Spectroscopic evidence for a pseudogap in the normal state of underdoped high-T-c superconductors

IT is well known that BCS mean-field theory is remarkably successful in describing conventional superconductors. A central concept of BCS theory is the energy gap in the electronic excitation spectrum below the superconducting transition temperature, T-c. The gap also serves as the order parameter: quite generally, long-range phase coherence and a non-zero gap go hand-in-hand(1). But in underdoped high-T-c superconductors there is considerable evidence that a pseudogap (a suppression of spectral weight) is already formed in the normal state above T-c-first, from studies of the spin excitation spectrum(2-5,24), which measure a 'spin gap', and later from a variety of other probes(6-10). Here we present a study of underdoped Bi2Sr2CaCu2O8+delta (Bi2212) using angle-resolved photoemission spectroscopy (ARPES), which directly measures the momentum-resolved electron excitation spectrum of the CuO2 planes. We find that a pseudogap with d-wave symmetry opens up in the normal state below a temperature T* > T-c, and develops into the d-wave superconducting gap once phase coherence is established below T-c.