We investigate the behavior of an N-component quantum rotor coupled to a bosonic dissipative bath having a sub-Ohmic spectral density J (ω) ∝ ωs with s < 1.With increasing dissipation strength, this system undergoes a quantum phase transition from a delocalized phase to a localized phase.We determine the exact critical behavior of this transition in the large-N limit. For 1 > s > 1/2, we find nontrivial critical behavior corresponding to an interacting renormalization group fixed point, while we find mean-field behavior for s < 1/2. The results agree with those of the corresponding long-range interacting classical model. The quantum-to-classical mapping is therefore valid for the sub-Ohmic rotor model.
M. Al-Ali and T. Vojta, "Quantum Phase Transition of the Sub-Ohmic Rotor Model," Physical Review B - Condensed Matter and Materials Physics, vol. 84, no. 19, pp. 195136-1-195136-6, American Physical Society (APS), Nov 2011.
The definitive version is available at http://dx.doi.org/10.1103/PhysRevB.84.195136
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