Surface states at the top of the bulk $d$ bands of Cu(100) and Cu(111) single crystals have been observed using angle-resolved photoemission spectroscopy. The measured dispersion of the surface-state bands falls in absolute energy gaps of the projected bulk band structure near the symmetry point $\overline{M}$ of the two-dimensional Brillouin zone, for both faces. Since the parent energy gaps are not caused by the hybridization of crossed energy bands, the observed states are of the Tamm rather than the Shockley type; surface states of the latter type have been observed before in $\mathrm{sp}$ gaps of several noble metals. Tamm states require a sufficiently strong surface "perturbation." Only the self-consistent calculations of Gay, Smith, and Arlinghaus recently reported for Cu(100) contain surface states in the energy-wave-vector region studied here. The observed peaks are very narrow, due to their energy location above the bulk $d$-band continuum as well as the good angle and energy resolution of our spectrometer.

• Received 29 May 1979

DOI:https://doi.org/10.1103/PhysRevB.20.3059