Optimization of alloy-analogy-based approaches to the infinite-dimensional Hubbard model

An analytical expression for the self-energy of the infinite-dimensional Hubbard model is proposed that interpolates between different exactly solvable limits. We profit by the combination of two recent approaches that are based on the alloy-analogy (Hubbard-III) solution: The modified alloy-analogy (MAA) which focuses on the strong-coupling regime, and the Edwards-Hertz approach (EHA) which correctly recovers the weak-coupling regime. Investigating the high-energy expansion of the EHA self-energy, it turns out that the EHA reproduces the first three exactly known moments of the spectral density only. This may be insufficient for the investigation of spontaneous magnetism. The analysis of the high-energy behavior of the CPA self-consistency equation allows for a new interpretation of the MAA: The MAA is the only (two-component) alloy-analogy that correctly takes into account the first four moments of the spectral density. For small U, however, the MAA does not reproduce Fermi-liquid properties. The defects of the MAA as well as of the EHA are avoided in the new approach. We discuss the prospects of the theory and present numerical results in comparison with essentially exact quantum Monte Carlo data. The correct high-energy behavior of the self-energy is proved to be a decisive ingredient for a reliable description of spontaneous magnetism.Comment: LaTeX, 18 pages, 12 eps figures include

Nonlinear structural vibrations by the linear acceleration method

Numerical integration method for calculating dynamic response of nonlinear elastic structure

Coil planet centrifugation as a means for small particle separation

The coil planet centrifuge uses a centrifugal force field to provide separation of particles based on differences in sedimentation rates by flow through a rotating coiled tube. Three main separations are considered: (1) single phase fresh sheep and human erythrocytes, (2) single phase fixed heep and human erythrocytes, and (3) electrophoretically enhanced single phase fresh sheep and human erythrocytes

On the magnetic stability at the surface in strongly correlated electron systems

The stability of ferromagnetism at the surface at finite temperatures is investigated within the strongly correlated Hubbard model on a semi-infinite lattice. Due to the reduced surface coordination number the effective Coulomb correlation is enhanced at the surface compared to the bulk. Therefore, within the well-known Stoner-picture of band ferromagnetism one would expect the magnetic stability at the surface to be enhanced as well. However, by taking electron correlations into account well beyond the Hartree-Fock (Stoner) level we find the opposite behavior: As a function of temperature the magnetization of the surface layer decreases faster than in the bulk. By varying the hopping integral within the surface layer this behavior becomes even more pronounced. A reduced hopping integral at the surface tends to destabilize surface ferromagnetism whereas the magnetic stability gets enhanced by an increased hopping integral. This behavior represents a pure correlation effect and can be understood in terms of general arguments which are based on exact results in the limit of strong Coulomb interaction.Comment: 6 pages, RevTeX, 4 eps figures, accepted (Phys. Rev. B), for related work and info see http://orion.physik.hu-berlin.d

A low-temperature dynamic mode scanning force microscope operating in high magnetic fields

A scanning force microscope was implemented operating at temperatures below 4.2K and in magnetic fields up to 8T. Piezoelectric quartz tuning forks were employed for non optical tip-sample distance control in the dynamic operation mode. Fast response was achieved by using a phase-locked loop for driving the mechanical oscillator. Possible applications of this setup for various scanning probe techniques are discussed.Comment: 5 pages, 5 figures, submitted to "Review of Scientific Instruments

A Simple Geometrical Model for Solid Friction

We present a simple model for the friction of two solid bodies moving against each other. In a self consistent way we can obtain the dependence of the macroscopic friction force as a function of the driving velocity, the normal force and the ruggedness of the surfaces in contact. Our results are discussed in the context of friction laws used in earthquake models.Comment: 9 pages, plain TeX, preprint HLRZ 24/9

Segregation in a fluidized binary granular mixture: Competition between buoyancy and geometric forces

Starting from the hydrodynamic equations of binary granular mixtures, we derive an evolution equation for the relative velocity of the intruders, which is shown to be coupled to the inertia of the smaller particles. The onset of Brazil-nut segregation is explained as a competition between the buoyancy and geometric forces: the Archimedean buoyancy force, a buoyancy force due to the difference between the energies of two granular species, and two geometric forces, one compressive and the other-one tensile in nature, due to the size-difference. We show that inelastic dissipation strongly affects the phase diagram of the Brazil nut phenomenon and our model is able to explain the experimental results of Breu et al. (PRL, 2003, vol. 90, p. 01402).Comment: 5 pages, 2 figure

Broad Histogram Monte Carlo

We propose a new Monte Carlo technique in which the degeneracy of energy states is obtained with a Markovian process analogous to that of Metropolis used currently in canonical simulations. The obtained histograms are much broader than those of the canonical histogram technique studied by Ferrenberg and Swendsen. Thus we can reliably reconstruct thermodynamic functions over a much larger temperature scale also away from the critical point. We show for the two-dimensional Ising model how our new method reproduces exact results more accurately and using less computer time than the conventional histogram method. We also show data in three dimensions for the Ising ferromagnet and the Edwards Anderson spin glass.Comment: 6 pages of a TeX file with 4 PS figures. Related papers at http://www.if.uff.br/~tjp