257 research outputs found
Disentangling multipole resonances through a full x-ray polarization analysis
Complete polarization analysis applied to resonant x-ray scattering at the Cr
K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted
into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994
eV). The physical mechanism behind this phenomenon is a subtle interference
effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions,
leading to a phase shift between the respective scattering amplitudes. This
effect may be exploited to disentangle two close-lying resonances that appear
as a single peak in a conventional energy scan, in this way allowing to single
out and identify the different multipole order parameters involved.Comment: 6 pages, 6 figure
Influence of static Jahn-Teller distortion on the magnetic excitation spectrum of PrO2: A synchrotron x-ray and neutron inelastic scattering study
A synchrotron x-ray diffraction study of the crystallographic structure of
PrO2 in the Jahn-Teller distorted phase is reported. The distortion of the
oxygen sublattice, which was previously ambiguous, is shown to be a chiral
structure in which neighbouring oxygen chains have opposite chiralities. A
temperature dependent study of the magnetic excitation spectrum, probed by
neutron inelastic scattering, is also reported. Changes in the energies and
relative intensities of the crystal field transitions provide an insight into
the interplay between the static and dynamic Jahn-Teller effects.Comment: 7 pages, 6 figure
KINETICS OF THE COMPUTER-SIMULATED TENNIS STROKE WITH DIFFERENT RACKETS
INTRODUCTION: The aim of this biomechanical analysis of the tennis stroke is the determination of the effects of the mass properties of different tennis rackets' on the kinetics of the striking arm. In contrast to experimental investigation the computer simulation gives an infinite temporal resolution so that the arm movements could be investigated especially during the racket-ball-contact phase. The planar model of the tennis stroke consisting of the immovable trunk, the upper arm, the lower arm, the hand and• the racket was derived from the mo ei of the human body by GLITSCH (1993). The striking arm with a racket tightly fixed to the hand was constructed as a pendulum of three rigid bodies which are connected with frictionless revolute joints. An elastic spring (0=45000 N/m) represents the ballracket-contact. The arm-racket-system• rotates around the shoulder joint and hits the resting ball in the respective" racket area centre. Considering rigid body mechanics the mass distributions of three different tennis rackets were measured and served as input for the model. The computer simulation with the initial conditions referring to real tennis strakes registered by KNUDSON (1990) was carried out with the software-packet DADS (Dynamic Analysis and Design System) by CADSI (Computer Aided Design Software Inc.). RESULTS: As it is shown in figure 1 the computer simulation has calculated an elbow flexion when rackets 1 or 2 are used. When the tennis forehand stroke is carried out with racket 3 the elbow is kept extended during the ball-racket-contact phase. The quite different arm movements during the impact with different tennis rackets are the result• of the separate locations of the centres of percussion with respect to the rigid handracket-system. The centre of percussion of racket 3 (5.3 cm) is located more distally than its centre of area because of the different mass distribution and finally because of its greater moment of inertia. In contrast to that the centre of percussion of the other two rackets (racket 1: -3.6 cm, racket 2: -3A cm) are located more proximally than the hitting point. CONCLUSION: The mechanical properties of tennis rackets, particularly the mass distribution, are responsible for different and movements during the ball-racket-contact phase. Obviously, there is no consensus of the preferable mass distribution of modern tennis rackets. This model can objectively assist in choosing one's individual favourable racket. REFERENCES: Glitsch U., Farkas R. (1993): Applications of a multi-body simulation model in human movement studies. Proc. Int. Soc. of Biomech., XIV1h congress, Paris. Knudson, DV (1990): Intrasubject variability of upper extremity angular kinematics on the tennis forehand drive. Int. J. of Sport Biomech., 6, 415-421
X-ray polarization: General formalism and polarization analysis
The polarization of x-rays plays an outstanding role in experimental
techniques such as non-resonant magnetic x-ray scattering and resonant x-ray
scattering of magnetic and multipolar order. Different instrumental methods
applied to synchrotron light can transform its natural polarization into an
arbitrary polarization state. Several synchrotron applications, in particular
in the field of magnetic and resonant scattering rely on the improvement in the
signal/noise ratio or the deeper insight into the ordered state and the
scattering process made possible through these polarization techniques. Here,
we present the mathematical framework for the description of fully and
partially polarized x-rays, with some applications such as linear x-ray
polarization analysis for the determination of the scattered beam's
polarization, and the Ge K-edge resonant scattering.Comment: 12 pages, 6 figures and 1 table. To be published in proceedings of
the workshop "Resonant Elastic X-ray Scattering", Aussois, France (2011
Mean-field model of the ferromagnetic ordering in the superconducting phase of ErNi_2B_2C
A mean-field model explaining most of the details in the magnetic phase
diagram of ErNi_2B_2C is presented. The low-temperature magnetic properties are
found to be dominated by the appearance of long-period commensurate structures.
The stable structure at low temperatures and zero field is found to have a
period of 40 layers along the a direction, and upon cooling it undergoes a
first-order transition at T_C = 2.3 K to a different 40-layered structure
having a net ferromagnetic component of about 0.4 mu_B/Er. The
neutron-diffraction patterns predicted by the two 40-layered structures, above
and below T_C, are in agreement with the observations of Choi et al.Comment: 4 pages, 3 figures (Revtex4
Lockin to Weak Ferromagnetism in TbNi2B2C and ErNi2B2C
This article describes a model in which ferromagnetism necessarily
accompanies a spin-density-wave lockin transition in the borocarbide structure
provided the commensurate phase wave vector satisfies Q = (m/n)a* with m even
and n odd. The results account for the magnetic properties of TbNi2B2C, and are
also possibly relevant also for those of ErNi2B2C.Comment: 4 page
High-field irreversible moment reorientation in the antiferromagnet FeTe
Magnetization measurements have been performed on single-crystalline
FeTe in pulsed magnetic fields up to 53 T
and temperatures from 4.2 to 65 K. At K, a non-reversible reorientation
of the antiferromagnetic moments is observed at T as the pulsed
field is on the rise. No anomaly is observed at during the fall of the
field and, as long as the temperature is unchanged, during both rises and falls
of additional field pulses. The transition at is reactivated if the
sample is warmed up above the N\'{e}el temperature K and cooled
down again. The magnetic field-temperature phase diagram of FeTe in
is also investigated. We present the temperature
dependence of , as well as that of the antiferromagnetic-to-paramagnetic
borderline in temperatures above 40 K.Comment: 5 pages, 4 figure
Magnetic ordering in GdNi2B2C revisited by resonant x-ray scattering: evidence for the double-q model
Recent theoretical efforts aimed at understanding the nature of
antiferromagnetic ordering in GdNi2B2C predicted double-q ordering. Here we
employ resonant elastic x-ray scattering to test this theory against the
formerly proposed, single-q ordering scenario. Our study reveals a satellite
reflection associated with a mixed-order component propagation wave vector,
viz., (q_a,2q_b,0) with q_b = q_a approx= 0.55 reciprocal lattice units, the
presence of which is incompatible with single-q ordering but is expected from
the double-q model. A (3q_a,0,0) wave vector (i.e., third-order) satellite is
also observed, again in line with the double-q model. The temperature
dependencies of these along with that of a first-order satellite are compared
with calculations based on the double-q model and reasonable qualitative
agreement is found. By examining the azimuthal dependence of first-order
satellite scattering, we show the magnetic order to be, as predicted,
elliptically polarized at base temperature and find the temperature dependence
of the "out of a-b plane" moment component to be in fairly good agreement with
calculation. Our results provide qualitative support for the double-q model and
thus in turn corroborate the explanation for the "magnetoelastic paradox"
offered by this model.Comment: 8 pages, 5 figures. Submitted to Phys. Rev.
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