5,004 research outputs found
Bremsstrahlung from relativistic bare heavy ions in single crystals
We calculate the bremsstrahlung emitted by heavy bare ions penetrating single crystals at highly relativistic energies. The main component, originating in scattering of the virtual photons of screened target nuclei on the projectile, appears with energies of, approximately, 25γ MeV for a lead ion (γ ≡ E/Mc2 where E and M denote projectile energy and mass). It shows dips in yield upon variation of the incidence angle to major crystallographic directions quite similar to those of other close-encounter processes. Incoherent interaction with single target electrons gives rise to two additional but quite different bremsstrahlung components, a moderate component distributed over the same frequencies as the main component, and a strong component confined to low energies. The channeling characteristics of the total bremsstrahlung spectrum vary substantially with photon energy
Doped coupled frustrated spin-1/2 chains with four-spin exchange
The role of various magnetic inter-chain couplings is investigated by
numerical methods in doped frustrated quantum spin chains. A non-magnetic
dopant introduced in a gapped spin chain releases a free spin-1/2 soliton. The
formation of a local magnetic moment is analyzed in term of soliton
confinement. A four-spin coupling which might originate from cyclic exchange is
shown to produce such a confinement in contrast to transverse magnetic
exchange. Dopants on different chains experience an effective space-extended
non-frustrating pairwise spin interaction.Comment: Few modifications and references added. Submitted to PR
The Ground State Energy of Heavy Atoms According to Brown and Ravenhall: Absence of Relativistic Effects in Leading Order
It is shown that the ground state energy of heavy atoms is, to leading order,
given by the non-relativistic Thomas-Fermi energy. The proof is based on the
relativistic Hamiltonian of Brown and Ravenhall which is derived from quantum
electrodynamics yielding energy levels correctly up to order Ry
The antiferromagnetic order in an F-AF random alternating quantum spin chain : (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3
A possibility of the uniform antiferromagnetic order is pointed out in an
S=1/2 ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg
quantum spin chain compound: (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3. The system
possesses the bond alternation of strong random bonds that take +/- 2J and weak
uniform AF bonds of -J. In the pure concentration limits, the model reduces to
the AF-AF alternation chain at x=0 and to the F-AF alternation chain at x=1.
The nonequilibrium relaxation of large-scale quantum Monte Carlo simulations
exhibits critical behaviors of the uniform AF order in the intermediate
concentration region, which explains the experimental observation of the
magnetic phase transition. The present results suggest that the uniform AF
order may survive even in the presence of the randomly located ferromagnetic
bonds.Comment: 4 pages, 3 figure
Final results from the EU project AVATAR: aerodynamic modelling of 10 MW wind turbines
This paper presents final results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Special attention is paid to the improvement of low fidelity engineering (BEM based) models with higher fidelity (CFD) models but also with intermediate fidelity free vortex wake (FVW) models. The latter methods were found to be a good basis for improvement of induction modelling in engineering methods amongst others for the prediction of yawed cases, which in AVATAR was found to be one of the most challenging subjects to model. FVW methods also helped to improve the prediction of tip losses. Aero-elastic calculations with BEM based and FVW based models showed that fatigue loads for normal production cases were over predicted with approximately 15% or even more. It should then be realised that the outcome of BEM based models does not only depend on the choice of engineering add-ons (as is often assumed) but it is also heavily dependent on the way the induced velocities are solved. To this end an annulus and element approach are discussed which are assessed with the aid of FVW methods. For the prediction of fatigue loads the so-called element approach is recommended but the derived yaw models rely on an annulus approach which pleads for a generalised solution method for the induced velocities
HortiBot: A System Design of a Robotic Tool Carrier for High-tech Plant Nursing
Danish organic outdoor gardeners today use 50-300 hours per hectare for manual weeding.
Through automatic controlling of an existing commercial machine this often heavy and costconsuming
weeding will be eliminated. At the same time, a fully-automatic registration of field
activities will contribute to the efficient implementation of EU directive 178/2002 concerning
traceability in the primary production and thereby enhance the food-safety in the production
chain. A radio controlled slope mower is equipped with a new robotic accessory kit. This
transforms it into a tool carrier (HortiBot) for high-tech plant nursing for e.g. organic grown
vegetables. The HortiBot is capable of passing over several parcels with visible rows
autonomously based on a new commercial row detection system from Eco-Dan a/s, Denmark.
This paper presents the solutions chosen for the HortiBot with regard to hardware, mechanicalelectrical
interfaces and software. Further, the principles from a Quality Function Deployment
(QFD) analysis was used to carry out the solicitation, evaluation and selection of most qualified
design parameters and specifications attained to a horticultural robotic tool carrier. The QFD
analysis provided a specific measure to evaluate each selected parameter in terms of satisfying
user requirements and operational performance aspects. Based on a combination of importance
rating and competitive priority ratings important user requirements include easy adaptation to
field conditions in terms of row distance and parcel size, profitability, minimum crop damage
during operation, and reliability. Lesser importance was attributed to affection value, attractive
look, the possibility of out of season usage, and the use of renewable energy
Early motor developmental milestones and level of neuroticism in young adulthood:a 23-year follow-up study of the Copenhagen Perinatal Cohort
Structure of boson systems beyond the mean-field
We investigate systems of identical bosons with the focus on two-body
correlations. We use the hyperspherical adiabatic method and a decomposition of
the wave function in two-body amplitudes. An analytic parametrization is used
for the adiabatic effective radial potential. We discuss the structure of a
condensate for arbitrary scattering length. Stability and time scales for
various decay processes are estimated. The previously predicted Efimov-like
states are found to be very narrow. We discuss the validity conditions and
formal connections between the zero- and finite-range mean-field
approximations, Faddeev-Yakubovskii formulation, Jastrow ansatz, and the
present method. We compare numerical results from present work with mean-field
calculations and discuss qualitatively the connection with measurements.Comment: 26 pages, 6 figures, submitted to J. Phys. B. Ver. 2 is 28 pages with
modified figures and discussion
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