Quantum criticality in a dissipative (2+1)-dimensional XY model of   circulating currents in high-Tc cuprates
  We present large-scale Monte Carlo results for the dynamical critical exponent z and the spatio-temporal two-point correlation function of a (2+1)-dimensional quantum XY model with bond dissipation, proposed to describe a quantum critical point in high-Tc cuprates near optimal doping. The phase variables of the model, originating with a parametrization of circulating currents within the CuO_2 unit cells in cuprates, are compact, {\theta_{r,\tau}} \in [-\pi,\pi>. The dynamical critical exponent is found to be z \approx 1, and the spatio-temporal correlation functions are explicitly demonstrated to be isotropic in space-imaginary time. The model thus has a fluctuation spectrum where momentum and frequency enter on equal footing, rather than having the essentially momentum-independent marginal Fermi liquid-like fluctuation spectrum previously reported for the same model. 
Iver Bakken Sperstad, Einar B. Stiansen, Asle Sudb{\o}
cond-mat.str-el cond-mat.stat-mech cond-mat.supr-con