nd - m at . s up r - co n ] 2 1 O ct 1 99 9 symmetry breaking in the superconducting state of Bi 2212

The paper compares the K-dependence of the superconducting gap in different doping ranges. The fine behavior of the leading edge gap indicates that the pairing susceptibility is peaked at special regions on the Fermi surface. These hot regions are found to be centered away from nominal " hot spots ". This indicates a symmetry breaking below Tc. I attribute this behavior to a signature of the pairing boson. Identification is made through comparison with neutron diffraction results. The mechanism of high temperature superconductivity is one of the most important current problems of condensed matter physics, and a detailed description of the interaction which leads to pairing is still lacking. The paper suggests a step in that direction. In common with most metals, the cuprate normal state exhibits a Fermi surface 1. Phase sensitive experiments, 2,3 indicate that the superconducting order parameter exhibits a change of sign around the Fermi surface 4. Ding et. al. have shown that for nearly optimally doped Bi2212, the energy gap, determined from the peak position of the spectral function at the Fermi level, varies with angle around the Fermi surface as ∆(φ) = ∆(0)Cos(2φ) (first order d-wave), 5 where φ is the Fermi surface angle measured from the Y-M direction of the Brillouin zone. In this report, the energy gap is determined from the measured Leading Edge Gap (LEG), for reasons to be detailed below. The doping dependence of the LEG results indicates a pattern of behavior that provides insight into the superconducting pairing mechanism.