The quantum phase transition in the sub-ohmic spin-boson model: Quantum   Monte-Carlo study with a continuous imaginary time cluster algorithm
  A continuous time cluster algorithm for two-level systems coupled to a dissipative bosonic bath is presented and applied to the sub-ohmic spin-Boson model. When the power s of the spectral function J(w) \propto w^s is smaller than 1/2, the critical exponents are found to be classical, mean-field like. Potential sources for the discrepancy with recent renormalization group predictions are traced back to the effect of a dangerously irrelevant variable. 
Andre Winter, Heiko Rieger, Matthias Vojta, Ralf Bulla