Interplay between symmetry breaking distortions and superconductivity in layered oxides

1986年，La2-xBaxCuO4 Ruddlesden-Popper （RP）体系中高温超导性（HTS）的发现激发了大量研究，以了解这种非常规超导性的起源。导致铜酸盐中这种现象的潜在机制无法用BCS理论来解释，该理论认为，传统的超导性是由于电子之间的小吸引力相互作用产生的，导致它们形成库珀对并凝结。尽管在了解这些铜酸盐表现出的丰富的不寻常性质方面进行了大量的研究，但目前还没有能够解释它们所有性质的微观理论。有人提出，超导性是由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及其与晶格的耦合仍然难以捉摸。RP铜酸盐的超导性似乎只出现在正交对称的单一空间群中，并在低温下结构相变到四方相时消失。在这个项目中，我们使用基于第一原理计算的理论和模拟来研究低温下从正交向四方的相变。这将有助于更好地了解晶格耦合的性质，从而在正交体系中产生超导性。

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.