Multi-particle excitations and spectral densities in quantum spin-systems

The excitation spectrum of the 2-leg S=1/2 Heisenberg ladder is examined perturbatively. Using an optimally chosen continuous unitary transformation we expand the Hamiltonian and the Raman operator about the limit of isolated rungs leading to high order series expansions allowing to calculate spectral densities quantitatively. The 2-particle sector is examined for total momentum k=0. We show that triplet-triplet interaction gives rise to a band splitting.Comment: 2 pages, 1 figure; submitted to the proceedings of the SCES2001 conference (Physica B

Construction and Tests of Modules for the ATLAS Pixel Detector

The ATLAS Pixel Detector is the innermost layer of the ATLAS tracking system and will contribute significantly to the ATLAS track and vertex reconstruction. The detector consists of identical sensor-chip-hybrid modules, arranged in three barrels in the centre and three disks on either side for the forward region. The position of the pixel detector near the interaction point requires excellent radiation hardness, mechanical and thermal robustness, good long-term stability, all combined with a low material budget. The pre-production phase of such pixel modules has nearly finished, yielding fully functional modules. Results are presented of tests with these modules.Comment: Poster at the XXIII Physics in Collisions Conference (PIC03), Zeuthen, Germany, June 2003, 3 pages, LaTeX, 2 eps figures. PSN FRAP1

Why are Retail Prices in Japan so High?: Evidence from German Export Prices

It is well documented that retail prices in Japan are higher than in other countries for similar products. The two main competing explanations for this finding are: (1) a relatively high degree of discriminatory practices against imports and (2) relatively high distribution costs associated with getting goods to the point of final sale in Japan. The first of these explanations implies that foreign exporters should charge higher prices on shipments to Japan than elsewhere, provided at least some of the rent associated with restrictive practices can be captured by the exporter. For the vast majority of the 37 7-digit German export industries studied here, the data are consistent with this implication. Prices on shipments to Japan appear to be significantly higher than prices on shipments to the United States, the United Kingdom, and Canada.

Perturbation Theory by Flow Equations: Dimerized and Frustrated S=1/2 Chain

The flow equation method (Wegner 1994) is used as continuous unitary transformation to construct perturbatively effective Hamiltonians. The method is illustrated in detail for dimerized and frustrated antiferromagnetic S=1/2 chains. The effective Hamiltonians conserve the number of elementary excitations which are S=1 magnons for the dimerized chains. The sectors of different number of excitations are clearly separated. Easy-to-use results for the gap, the dispersion and the ground state energies of the chains are provided.Comment: 18 pages, 15 figures included, to appear in Eur. Phys. J. B; Electronic data will be made available on appearance of articl

Unitary Gauge, Stueckelberg Formalism and Gauge Invariant Models for Effective Lagrangians

Within the framework of the path-integral formalism we reinvestigate the different methods of removing the unphysical degrees of freedom from spontanously broken gauge theories. These are: construction of the unitary gauge by gauge fixing; \rx -limiting procedure; decoupling of the unphysical fields by point transformations. In the unitary gauge there exists an extra quartic divergent Higgs self-interaction term, which cannot be neglected if perturbative calculations are performed in this gauge. Using the St\"uckelberg formalism this procedure can be reversed, i.~e., a gauge theory can be reconstructed from its unitary gauge. We also discuss the equivalence of effective-Lagrangian theories, containing arbitrary interactions, to (nonlinearly realized) spontanously broken gauge theories and we show how they can be extended to Higgs models.Comment: 20 pages LaTeX, 4 figures available on reques

The ATLAS Pixel Detector

The ATLAS Pixel Detector is the innermost layer of the ATLAS tracking system and will contribute significantly to the ATLAS track and vertex reconstruction. The detector consists of identical sensor-chip-hybrid modules, arranged in three barrels in the centre and three disks on either side for the forward region. The position of the Pixel Detector near the interaction point requires excellent radiation hardness, mechanical and thermal robustness, good long-term stability, all combined with a low material budget. The detector layout, results from final prototyping and the status of production are presented.Comment: Talk at the VERTEX2003 conference, Lake Windermere, UK, September 2003, 6 pages, LaTeX, 6 eps figure

Raman Response in Antiferromagnetic Two-Leg S=1/2 Heisenberg Ladders

The Raman response in the antiferromagnetic 2-leg S=1/2 Heisenberg ladder is calculated for various couplings by continuous unitary transformations. For leg couplings above 80% of the rung coupling a characteristic 2-peak structure occurs with a point of zero intensity within the continuum. Experimental data for CaV_2O_5 and La_yCa_(14-y)Cu_24O_41 are analyzed and the coupling constants are determined. Evidence is found that the Heisenberg model is not sufficient to describe cuprate ladders. We argue that a cyclic exchange term is the appropriate extension.Comment: 4 pages with 4 figures include

The Structure of Operators in Effective Particle-Conserving Models

For many-particle systems defined on lattices we investigate the global structure of effective Hamiltonians and observables obtained by means of a suitable basis transformation. We study transformations which lead to effective Hamiltonians conserving the number of excitations. The same transformation must be used to obtain effective observables. The analysis of the structure shows that effective operators give rise to a simple and intuitive perspective on the initial problem. The systematic calculation of n-particle irreducible quantities becomes possible constituting a significant progress. Details how to implement the approach perturbatively for a large class of systems are presented.Comment: 12 pages, 1 figure, accepted by J. Phys. A: Math. Ge