Quantum critical properties of the Bose-Fermi Kondo model in a large-N limit.

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)xSU(kappaN) 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 (/omega/(1-epsilon)sgnomega, with 0<epsilon<1). At the quantum-critical point, the Kondo scale vanishes and the local spin susceptibility (which is finite on the Kondo side for kappa<1) diverges. We also find an omega/T scaling for an extended range (15 decades) of omega/T. This scaling violates (for epsilon> or =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.