1,617 research outputs found
VLBI2010 - The TWIN radio telescope project at Wettzell, Germany
The Twin Telescope Wettzell (TTW) Project is funded to be exec
uted during the period of 2008-2011. The design of the TTW was based on the VLBI2010 vision of the corresponding IVS Working Group. In the first two project years the design passed the simulations with respect to its specifications and was approved for production. At the Geodetic Observatory Wettzell a thorough soil analysis was made in order to define the sites for the towers of the new radio telescopes. Meanwhile the construction work has begun and acceptance tests of several telescope parts, e.g. azimuth bearings, took place. The full assembly of the radio telescopes is scheduled for the next two years. In parallel to the construction work at the Wettzell site, the design work for the different feed options progressed
Direct Dynamics Simulations Using Hessian-Based Predictor-Corrector Integration Algorithms
In previous research [J. Chem. Phys.111, 3800 (1999)] a Hessian-based integration algorithm was derived for performing direct dynamics simulations. In the work presented here, improvements to this algorithm are described. The algorithm has a predictor step based on a local second-order Taylor expansion of the potential in Cartesian coordinates, within a trust radius, and a fifth-order correction to this predicted trajectory. The current algorithm determines the predicted trajectory in Cartesian coordinates, instead of the instantaneous normal mode coordinates used previously, to ensure angular momentumconservation. For the previous algorithm the corrected step was evaluated in rotated Cartesian coordinates. Since the local potential expanded in Cartesian coordinates is not invariant to rotation, the constants of motion are not necessarily conserved during the corrector step. An approximate correction to this shortcoming was made by projecting translation and rotation out of the rotated coordinates. For the current algorithm unrotated Cartesian coordinates are used for the corrected step to assure the constants of motion are conserved. An algorithm is proposed for updating the trust radius to enhance the accuracy and efficiency of the numerical integration. This modified Hessian-based integration algorithm, with its new components, has been implemented into the VENUS/NWChem software package and compared with the velocity-Verlet algorithm for the H2CO→H2+CO, O3+C3H6, and F−+CH3OOH chemical reactions
Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO
The microscopic description of the spin-Peierls transition in pure and doped
CuGeO_3 is developed taking into account realistic details of crystal
structure. It it shown that the presence of side-groups (here Ge) strongly
influences superexchange along Cu-O-Cu path, making it antiferromagnetic.
Nearest-neighbour and next-nearest neighbour exchange constants and
are calculated. Si doping effectively segments the CuO_2-chains
leading to or even slightly ferromagnetic. Strong
sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be
responsible for the spin-Peierls transition itself (``bond-bending mechanism''
of the transition). The nature of excitations in the isolated and coupled
spin-Peierls chains is studied and it is shown that topological excitations
(solitons) play crucial role. Such solitons appear in particular in doped
systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the
phase diagram.Comment: 7 pages, revtex, 7 Postscript figure
Temperature Dependence of Spin and Bond Ordering in a Spin-Peierls System
We investigate thermodynamic properties of a one-dimensional S=1/2
antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum
Monte Carlo method. In particular we study how spin and lattice dimerize as a
function of the temperature, which gives a fundamental process of the
spin-Peierls transition in higher dimensions. The degree of freedom of the
lattice is taken into account adiabatically and the thermal distribution of the
lattice distortion is obtained by the thermal bath algorithm. We find that the
dimerization develops as the temperature decreases and it converges to the
value of the dimerization of the ground state at T=0. Furthermore we find that
the coupling constants of spins fluctuate quite largly at high temperature and
there thermodynamic properties deviate from those of the uniform chain. Doping
of non-magnetic impurities causes cut of the chain into short chains with open
boundary. We investigate thermodynamic properties of open chains taking
relaxation of the lattice into consideration. We find that strong bonds locate
at the edges and a defect of the bond alternation appears in the chain with odd
number of sites, which causes enhancement of the staggered magnetic order. We
find a spreaded staggered structure which indicates that the defect moves
diffusively in the chain even at very low temperature.Comment: 7 pages, 17 figures; added comments on section 2 and 3, corrected
typo
Structural Critical Scattering Study of Mg-Doped CuGeO3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at
which the spin gap is established is significantly higher than the temperature
T_s at which the SP dimerization attains long-range order. The latter is
observed only for xx_c the SP correlation length
quickly decreases with increasing x. We argue that impurity-induced competing
interactions play a central role in these phenomena.Comment: 5 pages, 4 embedded eps figures, to appear in PR
Dynamics in the dimerised and high field incommensurate phase of CuGeO
Temperature (\ K) and magnetic field (\ T) dependent far
infrared absorption spectroscopy on the spin-Peierls coumpound CuGeO\ has
revealed several new absorption processes in both the dimerised and high field
phase of CuGeO. These results are discussed in terms of the modulation of
the CuGeO\ structure. At low fields this is the well known spin-Peierls
dimerisation. At high fields the data strongly suggests a field dependent
incommensurate modulation of the lattice as well as of the spin structure.Comment: 12 pages (revtex), 2 figures (eps), csh selfextracting .uu file, To
appear in PRB-Rapid Com
Mean-field theory of the spin-Peierls systems: Application to CuGeO3
A mean-field theory of the spin Peierls systems based on the two dimensional
dimerized Heisenberg model is proposed by introducing an alternating bond order
parameter. Improvements with respect to previous mean-field results are found
in the one-dimensional limit for the ground state and the gap energies. In two
dimensions, the analysis of the competition between antiferromagnetic long
range order and the spin-Peierls ordering is given as a function of the
coupling constants. We show that the lowest energy gap to be observed does not
have a singlet-triplet character in agreement with the low temperature
thermodynamic properties of CuGeO3.Comment: 3 Revtex pages. Submitted to Rapid Comm. Figures available upon
reques
Spin-Vacancy-Induced Long-Range Order in a New Haldane-Gap Antiferromagnet
Magnetic susceptibility, high-field magnetization and inelastic neutron
scattering experiments are used to study the magnetic properties of a new S=1
quasi-1-dimensional antiferromagnet PbNi2V2O8. Inter-chain interactions are
shown to be almost, but not quite, strong enough to destroy the nonmagnetic
singlet ground state and the energy gap in the magnetic excitation spectrum.
Substituting nonmagnetic Mg (S=0) ions for Ni (S=1) induces a
magnetically ordered state at low temperatures. To our knowledge this is the
first observation of doping-induced long-range order in a Haldane-gap system.Comment: 5 pages including 4 figure
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