Optically tuned dimensionality crossover in photocarrier-dopedSrTiO3: Onset of weak localization

We report magnetotransport properties of photogenerated electrons in undoped ${\mathrm{SrTiO}}_{3}$ single crystals under ultraviolet illumination down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. By tuning the light intensity, the steady-state carrier density can be controlled, while tuning the wavelength controls the effective electronic thickness by modulating the optical penetration depth. At short wavelengths, when the sheet conductance is close to the two-dimensional Mott minimum conductivity, we have observed critical behavior characteristic of weak localization. The negative magnetoresistance at low magnetic field is highly anisotropic, indicating quasi-two-dimensional electronic transport. The high mobility of photogenerated electrons in ${\mathrm{SrTiO}}_{3}$ allows continuous tuning of the effective electronic dimensionality by photoexcitation.