Local quantum criticality in the two-dimensional dissipative quantum XY model

We use quantum Monte Carlo simulations to calculate the phase diagram and the correlation functions for the quantum phase transitions in the two-dimensional dissipative quantum XY model with and without fourfold anisotropy. Without anisotropy, the model describes the superconductor-to-insulator transition in two-dimensional dirty superconductors. With anisotropy, the model represents the loop-current order observed in the underdoped cuprates and its fluctuations, as well as the fluctuations near the ordering vector in simple models of two-dimensional itinerant ferromagnets and itinerant antiferromagnets. These calculations test an analytic solution of the model which reexpressed it in terms of topological excitations: the vortices with interactions only in space but none in time, and warps with leading interactions only in time but none in space, as well as subleading interactions which are both space and time dependent. For parameters where the proliferation of warps dominates the phase transition, the critical fluctuations as functions of the deviation of the dissipation parameter α on the disordered side from its critical value αc are scale invariant in imaginary time τ as the correlation length in time ξ_τ=τce^[αc/(αc-α)]1/2 diverges, where τc is a short-time cutoff. On the other hand, the spatial correlations develop with a correlation length ξₓ≈ξ₀ln(ξ_τ), with ξ₀ of the order of a lattice constant. The dynamic correlation exponent z is therefore ∞. The Monte Carlo calculations also directly show warps and vortices. Their densities and correlations across the various transitions in the model are calculated and related to those of the order-parameter correlations in the dissipative quantum XY model.