Quantum Vorticity at positive temperature for spin systems with   continuous symmetry
  We propose a definition of vorticity at inverse temperature $\beta$ for Gibbs states in quantum XY or Heisenberg spin systems on the lattice by testing $\exp[-\beta H]$ on a complete set of observables ("one-point functions"). Imposing a compression of Pauli matrices at the boudary, which stands for the classical environment, we perform some numerical simulations on finite lattices in case of XY model, which exhibit usual vortex patterns. 
Dimitriy Minenkov, Michel Rouleux
math-ph math.MP