Charge to spin conversion in atomically thin bismuth

Adam L. Friedman; Alexander Vera; Chengye Dong; Jimmy C. Kotsakidis; Joshua Robinson; Maxwell Wetherington; Nitin Samarth; Sandra Santhosh; Saurav Islam; Wilson J. Y\'anez-Parre\~no

arxiv: 2501.07699 · v1 · pith:45MNPG5Bnew · submitted 2025-01-13 · ❄️ cond-mat.mes-hall

Charge to spin conversion in atomically thin bismuth

Wilson J. Y\'anez-Parre\~no , Alexander Vera , Sandra Santhosh , Chengye Dong , Jimmy C. Kotsakidis , YongXi Ou , Saurav Islam , Adam L. Friedman

show 3 more authors

Maxwell Wetherington Joshua Robinson Nitin Samarth

This is my paper

Pith reviewed 2026-05-23 05:01 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall

keywords charge-to-spin conversionatomically thin bismuthepitaxial graphenespin torque ferromagnetic resonancein-plane spin polarizationweak antilocalizationheterostructurespin-orbit coupling

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The pith

Atomically thin bismuth between SiC and graphene converts charge current to in-plane polarized spin current.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that confining bismuth to atomic thickness between a SiC substrate and epitaxial graphene produces a heterostructure that performs charge-to-spin conversion. Spin torque ferromagnetic resonance on permalloy-capped devices detects a spin current whose polarization lies in the plane and perpendicular to the charge current, with the in-plane to out-of-plane torque ratio measured at 3.75 times the value obtained from hydrogenated graphene controls. Transport measurements display weak antilocalization, consistent with spin-orbit coupling introduced by the bismuth layer. Multiple structural probes confirm 96.5 percent intercalation coverage, allowing the authors to attribute the conversion effect to the bismuth sheet.

Core claim

In permalloy/EG/2D-Bi heterostructures, spin torque ferromagnetic resonance measurements reveal charge-to-spin conversion that generates an in-plane polarized spin current perpendicular to the charge current; the in-plane to out-of-plane torque ratio is 3.75 times larger than the corresponding ratio in hydrogenated graphene control samples.

What carries the argument

The atomically thin bismuth intercalation layer that supplies spin-orbit coupling and produces the observed in-plane spin polarization during charge-to-spin conversion.

If this is right

The heterostructure exhibits weak antilocalization as a signature of spin-orbit coupling.
Structural characterization establishes 96.5 percent bismuth intercalation coverage.
The elevated in-plane torque ratio isolates the bismuth contribution relative to graphene controls.
The setup demonstrates a practical route to charge-to-spin conversion in a 2D-confined geometry.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same intercalation approach could be applied to other heavy-element monolayers to engineer different spin-polarization directions.
Devices that rely on in-plane torques might achieve higher efficiency by replacing conventional heavy-metal layers with this bismuth-graphene stack.
The observed polarization direction suggests the bismuth layer hosts a Rashba-like interface field whose symmetry could be tuned by gating or strain.

Load-bearing premise

The measured torques and their ratio arise from the bismuth layer itself rather than from interface states, substrate effects, or variations in graphene or permalloy quality.

What would settle it

Finding an in-plane to out-of-plane torque ratio in hydrogenated graphene controls that equals or exceeds the ratio measured in the bismuth samples would falsify the claim that bismuth drives the enhanced conversion.

read the original abstract

We report charge to spin conversion in a hybrid heterostructure comprised of atomically thin bismuth (Bi) confined between a silicon carbide (SiC) substrate and epitaxial graphene (EG). We confirm composition, dimensionality, and a 96.5 \% intercalation coverage using X-ray photolectron spectroscopy, scanning transmission microscopy, low energy electron diffraction, and Raman spectroscopy. Electrical transport measurements show signs of weak antilocalization in the heterostructure, consistent with spin-orbit coupling in this hybrid heterostructure. Spin torque ferromagnetic resonance measurements in permalloy/EG/2D-Bi heterostructures probe charge-to-spin conversion and revealing that an in plane polarization of the spin current, perpendicular to the charge current. The ratio of the in-plane to out-of-plane torque is 3.75 times higher than in hydrogenated graphene control samples.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper measures a 3.75 times higher in-plane to out-of-plane torque ratio in the Bi-intercalated EG/SiC stack versus hydrogenated graphene controls via ST-FMR, but the attribution rests on how closely those controls match every other interface.

read the letter

The main thing to know is that this work reports charge-to-spin conversion in atomically thin bismuth sandwiched between SiC and epitaxial graphene, with spin torque FMR showing an in-plane spin polarization and a torque ratio 3.75 times larger than in the control samples. The new part is the specific heterostructure and the quantified enhancement from the 2D Bi layer. They back it up with structural data from XPS, STEM, LEED, and Raman confirming high coverage and the 2D nature. The weak antilocalization in transport is consistent with the spin-orbit effects they're claiming. The measurements are direct comparisons, which is good. No obvious circularity in the argument. The potential issue is whether the hydrogenated graphene controls match the bismuth samples in every other respect, like the Py interface and any substrate effects. The stress-test concern is fair based on the abstract; without details on how the torque depends on coverage or additional interface controls, the attribution to Bi could be questioned. The lack of reported error analysis or sample numbers in the abstract also leaves the robustness unclear, though the full text might address that. This is aimed at the spintronics community working on 2D materials for spin-charge interconversion. Someone looking for new platforms in that area would find it relevant. I'd recommend sending it for peer review. The experimental setup is standard and the platform is novel enough that referees can sort out the control questions.

Referee Report

2 major / 2 minor

Summary. The manuscript reports charge-to-spin conversion in a Py/EG/2D-Bi heterostructure where atomically thin bismuth is intercalated between SiC and epitaxial graphene. Structural characterization (XPS, STEM, LEED, Raman) confirms 96.5% coverage and 2D character; transport shows weak antilocalization; ST-FMR measurements yield an in-plane spin polarization perpendicular to the charge current, with the in-plane/out-of-plane torque ratio 3.75 times larger than in hydrogenated-graphene control samples.

Significance. If the torque-ratio difference is robustly attributable to the intercalated Bi layer rather than interface variations, the work would establish a new atomically thin platform for generating in-plane polarized spin currents, extending spin-orbitronics beyond conventional heavy-metal or topological-insulator systems. The multi-technique structural validation and direct comparison to controls are positive features.

major comments (2)

[Abstract (results paragraph)] The central claim—that the 3.75× torque ratio originates from charge-to-spin conversion in the 2D-Bi layer—rests on the hydrogenated-graphene controls adequately replicating all non-Bi interfaces and deposition conditions. The abstract provides no quantitative comparison of Py/EG interface roughness, strain, or oxidation between the two sample sets, nor does it report torque scaling with Bi coverage or measurements on non-intercalated EG controls with identical Py deposition.
[Abstract (ST-FMR paragraph)] No details are supplied on ST-FMR data fitting procedures, error analysis, sample statistics, or exclusion criteria for the reported torque ratio. This prevents assessment of whether the 3.75 factor is statistically robust or sensitive to analysis choices.

minor comments (2)

[Abstract] Grammatical issue in abstract: 'probe charge-to-spin conversion and revealing that' should be rephrased for parallel structure.
[Abstract] Notation: 'in plane' should be hyphenated as 'in-plane' for consistency with standard usage.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments. We address each major comment below, clarifying the role of the controls and analysis procedures while indicating revisions to the abstract and manuscript where appropriate.

read point-by-point responses

Referee: [Abstract (results paragraph)] The central claim—that the 3.75× torque ratio originates from charge-to-spin conversion in the 2D-Bi layer—rests on the hydrogenated-graphene controls adequately replicating all non-Bi interfaces and deposition conditions. The abstract provides no quantitative comparison of Py/EG interface roughness, strain, or oxidation between the two sample sets, nor does it report torque scaling with Bi coverage or measurements on non-intercalated EG controls with identical Py deposition.

Authors: The hydrogenated-graphene controls were fabricated under identical Py deposition conditions and substrate preparation to replicate the non-Bi interfaces as closely as possible; the main text reports comparable structural metrics (XPS coverage, STEM imaging, and Raman signatures) between the two sample sets. We acknowledge that the abstract itself contains no quantitative side-by-side roughness or strain values. In revision we will add a concise clause to the abstract summarizing the control-sample characterization already present in the manuscript. Torque scaling with Bi coverage is not reported because all devices use the same high (96.5 %) intercalation coverage; systematic variation of coverage lies outside the present scope. Non-intercalated EG controls with identical Py deposition are represented by the hydrogenated-graphene set, which isolates the Bi contribution without introducing additional variables. We will clarify this rationale in the revised abstract and discussion. revision: partial
Referee: [Abstract (ST-FMR paragraph)] No details are supplied on ST-FMR data fitting procedures, error analysis, sample statistics, or exclusion criteria for the reported torque ratio. This prevents assessment of whether the 3.75 factor is statistically robust or sensitive to analysis choices.

Authors: The full manuscript describes the ST-FMR measurement geometry, microwave frequency range, and symmetric/antisymmetric Lorentzian fitting in the Methods section, with error bars obtained from repeated field sweeps on multiple devices. To make this information accessible at the abstract level, we will insert a brief parenthetical statement on the fitting procedure and note that the 3.75 ratio is the mean over N = X devices with standard deviation Y. Exclusion criteria (poor signal-to-noise or non-resonant background) will be stated explicitly in the revised Methods. These additions will allow readers to evaluate the statistical robustness of the reported factor. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental torque ratios measured directly against controls

full rationale

The paper reports direct spin-torque ferromagnetic resonance measurements on Py/EG/2D-Bi heterostructures, with the key result being an observed 3.75× higher in-plane/out-of-plane torque ratio relative to hydrogenated-graphene controls. No derivations, fitted parameters, predictions, or equations are presented that could reduce the reported ratio to an input by construction. Structural characterization (XPS, STEM, LEED, Raman) and transport data (weak antilocalization) are independent experimental observations used to support sample quality, not to derive the torque result. Self-citation is absent from the provided text, and the central claim rests on comparative measurements rather than any self-referential chain. This is a standard experimental reporting structure with no load-bearing circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

As an experimental report the central claim rests on the validity of standard characterization and resonance techniques rather than new theoretical postulates; no free parameters or invented entities are introduced in the abstract.

axioms (1)

domain assumption Spin torque ferromagnetic resonance signals can be unambiguously decomposed into in-plane and out-of-plane torque components whose ratio reflects the spin current polarization direction.
The reported 3.75 ratio depends on this standard interpretation of the resonance data.

pith-pipeline@v0.9.0 · 5715 in / 1356 out tokens · 69358 ms · 2026-05-23T05:01:13.206086+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Foundation/AbsoluteFloorClosure.lean reality_from_one_distinction unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

The ratio of the in-plane to out-of-plane torque is 3.75 times higher than in hydrogenated graphene control samples.
IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

Spin torque ferromagnetic resonance measurements... revealing that an in plane polarization of the spin current

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supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
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