Controlled crossover from 2D to quasi 1D correlated electron systems via surface engineering in oxide heterostructures
Final Report Abstract
In the funded project, the main objectives were related to first: a continuous tuning of the anisotropic electronic transport by varying the terrace width w of vicinal TiO2-terminated SrTiO3 (STO) substrates and second: the novel physics which may appear due to the symmetry lowering, especially with respect to the Rashba-type spin-orbit coupling (SOC). A systematic study on the influence of the vicinal substrate miscut on the anisotropic 2D electronic transport in AO/STO heterostructures was carried out. To this end, the vicinal miscut angle γ of the Ti-STO substrates, which were used for the AO film deposition, were varied from 0.1° to 6°. Measurements of the in-plane sheet resistance Rs(ϕ) of the 2D electron system were done for different current flow directions ϕ using patterned microbridges. Anisotropic transport evolves below 30 K. Analysis of Rs(ϕ) reveals impurity scattering by lattice dislocations of bulk STO and interfacial scattering by the step-edges due to the vicinal substrate miscut as the main reason for the anisotropic behavior. The anisotropic contribution to the resistance caused by interfacial scattering, Rt, systematically increases with increasing γ and decreasing terrace width w. In comparison to a standard substrate miscut of 0.1°(w ≈ 224 nm), the amplitude of Rt is increased by about one order of magnitude for γ = 6° (w ≈ 18 nm). However, the total anisotropy Rmax/Rmin only increases by a factor of 2.6. The influence of γ on Rmax/Rmin is notably reduced by the occurrence of step-bunching and lattice-dislocations in the STO substrate material. Step-bunching limits the terrace width, and anisotropic scattering by lattice dislocation may diminish or even overcompensate the influence of interfacial scattering. Therefore, an accurate tuning of the anisotropic transport by γ is hampered. The magnetoresistance MR increases with decreasing temperature T and increasing magnetic field B and reaches values up to about 20%. The positive MR is well described by classical Lorentz scattering and weak-antilocalization (WAL) of the corelated 2D electron system. In addition, below 30 K, MR becomes anisotropic at large fields (B > 3 T). With increasing γ the anisotropy increases and the field-dependence of MR becomes more linear, indicating enhanced disorder and anisotropic behavior of the 2D electron system. From the WAL, the inelastic field Bi and the spin-orbit field Bso which are related to the corresponding electron relaxation times were deduced. The Rashba-type SOC, which is characterized by the D`yakonov-Perel (DP) spin-relaxation mechanism, does not show significant anisotropic behavior with respect to the current flow direction ϕ or the anisotropic electronic transport in AO/STO. A distinct relation of the Rashba-type SOC to the miscut angle γ of the vicinal STO substrates is thus not evident. Even though there is an anisotropic behavior of Bso, the reason for that is primarily related to spin-flipping by impurity scattering, i. e., Elliott-Yafet (EY) spin-relaxation mechanism. The charge carrier mobility µ and Rs affect Bso which prevents a more detailed analysis of the Rashbatype SOC. Nevertheless, for γ > 2°, SOC displays deviations from standard behavior which might be taken as an indication for a beginning crossover from a 2D to a quasi 1D electronic transport. The values of Bso for the AO/STO heterostructures vary between 0.5 and 2 T and are well comparable to those observed for LAO/STO. Assuming similar SOC coupling in both kinds of heterostructures, the polar mismatch at the interface, responsible for the build-in electric field and Rashba effect, is expected to be comparable alike.
Publications
- Anisotropic electronic transport of the two-dimensional electron system in Al2O3/SrTiO3 heterostructures, Phys. Rev. B 95, 245132 (2017)
K. Wolff, R. Schäfer, R. Schneider, M. Meffert, D. Gerthsen, and D. Fuchs
(See online at https://doi.org/10.1103/PhysRevB.95.245132) - Patterning of twodimensional electron systems in SrTiO3 based heterostructures using a CeO2 template, AIP Advances 7, 056410 (2017)
D. Fuchs, K. Wolff, R. Schäfer, R. Thelen, M. Le Tacon and R. Schneider
(See online at https://doi.org/10.1063/1.4973696) - Anisotropic electronic transport and Rashba effect of the two-dimensional electron system in (110) SrTiO3-based heterostructures Phys. Rev. B 98, 125122 (2018)
K. Wolff, R. Eder, R. Schäfer, R. Schneider, and D. Fuchs
(See online at https://doi.org/10.1103/PhysRevB.98.125122) - Anisotropic electronic transport of the two-dimensional electron system in Al2O3-SrTiO3 heterostructures, Dissertation, Faculty of Physics, Karlsruhe Institute of Technology (2018)
K. Wolff
- Tuning the superconducting transition of SrTiO3- based 2DEGs with light, Appl. Phys. Lett. 115, 122601 (2019)
D. Arnold, D. Fuchs, K. Wolff and R. Schäfer
(See online at https://doi.org/10.1063/1.5119417)
