Superconductivity in an anomalously tetragonal YB2C3O6.62 single crystal: A possible singularity in the structural phase diagram
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During systematic studies of the structural phase diagram of YBa2Cu3O6+x (YBCO), a YBCO single crystal of oxygen concentration x=0.62 turned out to have a tetragonal crystal structure. By magneto-optical imaging of the expulsion of an applied magnetic field, it was shown that the sample was bulk superconducting. This makes the sample highly anomalous in two respects: with a stoichiometry of YBa2Cu3O6.62 the sample should have an orthorhombic symmetry, and a tetragonal undoped sample should not be superconducting at all. Our results corroborate previous findings of Topnikov [JETP Lett. 46, 577 (1987)] of a tetragonal superconducting YBCO crystal with x=0.62.