$PT$-Symmetry-Enabled Spin Circular Photogalvanic Effect in Antiferromagnetic Insulators

The short timescale spin dynamics in antiferromagnets is an attractive feature from the standpoint of ultrafast spintronics. Yet generating highly polarized spin current at room temperature remains a fundamental challenge for antiferromagnets. We propose a spin circular photogalvanic effect (spin CPGE), in which circularly polarized light can produce a highly spin-polarized current at room temperature, through an “injection-current-like” mechanism in parity-time ($PT$)-symmetric antiferromagnetic (AFM) insulators. We demonstrate this effect by first-principles simulations of bilayer ${\mathrm{CrI}}_3$ and room-temperature-AFM hematite. The spin CPGE is significant, and the magnitude of spin photocurrent is comparable with the widely observed charge photocurrent in ferroelectric materials. Interestingly, this spin photocurrent is not sensitive to spin-orbit interactions, which were regarded as fundamental mechanisms for generating spin current. Given the fast response of light-matter interactions, large energy scale, and insensitivity to spin-orbit interactions, our work gives hope to realizing fast-dynamic and temperature-robust pure spin current in a wide range of $PT$-symmetric AFM materials, including topological axion insulators and weak-relativistic magnetic insulators.

Figure 1

(a) Doubly degenerate bands due to $PT$ symmetry. (b) The transport direction of the two opposite directions of spins due to $PT$ symmetry after pumping. The blue and red ovals represent the contour plot of the two spin components’ conduction bands, and the black arrows show the overall spin-current direction. (c) The schematic of circularly polarized light-induced spin current under circularly polarized light. (d) The circularly polarized light generated electron and hole travel in opposite directions. Here, we define the light generated spin-down electron and -down hole pair. Solid and open circles represent electrons and holes, respectively. The arrows of the circles represent spin directions. (e) The proposed experimental setup. The spin-down hole will annihilate with a spin-down electron after being injected into the FM material.

Figure 2

The atomic structure (a) and band structure (b) of bilayer AFM ${\mathrm{CrI}}_3$. (c) The photodriven spin-current conductivity along the in-plane $x$ and $y$ direction for spin $S_z$. (d) The photodriven charge-current conductivity along the $x$ and $y$ direction. (e) and (f) The spin-current direction for AFM bilayer ${\mathrm{CrI}}_3$ (side view of Cr atoms) with opposite Néel vector directions.

Figure 3

(a) The $y$-direction photodriven spin current of the $S_z$ component with full SOC (gray solid line) and extremely weak SOC (dashed line), i.e., SOC strength $\lambda =0.001{\lambda }_{\mathrm{so}}$. The spin texture of valence bands at $E=-0.17\mathrm{eV}$ in Fig. 2 with full SOC (b) and extremely weak SOC (d). The spin photoconductivity distribution in $k$ space with full SOC (c) and extremely weak SOC (e).

Figure 4

(a) The atomic structure of bulk $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, the dashed circle being the $PT$-symmetry center. The spin $S_z$ component photoconductivity for the polarization of circularly polarized light in $yz$ plane (b) and in $xy$ plane (c). The spin photoconductivity distribution in $k$ space for (${\eta }_{xy,z}^{\circlearrowright ,Sz}$) (d).