W. von der Linden

Quasiparticle Dispersion of the 2D Hubbard Model: From an Insulator to a Metal

On the basis of Quantum-Monte-Carlo results the evolution of the spectral weight

Spin-correlations and low lying excited states of the spin-1/2 Heisenberg antiferromagnet on a square lattice

A(\vec k, \omega)

Ferromagnetism in the Hubbard model

of the two-dimensional Hubbard model is studied from insulating to metallic behavior. As observed in recent photoemission experiments for cuprates, the electronic excitations display essentially doping-independent features: a quasiparticle-like dispersive narrow band of width of the order of the exchange interaction

Maximum entropy based reconstruction of soft X-ray emissivity profiles in W7-AS

J

Spectral properties of the one-dimensional Hubbard model.

and a broad valence- and conduction-band background. The continuous evolution is traced back to one and the same many-body origin: the doping-dependent antiferromagnetic spin-spin correlation.

High-temperature superconductivity in the apex-oxygen model: a quantum Monte Carlo study

The ground state and the lowest excited states of the spin 1/2-Heisenberg model are investigated by exact diagonalization and variational Monte Carlo techniques. Our trial state represents a generalization of a wave function introduced by Hulthen, Kasteleijn and Marshall. The long range character of the spin-correlation function is in excellent agreement with exact diagonalization and also with recent neutron scattering results for La2CuO4. The asymptotic behavior of the spin-correlation function is found to differ from spin-wave theory. From the exact (N<=20 spins) and variational (N<=400) ground state energies we determine as asymptotic values 1.3025 and 1.288, respectively. We calculate the dispersion for the spin-wave excitations and identify an excited triplet which becomes degenerate with the ground state in the thermodynamic limit. This triplet state allows spontaneous symmetry breaking to occur atT=0 K. Quantum fluctuations reduce the sublattice magnetization to an effective value of 0.195 (3) as compared to the Néel-state value of 1/2.

Bayesian PIXE background subtraction

The stability of the ferromagnetic state with complete spin alignment against a single spin reversal is studied in the square-lattice Hubbard model with nearest-neighbour hopping. A variational ansatz is used, which is exact in one dimension and by comparison with accurate results for small clusters yields almost exact results in the two-dimensional case. The ferromagnetic region in the (W/U, delta ) phase diagram is mapped out, where W is the bandwidth, U the on-site electron interaction and delta is the number of holes per atom. This region is considerably smaller than in previous variational calculations and it is rigorously concluded that the state of complete spin alignment is unstable when delta >0.29, for all U, and when W/U>0.19, for all delta . The nature of the instability, and of the low-lying excitations in the ferromagnetic state for varying delta and W/V, is discussed.

Energy resolution enhancement in ion beam experiments with Bayesian probability theory

The reconstruction of 2-D emissivity profiles from soft X-ray tomography measurements constitutes a highly underdetermined and ill-posed inversion problem, because of the restricted viewing access, the number of chords and the increased noise level in most plasma devices. An unbiased and consistent probabilistic approach within the framework of Bayesian inference is provided by the maximum entropy method, which is independent of model assumptions, but allows any prior knowledge available to be incorporated. The formalism is applied to the reconstruction of emissivity profiles in an NBI heated plasma discharge to determine the dependence of the Shafranov shift on beta , the reduction of which was a particular objective in designing the advanced W7-AS stellarator

Local anharmonic vibrations strong correlations and superconductivity : a quantum simulation study

The spectral properties of the 1-D Hubbard model are obtained from quantum Monte Carlo simulations using the maximum entropy method. The one-particle excitations are characterized by dispersive cosine-like bands. Velocities for spin- and charge excitations are obtained that lead to a conformal charge c=0.98 +/- 0.05 for the largest system simulated (N=84). An exact sum-rule for the spin-excitations is fulfilled accurately with deviations of at most 10% only around 2 kF.

Deconvolution based on experimentally determined apparatus functions

The Projector Quantum Monte Carlo Method is applied to examine the existence of superconductivity in the Apex-Oxygen-Model which describes the physics of the correlated carriers in the CuO2-planes coupled to the anharmonic vibrations of the apex oxygen in the hightemperature superconductors. The simulations show clear evidence for extendeds-wave superconductivity in the 100 K temperature range for realistic choice of coupling parameters.