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. 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, which measure a 'spin gap', and later from a variety of other probes. Here we present a study of underdoped Bi₂Sr₂CaCu₂O(8+δ) (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).