REVIEW 3 major objections 5 minor 6 references
Experimental observation of coupled valley and spin Hall effect in p-doped WSe2 devices
T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read The paper reports an all-electrical WSe2/graphene device that produces a transverse out-of-plane spin current via the spin-locked valley Hall effect and detects it with a ferromagnetic nonlocal probe.
desk verdict Plausible first all-electrical spin-valley Hall effect in WSe2, but the evidence is thinner than the paper's wording—small signals, averaged without error bars, and linear background subtraction in some traces—so it deserves peer review with a demand for raw data. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is spin-valley locking in the WSe2 valence band: time-reversal symmetry enforces $\Omega(K)=-\Omega(-K)$ and $s(K)=-s(-K)$, and the ~450 meV spin splitting leaves only one spin sub-band at the Fermi level per valley. The anomalous velocity $\boldsymbol{v}=(e/\hbar)\,\boldsymbol{E}\times\boldsymbol{\Omega}(\boldsymbol{k})$ then sends opposite-spin holes to opposite transverse edges, converting a longitudinal charge current into a transverse out-of-plane spin current, the coupled valley and spin Hall effect. The detection harnesses a permalloy electrode with in-plane anisotropy: an external $B_z$ tilts its magnetization out of plane by a controllable fraction $m_z$, independently calibrated by anomalous Hall effect on a co-processed Hall bar, so the nonlocal voltage is a direct map of the out-of-plane spin chemical potential in the graphene channel.
What would settle it
Replace the in-plane-anisotropy probe with a perpendicularly magnetized electrode: the out-of-plane spin interpretation predicts a maximum nonlocal signal at zero field that reverses with current polarity, whereas an in-plane probe requires the field to cant the magnetization and so shows signal only for $|B_z|>0$. Observing the zero-field signal with a perpendicular detector would confirm the claim; failing to see it would refute the central interpretation.
Extended reading notes
Core claim
Under a bias applied to p-doped monolayer WSe2, holes in the valence band are spin-valley locked: the K valley carries one out-of-plane spin and the -K valley the opposite. The finite Berry curvature gives these valleys opposite anomalous velocities, so the electric field separates them transversely; the result is a spin current with polarization $\pm z$ flowing into the overlaying graphene. The nonlocal ferromagnetic detector measures the spin chemical potential only through its out-of-plane component, which is why the signal grows continuously as the applied field $B_z$ pulls the detector magnetization out of plane and saturates for $|B_z|>0.8$ T. Reversing the charge current reverses the spin polarization, and replacing the ferromagnetic probe with a nonmagnetic electrode removes the signal. A drift-diffusion model with valley chemical potentials and interface valley-memory loss reproduces the sign of the nonlocal voltage and puts the interfacial spin polarization at about 38%.
Load-bearing premise
The load-bearing assumption is that the magnetic probe in the nonlocal device tilts out of plane under $B_z$ exactly as the separate calibration Hall bar does; if the two rotate differently, the overlay between measured signal and $m_z(B_z)$ loses its meaning.
Editorial extensions
If this is right
- A monolayer WSe2 device can act as a purely electrical source of out-of-plane spin current, with no optical excitation and no external magnetic field required for generation.
- Because reversing the charge current reverses the spin polarization, the same device can serve as a switchable spin source for a perpendicularly magnetized electrode.
- The nonlocal-voltage-versus-$B_z$ method, calibrated by anomalous Hall effect on a co-processed Hall bar, can be applied to other two-dimensional materials to test for out-of-plane spin accumulation.
- The saturation value of the nonlocal signal, compared with the no-loss upper bound, gives a quantitative estimate of the spin/valley polarization surviving the WSe2/graphene interface, about 38% in these devices.
- The sign of the nonlocal voltage is tied to the Berry curvature sign and the valley/spin index, so the measurement is a direct electrical readout of the valley polarization flowing into the channel.
Reading between the lines
- If the claim holds, other monolayer transition metal dichalcogenides with large valence-band spin splitting, such as MoSe2 and WS2, should show the same effect, and the relative nonlocal signal sizes could be compared with their predicted spin splittings.
- Integrating the same WSe2 source with a perpendicularly magnetized electrode should enable an attempt at field-free spin-orbit torque switching; a successful switching experiment would be a direct device-level validation of the out-of-plane spin interpretation.
- Because conduction-band spin splitting in these materials is small, switching the doping from p-type to n-type should suppress the out-of-plane spin signal, providing a clean test that separates valley Hall spin generation from ordinary spin Hall effects.
- The interface valley-memory-loss model predicts that improving the WSe2/graphene interface, for example by a barrier layer or a cleaner transfer, should move the measured nonlocal voltage toward the calculated roughly three-times-larger upper bound, a quantitative target for materials engineering.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports an all-electrical nonlocal spin valve measurement in a monolayer WSe2/graphene hybrid device. A DC current through p-doped WSe2 is claimed to generate a transverse spin current with out-of-plane polarization via the spin-locked valley Hall effect; the spins diffuse into a graphene channel and are detected by a permalloy (Py) electrode whose magnetization is tilted out of plane by a perpendicular field. The authors support the claim with a nonmagnetic control, an independent AHE calibration of mz(Bz), a semiclassical sign analysis, and a drift-diffusion model.
Significance. If the observation is robust, this would be the first all-electrical generation and detection of out-of-plane spin polarization via the valley Hall effect in a TMD, with direct relevance for SOT-MRAM and valleytronics. The paper's strengths include the use of a nonmagnetic control, the sign analysis based on Berry curvature and spin-valley locking, and the observation that the nonlocal resistance saturation tracks the independently measured AHE curve. The device geometry and room-temperature operation are also notable. However, the experimental evidence currently rests on a small number of averaged traces without error bars, and the magnitude modelling is a consistency check rather than a parameter-free prediction.
major comments (3)
- [Main text Fig. 3a; Supplementary Section II] The central claim rests on the nonlocal resistance data in Fig. 3a, which are presented as an average of three datasets with no error bars or per-trace statistics. Supplementary Section II states that in two of the six traces (Fig. S7d,f) a linear background was subtracted. This is a load-bearing issue: if the individual traces do not each show a clear zero baseline at Bz=0, a monotonic mz-like increase, and a reversal with current polarity, the averaging and background subtraction could generate the apparent saturating signal from noise or drift. Please show the raw individual traces, quantify the signal-to-noise and the significance of the effect per trace, and specify the averaging procedure and the background subtraction criteria.
- [Main text Figs. 2-3a] The interpretation of Rnl(Bz) as tracking mz(Bz) relies on transferring the anomalous Hall curve measured on a standalone Py Hall bar to the actual Py probe on graphene. The calibration assumes that the small Py electrode on graphene has the same demagnetization field, anisotropy, and field history as the Hall bar. If the real probe rotates differently, the saturation field and zero-crossing of Rnl would not be expected to match the AHE curve. Please provide evidence that the calibration transfer is valid, for example a local magnetization measurement on a geometrically identical probe, or, failing that, soften the claim that Rnl mirrors the independently measured mz.
- [Supplementary Section III] The sign derivation contains an internal inconsistency in the definition of Vnl. The text defines Vnl = V_NM - V_FM for magnetization along +z, but for magnetization along -z it writes Vnl = V_FM - V_NM > 0, which is the opposite sign convention. Since the sign of the nonlocal voltage is a central claim that distinguishes the spin polarization direction, this inconsistency needs to be resolved and the signs of all four quadrants in Fig. 4 re-derived consistently.
minor comments (5)
- [Supplementary Section VII heading] The heading 'Upper Bond' should be 'Upper Bound'.
- [Fig. 4 caption] The caption is truncated (it ends with 'lch=1.2um, w1'); please complete the figure caption.
- [Main text and Fig. 4] The symbols sz and mz are used without formal definition; please define those quantities when they first appear.
- [Supplementary Sections VI-VII] The model contains free parameters (alpha, beta, lv, lv', and sigma_xy_VH), so the 'upper bound' estimate is not parameter-free. The factor-of-3 agreement with experiment should be presented as a consistency check, not as a parameter-free prediction.
- [Main text and Supplementary Section II] The control device in Fig. 3b consists of a single trace; please state whether the control experiment was reproduced in multiple devices or provide additional control datasets.
Circularity Check
No circularity: the central observation is supported by an external AHE calibration, a nonmagnetic control, and sign/magnitude estimates from independent semiclassical theory.
full rationale
The paper's central claim is an experimental observation of out-of-plane spin accumulation from spin-locked valley Hall effect in WSe2. The derivation chain is not circular. The non-local signal is compared with an independently measured anomalous Hall effect curve on a separate Py Hall bar, and the non-magnetic control device rules out non-spin artifacts. The sign of the expected non-local voltage is derived in Supplementary Section III from the semiclassical Berry-curvature equation of motion and the spin-valley locking relation s(K) = -s(-K), not from the measured data; this is an external theoretical prediction. The magnitude estimate in Supplementary Sections VI-VII uses a drift-diffusion model from Shimazaki et al. and literature/assumed parameters, and is explicitly labelled an upper bound. The discrepancy between the upper bound and the data is attributed after the fact to interface valley loss; it is not a parameter fitted to force agreement. The self-citations (refs 17, 26, 28) are to prior fabrication, valley-Hall, and graphene spin-valve results; the PD efficiency value from ref 28 is from an independent graphene spin-valve experiment and is not a fitted input to the observation. No equation is defined in terms of the claimed result, and no fitted parameter is renamed as a prediction. Therefore no circular step is present.
Assumptions & free parameters
free parameters (3)
- WSe2/graphene interface opacity parameters alpha and beta =
not directly measured; set to 0 for the upper-bound estimate, then invoked to account for the roughly 3x discrepancy…
- Valley Hall conductivity sigma_xy_VH =
about 2 e^2/h
- Valley diffusion lengths lv and lv' =
0.6 um and 1 um
assumptions (5)
- domain assumption Monolayer WSe2 valence band has spin-valley locking with out-of-plane spin polarization and large spin splitting (~450 meV).
- standard math Semiclassical Berry-curvature equation of motion r_dot = (1/hbar) dE/dk - k_dot x Omega describes carrier motion in WSe2.
- domain assumption Py magnetization orientation under an out-of-plane field follows a macrospin free-energy model with demagnetization field 4*pi*Ms.
- domain assumption Valley chemical potential difference in graphene obeys a one-dimensional diffusion equation with no valley Hall term.
- domain assumption The O2 plasma treatment leaves a clean monolayer WSe2 under a WOx layer.
Cite this review
Pith. "Pith review of Experimental observation of coupled valley and spin Hall effect in p-doped WSe2 devices." pith.science (2026). https://pith.science/paper/BINQMPZ5
@misc{pith2026190801396,
author = {Pith},
title = {Pith review of: Experimental observation of coupled valley and spin Hall effect in p-doped WSe2 devices},
year = {2026},
howpublished = {\url{https://pith.science/paper/BINQMPZ5}},
note = {Machine review of arXiv:1908.01396}
}
read the original abstract
Giant spin Hall effect (GSHE) has been observed in heavy metal materials such as Ta, Pt, and W, where spins are polarized in the surface plane and perpendicular to the charge current direction. Spins generated in these materials have successfully switched magnets with in-plane magnetic anisotropy (IMA) and perpendicular magnetic anisotropy (PMA) through spin orbit torque (SOT) mechanism. It is generally accepted that PMA magnets are preferred over IMA magnets in data storage applications owing to their large thermal stability even at ultra scaled dimensions. However, SOT switching of PMA magnets by conventional GSHE materials requires either a small external magnetic field, a local dipolar field, or introducing tilted anisotropy to break the symmetry with respect to the magnetization. To deterministically switch a PMA without any additional assistance, nonconventional GSHE materials that can generate spins with polarization perpendicular to the surfaces are needed. Several monolayer transition metal dichalcogenides (TMDs) have been predicted to generate such out of plane spins due to their 2D nature and unique band structures. Interestingly, opposite spins are locked to their respective sub-band in each K valley of the TMD valence band with substantially large energy splitting, which enables polarized spins to be accessible through electrical gating and spatially separated by electric field through the valley Hall effect (VHE). Therefore, spatial separation and accumulation of spins in these 2D TMDs are uniquely referred to as coupled valley and spin Hall effect. Here, we report an experiment of electrical generation of spin current with out of plane polarization in monolayer WSe2 and detection of spin signals through a nonlocal spin valve structure built on a lateral graphene spin diffusion channel that partially overlaps with WSe2.
Reference graph
Works this paper leans on
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Reviewed August 14, 2026 · model on record in the stance chip above.
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