Query: WSEAS  University of Bremen

Candidate papers:

abstract: Strain engineering vanadium dioxide thin films is one way to alter this material's characteristic first order transition from semiconductor to metal. In this study we extend the exploitable strain
author_names: Simon Fischer

Jon-Olaf Krisponeit

Michael Foerster

Lucia Aballe

Jens Falta

Jan Ingo Flege
title: Massively Strained VO2 Thin Film Growth on RuO2
year: 2020

abstract: We have investigated the toroidal analog of ellipsoidal shells of matter, which are of great significance in Astrophysics. The exact formula for the gravitational potential \Psi(R,Z) of a shell with a circular section at the pole of toroidal coordinates is first established. It depends on the mass of the shell, its main radius and axis ratio $e$ (i.e. core to main radius ratio) and involves the product of the complete elliptic integrals of the first and second kinds. Next, we show that successive partial derivatives \partial^{n +m} \Psi/\partial_{R^n} \partial_{Z^m} are also accessible by analytical means at that singular point, thereby enabling the expansion of the interior potential as a bivariate series. Then, we have generated approximations at orders $0$ $1$ $2$ and $3$ corresponding to increasing accuracy. Numerical experiments confirm the great reliability of the approach, in particular for small to moderate axis ratios $e^2 \lesssim 0.1$ typically) In contrast with the ellipsoidal case (Newton's theorem) the potential is not uniform inside the shell cavity as a consequence of the curvature. We explain how to construct the interior potential of toroidal shells with a thick edge (i.e. tubes) and how a core stratification can be accounted for. This is a new step towards the full description of the gravitating potential and forces of tori and rings. Applications also concern electrically charged systems, and thus go beyond the context of gravitation.
author_names: Jean-Marc Hur'e

Audrey Trova

Vladim R Karas

CNRSUniv Bordeaux

University of Bremen

Center of Applied Space Technology

Microgravity Germany

A -M Dumont Astronomical Institute

Academy of Sciences

Czech Republic
title: Interior potential of a toroidal shell from pole values
venue: Monthly Notices of the Royal Astronomical Society
year: 2019

abstract: We present results from two ~30 ks Chandra observations of the hot atmospheres of the merging galaxy groups centered around NGC 7618 and UGC 12491. Our images show the presence of arc like sloshing cold fronts (CFs) wrapped around each group center and ~100 kpc long spiral tails in both groups. Most interestingly, the CFs are highly distorted in both groups, exhibiting "wings" along the fronts. These features resemble the structures predicted from non viscous hydrodynamic simulations of gas sloshing, where Kelvin Helmholtz instabilities (KHIs) distort the CFs. This is in contrast to the structure seen in many other sloshing and merger CFs, which are smooth and featureless at the current observational resolution. Both magnetic fields and viscosity have been invoked to explain the absence of KHIs in these smooth CFs, but the NGC 7618/UGC 12491 pair are two in a growing number of both sloshing and merger CFs that appear distorted. Magnetic fields and/or viscosity may be able to suppress the growth of KHIs at the CFs in some clusters and groups, but clearly not in all. We propose that the presence or absence of KHI distortions in CFs can be used as a measure of the effective viscosity and/or magnetic field strengths in the intracluster medium.
author_names: Elke Roediger

Ralph P Kraft

Marie E Machacek

William R Forman

P E J Nulsen

C Jones

S Heinz Jacobs University Bremen

Harvard--Smithsonian Center for Astrophysics

Department of Engineering Physics

The Johns Hopkins University
title: Irregular Sloshing Cold Fronts in the nearby Merging Groups NGC 7618 and UGC 12491: Evidence for Kelvin Helmholtz Instabilities
year: 2012

abstract: In this paper we present results of two novel experimental methods to investigate the collisional behavior of individual macroscopic icy bodies. The experiments reported here were conducted in the microgravity environments of parabolic flights and the Bremen drop tower facility. Using a cryogenic parabolic flight setup, we were able to capture 41 near central collisions of 1.5 cm sized ice spheres at relative velocities between 6 and 22 cm s 1 The analysis of the image sequences provides a uniform distribution of coefficients of restitution with a mean value of e 0.45 and values ranging from e 0.06 to 0.84. Additionally, we designed a prototype drop tower experiment for collisions within an ensemble of up to one hundred cm sized projectiles and performed the first experiments with solid glass beads. We were able to statistically analyze the development of the kinetic energy of the entire system, which can be well explained by assuming a granular'fluid' following Haff's law with a constant coefficient of restitution of e 0.64. We could also show that the setup is suitable for studying collisions at velocities of <5 mm s 1 appropriate for collisions between particles in Saturn's dense main rings.
author_names: Daniel Heisselmann

Jurgen Blum

Helen Jane Fraser

K Maltman Institut fur Theor Physik

Technische Universitat Braunschweig

University of Strathclyde

United Kingdom
title: Microgravity experiments on the collisional behavior of Saturnian ring particles
year: 2010

abstract: Real time electron microscopy observation on morphological changes in gold nanostructures deposited on Si (1?0?0) surfaces as a function of annealing temperatures has been reported. Two types of interfaces with silicon substrates were used prior to gold thin film deposition: (i) without native oxide and on ultra clean reconstructed Si surfaces and (ii) with native oxide covered Si surfaces. For ?2.0?nm thick Au films deposited on reconstructed Si (1?0?0) surfaces using the molecular beam epitaxy method under ultra high vacuum conditions, aligned four fold symmetric nanogold silicide structures formed at relatively lower temperatures (compared with the one with native oxide at the interface) For this system, 82% of the nanostructures were found to be nanorectangle like structures with an average length of ?27?nm and aspect ratio of 1.13 at ?700??C. For ?5.0?nm thick Au films deposited on Si (1?0?0) surface with native oxide at the interface, the formation of a rectangular structure was observed at higher temperatures ?850??C) At these high temperatures, desorption of gold silicide followed the symmetry of the substrate. Native oxide at the interface was found to act like a barrier for the inter diffusion phenomena. Structural characterization was carried out using advanced electron microscopy methods.
author_names: Ashutosh Rath

Jatis Kumar Dash

Raghavendra Rao Juluri

Andreas Rosenauer

P V Satyam Institute of Physics

Sachivalaya Marg

India Department of Physics

University of Bremen
title: Temperature dependent electron microscopy study of Au thin films on Si (1 0 0) with and without a native oxide layer as barrier at the interface
year: 2011

abstract: Solar UV variability is extremely relevant for the stratospheric ozone. It has an impact on Earth's atmospheric structure and dynamics through radiative heating and ozone photochemistry. Our goal is to study the slope of the solar UV spectrum in two UV bands important to the stratospheric ozone production. In order to investigate the solar spectral variability, we use data from SOLSTICE (the Solar Stellar Irradiance Comparison Experiment) on board the Solar Radiation and Climate Experiment (SORCE) satellite. Datasets used are far UV (115 180 nm) and middle UV (180 310 nm) as well as the Mg II index (the Bremen composite) We introduce the SOLSTICE [FUV MUV] colour to study the solar spectral characteristics, as well as to analyse the colour versus Mg II index. To isolate the 11 year scale variation, we used Empirical Mode Decomposition (EMD) on the datasets. The [FUV MUV] colour strongly correlates with the Mg II index. The [FUV MUV] colour shows a time dependent behaviour when plotted versus the Mg II index. To explain this dependence we hypothesize an efficiency reduction of SOLSTICE FUV irradiance using an exponential ageing law.
author_names: Milan Lovric

Federico Tosone

Ermanno Pietropaolo

Dario Del Moro

Luca Giovannelli

Chiara Cagnazzo

Finland Department of Physics

Univ Roma Tor Vergata

Department of Engineering Physics

University of L'Aquila

Institute for Nuclear Research of the Russian Academy of Sciences
title: The dependence of the [FUV MUV] colour on solar cycle
year: 2016

abstract: We report on the formation of oriented gold nanostructures on Si(100) substrate by annealing procedures in low vacuum (10 2 mbar) and at high temperature (975 degC) Various thicknesses of gold films have been deposited with SiOx (using high vacuum thermal evaporation) and without SiOx (using molecular beam epitaxy) at the interface on Si(100) Electron microscopy measurements were performed to determine the morphology, orientation of the structures and the nature of oxide layer. Interfacial oxide layer, low vacuum and high temperature annealing conditions are found to be necessary to grow oriented gold structures. These gold structures can be transferred by simple scratching method.
author_names: Ashutosh Rath

Jatis Kumar Dash

Raghavendra Rao Juluri

Andreas Rosenauer

Marcos Schoewalter

P V Satyam Institute of Physics

Sachivalaya Marg

India Department of Physics

University of Bremen
title: Growth of oriented Au nanostructures: Role of oxide at the interface
year: 2012

abstract: Institut fu"r Theoretische Physik, Universita"t Bremen Otto Hahn Allee 1, D 28359 Bremen, GermanyReceived December, 27th 2006/ accepted ?ABSTRACTContext.We study the planet populations in the discovery window of the CoRoT space telescope scheduled for launch on December27th. We base the prediction on 'first principles' calculati ons of planet formation in the framework of the planetesimal hypothesis.Aims.To provide a priori planetary initial mass functions for confrontation with the CoRoT planet discoveries in the entire range ofsensitivity of the CoRoT instrument, i.e. for all giant planets and down to terrestrial planet masses.Methods. We construct a comprehensive set of static complete equilibrium core envelope protoplanets with detailed equationsofstate and opacity and radiative transfer by convection and radiation. Protoplanets are calculated for host star masses of 0.8 to 2 solarmasses and orbital periods of 1 to 16 days. We subsequently check the stability of the planetary population by a series of methods.Results.We find the static planetary populations to be stable and thus a plausible ensemble to predict the planetary IMF for orbitalperiods in the specified range.Conclusions.We predict bimodal planetary initial mass functions with shapes depending on orbital period. The two main maximaare around a Jupiter mass and about 50 earth masses. We predict an abundant population of Hot Neptunes and a large population ofplanets that fill the solar system gap of planetary masses between Neptune and Saturn.Key words. planets:formation exoplanets planets: mass solar syst em:formation initial mass function
author_names: Gunther Wuchterl

Christopher Broeg

S W Krause

Bojan Pecnik

Jochen Eisloeffel Thueringer Landessternwarte Tautenburg

Department of Engineering Physics

University of Split

Institut fuer Theoretische Physik

Bremen University
title: Properties of the short period CoRoT planet population I: Theoretical planetary mass spectra for a population of stars of 0.8 to 2 solar masses and orbital periods of less then 20 days.
year: 2008

abstract: We report on the phase separation in Au Ge system leading to the formation of lobe lobe (bi lobed) Au Ge nanostructures under ultra high vacuum (UHV) conditions (3 x 10 10 mbar) on clean Si(100) surfaces. For this study, 2.0 nm thick Au samples were grown on the substrate surface by molecular beam epitaxy. Thermal annealing was carried out inside the UHV chamber at temperature 500 degC and following this, nearly square shaped AuxSi1 x nano structures of average length 48 nm were formed. A 2 nm Ge film was further deposited on the above surface while the substrate was kept at a temperature of 500 degC. Well ordered Au Ge nanostructures where Au and Ge residing side by side (lobe lobe structures) were formed. In our systematic studies, we show that, gold silicide nanoalloy formation at the substrate (Si) surface is necessary for forming phase separated Au Ge bilobed nanostructures. These results show that the Au Ge bonding is unstable in nature. Electron microscopy (TEM, STEM EDS, SEM) studies were carrie.
author_names: Ashutosh Rath

Jatis Kumar Dash

Raghavendra Rao Juluri

Marco Schowalter

K N Mueller

Andreas Rosenauer

P V Satyam Institute of Physics

Sachivalaya Marg

India Department of Physics

University of Bremen
title: Nano scale phase separation in Au Ge system on ultra clean Si(100) surfaces
year: 2012

abstract: Two questions on the topology of compact energy surfaces of natural two degrees of freedom Hamiltonian systems in a magnetic field are discussed. We show that the topology of this 3 manifold (if it is not a unit tangent bundle) is uniquely determined by the Euler characteristic of the accessible region in configuration space. In this class of 3 manifolds for most cases there does not
author_names: Alexey Bolsinov

Holger R Dullin

Andrea E Schweitzer Ohio State University

University of Bremen
title: Topology of energy surfaces and existence of transversal Poincare sections
year: 1996