Excitation Spectrum of the Attractive Hubbard Model

We study excitation-spectrum and normal-state properties of the 2D attractive Hubbard model using the conserving, self-consistent T-matrix formalism in the intermediate coupling regime and at low electron concentration. Numerical results are presented for one-particle and two-particle excitation spectra, the one-particle momentum distribution, the chemical potential, and the static spin-susceptibility. For a coupling strength of U/t = 4.0 the one-particle spectral function, A(k,omega), shows two peaks of different weights. One peak can be associated with pair formation, whereas the other corresponds to renormalized quasiparticle excitation. It turns out that the two-band feature is reasonably well described by an ansatz for A(k,omega), which satisfies the first four frequency moments.