Quasi-particle spectra of perovskites: Enhanced Coulomb correlations at surfaces

Photoemission spectra of the perovskites Ca_xSr_1-xVO₃, Ca_xLa_1-xVO₃, and SrRuO₃ indicate that Coulomb correlations are more pronounced at the surface than in the bulk. To investigate this effect we use the dynamical mean field theory combined with the Quantum Monte Carlo technique and evaluate the multi-orbital self-energy. These systems exhibit different degrees of band filling and range from metallic to insulating. The key input in the calculations is the layer dependent local density of states which we obtain from a tight-binding approach for semi-infinite cubic systems. As a result of the planar character of the perovskite t_2g bands near the Fermi level, the reduced coordination number of surface atoms gives rise to a significant narrowing of the surface density of those subbands which hybridize preferentially in planes normal to the surface. Although the total band width coincides with the one in the bulk, the effective band narrowing at the surface leads to stronger correlation features in the quasi-particle spectra. In particular, the weight of the quasi-particle peak near E_F is reduced and the amplitude of the lower and upper Hubbard bands is enhanced, in agreement with experiments.