Dynamical spin and charge response functions in the doped two-dimensional Hubbard model

Dynamical properties of the spin and charge response functions in the doped two-dimensional Hubbard model are calculated by taking into account the drastic separation of the single-particle spectral function into the low-energy coherent and high-energy incoherent parts due to the strong Coulomb interaction. We show that this evolution of the electronic states is the origin of the broad and structureless feature in the charge response function. In the weak coupling regime the low-energy enhancement of the spin excitation is produced which can be explained within the random phase approximation. However, for the larger interaction close to the antiferromagnetic Stoner condition, the low-energy intensity of the spin excitation is suppressed. Received: 25 September 1997 / Revised: 19 December 1997 / Accepted: 9 January 1998     

Mott transition in the two-dimensional Hubbard model

Spectral properties of the two-dimensional Hubbard model near the Mott transition are investigated by using cluster perturbation theory. The Mott transition is characterized by freezing of the charge degrees of freedom in a single-particle excitation that leads continuously to the magnetic excitation of the Mott insulator. Various anomalous spectral features observed in cuprate high-temperature superconductors are explained in a unified manner as properties near the Mott transition.     

Magnetization in the two-dimensional Hubbard model: a numerical study

The Projector Quantum Monte Carlo method is used to study the two-dimensional Hubbard model with generalized boundary conditions at half-filling. The convergence of the algorithm depends strongly on the initial trial state. Spin-density waves provide an excellent trial state for the case of weak and of strong correlations. This choice of a trial state with broken symmetry allows us to calculate directly the staggered (or sublattice) magnetization m ₀ as a function of the on-site repulsion U. The use of general boundary conditions strongly reduces finite size effects in m ₀.     

Nagaoka ferromagnetism in the two-dimensional infinite-U Hubbard model

We present different numerical calculations based on variational quantum Monte Carlo simulations supporting a ferromagnetic ground state for finite and small hole densities in the two-dimensional infinite-U Hubbard model. Moreover, by studying the energies of different total spin sectors, these calculations strongly suggest that the paramagnetic phase is unstable against a phase with a partial polarization for large hole densities delta approximately 0.40 with evidence for a second-order transition to the paramagnetic large doping phase.     

Finite-temperature phase transitions in a two-dimensional boson Hubbard model

We study finite-temperature phase transitions in a two-dimensional boson Hubbard model with zero-point quantum fluctuations via Monte Carlo simulations of a quantum rotor model and construct the corresponding phase diagram. Compressibility shows a thermally activated gapped behavior in the insulating regime. Finite-size scaling of the superfluid stiffness clearly shows the nature of the Kosterlitz-Thouless transition. The transition temperature T(c) confirms a scaling relation T(c) proportional, rho(0)(x), with x=1.0. Some evidence of anomalous quantum behavior at low temperatures is presented.