Evidence for Quantum Critical Behavior in the Optimally Doped Cuprate Bi 2 Sr 2 CaCu 2 O 8+δ

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Abstract
The photoemission line shapes of the optimally doped cuprate Bi 2 Sr 2 CaCu 2 O 8+δ were studied in the direction of a node in the superconducting order parameter by means of very high resolution photoemission spectroscopy. The peak width or inverse lifetime of the excitation displays a linear temperature dependence, independent of binding energy, for small energies, and a linear energy dependence, independent of temperature, for large binding energies. This behavior is unaffected by the superconducting transition, which is an indication that the nodal states play no role in the superconductivity. Temperature-dependent scaling suggests that the system displays quantum critical behavior.