Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi2Sr2CaCu2O8+delta. This gap exists only in underdoped samples and closes around the doping level at which the superconducting transition temperature Tc is a maximum. The momentum dependence and magnitude of the gap closely resemble those of the dx2-y2 gap observed in the superconducting state. This observation is consistent with results from several other experimental techniques, which also indicate the presence of a gap in the normal state. Some possible theoretical explanations for this effect are reviewed.

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An angle-resolved photoemission study is reported on Ca 2 CuO 2 Cl 2 , a parent compound of high- T c superconductors. Analysis of the electron occupation probability, n( k ), from the spectra shows a steep drop in spectral intensity across a contour that is close to the Fermi surface predicted by the band calculation. This analysis reveals a Fermi surface remnant, even though Ca 2 CuO 2 Cl 2 is a Mott insulator. The lowest energy peak exhibits a dispersion with approximately the |cos k x a – cos k y a | form along this remnant Fermi surface. Together with the data from Dy-doped Bi 2 Sr 2 CaCu 2 O 8+δ , these results suggest that this d -wave–like dispersion of the insulator is the underlying reason for the pseudo gap in the underdoped regime.