Abstract
Within the cuprate constellation, one fixed star has been the superconducting dome in the quantum phase diagram of transition temperature vs. the excess charge on the Cu in the CuO2-planes, p, resulting from O-doping or cation substitution. However, a more extensive search of the literature shows that the loss of the superconductivity in favor of a normal Fermi liquid on the overdoped side should not be assumed. Many experimental results from cuprates prepared by high-pressure oxygenation show Tc converging to a fixed value or continuing to slowly increase past the upper limit of the dome of p = 0.26–0.27, up to the maximum amounts of excess oxygen corresponding to p values of 0.3 to > 0.6. These reports have been met with disinterest or disregard. Our review shows that dome-breaking trends for Tc are, in fact, the result of careful, accurate experimental work on a large number of compounds. This behavior most likely mandates a revision of the theoretical basis for high-temperature superconductivity. That excess O atoms located in specific, metastable sites in the crystal, attainable only with extreme O chemical activity under HPO conditions, cause such a radical extension of the superconductivity points to a much more substantial role for the lattice in terms of internal chemistry and bonding.
Highlights
These findings demonstrate that the insertion of extra O atoms by the High-pressure oxygenation (HPO) process gives different arrangements of the O sublattice compared to conventional methods
HPO methods produce a large number of extremely overdoped cuprates of both the La2 CuO4 (LCO) and YBCO classe, which remain superconducting without any reduction of their
They, produce a second phase diagram that may be the correct one since it derives from the intrinsic behavior of the superconductivity without dephasing or other loss mechanisms originating in defects of the material
Summary
(giving Cu a calculated valency near 3+) [25] with Tc remaining above 90 K across the full range of O stoichiometry challenges much of our understanding of high-temperature superconductivity This is further established by the unique behaviors of these materials that provide counterexamples to most of the listed criteria as well as others so taken for granted that there is no need to cite them: Superconductivity maintained with Tc up to 50–115 K, increasing or constant through, and well past, the conventional p = 0.27 upper limit of the dome [27,28]; In tetragonal Sr2 CuO3.3 , CuO~1.5 planes with all of the O positions filled in the b direction and half vacant in the a direction, which is the orientation that aligns with an applied magnetic field [29,30]; Cu-Oap distances < 2.2 Å in many HPO YBCO-type compounds; In Ba2 CuO3.2 , the square planar geometry of the Cu is inverted so that the Cu-Oap distances are longer than the Cu-O (planar) ones, which results in an inversion of the 3d(x2 −y2 ) and 3d(z2 ) energies and a substantial reduction in the two-dimensional character of the electronic states [31]; in Sr2 CuO3.3 , a transformation of its dynamic structure concomitant with its superconducting transition [30]; Evidence for Fermi-liquid-type carriers coexisting with the superconducting ones [32]. We anticipate that a new consideration of these results can provide new insights into the understanding of the superconductivity mechanism and its complexity
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