Self-energy effects in the optical anisotropy of GaP(001)

We calculate the reflectance anisotropy for GaP(001)(2x4) surfaces using a real-space multigrid method and ab initio pseudopotentials. Our results obt ained within DFT-LDA show good qualitative agree ment with recent experimen ts. This holds in particular for the stoichiometric trends. A strong negati ve anisotropy at low photon energies is linked to the formation of Ga-Ga bo nds along the [110] direction. There are discrepancies, however, with respe ct to the line shape and the energetic positions of characteristic peaks. S ubstantial improvement is achieved by using a numerically efficient GW appr oach with approximations for local-field effects and dynamical screening. W e find that the spectral features related to transitions between surface pe rturbed bulk wave functions are more strongly affected by self-energy corre ctions than anisotropies directly linked to surface electronic states.