Transmission phase lapses in quantum dots: The role of dot-lead coupling asymmetry

Lapses of transmission phase in transport through quantum dots (QDs) are ubiquitous already in the absence of interaction, in which case their precise location is determined by the signs and magnitudes of the tunnelling matrix elements. However, actual measurements for a QD embedded in an Aharonov-Bohm interferometer show systematic sequences of phase lapses separated by Coulomb peaks - an issue that has attracted much attention and generated controversy. Using a two-level QD as an example we show that this phenomenon can be accounted for by the combined effect of asymmetric dot-lead couplings (left lead/right lead asymmetry as well as different level broadening for different levels) and interaction-induced 'population switching' of the levels, rendering this behaviour generic. We construct and analyse a mean-field scheme for an interacting QD, and investigate the properties of the mean-field solution, paying special attention to the character of its dependence (continuous versus discontinuous) on the chemical potential or gate voltage.