21,803 research outputs found
The spin-half Heisenberg antiferromagnet on two Archimedian lattices: From the bounce lattice to the maple-leaf lattice and beyond
We investigate the ground state of the two-dimensional Heisenberg
antiferromagnet on two Archimedean lattices, namely, the maple-leaf and bounce
lattices as well as a generalized - model interpolating between both
systems by varying from (bounce limit) to (maple-leaf
limit) and beyond. We use the coupled cluster method to high orders of
approximation and also exact diagonalization of finite-sized lattices to
discuss the ground-state magnetic long-range order based on data for the
ground-state energy, the magnetic order parameter, the spin-spin correlation
functions as well as the pitch angle between neighboring spins. Our results
indicate that the "pure" bounce () and maple-leaf () Heisenberg
antiferromagnets are magnetically ordered, however, with a sublattice
magnetization drastically reduced by frustration and quantum fluctuations. We
found that magnetic long-range order is present in a wide parameter range and that the magnetic order parameter varies only
weakly with . At a direct first-order transition to
a quantum orthogonal-dimer singlet ground state without magnetic long-range
order takes place. The orthogonal-dimer state is the exact ground state in this
large- regime, and so our model has similarities to the Shastry-Sutherland
model. Finally, we use the exact diagonalization to investigate the
magnetization curve. We a find a 1/3 magnetization plateau for and another one at 2/3 of saturation emerging only at large .Comment: 9 pages, 10 figure
Experimental vs. Numerical Eigenvalues of a Bunimovich Stadium Billiard -- A Comparison
We compare the statistical properties of eigenvalue sequences for a gamma=1
Bunimovich stadium billiard. The eigenvalues have been obtained by two ways:
one set results from a measurement of the eigenfrequencies of a superconducting
microwave resonator (real system) and the other set is calculated numerically
(ideal system). The influence of the mechanical imperfections of the real
system in the analysis of the spectral fluctuations and in the length spectra
compared to the exact data of the ideal system are shown. We also discuss the
influence of a family of marginally stable orbits, the bouncing ball orbits, in
two microwave stadium billiards with different geometrical dimensions.Comment: RevTex, 8 pages, 8 figures (postscript), to be published in Phys.
Rev.
Large magnetoresistance effect due to spin-injection into a non-magnetic semiconductor
A novel magnetoresistance effect, due to the injection of a spin-polarized
electron current from a dilute magnetic into a non-magnetic semiconductor, is
presented. The effect results from the suppression of a spin channel in the
non-magnetic semiconductor and can theoretically yield a positive
magnetoresistance of 100%, when the spin flip length in the non-magnetic
semiconductor is sufficiently large. Experimentally, our devices exhibit up to
25% magnetoresistance.Comment: 3 figures, submitted for publicatio
Nb3Sn wire shape and cross sectional area inhomogeneity in Rutherford cables
During Rutherford cable production the wires are plastically deformed and
their initially round shape is distorted. Using X-ray absorption tomography we
have determined the 3D shape of an unreacted Nb3Sn 11 T dipole Rutherford
cable, and of a reacted and impregnated Nb3Sn cable double stack.
State-of-the-art image processing was applied to correct for tomographic
artefacts caused by the large cable aspect ratio, for the segmentation of the
individual wires and subelement bundles inside the wires, and for the
calculation of the wire cross sectional area and shape variations. The 11 T
dipole cable cross section oscillates by 2% with a frequency of 1.24 mm (1/80
of the transposition pitch length of the 40 wire cable). A comparatively
stronger cross sectional area variation is observed in the individual wires at
the thin edge of the keystoned cable where the wire aspect ratio is largest.Comment: 6 pages, 11 figures, presented at EUCAS 201
Anderson Localization in a String of Microwave Cavities
The field distributions and eigenfrequencies of a microwave resonator which
is composed of 20 identical cells have been measured. With external screws the
periodicity of the cavity can be perturbed arbitrarily. If the perturbation is
increased a transition from extended to localized field distributions is
observed. For very large perturbations the field distributions show signatures
of Anderson localization, while for smaller perturbations the field
distribution is extended or weakly localized. The localization length of a
strongly localized field distribution can be varied by adjusting the
penetration depth of the screws. Shifts in the frequency spectrum of the
resonator provide further evidence for Anderson localization.Comment: 7 pages RevTex, to be published in Phys. Rev.
Molecular observation of contour-length fluctuations limiting topological confinement in polymer melts
In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron-spin-echo investigations of the single-chain dynamic-structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic-structure factors are quantitatively described by the effect of contour-length fluctuations on the confining tube, establishing this mechanism on a molecular level in space and time
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