Enhanced spin-orbit coupling and charge carrier density suppression in LaAl_1-xCr_xO₃/SrTiO₃ hetero-interfaces

We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO₃/SrTiO₃ interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (n_□) drops monotonically from ∼2.2 × 10¹⁴ cm^-2 to ∼2.5 × 10¹³ cm^-2 on replacing ≈60% of the Al sites by Cr and the sheet resistance (R_□) exceeds the quantum limit for localization (h/2e²) in the concentrating range 40-60% of Cr. The samples with Cr ≤40% show a distinct minimum (T_m) in metallic R_□(T) whose position shifts to higher temperatures on increasing the substitution. Distinct signatures of Rashba spin-orbit interaction (SOI) induced magnetoresistance (MR) are seen in R_□ measured in out of plane field (H_⊥) geometry at T ≤ 8 K. Analysis of these data in the framework of Maekawa-Fukuyama theory allows extraction of the SOI critical field (H_SO) and time scale (τ_SO) whose evolution with Cr concentration is similar as with the increasing negative gate voltage in LAO/STO interface. The MR in the temperature range 8 K ≤ T ≤ T_m is quadratic in the field with a +ve sign for H_⊥ and -ve sign for H_∥. The behaviour of H_∥ MR is consistent with Kondo theory which in the present case is renormalized by the strong Rashba SOI at T < 8 K.