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Topological materials

Can a topological insulator turn heat into spin current?

Jain R, Stanley M, Bose A, et al. · Science advances · 2023

Open access · cc by · source: Europe PMC

A thin film of the topological insulator bismuth selenide turns a temperature gradient into a flow of electron spin far more efficiently than platinum or tungsten.

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Study at a glance

Design
Other — Thin-film Bi2Se3/CoFeB Hall-bar devices with on-chip heaters; compares thermally driven (spin Nernst) and electrically driven (spin Hall magnetoresistance) signals, plus second-harmonic Hall torque measurements.
N
No participant count; main bilayer device plus a second bilayer with 6 nm CoFeB and single-layer control samples.
Population
MBE-grown 8 nm Bi2Se3 films capped with sputtered CoFeB, measured at room temperature
Outcome
Spin Nernst magneto-thermopower, spin Hall magnetoresistance, Seebeck coefficient, and derived spin Nernst ratio

Structured fields used in claim comparison tables when every cited study has a complete layer.

Key findings

The heat-driven signal had the angular pattern expected for the spin Nernst effect and flipped sign when the gradient was reversed. The ratio of the spin Nernst to spin Hall efficiency was about −0.83, and the spin Nernst ratio had a lower bound of about −0.62 in magnitude, roughly three times that of platinum and two to three times that of tungsten. Spin current per unit thermal gradient was higher than in tungsten and similar to platinum despite bismuth selenide's much higher resistivity.

Methodology

The authors grew 8 nm films of bismuth selenide, confirmed their surface states, and topped them with a thin magnetic CoFeB layer patterned into long Hall bars with small heaters at each end. They measured how the bar's resistance and heat-driven voltage changed as a strong magnetic field was rotated, separating the spin-related parts from ordinary magnetoresistance by their field dependence and by control samples of each layer alone. Combining these with a measured Seebeck coefficient and a spin-torque estimate, they calculated how efficiently heat generates spin current.

Limitations

The key efficiencies are lower bounds, because the spin Hall ratio was estimated from a torque measurement that includes an unknown interface transparency factor. The analysis assumes heat-driven and electrically driven spin currents are converted to voltage the same way. The study used one composition at room temperature, so the proposed tuning of the Fermi level is untested, and it cannot tell whether topological surface states survive under the magnetic layer or are responsible for the effect.

How this study connects

Role on claims

Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.

  • SupportsTopological materialsconcept

    Surface states in topological insulators matter for real device signals, not just theory.

    Topological surface electrons can dominate useful responses: a Bi2Se3 film kept producing more terahertz third harmonic as pump power rose while graphene saturated, reaching about 8% field conversion (about 0.5 mW), and a Bi2Se3 film showed a thermally driven spin current roughly two to three times more efficient (as a ratio) than tungsten or platinum.

    Evidence for the claim as stated.

  • QualifiesTopological materialsconcept

    Surface states in topological insulators matter for real device signals, not just theory.

    Topological surface electrons can dominate useful responses: a Bi2Se3 film kept producing more terahertz third harmonic as pump power rose while graphene saturated, reaching about 8% field conversion (about 0.5 mW), and a Bi2Se3 film showed a thermally driven spin current roughly two to three times more efficient (as a ratio) than tungsten or platinum.

    Scope note — Efficiencies are lower bounds, and the study cannot say whether surface states survive under the magnetic layer.

    Limits the claim's scope: a different population, assay, or outcome.

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