Abstract
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 as (LCO) with Sr (LSCO), or electron doped, such as (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 in -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 between and the charge transfer gap, while also providing a guide towards the design of optimized high- superconductors. Further, our results suggest that strong correlation, enough to induce a Mott gap, may not be a prerequisite for high- superconductivity.
- Received 11 December 2017
- Revised 24 March 2018
DOI:https://doi.org/10.1103/PhysRevX.8.021038
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
High-temperature superconductors conduct electricity with zero resistance at temperatures well above absolute zero but below some critical temperature (). Finding a way to push to higher values opens a path to superconductors that operate at more practical temperatures. However, finding a single parameter that can control continues to elude physicists. Here, we show how the bond length between copper and oxygen atoms in copper-oxide-based superconductors can alter the critical temperature.
We focus on , a parent compound of some copper-based high-temperature superconductors. Taking a realistic state-of-the-art theoretical approach, we look at how the electronic properties change as one of the oxygen atoms (known as the apical oxygen, based on its geometric position) is displaced from its equilibrium position. We find that this alteration changes the material from an insulator to a metal. At this transition, the spin fluctuates most violently, which in turn increases to its maximum.
Our results open new avenues for controlling by inducing local distortions via pump-probe optics or terahertz radiation. Future calculations will provide a more substantial platform for this goal by including effects that come into play when the system is far from equilibrium.
