🔬 mesoscale physics

Floquet engineering of spin-valley selective transport in jacutingaite

This paper demonstrates that irradiating the barrier of a monolayer jacutingaite tunnel junction with off-resonant circularly polarized light enables highly efficient, tunable spin-valley selective transport, achieving near-perfect valley filtering and substantial spin polarization through photon-dressed mass terms and Fabry-Pérot interference.

Original authors: Otman Bouladiane, Kamal Azaidaoui, Clarence Cortes, David Laroze, Ahmed Jellal

Published 2026-08-26

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CC BY 4.0

Original authors: Otman Bouladiane, Kamal Azaidaoui, Clarence Cortes, David Laroze, Ahmed Jellal

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, scientists are constantly searching for materials that can carry information not just as a flow of electricity, but as a specific type of spin or a specific location within the material's atomic structure. This pursuit is driven by the need for faster, more efficient devices that consume less power. A major challenge in this field has been finding materials that are stable enough to work at room temperature while still possessing a large energy gap, a feature that allows them to act as insulators in their interior but conductors on their edges. While some materials like graphene are famous for their electronic properties, they lack the necessary internal gap to function effectively in these advanced roles. Others, like certain silicon-based structures, offer better performance but remain fragile or difficult to manufacture. Researchers are therefore turning their attention to a naturally occurring mineral called jacutingaite, which promises a robust energy gap and the ability to be peeled into thin, stable sheets, making it a promising candidate for the next generation of electronic components.

Building on this potential, a team of researchers has explored how to control the flow of electrons through a thin layer of jacutingaite using light. They set up a theoretical model of a tunnel junction, which is essentially a narrow barrier sandwiched between two leads. In their setup, the leads remain in a quiet, undisturbed state, while the central barrier is bathed in a specific type of light: circularly polarized light that is tuned to a frequency that does not directly excite the electrons but instead influences their behavior from a distance. By shining this light only on the barrier, the scientists created a situation where the electrons passing through the middle experience a different environment than those in the leads. This difference allows the light to act as a precise filter, sorting electrons based on two distinct characteristics: their spin, which is an intrinsic form of angular momentum, and their valley, which refers to the specific momentum state they occupy within the crystal lattice.

The researchers found that this light-induced filtering is remarkably effective. When the light hits the barrier, it modifies the energy landscape for the electrons in a way that depends on which valley they are in and which way their spin is pointing. Because the light interacts differently with the two valleys, it creates a situation where electrons from one valley can pass through the barrier while those from the other are blocked. Simultaneously, the material's own internal properties, combined with an external magnetic influence, allow the system to distinguish between electrons with different spins. The result is a device that can produce a current where nearly all the electrons share the same spin and the same valley, achieving a level of purity that approaches one hundred percent for the valley and reaches about seventy percent for the spin.

What makes this discovery particularly powerful is the ability to switch which type of electron is allowed to pass. By adjusting the strength of the light, the intensity of the magnetic influence, or the electric field applied to the material, the researchers showed that they could toggle the device to favor different combinations of spin and valley. This switching happens because the light effectively changes the "mass" of the electrons in the barrier, making it easier for some to cross and harder for others. The width of the barrier also plays a crucial role; as electrons travel through it, they create interference patterns similar to ripples in a pond. By fine-tuning the width of the barrier, the scientists can enhance the transmission of a specific type of electron while suppressing the others, much like tuning a radio to a specific station.

The study, which relies on detailed computer simulations of how electrons behave in this specific material, suggests that jacutingaite offers a unique platform for building future spintronic devices. Unlike other materials where light might affect all electrons equally, the specific atomic structure of jacutingaite allows the light to target specific groups of electrons with high precision. The researchers demonstrated that by combining the light with other controllable fields, they could create broad ranges of conditions where the device acts as a highly selective valve for electron flow. This work does not claim to have built a physical device yet, but it provides a clear roadmap for how such a device could function. It shows that by using light to dress the barrier in a tunnel junction, it is possible to engineer a system that sorts electrons with a degree of control that was previously difficult to achieve, opening the door to new types of electronic components that rely on the manipulation of both spin and valley degrees of freedom.

Technical Summary: Floquet Engineering of Spin-Valley Selective Transport in Jacutingaite

Problem Statement
The paper addresses the challenge of achieving tunable, high-efficiency spin and valley filtering in two-dimensional topological materials. While the Quantum Spin Hall (QSH) effect offers protected edge states, practical device applications often require bulk transport mechanisms that can be dynamically controlled. Existing materials like graphene lack sufficient spin-orbit coupling (SOC) to open a substantial gap, while others like silicene offer small gaps or require specific substrate conditions. Jacutingaite ( $Pt_2HgSe_3$ ) is identified as a promising candidate due to its predicted large bulk gap ( $\sim 0.5$ eV) and intrinsic Kane-Mele SOC. However, a systematic understanding of how to dynamically control spin-valley currents in a tunnel junction geometry—specifically where only the barrier is driven by light while leads remain in equilibrium—remains an open question. The study focuses on how off-resonant circularly polarized light can engineer effective mass terms to create selective transport channels without relying solely on static gating or edge-state conduction.

Methodology
The authors employ a theoretical framework combining Floquet theory with scattering theory to model electron transport through a monolayer jacutingaite tunnel junction.

System Geometry: A planar junction consisting of undriven source (Region I) and drain (Region III) leads sandwiching a finite barrier region (Region II, width $L$ ). The barrier is irradiated by off-resonant circularly polarized light and subjected to a scalar potential $V_0$ , a staggered sublattice potential $V_z$ , and a substrate-induced exchange field $m_s$ .
Hamiltonian: The low-energy physics is described by a Dirac Hamiltonian around the $K$ and $K'$ valleys. In the high-frequency limit, the time-dependent drive is treated using the van Vleck expansion to derive an effective static Floquet Hamiltonian. This results in a photon-dressed, valley-dependent mass term $\lambda_\omega$ that renormalizes the intrinsic Dirac mass.
Transport Calculation: Using scattering theory, the authors solve for the spinor wavefunctions in each region, matching boundary conditions at the interfaces ( $x=0$ and $x=L$ ). They derive analytical expressions for the transmission ( $T_{\eta sz}$ ) and reflection ( $R_{\eta sz}$ ) coefficients for each spin-valley channel ( $\eta = \pm 1$ for valleys, $s_z = \pm 1$ for spin).
Observables: The study computes channel-resolved transmission, reflection, and Landauer conductance ( $G_{\eta sz}$ ). Spin ( $P_s$ ) and valley ( $P_v$ ) polarizations are defined based on the conductance differences between channels.

Key Contributions and Results

Photon-Dressed Mass Engineering: The study demonstrates that off-resonant circularly polarized light induces an effective mass term $\lambda_\omega$ that shifts the Dirac masses in the barrier with opposite signs for the $K$ and $K'$ valleys. This creates a mass mismatch at the interfaces between the undriven leads and the driven barrier, which is the primary mechanism for scattering and selectivity.
Four-Channel Selectivity: Unlike systems with spin-degenerate bands (e.g., $\alpha-T_3$ lattices), jacutingaite's intrinsic Kane-Mele SOC couples the valley index to the physical electron spin. This allows the drive to produce genuine spin polarization even without an exchange field, enabling a four-channel ( $spin \times valley$ ) selectivity mechanism.
Tunable Filtering Regimes:
- Threshold Filtering: By tuning the drive amplitude ( $A_0$ ), staggered potential ( $V_z$ ), and exchange field ( $m_s$ ), the authors identify broad parameter windows where specific spin-valley channels are propagating while others are evanescent.
- Interference Control: The finite barrier width $L$ introduces channel-dependent Fabry-Pérot interference phases ( $q_{\eta sz}^x L$ ). This allows for geometric tuning where a target channel can be placed on resonance while competing channels are suppressed.
Quantitative Performance: The simulations reveal regimes with near-perfect valley filtering ( $|P_v| \simeq 100\%$ $∣ P_{v} ∣ ≃ 100%$ ) and substantial spin polarization ( $|P_s| \sim 70\%$ $∣ P_{s} ∣ \sim 70%$ ).
- Valley polarization is primarily controlled by the photon dressing and $V_z$ , which can switch the dominant valley from $K$ to $K'$ .
- Spin polarization is enhanced by the exchange field $m_s$ and the intrinsic SOC, with the sign of polarization determined by the interplay of $V_z$ and the drive.
Robustness: The selectivity persists after angular averaging (integration over transverse momentum $k_y$ ) in the conductance calculations, indicating that the effect is robust and not merely a fine-interference artifact.

Significance and Claims
The paper claims that the proposed setup offers a practical platform for dynamically controlled, highly selective spin-valley transport. The significance lies in the ability to decouple and independently tune spin and valley currents using external knobs (light amplitude, electric field, and exchange field) within a single material system.

Mechanism: The work highlights that the selectivity arises from the interplay of photon-dressed mass shifts (creating valley contrast) and exchange fields (creating spin contrast), mediated by Fabry-Pérot interference in the barrier.
Material Advantage: The authors emphasize that jacutingaite's large intrinsic SOC ( $\lambda_{so} \approx 81$ meV) is crucial, as it provides a mass splitting between spin channels that is significantly larger than in silicene, leading to broader operating windows for polarization.
Device Implications: The study positions the junction as a potential "opto-spintronic element" where light and substrate engineering steer the dominant transport channel. The authors note that while the current analysis is in the off-resonant, prethermal regime, the findings provide a foundation for future device-level spin-valley valve operations.

The paper concludes by acknowledging that future work should address finite temperature, disorder, and intermediate-frequency regimes where Floquet sidebands become populated, but maintains that the current high-frequency, static-mass approximation successfully captures the essential physics of tunable spin-valley filtering in jacutingaite.

Technical Summary: Floquet Engineering of Spin-Valley Selective Transport in Jacutingaite

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