Competition between the pseudogap and superconductivity in the high-Tc copper oxides

In a classical Bardeen - Cooper - Schrieffer superconductor, pairing and coherence of electrons are established simultaneously below the critical transition temperature ( T-c), giving rise to a gap in the electronic energy spectrum. In the high- T-c copper oxide superconductors, however, a pseudogap(1-8) extends above T-c. The relationship between the pseudogap and superconductivity is one of the central issues in this field(9-17). Spectral gaps arising from pairing precursors are qualitatively similar to those caused by competing electronic states, rendering a standard approach to their analysis inconclusive(10-16). The issue can be settled, however, by studying the correlation between the weights associated with the pseudogap and superconductivity spectral features. Here we report a study of two spectral weights using angle- resolved photo-emission spectroscopy. The weight of the superconducting coherent peak increases away from the node following the trend of the superconducting gap, but starts to decrease in the antinodal region. This striking non- monotonicity reveals the presence of a competing state. We emonstrate a direct correlation, for different values of momenta and doping, between the loss in the low- energy spectral weight arising from the opening of the pseudogap and a decrease in the spectral weight associated with superconductivity. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing.