Pure Spin Edge Photocurrent

• Pure spin edge photocurrent is defined as the generation of a steady-state, spin-polarized current carried exclusively by edge states under optical excitation, with negligible net charge flow.
• The phenomenon arises from the interplay of edge state dispersion, symmetry, and spin–orbit coupling in materials like topological insulators, 2D semiconductors, and altermagnets.
• Advanced computational and experimental methods, including NEGF–DFT and Floquet theory, have been employed to optimize and detect these spin currents for opto-spintronic applications.

Pure spin edge photocurrent refers to the generation of a steady-state, spin-polarized current carried exclusively by edge-localized electronic states, with vanishing net charge current, induced via optical excitation (typically by polarized light). This phenomenon is central to the non-local conversion of light into spin information at the boundary of quantum materials, including topological insulators, 2D semiconductors, and altermagnets. Distinct from bulk effects, pure spin edge photocurrents are governed by the interplay of edge state dispersion, symmetry, spin–orbit coupling, and the polarization/frequency of the incident radiation. Control and detection of such currents underpin the development of opto-spintronic interfaces, spin logic, and nonvolatile photodetectors.

1. Fundamental Mechanisms and Theoretical Frameworks

Multiple theoretical frameworks describe pure spin edge photocurrent, contingent on the underlying material class.

• Topological Insulators In quantum spin Hall (QSH) and 2D topological insulators, helical edge states with locked spin-momentum relation underlie the response. The Volkov–Pankratov Hamiltonian models a Dirac-like band inversion at an edge, supporting counterpropagating states with spins $s_z = \pm 1/2$. Electric dipole transitions between edge and bulk states are allowed only for in-plane fields orthogonal to the edge, producing pure spin current without net charge flow when the Fermi energy lies within the gap and the photon polarization conditions are met (Mahmoodian et al., 2017).
• Monolayer Semiconductors and the Photogalvanic Effect In group-V monolayer systems such as Sb and Bi, linearly polarized light in the photogalvanic regime generates spin-up and spin-down carriers with equal magnitude and opposite directions along specific crystallographic edges (e.g., zigzag). Symmetry constraints forbid charge flow but allow finite pure spin current; this is described by non-equilibrium Green's function (NEGF)–density functional theory (DFT) approaches and quantified by a bulk spin-photovoltaic tensor (Zhang et al., 2023).
• Altermagnets In $d$-wave altermagnets, the edge forms due to non-relativistic spin–splitting of the bands, and the photocurrent arises from second-order (in electric field) corrections to the distribution function. The edge breaks inversion symmetry, converting momentum alignment into a pure spin flow with analytic dependence on edge orientation, polarization angle, and frequency (Golub, 4 Dec 2025).
• Floquet Topological Systems When a QSH insulator or similar honeycomb system is irradiated with periodic drive (e.g., a mid-IR laser), Floquet–scattering theory predicts that linearly polarized light generates purely spin-polarized edge currents at zero bias due to the symmetry of the driving field—charge current vanishes under time-reversal symmetry, but spin currents can be finite if $I_\uparrow = -I_\downarrow \neq 0$ (Berdakin et al., 2020).
• Transition Metal Dichalcogenides (TMDs) In TMD monolayers, edge states with spin–valley locking support edge-localized photocurrent. Analytical $k\cdot p$ Hamiltonians with edge boundary conditions determine that specific valley-contrasting transitions under circular or linear polarization yield pure spin or spin–valley edge currents, depending on polarization and boundary parameters (Enaldiev, 2017).

2. Experimental Realization and Device Architectures

Device architectures for pure spin edge photocurrent leverage geometry and heterointerface engineering:

• Lateral Spin-Photodiodes Devices based on Fe/AlOx/p-InGaAs, incorporating a refracting-facet etched into the device sidewall, deliver circularly polarized light directly into a thin InGaAs layer beneath the Fe spin-detecting contact. Photogenerated spin-polarized carriers tunnel through crystalline AlOx into Fe, with the measurable helicity-dependent photocurrent determined by the asymmetry in unoccupied spin-resolved density of states of Fe at the injection energy (Roca et al., 2018).
• Edge-Selective Optical Pumping In 2D TIs, precise in-plane polarization of light orthogonal to a specific edge or masking techniques ensure that only one edge is pumped. This enables the isolation and detection of pure spin currents, typically by coupling the edge to a lateral quantum dot or via nonlocal electrical detection schemes (Mahmoodian et al., 2017).
• 2D Monolayer Devices Armchair and zigzag transport directions in group-V monolayers are discriminated using photoresponse anisotropy, with detection via inverse spin Hall effect or optical (Kerr rotation) imaging to resolve edge spin accumulation (Zhang et al., 2023).

3. Computational and Analytical Techniques

• Quantum Transport (NEGF–DFT) In group-V monolayers, first-principles calculations yield spin-resolved photocurrent by explicitly evaluating electron–photon self-energies and their effect on the spin-up and spin-down photocarrier distribution, with polarization encoded in the light–matter vertex (Zhang et al., 2023).
• Drift–Diffusion and Quantum Tunneling In Fe/AlOx/p-InGaAs photodiodes, a 1D drift–diffusion model for spin and charge carriers in the semiconductor, coupled with an energy-resolved quantum tunneling calculation at the ferromagnet interface, captures the magnitude and spin selectivity of the edge photocurrent. This framework self-consistently imposes boundary-matching at the tunnel barrier (Roca et al., 2018).
• Floquet Scattering Theory The effect of periodic optical driving on quantum spin Hall edge states is analyzed by constructing the Floquet Hamiltonian in extended Hilbert space (Sambe space), with scattering matrices computed between photon-dressed edge channels. This allows calculation of spin and charge pumping currents for various polarizations and drive parameters (Berdakin et al., 2020).
• Analytical $k\cdot p$ and Boltzmann Equation Methods For TMDs, analytic $k\cdot p$ models with boundary conditions classify edge states. For altermagnets, linearized Boltzmann equations with symmetry-informed velocities and distribution functions capture the pure spin edge response and its dependence on edge orientation and polarization (Golub, 4 Dec 2025, Enaldiev, 2017).

4. Symmetry, Polarization, and Material Dependence

The key determinant of pure spin edge photocurrent is symmetry:

• Symmetry Selection Rules Mirror symmetry orthogonal to the edge (group-V monolayers) forbids charge but permits spin current for certain polarizations. Time-reversal symmetry (TRS) in QSH insulators suppresses net charge current under linear polarization but allows pure spin pumping due to Kramers degenerate edge counterpropagation (Berdakin et al., 2020, Zhang et al., 2023).
• Polarization Dependence Pure spin currents require the polarization of the electromagnetic field to have a component orthogonal to the edge (QSH, altermagnets, group-V monolayers). For TMDs, linearly polarized light induces net spin current only if valley populations are equally addressed; circular polarization can be tuned to manipulate valley and spin selectivity (Enaldiev, 2017).
• Material and Edge Engineering The magnitude and direction of the photocurrent are sensitive to the type of edge termination (zigzag vs. armchair in 2D materials), the choice of interface barrier (AlOx vs. MgO), the density of states asymmetry in the ferromagnet, and the orientation of the edge with respect to principal axes in altermagnets (Roca et al., 2018, Golub, 4 Dec 2025).

5. Magnitude, Optimization, and Detection

Table: Pure Spin Edge Photocurrent in Different Systems

Material/Platform	Max Current (order)	Polarization Control
Fe/AlOx/p-InGaAs spin-PD	F ≈ 0.4% (I_ph ~ 8 μA)	Circular (σ+, σ−), Side Window
2D TI (VP model)	10⁷–10⁸ spins/s	In-plane E ⟂ edge
QSH insulator Floquet drive	~50 nA (mid-IR)	Linear (pure spin), Circular (spin-polarized charge)
Group-V monolayers	~0.07 a₀²/photon	Linear, along zigzag
Altermagnet (d-wave)	10⁻⁷–10⁻⁶ A/m	Linear (E ⟂ edge), θ-dep
TMDs (MoS₂, etc.)	μA (predicted)	Linear (J_s), Circular (J_v, J_s)

Magnitude is controlled by several design variables:

• Edge and bulk DOS asymmetries (interface and ferromagnet selection)
• Spin relaxation and recombination times
• Doping and electrostatics
• Edge orientation (especially in non-symmorphic or d-wave systems)
• Polarization and frequency of incident light

Detection strategies include electrical readout under magnetic field (enabling spin–charge conversion), optical detection of edge spin accumulation (Kerr rotation, polarized luminescence), and spin Hall voltage measurement across heavy-metal contacts.

6. Applications and Technological Relevance

Pure spin edge photocurrent fundamentally enables on-chip, all-optical spin injection and detection with no net charge transfer, crucial for nonvolatile memory, opto-spintronic logic, and photodetector design in two-dimensional materials. Spin–current manipulation via light polarization provides dynamic control and switching capabilities. Materials such as half-metallic Heusler alloys, 2D ferroelectrics, and high–Curie temperature group-V monolayers are promising candidates for device implementation, given their sizeable spin photovoltages and robust operation at ambient conditions (Roca et al., 2018, Zhang et al., 2023).

7. Perspectives and Future Directions

Current limitations include the modest spin–charge conversion efficiency—dominated by the low unoccupied spin polarization in conventional ferromagnets (e.g., Fe)—and constraints imposed by edge disorder or intervalley scattering. Future development hinges on exploiting materials with large spin polarization at the relevant chemical potentials (e.g., Heusler alloys in photodiodes), maximizing interface quality, and integrating complementary detection schemes (e.g., heavy-metal contacts and optical probes). Floquet engineering and stackable 2D heterostructures offer pathways to tune and enhance pure spin edge photocurrents, with targeted applications in ultrafast, energy-efficient optospintronics (Berdakin et al., 2020, Golub, 4 Dec 2025).