Abstract
In this review article, we show our recent results relating to the undoped (Ce-free) superconductivity in the electron-doped high- T c 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- T c cuprates. We show our proposed electronic structure model to understand the undoped superconductivity. Finally, unsolved future issues of the T’-cuprates are discussed.
Highlights
In the past three decades, cuprates exhibiting high-temperature superconductivity have been intensively studied
Following observations of the undoped (Ce-free) superconductivity in the parent compounds [18,19] and the suggestion of a new phase diagram shown in Figure 1b [20] opened a new era of research in the high-Tc superconductivity
Under the strong electron correlation generating the Mott–Hubbard gap between the upper Hubbard band (UHB) and the lower Hubbard band (LHB) of the Cu 3dx2−y2 orbital, calculations based on the local density approximation (LDA) combined with the dynamical mean-field theory (DMFT) revealed the possibility of the closing of the so-called charge-transfer (CT) gap between UHB of the Cu 3dx2−y2 orbital and the O 2p band observed in the hole-doped cuprates [41,42]
Summary
In the past three decades, cuprates exhibiting high-temperature superconductivity have been intensively studied. The doping of electrons into the parent compounds weakens the AF order, but the AF order is maintained around the optimally doped regime. Following observations of the undoped (Ce-free) superconductivity in the parent compounds [18,19] and the suggestion of a new phase diagram shown in Figure 1b [20] opened a new era of research in the high-Tc superconductivity. We present the characteristics of the electron-doped T’-cuprates including the reduction annealing, conventional phase diagram and undoped superconductivity. Note that there are already excellent and complete reviews on the materials and physics focusing on the conventional electron-doped cuprates [24,25]
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