Novel Electronic State and Superconductivity in the Electron-Doped High-Tc T'-Superconductors

TL;DR

This review discusses recent findings on undoped superconductivity in electron-doped T'-cuprates, highlighting the transition from localized to metallic states and proposing an electronic structure model for understanding this phenomenon.

Contribution

It introduces a new electronic structure model explaining undoped superconductivity in T'-cuprates, emphasizing the role of reduction annealing and magnetic order.

Findings

Reduction annealing transforms localized carriers into a metallic, superconducting state.

Undoped T'-cuprates exhibit short-range magnetic order linked to excess oxygen.

The electronic structure model accounts for undoped superconductivity in these materials.

Abstract

In this review article, we show our recent results relating to the undoped (Ce-free) superconductivity in the electron-doped high-Tc cuprates with the so-called T' structure. For an introduction, we briefly mention the characteristics of the electron-doped T'-cuprates, including the reduction annealing, conventional phase diagram and undoped superconductivity. Then, our transport and magnetic results and results relating to the superconducting pairing symmetry of the undoped and underdoped T'-cuprates are shown. Collaborating spectroscopic and nuclear magnetic resonance results are also shown briefly. It has been found that, through the reduction annealing, a strongly localized state of carriers accompanied by an antiferromagnetic pseudogap in the as-grown samples changes to a metallic and superconducting state with a short-range magnetic order in the reduced superconducting samples. The formation of the short-range magnetic order due to a very small amount of excess oxygen in the reduced superconducting samples suggests that the T'-cuprates exhibiting the undoped superconductivity in the parent compounds are regarded as strongly correlated electron systems, as well as the hole-doped high-Tc cuprates. We show our proposed electronic structure model to understand the undoped superconductivity. Finally, unsolved future issues of the T'-cuprates are discussed.