Quantum Critical Properties of the Bose-Fermi Kondo Model in a Large-NLimit

Studies of non-Fermi-liquid properties in heavy fermions have led to the current interest in the Bose-Fermi Kondo model. Here we use a dynamical large-N approach to analyze an SU(N)×SU(κN) generalization of the model. We establish the existence in this limit of an unstable fixed point when the bosonic bath has a sub-Ohmic spectrum (|ω|^1-ϵsgnω, with $0<ϵ<1$). At the quantum-critical point, the Kondo scale vanishes and the local spin susceptibility (which is finite on the Kondo side for κ<1) diverges. We also find an ω/T scaling for an extended range (15 decades) of ω/T. This scaling violates (for ϵ≥1/2) the expectation of a naive mapping to certain classical models in an extra dimension; it reflects the inherent quantum nature of the critical point.