Ordering Phenomena in Transition Metal Oxides

Magnetotransport in Photocarrier-Doped Quantum Paraelectric Semiconductors: Localization, Dimensionality, and the Extreme Quantum Limit   

We report magnetotransport properties of photogenerated electrons in undoped SrTiO₃ single crystals under ultraviolet illumination down to 2 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 SrTiO₃ allows continuous tuning of the effective electronic dimensionality by photoexcitation.

At high magnetic field, for three-dimensional photo-generated electrons, we have investigated the extreme quantum limit. This regime is distinct from conventional semiconductors, due to the large electron effective mass and large lattice dielectric constant. At low temperature, the magnetoresistance and Hall resistivity saturate at high magnetic field, deviating from conventional behavior. As a result, the Hall coefficient vanishes on the scale of the ratio of the Landau level splitting to the thermal energy, indicating the essential role of lowest Landau level occupancy, as limited by thermal broadening. Similar results found for KTaO₃ suggest that these features are robust to the details of the spin-orbit coupling or the conduction band degeneracy.