Metal-Insulator Transition in Copper Oxides Induced by Apex Displacements

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 La 2 CuO 4 (LCO) with Sr (LSCO), or electron doped, such as Nd 2 CuO 4 (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 oxygens 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 (A-O) in LCO affects the optical gap, spin and charge susceptibilities, and superconducting order parameter. By combining quasiparticle self-consistent GW (QSGW ) and dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small displacements of the apical oxygens drive the compound to a metallic state through a localization/delocalization transition, with a concomitant maximum d-wave order parameter at the transition. We address the question whether NCO can be seen as the limit of LCO with large apical displacements, and elucidate the deep physical reasons why the behaviour of NCO is so different than the hole doped materials. We shed new light on the recent correlation observed between T c and the charge transfer gap, while also providing a guide towards the design of optimized high-Tc superconductors. Further our results suggest that strong correlation, enough to induce Mott gap, may not be a prerequisite for high-Tc superconductivity.

Electron–phonon-driven three-dimensional metallicity in an insulating cuprate

“…The compound with undisplaced unit cell has been recently characterized by the local density approximation (LDA) and the QSGW +DMFT methods (see Ref. [5]). The LDA bands at the Fermi level consist of strongly mixed (3:2) Cu-3d x 2 −y 2 and O-2p x , p y orbitals.…”

“…We modelled the optical data of LCO by combining the QSGW theory and DMFT calculations [5], as implemented in the Questaal package [65,66].…”

“…Recently, this purely electronic scenario has been challenged by a body of work. On the theory side, it is believed that the selective modification of bond lengths and angles can trigger a localization-delocalization transition in the undoped parent compounds [3,4], or even lead to a concomitant increase of the superconducting T C [5]. On the experimental side, the interplay between the electron-electron and the electron-phonon interaction has been proposed as an efficient pathway to stabilize superconductivity [6][7][8][9].…”

“…While this intertwined character of different interactions makes cuprates excellent candidates to benchmark new theories in correlated-electron physics, it also renders these solids a puzzling case to understand [10]. First-principles theoretical descriptions that deal with strong correlations are notoriously difficult to handle, and only now powerful methods are becoming available that render the problem tractable on modern computers [5,11]. At the same time, experimental progress in disentangling intricate interactions relies on the development of novel spectroscopic techniques.…”

Quantum criticality associated with dimensional crossover in the iso-electronic series Ca2−x Sr x RuO4

“…We fit ImΣ(iω) to a fourth order polynomial in iω. The mass enhancement, related to the coefficient (s 1 ) of the linear term in the expansion * = + m m s 1 1 [43,44], increases with lowering of temperature (figure 2(c)). It can be seen that the xy orbital has a larger mass enhancement than the xz and yz orbitals [13,16], and concomitantly, the magnitude of the intercept of ImΣ(iω) at ω=0 for xy orbital is larger than that for the other two (figure 2(c)).…”