arXiv subjects

W. Hanke

Publications and source records attributed to W. Hanke.

64 records · Page 4Linked to original sources

arXiv1995

T-Matrix Formulation of Impurity Scattering in Correlated Systems

Using a generalized T-matrix description which, in principle, exactly includes Coulomb correlations and potential scattering events, resonant and bound impurity states are discussed. Like in the non-interacting case, the effects of the scattering potential can be divided into different partial wave channels, exploiting the symmetry of the underlying lattice. Due to Coulomb correlations bare local (i.e.\ s-wave) potentials become dynamic and extended, being responsible also for p-, d-wave etc. scattering effects. Numerically exact results for both the two-dimensional t--J and Hubbard models are used to construct a simple (static) approximation to the effective impurity potential which is shown to reproduce the exact resonant scattering and bound states in the relevant symmetry channels.

cond-mat↗

arXiv1995

Evidence for a superfluid density in t--J ladders

Applying three independent techniques, we give numerical evidence for a finite superfluid density in isotropic hole-doped t--J ladders: We show the existence of anomalous flux quantization, emphasising the contrasting behaviour to that found in the `Luttinger liquid' regime stabilised at low electron densities; We consider the nature of the low-lying excitation modes, finding the 1-D analog of the superconducting state; And using a density matrix renormalization group approach, we find long range pairing correlations and exponentially decaying spin-spin correlations.

cond-mat↗

arXiv1995

Consistent Application of Maximum Entropy to Quantum-Monte-Carlo Data

Bayesian statistics in the frame of the maximum entropy concept has widely been used for inferential problems, particularly, to infer dynamic properties of strongly correlated fermion systems from Quantum-Monte-Carlo (QMC) imaginary time data. In current applications, however, a consistent treatment of the error-covariance of the QMC data is missing. Here we present a closed Bayesian approach to account consistently for the QMC-data.

cond-mat↗

arXiv1995

SPIN AND CHARGE MODES OF THE t-J LADDER

The spin and charge excitations of the t--J ladder are studied by exact diagonalization techniques for several electron densities. The various modes are classified according to their spin (singlet or triplet excitations) and their parity under a reflection with respect to the symmetry axis along the chains and a finite size scaling of the related gaps is performed. At low doping, formation of hole pairs leads to a spin gap for all $J/t$ ratios. This phase is characterized by (at least) one vanishing energy mode {\it only} in the charge bonding channel when $K_x\rightarrow 0$ consistent with the existence of superconducting pairing correlations. At larger doping the spin gap disappears. Although the anti-bonding modes remain gapped, low energy $K_x\sim 0$ and $K_x\sim 2k_F$ spin and charge bonding modes are found consistent with a single band Luttinger scenario. At sufficient low electron density and above a critical value of J/t we also expect another phase of electron pairs with gapped spin excitations.

cond-mat↗

arXiv1994

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 $A(\vec k, ω)$ 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 $J$ 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.

cond-mat↗

arXiv1994

Conserving approximation for the three-band Hubbard model: flat quasiparticle dispersion

It is shown that the low-energy single-particle excitation-spectrum of the three-band Hubbard model at hole-dopings away from half-filling agrees remarkably well with Quantum Monte Carlo data and spectroscopic experiments within the framework of a conserving approximation that includes self-consistently the interaction with charge, spin, and two-particle fluctuations. The dispersion of the quasiparticle-peak obtained from the spectral-weight function is flat around the $(π,0)$ and $(0,π)$ points as has been observed in recent angle-resolved photoemission measurement. The significant reduction of the quasiparticle-dispersion near the Fermi-energy is due to spin-fluctuations, rather than being induced by band effects.

cond-mat↗

arXiv1994

Binding of Holes to Magnetic Impurities in a Strongly Correlated System

The effect of a magnetic (S=1/2) impurity coupled to a 2D system of correlated electrons (described by the t--J model) is studied by exact diagonalisations. It is found that, if the exchange coupling of the impurity with the neighboring spins is ferromagnetic or weakly antiferromagnetic, an extra hole can form bound states of different spatial symmetries with the impurity extending to a few lattice spacings. The binding energy is maximum when the impurity is completely decoupled (vacancy) and vanishes for an antiferromagnetic coupling exceeding $\sim 0.3 J$. Several peaks appear in the single hole spectral function below the lower edge of the quasiparticle band as signatures of the d-, s- and p-wave boundstates.

cond-mat↗

arXiv1993

Spectral properties of the one-dimensional Hubbard model

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.

cond-mat↗

arXiv1993

Resonant Impurity Scattering in a Strongly Correlated Electron Model

Scattering by a single impurity introduced in a strongly correlated electronic system is studied by exact diagonalization of small clusters. It is shown that an inert site which is spinless and unable to accomodate holes can give rise to strong resonant scattering. A calculation of the local density of state reveals that, for increasing antiferromagnetic exchange coupling, d, s and p-wave symmetry bound states in which a mobile hole is trapped by the impurity potential induced by a local distortion of the antiferromagnetic background successively pull out from the continuum.

cond-mat↗

arXiv1993

Temperature Derivative of the Superfluid Density in the Attractive Hubbard model

Based on extensions of the grand-canonical Quantum Monte-Carlo algorithm to incorporate magnetic fields, we provide numerical data confirming the existence of a Kosterlitz-Thouless transition in the attractive Hubbard model. Here, we calculate the temperature derivative of the superfluid density, to pin down the transition. Away from half-band filling, the above quantity, shows a response which increases with lattice size at the transition temperature. In contrast, such a signal is not observed for the case of a half-band filling.

cond-mat↗