Quasiparticle dispersion in Bi2Sr2CaCu2O8 is investigated with improved angular resolution as a function of temperature and doping. Unlike the linear dispersion predicted by the band calculation, the data show a sharp break in dispersion at 50+/-15 meV binding energy where the velocity changes by a factor of 2 or more. This change provides an energy scale in the quasiparticle self-energy. This break in dispersion is evident at and away from the d-wave node line, but the magnitude of the dispersion change decreases with temperature and with increasing doping.
An inelastic neutron scattering study of overdoped Bi(2)Sr(2)CaCu(2)O(8+delta) ( T(c) = 83 K) has revealed a resonant spin excitation in the superconducting state. The mode energy is E(res) = 38.0 meV, significantly lower than in optimally doped Bi(2)Sr(2)CaCu(2)O(8+delta) ( T(c) = 91 K, E(res) = 42.4 meV). This observation, which indicates a constant ratio E(res)/k(B)T(c) approximately 5.4, helps resolve a long-standing controversy about the origin of the resonant spin excitation in high temperature superconductors.
Electronic Raman scattering has been studied in single-crystal Bi 2 Sr 2 Ca(Cu 1Ϫx Fe y ) 2 O ␦ for different Fe concentrations. The 2⌬ peak positions of the B 1g and A 1g channels in the superconducting state are only slightly affected by impurity scattering with the ratio B1g / A1g unchanged. There is also no low-frequency threshold at any impurity concentration. These two features are consistent with unconventional pairing. However, the crossover scale to a cubic dependence in the B 1g channel, contrary to the d-pairing expectations, decreases with increasing Fe concentrations.
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