Direct evidence for the pseudogap
Temperature dependence of the superconducting energy gap and the pseudogap for the underdoped compound Bi2Sr2CaCu2O8 at three different points on the "expected" Fermi surface (marked a, b and c opposite) measured with angle-resolved photoemission spectroscopy (ARPES). The gap can be seen as the shift (to the left) of the leading edge of the spectrum
(red lines) with respect to the reference spectrum of a normal metal (green lines). The spectra at the lowest temperature indicate that the magnitude of the superconducting gap is angle-dependent and they are consistent with d-wave symmetry being predominant. The superconducting energy gap persists as a pseudogap when the temperature is raised above the superconducting transition temperature for this sample (Tc = 85 K). The "expected" Fermi surface is determined from the Fermi surface near or above optimal doping: it is plotted in the plane defined by the momentum in the x- and y-directions in the copper-oxide planes. (J W Loram et al. 1997 Physica C 282287 1405; and references in Timusk and Statt in further reading).