Berry Phase and the Breakdown of the Quantum to Classical Mapping for   the Quantum Critical Point of the Bose-Fermi Kondo model
  The phase diagram of the Bose-Fermi Kondo model contains an SU(2)-invariant Kondo-screened phase separated by a continuous quantum phase transition from a Kondo-destroyed local moment phase. We analyze the effect of the Berry phase term of the spin path integral on the quantum critical properties of this quantum impurity model. For a range of the power-law exponent characterizing the spectral density of the dissipative bosonic bath, neglecting the influence of the Berry phase term makes the fixed point Gaussian. For the same range of the spectral density exponent, incorporating the Berry phase term leads instead to an interacting fixed point, for which a quantum to classical mapping breaks down. Some general implications of our results are discussed. 
Stefan Kirchner and Qimiao Si