arXiv subjects

M. Böhm

Publications and source records attributed to M. Böhm.

21 records · Page 2Linked to original sources

arXiv1998

Subtle interface magnetism of Fe/Au multilayers

By ab initio LMTO calculations in atomic-sphere approximation we have studied the interlayer exchange coupling between Fe films separated by Au spacers in infinite Fe/Au multilayers with (001) interface orientation. We also performed detailed calculations of the magnetization and charge profiles across the system. We find an enhancement of the Fe moments at the interface, which amount to $\approx 2.8$ $μ_B$ instead of the bulk value of 2.2 $μ_B$, and we also find a slight magnetic polarization of the order of 0.01 $μ_B$ at the Au interface layers. These results do not depend sensitively on the lattice constants assumed in the calculation. When we try to optimize our results with respect to the ratio $R_{Fe}/R_{Au}$ of the Wigner-Seitz spheres for the mutual components, we often find the optimum near charge neutrality of the interface monolayers: However, this is not always the case, and usually the sign and the magnitude of the exchange coupling depend sensitively on the optimal choice of the above-mentioned ratio. In particular, for Fe$_5$/Au$_3$ we find an anomaly with a large dipole moment at the interface and a related anomaly of the exchange coupling.

cond-mat.mtrl-sci↗

arXiv1994

Calculation of two-loop self-energies in the electroweak Standard Model

Motivated by the results of the electroweak precision experiments, studies of two-loop self-energy Feynman diagrams are performed. An algebraic method for the reduction of all two-loop self-energies to a set of standard scalar integrals is presented. The gauge dependence of the self-energies is discussed and an extension of the pinch technique to the two-loop level is worked out. It is shown to yield a special case of the background-field method which provides a general framework for deriving Green functions with desirable theoretical properties. The massive scalar integrals of self-energy type are expressed in terms of generalized multivariable hypergeometric functions. The imaginary parts of these integrals yield complete elliptic integrals. Finally, one-dimensional integral representations with elementary integrands are derived which are well suited for numerical evaluation.

hep-ph↗

arXiv1993

Reduction of general two-loop self-energies to standard scalar integrals

We present a method for reducing general two-loop self-energies to standard scalar integrals in massive gauge theories with special emphasis on the electroweak Standard Model. It includes the tensor integral reduction for all two-loop integrals appearing in self-energy calculations. The results are valid for arbitrary values of the invariant momentum $p^2$, all particle masses, the space-time dimension $D$ and the gauge parameters $ξ_i \; (i = γ, Z, W)$. The algebraic structure of the results clearly displays the gauge dependence of the considered quantities and allows to perform very stringent checks. We explicitly verify Slavnov-Taylor identities by calculating several thousand Feynman-diagrams and adding them up algebraically. As an application we calculate the light fermion contributions to the two-loop gauge boson self-energies of the electroweak SM. We study their gauge dependence and discuss the occurring standard integrals.

hep-ph↗