Interplay between symmetry breaking distortions and superconductivity in layered oxides

1986年在La2-xBaxCuO4 Ruddlesden-Popper(RP)体系中发现了高温超导电性(HTS)，这激发了人们对这种非常规超导电性起源的大量研究。导致铜酸盐中这种现象的潜在机制不能用BardeenCooper-Schrieffer(BCS)理论来解释，该理论认为传统的超导电性是由于电子之间的小吸引相互作用而产生的，使它们形成库珀对并凝聚。尽管进行了大量的研究工作来了解这些铜酸盐所展示的丰富的不同寻常的性质，但目前还没有微观理论可以解释它们的所有性质。超导电性是由Jahn-Teller极化子介导的电子-声子耦合引起的，但这些准粒子的确切性质以及它们与晶格的耦合仍然难以捉摸。在低温下，RP铜酸盐的超导电性似乎只出现在正交对称的单一空间群中，当结构相变为四方相时，超导电性消失。在这个项目中，我们使用理论和基于第一性原理计算的模拟来研究低温下从正交到四方相的相变。这应该有助于更好地洞察在正交体系中引起超导的晶格耦合的本质。

The discovery of high-temperature superconductivity (HTS) in the La2-xBaxCuO4 Ruddlesden-Popper (RP) system in 1986 has stimulated a tremendous amount of research in understanding the origin of this unconventional superconductivity. The underlying mechanisms leading to such phenomena in the cuprates cannot be explained by BardeenCooper-Schrieffer (BCS) theory, which argues that conventional superconductivity arises due to small attractive interactions between electrons, causing them to form Cooper pairs and condense. Despite intensive research efforts in understanding the wealth of unusual properties these cuprates exhibit, there currently exists no microscopic theory which can account for all of their properties. It has been proposed that superconductivity arises due to electron-phonon coupling mediated by Jahn-Teller polarons, but the exact nature of these quasiparticles and their coupling to the lattice remains elusive. Superconductivity in the RP cuprates only seems to arise in a single space group of orthorhombic symmetry and vanishes upon a structural phase transition to a tetragonal phase at low temperatures. In this project, we use theory and simulation based on firstprinciples calculations to investigate the phase transition from orthorhombic to tetragonal at low temperatures. This should help gain better insight into the nature of the lattice coupling giving rise to superconductivity in the orthorhombic system.