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 i n describing conventional superconductors. A central concept of BCS th eory is the energy gap in the electronic excitation spectrum below the superconducting transition temperature, T-c. The gap also serves as t he order parameter: quite generally, long-range phase coherence and a non-zero gap go hand-in-hand(1). But in underdoped high-T-c supercondu ctors there is considerable evidence that a pseudogap (a suppression o f spectral weight) is already formed in the normal state above T-c-fir st, from studies of the spin excitation spectrum(2-5,24), which measur e a 'spin gap', and later from a variety of other probes(6-10). Here w e present a study of underdoped Bi2Sr2CaCu2O8+delta (Bi2212) using ang le-resolved photoemission spectroscopy (ARPES), which directly measure s the momentum-resolved electron excitation spectrum of the CuO2 plane s. We find that a pseudogap with d-wave symmetry opens up in the norma l state below a temperature T > T-c, and develops into the d-wave sup erconducting gap once phase coherence is established below T-c.