DYNAMICS OF SINGLE-PARTICLE AND COLLECTIVE EXCITATIONS FROM QUANTUM MONTE-CARLO

Singular value decomposition is applied to the analytic continuation i nto real time of the Matsubara Green's functions obtained in Monte Car lo simulations. Tile spectral density of single-particle excitations, and dynamical spill susceptibility are calculated for a two-dimensiona l Hubbard model. Tile Quantum Monte Carlo results reveal a two-band si ngle-particle spectrum with an extended saddle point. The dynamical sp ill susceptibility while contradicting tile prediction of long-range a ntiferromagnetic order based on tile random-phase approximation, is in qualitative agreement with self-consistent renormalization results fo r the same lattice size and temperature as the simulations.