Reassessment of the electronic state, magnetism, and superconductivity in high-Tc cuprates with the Nd2CuO4 structure

Abstract

The electronic phase diagram of the cuprates remains enigmatic and is still a key ingredient to understand the mechanism of high-T c superconductivity. It has been believed for a long time that parent compounds of cuprates were universally antiferromagnetic Mott insulators (charge-transfer insulators) and that high-T c superconductivity would develop upon doping holes or electrons in a Mott-Hubbard insulator ("doped Mott-insulator scenario"). However, our recent discovery of superconductivity in the parent compounds of square-planar cuprates with the Nd 2 CuO 4 (T') structure and the revised electronic phase diagram in T' cuprates urged a serious reassessment to the above scenario. In this review, we present the main results derived from our synthesis and experiments on T' cuprates in the undoped or heavily underdoped regime over 20 years, including material issues and basic physics. The key material issue is how to remove excess oxygen ions at O ap without introducing oxygen vacancies in the CuO 2 planes. In order to put this into practice, the basic knowledge of complex solid-state chemistry in T' cuprates is required, which is also included in this review.

Evolution of charge order topology across a magnetic phase transition in cuprate superconductors

“…In contrast to widely studied Ce-doped superconductors (T -Ln 2−x Ce x CuO 4 ), thin films of T -Ln 2 CuO 4 can host superconductivity without cation substitution, when subjected to post-growth reduction annealing procedures [26]. The possibility of superconductivity in the undoped limit questions the very Mott nature of the parent T -cuprates, and alternative scenarios involving a Slater mechanism have been proposed [26,27]. On the other hand, recent angle-resolved photoemission spectroscopy (ARPES), X-ray photoemission spectroscopy (XPS), and Xray absorption spectroscopy (XAS) studies concluded that superconducting T -Ln 2 CuO 4 are actually electron-doped, presumably due to oxygen vacancies [28][29][30].…”

“…In this paper, we investigate the evolution of the Q-space topology of charge correlations in T -Ln 2 CuO 4 (Ln=Nd, Pr) thin films as a function of electron doping. In contrast to widely studied Ce-doped superconductors (T -Ln 2−x Ce x CuO 4 ), thin films of T -Ln 2 CuO 4 can host superconductivity without cation substitution, when subjected to post-growth reduction annealing procedures [26]. The possibility of superconductivity in the undoped limit questions the very Mott nature of the parent T -cuprates, and alternative scenarios involving a Slater mechanism have been proposed [26,27].…”

Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxides

“…Several recent reviews have given excellent discussions of the experimental developments on the electron-doped cuprates [54][55][56] . There is general agreement that the difference between the electron-and hole-doped cuprates originates from their different crystal structures (T in the former versus predominantly T in the latter).…”

“…Developing a theory for the electron-doped systems has been difficult, as some of the experimental observations are universally shared between electron and hole-doped materials, while others are unique to one or the other family. The situation has become more confusing in recent years, as with the development of specialized reduction annealing processes the boundary between the AFM and the superconducting phase has shifted to smaller and smaller doping concentrations, and even the completely undoped T compounds have been found to be superconducting 55 . In the following we make distinction between "conventionally annealed" and "specially annealed" T compounds.…”

Природа Псевдощелевой Фазы ВТСП Купратов

“…Очевидно, что эти родительские T ′ -купраты находятся вблизи порога диспропорционирования и перехода к системе EH-димеров. Однако, диэлектрическое антиферромагнитное состояние в объемных образцах этих T ′ -купратов стабилизируется остаточным " апикальным" кислородом, который не удается полностью удалить [25]. Усовершенствование техники осаждения и отжига позволило недавно получить тонкие пленки родительских T ′ -купратов R 2 CuO 4 (R = Pr, Nd, Sm, Eu, Gd) без " апикального" кислорода, которые являются металлическими и сверхпроводящими при температурах ≤ 30 K [25].…”