nonmagnetic and long-range magnetic disorder but collapse under short-range magnetic disorder that induces spin flips and intervalley scattering, whereas the chiral phase remains quantized under every disorder type (Chen et al., 6 Mar 2026). In edge-engineered honeycomb models, immunity to short-range and smooth long-range scatterers follows from chirality and spatial separation of the relevant modes (Liu et al., 2016).

A recurrent misconception is that any valley-selective or spin-selective response is already “perfect.” The MoS$P_v=\pm1$44 nanoribbon study explicitly reports that an external magnetic field alone gives only a very small valley polarization, while a valley polarization equal to $P_v=\pm1$45 can be achieved using an exchange field of $P_v=\pm1$46; perfect selection of a particular spin-valley combination requires a specific set of exchange field and gate voltage as input parameters (Heshmati et al., 2016). Likewise, in superconducting switches the pure CT/CAR or pure EC/CAR regimes are confined to restricted chemical-potential or electric-field windows (Majidi et al., 2014, Lu et al., 4 Sep 2025). This suggests that a perfect spin-valley switch is best understood as a sharply defined operating point in a multi-parameter phase space, established by gap closure for the desired channel and simultaneous exclusion, hybridization, or topological suppression of the others.