Metal-Insulator Transition in Copper Oxides Induced by Apex Displacements

• Published in: Physical review. X Vol. 8; no. 2; p. 021038
• Main Authors: Acharya, Swagata, Weber, Cédric, Plekhanov, Evgeny, Pashov, Dimitar, Taraphder, A., Van Schilfgaarde, Mark
• Format: Journal Article
• Language: English
• Published: College Park American Physical Society 10.05.2018
• Subjects: Absolute zero; Antiferromagnetism; Charge transfer; Copper; Copper oxides; Cuprates; Design optimization; Displacement; Electronic properties; Elementary excitations; First principles; High temperature; High temperature superconductors; Insulators; Mean field theory; Metal-insulator transition; Order parameters; Oxygen atoms; Phase diagrams; Physicists; Superconductivity; Transition temperature
• ISSN: 2160-3308; 2160-3308
• DOI: 10.1103/PhysRevX.8.021038

High temperature superconductivity has been found in many kinds of compounds built from planes of Cu and O, separated by spacer layers. Understanding why critical temperatures are so high has been the subject of numerous investigations and extensive controversy. To realize high temperature superconductivity, parent compounds are either hole doped, such asLa2CuO4(LCO) with Sr (LSCO), or electron doped, such asNd2CuO4(NCO) with Ce (NCCO). In the electron-doped cuprates, the antiferromagnetic phase is much more robust than the superconducting phase. However, it was recently found that the reduction of residual out-of-plane apical oxygen dramatically affects the phase diagram, driving those compounds to a superconducting phase. Here we use a recently developed first-principles method to explore how displacement of the apical oxygen (AO) in LCO affects the optical gap, spin and charge susceptibilities, and superconducting order parameter. By combining quasiparticle self-consistent GW (QS GW) and dynamical mean-field theory (DMFT), we show that LCO is a Mott insulator, but small displacements of the apical oxygen drive the compound to a metallic state through a localization-delocalization transition, with a concomitant maximum ind-wave order parameter at the transition. We address the question of whether NCO can be seen as the limit of LCO with large apical displacements, and we elucidate the deep physical reasons why the behavior of NCO is so different from the hole-doped materials. We shed new light on the recent correlation observed betweenTcand the charge transfer gap, while also providing a guide towards the design of optimized high-Tcsuperconductors. Further, our results suggest that strong correlation, enough to induce a Mott gap, may not be a prerequisite for high-Tcsuperconductivity.