Magnetic frustration in high-T c superconductors

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Abstract
The breakdown of antiferromagnetic order and its coincidence with the onset of the superconducting state (the TN, Tc→0 condition) is explained by the itinerancy of polarons that are created by ‘‘impurity’’ cations or by oxygen vacancies. In both La2−xSrxCuO4and YBa2Cu3Oy systems, the electrostatic balance is maintained in a traditional sense by the formation of Cu3+(d8) ions, here in low‐spin (S=0) electron configurations because of tetragonal crystal fields at the Cu sites. Since the spins of the host Cu2+(d9) ions (S=1/2) surrounding the diamagnetic Cu3+ p‐type polaron ion will undergo canting that results from the magnetic dilution effects, a dynamic region of frustration will exist within the confines of the polaron cell boundaries. As cells begin to merge through increased polaron concentration, percolation of cells will destroy magnetic order at a threshold determined by the product of cell radius and carrier concentration. Analysis of the cell radii indicate that frustration should set in at a Cu3+ concentration (x≊0.07) that is consistent with the TN, Tc→0 condition observed in both of the above superconductor families. From these concepts, a rationale emerges for a superconductor phase diagram that links the magnetic, insulating, superconducting, and normal metallic regions as functions of temperature and composition.