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

低维系统的输运性质一直是凝聚态物理学家的研究热点。在降低的维度中，由于例如波动、电子相关和无序的重要性增加，新的量子电子态出现。二维超导与磁性共存，以及在两个带绝缘体LaAlO_3（LAO）和SrTiO_3（STO）之间的界面处的多量子临界性的发现是近年来引起广泛关注的一个突出例子。在该界面处形成的低维关联电子系统中的电输运在低温下通常表现出各向异性行为，这是目前激烈争论的问题，远未被理解。除了非本征缺陷散射外，本征相分离也会导致输运系数的显著各向异性。我们小组的初步结果表明，由于STO衬底的不可避免的误切，TiO 2终止表面的平台结构的取向与超导性能具有很强的相关性。沿着台阶边缘发现了上级超导行为，并表明进一步减少维数向准一维超导。在这里，维度是由超导相干长度和台阶的宽度w之间的比率控制。后者与STO衬底的错切角直接相关。我们的目标是通过改变STO衬底的错切角来准连续地调节基于STO的2D电子系统中的电子各向异性。从2D到准1D的预期交叉预计会影响界面处的对称性和量子涨落，从而影响与相关电子系统的超导行为密切相关的Rashba型自旋轨道耦合。我们建议的长期愿景是详细研究这些方面的极性LAO/STO系统和近非极性的a-Al 2 O3/STO系统（STO上的无定形Al 2 O3），我们最近建立了二维超导。通过控制各向异性，我们对关联电子系统中从二维到准一维的电子输运交叉进行了系统的研究。的可能性，以调整维度的梯田宽度将使我们能够探索电阻状态的交叉制度，从2D到准1D超导样品，并阐明其中的基本拓扑激发，这是目前正在辩论中。

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.