A note on the monomer-dimer model

We consider the monomer-dimer model, whose realisations are spanning sub-graphs of a given graph such that every vertex has degree zero or one. The measure depends on a parameter, the monomer activity, which rewards the total number of monomers. We consider general correlation functions including monomer-monomer correlations and dimer-dimer covariances. We show that these correlations decay exponentially fast with the distance if the monomer activity is strictly positive. Our result improves a previous upper bound from van den Berg and is of interest due to its relation to truncated spin-spin correlations in classical spin systems. Our proof is based on the cluster expansion technique.Comment: 7 pages, 1 figur

Coherent phonon dynamics at the martensitic phase transition of Ni_2MnGa

We use time-resolved optical reflectivity to study the laser stimulated dynamics in the magnetic shape memory alloy Ni_2MnGa. We observe two coherent optical phonons, at 1.2 THz in the martensite phase and at 0.7 THz in the pre-martensite phase, which we interpret as a zone-folded acoustic phonon and a heavily damped amplitudon respectively. In the martensite phase the martensitic phase transition can be induced by a fs laser pulse on a timescale of a few ps.Comment: 3 figure

Temperature dependence of the superconducting gap anisotropy in Bi2_{2}Sr2_{2}Ca1_{1}Cu2_{2}O8+x_{8+x}

We present the first detailed data of the momentum-resolved, temperature dependence of the superconducting gap of $Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x}$, complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the $\Gamma-\bar{M}$ and $\Gamma-X$ directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors such as lead. Specifically, the size of the superconducting gap along the $\Gamma-X$ direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the $\Gamma-\bar{M}$ direction.Comment: APS_REVTEX. 19 pages, including 8 figures, available upon request. UW-Madison preprin

Localization Effects in Bi2Sr2Ca(Cu,Co)2O8+y High Temperature Superconductors

Doping Bi2Sr2Ca1Cu2O8+y with Co causes a superconductor-insulator transition. We study correlations between changes in the electrical resistivity RHOab(T) and the electronic bandstructure using identical single crystalline samples. For undoped samples the resistivity is linear in temperature and has a vanishing residual resistivity. In angle resolved photoemission these samples show dispersing band-like states. Co-doping decreases TC and causes and increase in the residual resistivity. Above a threshold Co-concentration the resistivity is metallic (drab/dT >0) at room temperature, turns insulating below a characteristic temperature Tmin and becomes super- conducting at even lower temperature. These changes in the resistivity correlate with the disappearance of the dispersing band-like states in angle resolved photoemission. We show that Anderson localization caused by the impurity potential of the doped Co-atoms provides a consistent explanation of all experimental features. Therefore the TC reduction in 3d-metal doped high- temperature superconductors is not caused by Abrikosov Gor'kov pair- breaking but by spatial localization of the carriers. The observed suppression of TC indicates that the system is in the homogenous limit of the superconductor-insulator transition. The coexistance of insulating (dRHOab/dT <0) normal state behavior and super- conductivity indicates that the superconducting ground state is formed out of spatially almost localized carriers.Comment: Postscript file 11 pages plus 4 figures available on reques

Observation of a van Hove Singularity in Bi2Sr2Ca1Cu2O8+xBi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x} with Angle Resolved Photoemission

We have performed high energy resolution angle-resolved photoemission studies of the normal state band structure of oxygen overdoped $Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x}$. We find that there is an extended saddle point singularity in the density of states along $\Gamma-\bar{M}-Z$ direction. The data also indicate that there is an asymmetry in the Fermi surface for both the $\Gamma-\bar{M}-Z$ and perpendicular directions.Comment: APS_Revtex. 28 pages, including 16 figures, available upon request. UW-Madison preprint#

Coexistence, enhancements and short loops in random walk loop soups

oai:archive.wias-berlin.de:wias_mods_00007475We consider a general random walk loop soup which includes, or is related to, several models of interest, such as the Spin O(N) model, the double dimer model and the Bose gas. The analysis of this model is challenging because of the presence of spatial interactions between the loops. For this model it is known from [30] that macroscopic loops occur in dimension three and higher when the inverse temperature is large enough. Our first result is that, on the d dimensional lattice, the presence of repulsive interactions is responsible for a shift of the critical inverse temperature, which is textitstrictly greater than (1/2d), the critical value in the non interacting case. Our second result is that a positive density of microscopic loops exists for all values of the inverse temperature. This implies that, in the regime in which macroscopic loops are present, microscopic and macroscopic loops textitcoexist. Moreover, we show that, even though the increase of the inverse temperature leads to an increase of the total loop length, the density of microscopic loops textitis uniformly bounded from above in the inverse temperature. Our last result is confined to the special case in which the random walk loop soup is the one associated to the Spin O(N) model with arbitrary integer values of N ≥2 and states that, on ℤ 2, the probability that two vertices are connected by a loop textitdecays at least polynomially fast with their distance

INTERLAYER COUPLING AND THE METAL-INSULATOR TRANSITION IN Pr-SUBSTITUTED Bi(2)Sr(2)CaCu(2)O(8+y)

Substitution of rare-earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a metal-insulator transition. Using resonant photoemission we study how this chemical substitution affects the electronic structure of the material. For the partial Cu-density of states at E_F and in the region of the valence band we observe no significant difference between a pure superconducting sample and an insulating sample with 60% Pr for Ca. This suggests that the states responsible for superconductivity are predomi- nately O-states. The partial Pr-4f density of states was extracted utilizing the Super- Koster-Kronig Pr 4d-4f resonance. It consists of a single peak at 1.36eV binding energy. The peak shows a strongly assymetric Doniach-Sunjic line- shape indicating the presence of a continuum of electronic states with sharp cut off at E_F even in this insulating sample. This finding excludes a bandgap in the insulating sample and supports the existance of a mobility gap caused by spatial localization of the carriers. The presence of such carriers at the Pr-site, between the CuO_2 planes shows that the electronic structure is not purely 2-dimensional but that there is a finite interlayer coupling. The resonance enhancement of the photoemission cross section, at the Pr-4d threshold, was studied for the Pr-4f and for Cu-states. Both the Pr-4f and the Cu-states show a Fano-like resonance. This resonance of Cu-states with Pr-states is another indication of coupling between the the Pr-states and those in the CuO_2 plane. Because of the statistical distribution of the Pr-ions this coupling leads to a non-periodic potential for the states in the CuO_2 plane which can lead to localization and thus to the observed metal-insulator transition.Comment: Gziped uuencoded postscript file including 7 figures Scheduled for publication in Physical Review B, May 1, 1995

Ungewöhnliche Stickstoff-Koordinationen der Selten-Erd-Elemente

Die Arbeit befasst sich mit der Darstellung und Untersuchung von homoleptischen Koordinationsverbindungen der Selten-Erd-Elemente mit einer weitgehend vollständigen Stickstoff-Koordination mit festkörperchemischen Methoden. Bei diesen Arbeiten wurden gängige Stickstoff-Heterozyklen als Amine eingesetzt. Die verwendeten Reaktionsbedingungen wurden gezielt außerhalb des Bereiches der klassischen Lösungsmittelsynthese gewählt, da sich so zahlreiche Möglichkeiten zur Synthese von Verbindungen mit bisher nicht bekannten SE-Stickstoff-Koordinationen bieten. Im Einzelnen umfassen sie festkörperchemische Synthesemethoden, d.h. solvensfreie Synthesemethoden mit "Hochtemperatur"- (Schmelz- und solvothermalen Reaktionen), sowie "Tieftemperatur"-Reaktionen, in denen unter Einsatz flüssigen Ammoniaks als Reaktionsmedium und -partner gearbeitet wird. Bei beiden handelt es sich um Oxidationsreaktionen von Selten-Erd-Metallen mit einem Amin, wobei abhängig von den Ligandeigenschaften der eine und / oder andere Syntheseweg beschritten werden kann. Die Ergebnisse der "Hoch"- und "Tieftemperatur"-Oxidationen unterscheiden sich dabei erheblich. In allen Fällen gelang die Kristallisation der Produkte unter Reaktionsbedingungen. Es wurden erste unsubstituierte Pyrazolate der Selten-Erd-Elemente, neue Strukturtypen von SE-Dipyridylamiden ([SE(N(C5H4N)2)3]2 SE = Sm, Ce) und Pyridylbenzimidazolaten (NC12H8(NH)2[Gd(N3C12H8)4] (Typ IV)), erste unsubstituierte Selten-Erd-Pyrrolate wie z.B. in 1∞[Yb5(C4H4N)12(NH3)4(NH2)]∙2(C4H5N) unter Einbau von NH3 und seinen Ammonolyse-Produkten in die Koordinationssphäre des jeweiligen Zentralteilchens erhalten. So konnten auch [Sm(NH3)9][Sm(Pyr)6], das zwei homoleptische Ionen enthält, sowie das erste molekulare Nitrid eines Selten-Erd-Elementes, [Yb3N(Dpa)6][Yb(Dpa)3] dargestellt werden. Desweiteren wurden im Rahmen dieser Arbeit verschiedene Konkurrenz- und Weiterreaktionen mit zuvor dargestellten Selten-Erd-Amiden durchgeführt, die zum Erhalt von Verbindungen wie [Yb2(Cbz)4(Phpip)4]·2Phpip und [Yb(Bipy)2(Pyr)3] führten und mit den Untersuchungen der thermischen Eigenschaften ausgewählter Verbindungen das Verständnis der Reaktionsabläufe bzw. über die Triebkräfte der Bildung der eingesetzten Selten-Erd-Amide erweitern. Arbeiten mit SE-Precursoren führten im Fall der SE-Halogenide ebenfalls zu neuen, heteroleptischen SE-N-Verbindungen. Aus den in dieser Arbeit vorgestellten Verbindungen lässt sich der Schluss ziehen, dass die Koordinationssphäre um verschiedene Selten-Erd-Atome in SE-Amiden durch Auswahl geeigneter Amine und Reaktionsbedingungen gezielt beeinflussbar ist. Insgesamt konnten im Rahmen dieser Arbeit 20 neue Verbindungen synthetisiert und charakterisiert werden

Diffraction-limited storage rings - a window to the science of tomorrow.

This article summarizes the contributions in this special issue on Diffraction-Limited Storage Rings. It analyses the progress in accelerator technology enabling a significant increase in brightness and coherent fraction of the X-ray light provided by storage rings. With MAX IV and Sirius there are two facilities under construction that already exploit these advantages. Several other projects are in the design stage and these will probably enhance the performance further. To translate the progress in light source quality into new science requires similar progress in aspects such as optics, beamline technology, detectors and data analysis. The quality of new science will be limited by the weakest component in this value chain. Breakthroughs can be expected in high-resolution imaging, microscopy and spectroscopy. These techniques are relevant for many fields of science; for example, for the fundamental understanding of the properties of correlated electron materials, the development and characterization of materials for data and energy storage, environmental applications and bio-medicine