21,058 research outputs found
Field-free molecular orientation by THz laser pulses at high temperature
We investigate to which extend a THz laser pulse can be used to produce
field-free molecular orientation at high temperature. We consider laser pulses
that can be implemented with the state of the art technology and we show that
the efficiency of the control scheme crucially depends on the parameters of the
molecule. We analyze the temperature effects on molecular dynamics and we
demonstrate that, for some molecules, a noticeable orientation can be achieved
at high temperature.Comment: 13 pages, 7 figure
A theoretical study of microwave beam absorption by a rectenna
The rectenna's microwave power beam absorption limit was theoretically confirmed by two mathematical models descriptive of the microwave absorption process; first one model was based on the current sheet equivalency of a large planar array above a reflector and the second model, which was based on the properties of a waveguide with special imaging characteristics, quantified the electromagnetic modes (field configurations) in the immediate vicinity of a Rectenna element spacing which permit total power beam absorption by preventing unwanted modes from propagating (scattering) were derived using these models. Several factors causing unwanted scattering are discussed
Spin Gaps in Coupled t-J Ladders
Spin gaps in coupled - ladders are investigated by exact
diagonalization of small clusters up to 48 sites. At half-filling, the
numerical results for the triplet excitation spectrum are in very good
agreement with a second order perturbation expansion in term of small
inter-ladder and intra-ladder exchange couplings between rungs
(). The band of local triplet excitations moving
coherently along the ladder (with momenta close to ) is split by the
inter-ladder coupling. For intermediate couplings finite size scaling is used
to estimate the spin gap. In the isotropic infinite 4-chain system (two coupled
ladders) we find a spin gap of , roughly half of the single ladder
spin gap. When the system is hole doped, bonding and anti-bonding bound pairs
of holes can propagate coherently along the chains and the spin gap remains
finite.Comment: 11 pages, 5 figures, uuencoded form of postscript files of figures
and text, LPQTH-94/
Can crack front waves explain the roughness of cracks ?
We review recent theoretical progress on the dynamics of brittle crack fronts
and its relationship to the roughness of fracture surfaces. We discuss the
possibility that the intermediate scale roughness of cracks, which is
characterized by a roughness exponent approximately equal to 0.5, could be
caused by the generation, during local instabilities by depinning, of
diffusively broadened corrugation waves, which have recently been observed to
propagate elastically along moving crack fronts. We find that the theory agrees
plausibly with the orders of magnitude observed. Various consequences and
limitations, as well as alternative explanations, are discussed. We argue that
another mechanism, possibly related to damage cavity coalescence, is needed to
account for the observed large scale roughness of cracks that is characterized
by a roughness exponent approximately equal to 0.8Comment: 26 pages, 3 .eps figure. Submitted to J. Mech. Phys. Solid
Overlapping resonances in the control of intramolecular vibrational redistribution
Coherent control of bound state processes via the interfering overlapping
resonances scenario [Christopher et al., J. Chem. Phys. 123, 064313 (2006)] is
developed to control intramolecular vibrational redistribution (IVR). The
approach is applied to the flow of population between bonds in a model of
chaotic OCS vibrational dynamics, showing the ability to significantly alter
the extent and rate of IVR by varying quantum interference contributions.Comment: 10 pages, 7 figure
Strong Coupling Expansions for Antiferromagnetic Heisenberg S=1/2 Ladders
The properties of antiferromagnetic Heisenberg ladders with
2, 3, and 4 chains are expanded in the ratio of the intra- and interchain
coupling constants. A simple mapping procedure is introduced to relate the 4
and 2-chain ladders which holds down to moderate values of the expansion
parameters. A second order calculation of the spin gap to the lowest triplet
excitation in the 2- and 4-chain ladders is found to be quite accurate even at
the isotropic point where the couplings are equal. Similar expansions and
mapping procedures are presented for the 3-chain ladders which are in the same
universality class as single chains.Comment: 10 physical pages, uuencoded compressed PostScript file including 12
figures, ETH-TH/942
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