arXiv subjects

Gerd Bergmann

Publications and source records attributed to Gerd Bergmann.

26 records · Page 2Linked to original sources

arXiv2006

A Compact Approximate Solution to the Friedel-Anderson Impuriy Problem

An approximate groundstate of the Anderson-Friedel impurity problem is presented in a very compact form. It requires solely the optimization of two localized electron states and consists of four Slater states (Slater determinants). The resulting singlet ground state energy lies far below the Anderson mean field solution and agrees well with the numerical results by Gunnarsson and Schoenhammer, who used an extensive 1/N_{f}-expansion for a spin 1/2 impurity with double occupancy of the impurity level. PACS: 85.20.Hr, 72.15.Rn

cond-mat.mes-hall↗

arXiv2005

A Critical Analysis of the Mean-Field Approximation for the Calculation of the Magnetic Moment in the Friedel-Anderson Impurity Model

It is shown that the calculation of the magnetic moment of a Friedel-Anderson impurity in mean-field theory is unreliable. A class of approximate solutions, which contains the mean-field solution as an element, is expressed in rotated Hilbert space and optimized. The optimal state has considerably lower energy than the mean field solution and requires almost twice the Coulomb exchange U to become magnetic. Since most moment calculations of magnetic impurities, for example the spin-density-functional theory, use the mean-field approximation the resulting magnetic moments have to be critically reexamened.(After publication the reference can be found at "http://physics.usc.edu/~bergmann/") PACS: 75.20.Hr

cond-mat.mes-hall↗

arXiv2005

Quantitative Simulation of the Superconducting Proximity Effect

A numerical method is developed to calculate the transition temperature of double or multi-layers consisting of films of super- and normal conductors. The approach is based on a dynamic interpretation of Gorkov's linear gap equation and is very flexible. The mean free path of the different metals, transmission through the interface, ratio of specular reflection to diffusive scattering at the surfaces, and fraction of diffusive scattering at the interface can be included. Furthermore it is possible to vary the mean free path and the BCS interaction NV in the vicinity of the interface. The numerical results show that the normalized initial slope of an SN double layer is independent of almost all film parameters except the ratio of the density of states. There are only very few experimental investigations of this initial slope and they consist of Pb/Nn double layers (Nn stands for a normal metal). Surprisingly the coefficient of the initial slope in these experiments is of the order or less than 2 while the (weak coupling) theory predicts a value of about 4.5. This discrepancy has not been recognized in the past. The autor suggests that it is due to strong coupling behavior of Pb in the double layers. The strong coupling gap equation is evaluated in the thin film limit and yields the value of 1.6 for the coefficient. This agrees much better with the few experimental results that are available. PACS: 74.45.+r, 74.62.-c, 74.20.Fg

cond-mat.mes-hall↗

arXiv2005

The Conductance of a Perfect Thin Film with Diffuse Surface Scattering

The conductance of thin films with diffusive surface scattering was solved semi-classically by Fuchs and Sondheimer. However, when the intrinsic electron mean free path is very large or infinite their conductance diverges. In this letter a simple diffraction picture is presented. It yields a conductance which corresponds to a limiting mean free path. PACS: 73.50.-h, 73.50.Bk, 73.23.-b, 73.25.+i, B146

cond-mat.mes-hall↗

arXiv2005

The anomalous Hall effect in ferromagnetic Fe: Skew scattering or side jump?

The question is investigated whether the anomalous Hall effect (AHE) in Fe films is due to skew scattering or side jump. For this purpose sandwiches of FeIn are investigated in which the conduction electrons carry their drift velocity across the interface. This yields an additional AHE conductance $ΔG_{xy}$ whose dependence on the In mean free path is used to determine the mechanism of the AHE in the Fe film. The structure of the Fe film is kept constant.

cond-mat.mes-hall↗

arXiv2004

The Meservey-Tedrov effect in FSF double tunneling junctions

Double tunneling junctions of ferromagnet-superconductor-ferromagnet electrodes (FSF) show a jump in the conductance when a parallel magnetic field reverses the magnetization of one of the ferromagnetic electrodes. This change is generally attributed to the spin-valve effect or to pair breaking in the superconductor because of spin accumulation. In this paper it is shown that the Meservey-Tedrov effect causes a similar change in the conductance since the magnetic field changes the energy spectrum of the quasi-particles in the superconductor. A reversal of the bias reverses the sign in the conductance jump.

cond-mat.mes-hall↗

arXiv2004

Localization and superconducting proximity effect in sandwiched potassium films

Thin films of alkali metals when sandwiched at both surfaces by thin metal films loose their conductance. The superconducting proximity effect is used to investigate the change in the alkali film. On the length scale of the film thickness the electronic properties of the alkali film do not change noticeably although its conductance is dramatically reduced, corresponding to localized electrons.

cond-mat.mes-hall↗

arXiv2004

Induced Spin-Currents in Alkali-Films

In sandwiches of FeK and FeCs the conduction electrons in the alkali metals have a large mean free path. The experiments suggest that the specular reflection for spin up and down electrons is different at the interface yielding a spin current in the alkali film. The spin current is detected by the anomalous Hall effect of Pb surface impurities.

cond-mat.mtrl-sci↗