| speaker: | Kevin Ingersent Gainesville, USA |
| time: | Th., 23.08, 14:00-14:50 |
Models of a local degree of freedom coupled both to a fermionic band and to a bath of dispersive bosons have been proposed to describe magnetic impurities or quantum dots subjected to various dissipative mechanisms. They also arise in the extended dynamical mean-field theory of heavy-fermion quantum criticality. In cases where the bosonic spectral density vanishes at zero frequency according to an exponent 0 < s <= 1, such models typically exhibit a continuous boundary quantum phase transition (QPT) between (1) a "delocalized" phase in which the impurity is absorbed into, or Kondo-screened by, the fermionic band and (2) a "localized" phase in which the impurity is locked into one of its internal states via its coupling to the bosonic bath. This talk will report renormalization-group results on the nature of this QPT in several different models. For sufficiently small values of the exponent s, each of the models exhibits an interacting quantum critical point in the universality class of the sub-ohmic spin-bosonic model. However, when bosonic localization of the impurity is assisted by the presence of a pseudogap in the fermionic density of states, new classes of quantum criticality are found.