Superconductivity in novel surface and interface electronic systems

定制相关的界面和表面系统的可能性打开了一系列新的和复杂的电子行为，包括超导的新的迷人的例子。最近的例子有氧化物异质结构中300mK附近的超导电性[1，2]或绝缘和金属镧铜氧化物之间的高温界面超导[3]。显然，两个材料系统的结合不仅允许访问以前未探索的参数区域--例如异质结构中的高迁移率低密度电子气--而且还可能产生在单独的组成材料中找不到的多体状态--例如观察到的超导电性。这就提出了一个问题，在这种不均匀的情况下，人们是否可以想到新的例子，也可能想到新的超导物理机制，这可能会导致更高的转变温度或其他改进，如更高的临界电流密度。为了进一步探讨这些问题，为整体或孤立的二维(2D)问题开发的理论方法必须适用于新的系统。在这里，我们计划为两个现代而强大的理论框架实现这一点：(I)在变分团簇近似(VCA)中评估的强关联电子的自能泛函方法(SFA)，以及(Ii)用于研究长程有序趋势的弱到中等相互作用的泛函重整化群(FRG)。有了这些资源，我们计划在异质结构、表面系统和其他层状结构中寻找新的超导现象和机制。特别是，我们计划在这一背景下仔细审查最近的两个理论提议，并旨在提出实现新颖的、可能是高温超导的其他想法。

The possibility to tailor correlated interface and surface systems opens a wide range of novel and complex electronic behavior, including new fascinating examples of superconductivity. Recent cases are the superconductivity around 300mK in oxide heterostructures [1, 2] or the high-temperature interface superconductivity between insulating and metallic lanthanum copper oxides [3]. Apparently, the combination of two material systems not only allows to access previously unexplored parameter regions - such as high-mobility low-density electron gases in heterostructures - but can also give rise to many-body states not found in the separate constituent materials - such as the observed superconductivity. This raises the question whether one can think of new examples and possibly also of new physical mechanisms of superconductivity in such inhomogeneous situations that could lead to higher transition temperatures or other improvements such as higher critical current densities.In order to pursue these questions further, theoretical methods developed for bulk or isolated two-dimensional (2D) problems have to be adapted for the new systems. Here we plan to accomplish this for two modern and powerful theoretical frameworks: (i) the self-energy functional approach (SFA) for strongly correlated electrons, evaluated within the variational cluster approximation (VCA), and (ii) the functional renormalization group (fRG) for weak to moderate interactions for the study of long-range ordering tendencies. With these resources at hand, we plan to search for novel occurrences and mechanisms of superconductivity in heterostructures, surface systems and other layered structures. In particular, we plan to scrutinize two recent theoretical proposals in this context, and aim to come up with additional ideas for realizations of novel, possibly high-temperature superconductivity.