Electric-field control of pure spin photocurrent in germanene

TL;DR

The paper addresses electrically controlling pure spin photocurrents in time-reversal symmetric, nonmagnetic 2D materials with mirror symmetry. It develops a theoretical framework combining a Dirac Hamiltonian and nonlinear optical response theory and validates it with NEGF-DFT calculations on germanene. The authors show that an out-of-plane electric field can modulate and even reverse the pure spin photocurrent $J^{a,s^z}$ by inducing spin splitting near the Fermi energy, with further tunability via photon energy and incident angle. This work provides a viable route to electric-field-controlled spin transport in 2D nonmagnetic systems and could enable new optospintronic devices and volatile spin memories.

Abstract

The electrical control of pure spin current remains a central challenge in spintronics, particularly in time-reversal symmetric systems composed of nonmagnetic elements, where spin and electric fields interact only indirectly. In this work, we develop a theoretical framework for electrically tuning pure spin photocurrent in two-dimensional materials with time-reversal symmetry via a gate electric field. Through theoretical analysis, we demonstrate that in systems with spin-orbit coupling and in-plane mirror symmetry, an out-of-plane electric field induces spin splitting and reversal in the band structure near the Fermi energy, enabling magnitude control and direction reversal of the pure spin photocurrent. To validate this mechanism, we perform first-principles calculations on germanene, an experimentally realized two-dimensional material. Beyond amplitude modulation, we reveal that reversing the direction of the applied electric field leads to a corresponding reversal of the pure spin photocurrent. Furthermore, we show that the pure spin photocurrent can be tuned by varying the photon energy and the incident angle of light, providing additional degrees of control over spin transport. These findings establish a robust strategy for electric-field-controlled pure spin transport in two-dimensional materials, offering new possibilities for the development of optospintronic devices.

Figures (4)

• Figure 1: (a) Schematic illustration of electric-field-tuned pure spin photocurrent generation via nonlinear optical effects in two-dimensional systems with mirror symmetry $M^x$. (b) Schematic representation of a two-probe device based on germanene. The system comprises three regions: left and right leads, and a central scattering region where incident light with photon energy $E=\hbar\omega$ is applied.
• Figure 2: (a, b) Band structure of germanene for gate voltages $V_g=\pm 2$V, respectively. The color scale represents the spin polarization angle $\Phi$, with blue and red indicating spin-up and spin-down states, respectively.
• Figure 3: The spin-dependent photoresponse versus the photon energy $E_{ph}$ under vertically incident linearly polarized light irradiation when a $V_g=\pm 2$V electric field is applied. (a, b) The calculated spin-dependent photoresponse under photon polarization along the x-axis and y-axis when applying a gate voltage of $2$ V. (c, d) The calculated spin-dependent photoresponse for photon polarization along the x-axis and y-axis when applying a gate voltage of $-2$ V.
• Figure 4: (a, b) The spin-dependent photoresponse versus the gate voltage $V_g$. Here, the incident photon energy is fixed at $E_{ph}$=0.04eV, and the linearly polarized light is polarized along the (a) x-axis and (b) y-axis, respectively. (c, d) The spin-dependent photoresponse versus photon incident angle $\beta$ when applying a gate voltage of $\pm2$ V. Here, the photon energy $E_{ph}$ is equal to 0.04 eV and the linearly polarized light is polarized along the x-axis.