🔬 materials science

Electron spin resonance driven photogalvanic effect in graphene-based structures

This paper reports and theoretically explains the observation of an electron-spin-resonance-driven linear photogalvanic effect in unbiased monolayer graphene and WSe₂/graphene heterostructures, demonstrating that resonant photocurrents generated by spin transitions can serve as a sensitive probe for electron spin resonance in two-dimensional devices.

Original authors: C. Bray, I. Yahniuk, L. E. Golub, M. Marocko, C. Consejo, B. Benhamou-Bui, Ziyang Gan, A. George, A. Turchanin, P. Sadovyi, K. Watanabe, T. Taniguchi, J. Eroms, J. Fabian, F. Teppe, S. D. Ganichev

Published 2026-08-27

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CC BY 4.0

Original authors: C. Bray, I. Yahniuk, L. E. Golub, M. Marocko, C. Consejo, B. Benhamou-Bui, Ziyang Gan, A. George, A. Turchanin, P. Sadovyi, K. Watanabe, T. Taniguchi, J. Eroms, J. Fabian, F. Teppe, S. D. Ganichev

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of modern electronics, the ability to control the flow of electricity is fundamental, but a more subtle property of electrons is gaining attention: their spin. Imagine an electron not just as a tiny particle of charge, but as a microscopic magnet spinning on its own axis. This spin gives the electron a magnetic personality, pointing either "up" or "down." Scientists have long used a technique called electron spin resonance to study these spins, similar to how a radio tuner finds a specific station by matching a frequency. When the right magnetic field and energy are applied, the spins flip, revealing details about the material they inhabit. However, detecting this flip in tiny, microscopic devices has been difficult because traditional methods require bulky equipment that struggles to sense such small amounts of matter. Researchers have been searching for a way to "listen" to these spins using electricity alone, without needing large external sensors, to better understand and control the materials that power our future technologies.

A team of physicists has now found a way to do exactly that using a material as thin as a single atom: graphene. In a new study, they demonstrated that they can detect the flipping of electron spins in graphene and a related sandwich-like structure made of graphene and another material called tungsten diselenide, simply by measuring a small voltage generated when the material is hit with light. The researchers did not apply any external battery or electrical push to the samples; instead, they shone a specific type of invisible light, vibrating at frequencies between 45 and 75 gigahertz, onto the material while placing it in a magnetic field. As they adjusted the strength of the magnetic field, they watched for a specific reaction: a sudden change in the voltage produced by the light.

What they found was a clear, repeating pattern. At certain precise magnetic field strengths, the voltage dropped sharply, creating a dip that signaled the electrons were resonating, or flipping their spins, in sync with the light. This happened in two different setups: one where the magnetic field pointed straight down through the material, and another where it ran parallel to the surface. In both cases, the voltage signal behaved in a very specific way. The resonant signal, which indicated the spin flip, always pointed in the opposite direction to the background voltage caused by the light hitting the material normally. Furthermore, when the researchers changed the type of electrical charge flowing through the material from positive to negative, the entire signal flipped its direction. This reversal confirmed that the effect was deeply tied to the nature of the electrons themselves.

The scientists observed multiple distinct dips in the voltage, suggesting that the electrons were resonating in more than one way. They measured how long it took for these spins to relax or settle back down after being flipped, finding that the process happened incredibly fast, within a few tens of picoseconds. This speed is a crucial piece of information for anyone hoping to use electron spins for ultra-fast computing. The team also developed a detailed theory to explain why this voltage was generated. They proposed that when the light hits the electrons, it pushes them into a specific alignment. As these aligned electrons bounce off imperfections in the material, they are scattered in a way that creates a net flow of charge, generating the voltage. The theory explains why the spin-flip signal opposes the normal signal: the light pushes the electrons into a different alignment when they are flipping their spins compared to when they are just absorbing energy normally.

This discovery is significant because it proves that electron spin resonance can be detected electrically in unbiased, microscopic devices without the need for large, sensitive magnetic detectors. The method works reliably in both graphene and complex structures where graphene is paired with other materials. While the researchers successfully identified the spin-flip signals and measured their speed, they noted that the exact reason for the multiple distinct resonances they observed remains a puzzle. The signals were consistent and reproducible, but the specific microscopic cause of the extra peaks is still unclear and requires further study. Nevertheless, the work establishes a powerful new tool for probing the magnetic properties of two-dimensional materials, opening a path to understanding spin behavior in the tiny devices that may one day power our world.

Technical Summary: Electron Spin Resonance Driven Photogalvanic Effect in Graphene-Based Structures

Problem and Motivation
Electron spin resonance (ESR) is a primary spectroscopic tool for probing spin states in condensed matter, traditionally detected via microwave absorption in resonant cavities. However, this method suffers from reduced sensitivity in micron-scale samples due to the signal scaling with the number of spins coupled to the cavity mode. While electrical detection of ESR (EDESR) has been developed to circumvent this limitation, it typically requires an applied bias, which can itself alter the measured response. Consequently, there is a need for an unbiased detection scheme based on dc photocurrent or photovoltage to probe spin resonances in micron-scale two-dimensional (2D) devices, particularly in graphene and transition metal dichalcogenide (TMDC)/graphene heterostructures.

Methodology
The authors investigated unbiased monolayer graphene (MLG) and WSe2/MLG heterostructures encapsulated in hexagonal boron nitride (hBN). The devices were fabricated into Hall-bar geometries on Si++/SiO2 substrates.

Experimental Setup: Measurements were conducted in a cryogen-free magnet system at temperatures around 1.7–2.3 K. The samples were irradiated with linearly polarized continuous-wave (cw) radiation in the 45–75 GHz frequency range.
Geometries: Experiments were performed in two configurations:
- Faraday geometry: Magnetic field ( $B$ ) perpendicular to the sample surface ( $B \parallel z$ ).
- Voigt geometry: Magnetic field applied in-plane along the Hall bar ( $B \parallel x$ ).
Detection: The photovoltage ( $U_{ph}$ ) was measured across source-drain contacts under unbiased conditions. The electric field of the radiation was oriented either parallel or perpendicular to the current path. Gate voltages were swept to modulate carrier density and type (n-type vs. p-type).

Key Contributions and Theoretical Framework
The paper reports the observation of an ESR-driven linear photogalvanic effect (LPGE) and develops a microscopic theory to explain the mechanism.

Microscopic Theory: The authors propose that radiation-induced momentum alignment, followed by skew scattering, generates the photocurrent.
- Drude Background: For indirect Drude-like absorption, momentum alignment occurs along the radiation electric field ( $E$ ).
- ESR Contribution: For direct spin-resonant transitions (mediated by Rashba spin-orbit coupling enabling electric dipole spin resonance), the momentum alignment occurs perpendicular to $E$ .
Opposite Signs: Because the momentum alignments for Drude absorption and ESR transitions are orthogonal, the resulting skew-scattering currents have opposite signs. The theory predicts that the resonant ESR contribution should have the opposite sign to the non-resonant Drude background.
Symmetry: The theory accounts for the $C_1$ point symmetry of the structures, which allows for skew-scattering-induced photocurrents.

Experimental Results

Resonant Features: Both MLG and WSe2/MLG structures exhibited pronounced resonant dips in the photovoltage under linearly polarized GHz radiation. These resonances were observed in both Faraday and Voigt geometries.
Linearity and Intercepts: The magnetic field positions of the resonances varied linearly with radiation frequency ( $\hbar\omega = g\mu_B B + \Delta$ ). The Landé factor was determined to be $g \approx 1.98-1.99$ . Multiple resonances were observed with different zero-field intercepts ( $\Delta$ ): approximately $-5$ to $-7 $GHz,$ 0$ GHz, and $+10.2$ GHz.
Sign Reversal:
- The resonant photocurrent consistently had the opposite sign to the non-resonant Drude background.
- Both resonant and non-resonant photocurrents reversed sign upon crossing the charge-neutrality point (CNP), consistent with the dependence on carrier charge polarity.
Spin Relaxation: From the linewidths of the resonances, spin relaxation times ( $\tau_s$ ) were extracted, ranging from approximately 18 ps to 130 ps.
Isotropy: The resonances and their characteristics were found to be nearly isotropic with respect to magnetic field orientation (Faraday vs. Voigt), with similar resonance positions and widths in both geometries.

Significance and Claims
The authors establish the photogalvanic response as a viable, unbiased probe for ESR in micron-scale graphene-based devices.

Unbiased Detection: The work demonstrates that spin resonances can be detected without an applied bias, avoiding potential perturbations to the spin system.
Mechanism Identification: The observation of opposite signs between resonant and non-resonant currents provides strong evidence for the proposed microscopic mechanism involving orthogonal momentum alignments and skew scattering.
Limitations on Origin: While the paper successfully describes the generation of the photocurrent, it remains modest regarding the microscopic origin of the multiple resonances and their specific widths. The authors note that standard explanations involving valley-Zeeman or Rashba splitting are insufficient to explain the isotropic nature and specific splitting patterns observed at high Fermi energies. They suggest that the origin of the splitting and the relatively broad linewidths (short spin lifetimes compared to state-of-the-art transport measurements) remains puzzling and requires further study, potentially involving inhomogeneous broadening or Moiré superlattice effects, but do not definitively conclude the specific cause in this work.

In conclusion, the paper provides a robust experimental and theoretical framework for detecting ESR via LPGE in unbiased 2D systems, highlighting the utility of this technique for characterizing spin dynamics in device-scale structures.

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