Coherent quasiparticle weight and its connection to high-T_c superconductivity from angle-resolved photoemission

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Abstract
One of the outstanding puzzles of high temperature superconductors (HTS) is what determines the unusually high superconducting transition temperature T_c. In ordinary superconductors, T_c is solely determined by the energy gap \Delta, which is the binding energy for pairing two electrons in the superconducting state. However, this picture, fails completely for high-T_c superconductors where T_c is not directly related to \Delta. For example, changing the doping (chemical) concentration can increase \Delta while decreasing T_c. In fact, \Delta can remain nonzero in the normal state above T_c in HTS, while it is zero above T_c in ordinary superconductors. Here we report that, for HTS, T_c is determined by a new quantity Z\Delta through a fundamental relation T_c=const.\times Z\Delta. The quantity Z is the measure of the integrity (coherence) of an electron --- it is the extent to which an electron inside the material behaves like a free electron. We find that Z grows linearly with doping at low temperatures, and the reduction of Z upon heating resembles that of the superfluid density. We interpret these results to indicate that the missing element of the conventional picture of superconductivity for HTS is the role played by the one-electron coherence factor Z.

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