Scattering phases in quantum dots:: An analysis based on lattice models

The properties of scattering phases in quantum dots are analyzed with the help of lattice models. We first derive expressions relating the different scattering phases and the dot Green functions. We analyze the Friedel sum rule in detail, and discuss the deviation of the phase of the transmission amplitude from the Friedel phase at the zeros of the transmission. The occurrence of such zeros is related to the parity of the isolated dot levels. A statistical analysis of the isolated dot wave functions reveals the absence of significant correlations in the parity for large disorder, and the appearance, for weak disorder, of certain dot states which are strongly coupled to the leads. It is shown that large differences in the coupling to the leads give rise to an anomalous charging of the dot levels. A mechanism for the phase lapse observed experimentally based on this property is discussed, and illustrated with model calculations.