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VOLUME 61 (1995) | ISSUE 4 | PAGE 264
Gap anisotropy in high-Tc superconductors produced as a result of s-wave Cooper pairing of electrons on a multiply connected Fermi surface with spontaneous symmetry breaking
The "butterfly" anisotropy of the gap observed in the plane of the Cu02 layers in the high Tc superconductor Bi2Sr2CaCu208+<5 is not a consequence of d- wave Cooper pairing. It is instead due to an anisotropic s-wave pairing of electrons on a multiply connected (multivalley) Fermi surface with flat (congruent) regions and a spontaneous breaking of the symmetry of the electron spectrum as the result of a Peierls structural instability, accompanied by the formation of an insulating gap along one diagonal of the Brillouin zone. The phase shift of Josephson currents along the a and b axes observed in YBa2Cu307_a single crystals may be due to a difference between the signs of the gap parameters in the Cu02 layers and the CuO chains. This circumstance might simulate a d-wave pairing. © 1995 American Institute of Physics.