Circular Photogalvanic Effect in Oxide Two-Dimensional Electron Gases

Two-dimensional electron gases (2DEGs) at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface have attracted wide interest, and some exotic phenomena are observed, including 2D superconductivity, 2D magnetism, and diverse effects associated with Rashba spin-orbit coupling. Despite the intensive investigations, however, there are still hidden aspects that remain unexplored. For the first time, here we report on the circular photogalvanic effect (CPGE) for the oxide 2DEG. Spin polarized electrons are selectively excited by circular polarized light from the in-gap states of ${\mathrm{SrTiO}}_3$ to 2DEG and are converted into electric current via the mechanism of spin-momentum locking arising from Rashba spin-orbit coupling. Moreover, the CPGE can be effectively modified by the density and distribution of oxygen vacancies. This Letter presents an effective approach to generate and manipulate the spin polarized current, paving the way toward oxide spintronics.

Figure 1

(a) Sketches for the CPGE measurements. Circularly polarized incident light is introduced into the 2DEG sample with two electrodes arranged along the $x$ axis. The polar angle ${\theta }_0$ and azimuth angle $\varphi$ of the incident light are defined in the figure. (b) A schematic diagram for the mechanism of CPGE. The Rashba effect occurs at the $\mathrm{LAO}/\mathrm{STO}$ interfaces where the broken spatial symmetry removes $\mathit{k}$-space spin degeneracy of the $3d$ band structure. The energy level of oxygen vacancies locates in the band gap of STO. The circularly polarized light will selectively excite electrons in defect levels to the specific conduction band (black arrow), generating a transverse spin polarized electric current along the $x$ axis in real space. (c),(g) Photovoltage as a function of laser polarization for (111)-$\mathrm{LAO}/\mathrm{STO}$ 2DEG with a carrier density of $4.9\times {10}^{14}{\mathrm{cm}}^{-2}$, with differently directed incident light. Dots are experimental data, and lines are data fitting to Eq. (1). The power of the light is 27 mW. All measurements were conducted at room temperature. (d)–(f) illustrate the distribution of photogenerated electrons, which are excited by left-circular light from different directions, in reciprocal (upper panel) and real (bottom panel) spaces.

Figure 2

$V_{\mathrm{CPGE}}$ as a function of incident angle ${\theta }_0$. $V_{\mathrm{CPGE}}$ is different oriented samples. Solid symbols are experimental data and solid curves are results of curve fitting based Eq. (7).

Figure 3

(a) CPGE as a function of carrier densities for (111)-$\mathrm{LAO}/\mathrm{STO}$. The obvious increase in $V_{\mathrm{CPGE}}$ takes place when $n_s$ exceeds ${10}^{15}{\mathrm{cm}}^{-2}$. (b) Photovoltages as a function of $\alpha$, collected for two samples with different carrier densities. Light power is 27 mW.

Figure 4

(a) $V_{\mathrm{CPGE}}$ as a function of gate voltage. Inset: a schematic diagram for experiment setup. (b) Photovoltages as functions of α, collected under the gate voltages of $-20$ and 40 V, respectively. Solid symbols are experimental results and solid curves are results of curve fitting based on Eq. (1).