Controlled crossover from 2D to quasi 1D correlated electron systems via surface engineering in oxide heterostructures

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Transport properties of low-dimensional systems hold a great fascination to condensed matter physicists. In reduced dimensions novel quantum electronic states emerge due to for instance the increasing importance of fluctuations, electron correlations and disorder. The discovery of two-dimensional (2D) superconductivity coexisting with magnetism, and multiple quantum criticality at the interface between the two band insulators LaAlO3 (LAO) and SrTiO3 (STO) serve as a prominent example which attracted a lot of attention recently. Electrical transport in the low-dimensional correlated electron system forming at this interface often displays anisotropic behavior at low temperatures which is currently under intense debate and far from being understood. Beside extrinsic defect scattering an intrinsic phase separation may lead to significant anisotropy of transport coefficients. Preliminary results in our group indicate a strong correlation of the superconducting properties with the orientation of the terrace structure of the TiO2-terminated surfaces which is always present due to the inevitable miscut of the STO substrates. Superior superconducting behavior was found along the step edges between terraces and indicates a further reduction of dimensionality towards quasi 1D superconductivity. Here, the dimensionality is controlled by the ratio between the superconducting coherence length and the width w of terraces. The latter is directly related to the miscut angle of the STO substrates.The objective of our proposal is to tune quasi continuously the electronic anisotropy in STO-based 2D electron systems by varying w via the miscut angle of the STO substrates. The anticipated crossover from 2D to quasi 1D is expected to affect symmetry and quantum fluctuations at the interface and hence the Rashba-type spin-orbit coupling which is closely tied to the superconducting behavior of the correlated electron system. The long term vision of our proposal is to study these aspects in detail for the polar LAO/STO system and the nearly non-polar a-Al2O3/STO system (amorphous Al2O3 on STO) for which we established 2D superconductivity recently. By controlling the anisotropy we carry out a systematic study of the crossover from 2D to quasi 1D electronic transport in correlated electron systems. The possibility to tune dimensionality by the terrace width will allow us exploring resistive states in the crossover regime from 2D to quasi 1D superconducting samples and to shed light on the elementary topological excitations therein which is currently under debate.