587 research outputs found
Knot undulator to generate linearly polarized photons with low on-axis power density
Heat load on beamline optics is a serious problem to generate pure linearly
polarized photons in the third generation synchrotron radiation facilities. For
permanent magnet undulators, this problem can be overcome by a figure-8
operating mode. But there is still no good method to tackle this problem for
electromagnetic elliptical undulators. Here, a novel operating mode is
suggested, which can generate pure linearly polarized photons with very low
on-axis heat load. Also the available minimum photon energy of linearly
polarized photons can be extended much by this method
Upper critical field study in the organic superconductor -(ET)SFCHCFSO : Possibility of Fulde-Ferrell-Larkin-Ovchinnikov state
We report upper critical field measurements in the metal-free-all-organic
superconductor -(ET)SFCHCFSO obtained
from measuring the in-plane penetration depth using the tunnel diode oscillator
technique. For magnetic field applied parallel to the conducting planes the low
temperature upper critical fields are found to exceed the Pauli limiting field
calculated by using a semi-empirical method. Furthermore, we found a signature
that could be the phase transition between the superconducting vortex state and
the Fulde-Ferrell-Larkin-Ovchinnikov state in the form of a kink just below the
upper critical field and only at temperatures below 1.23 K.Comment: 4 pages, 6 figure
Weak ferromagnetism and glassy state in kappa-(BEDT-TTF)2Hg(SCN)2Br
Since the first observation of weak ferromagnetism in the charge-transfer
salt kappa-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840
(1992)], no further evidence of ferromagnetism in this class of organic
materials has been reported. Here we present static and dynamic spin
susceptibility measurements on kappa-(BEDT-TTF)2Hg(SCN)2Br revealing weak
ferromagnetism below about TWF = 20 K. We suggest that frustrated spins in the
molecular dimers suppress long-range order, forming a spin-glass ground state
in the insulating phase
Associations between Proprioceptive Neural Pathway Structural Connectivity and Balance in People with Multiple Sclerosis
Mobility and balance impairments are a hallmark of multiple sclerosis (MS), affecting nearly half of patients at presentation and resulting in decreased activity and participation, falls, injuries, and reduced quality of life. A growing body of work suggests that balance impairments in people with mild MS are primarily the result of deficits in proprioception, the ability to determine body position in space in the absence of vision. A better understanding of the pathophysiology of balance disturbances in MS is needed to develop evidence-based rehabilitation approaches. The purpose of the current study was to (1) map the cortical proprioceptive pathway in vivo using diffusion-weighted imaging and (2) assess associations between proprioceptive pathway white matter microstructural integrity and performance on clinical and behavioral balance tasks. We hypothesized that people with MS (PwMS) would have reduced integrity of cerebral proprioceptive pathways, and that reduced white matter microstructure within these tracts would be strongly related to proprioceptive-based balance deficits. We found poorer balance control on proprioceptive-based tasks and reduced white matter microstructural integrity of the cortical proprioceptive tracts in PwMS compared with age-matched healthy controls (HC). Microstructural integrity of this pathway in the right hemisphere was also strongly associated with proprioceptive-based balance control in PwMS and controls. Conversely, while white matter integrity of the right hemisphere’s proprioceptive pathway was significantly correlated with overall balance performance in HC, there was no such relationship in PwMS. These results augment existing literature suggesting that balance control in PwMS may become more dependent upon (1) cerebellar-regulated proprioceptive control, (2) the vestibular system, and/or (3) the visual system
Anisotropic charge dynamics in the quantum spin-liquid candidate -(BEDT-TTF)Cu(CN)
We have in detail characterized the anisotropic charge response of the dimer
Mott insulator -(BEDT-TTF)\-Cu(CN) by dc conductivity, Hall
effect and dielectric spectroscopy. At room temperature the Hall coefficient is
positive and close to the value expected from stoichiometry; the temperature
behavior follows the dc resistivity . Within the planes the dc
conductivity is well described by variable-range hopping in two dimensions;
this model, however, fails for the out-of-plane direction. An unusually broad
in-plane dielectric relaxation is detected below about 60 K; it slows down much
faster than the dc conductivity following an Arrhenius law. At around 17 K we
can identify a pronounced dielectric anomaly concomitantly with anomalous
features in the mean relaxation time and spectral broadening. The out-of-plane
relaxation, on the other hand, shows a much weaker dielectric anomaly; it
closely follows the temperature behavior of the respective dc resistivity. At
lower temperatures, the dielectric constant becomes smaller both within and
perpendicular to the planes; also the relaxation levels off. The observed
behavior bears features of relaxor-like ferroelectricity. Because
heterogeneities impede its long-range development, only a weak tunneling-like
dynamics persists at low temperatures. We suggest that the random potential and
domain structure gradually emerge due to the coupling to the anion network.Comment: 14 pages, 13 figure
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Design and evaluation of a short period Nb3Sn superconducting undulator prototype
The design of a class of short period superconducting undulators is presented. We begin with a parameter-based analysis that provides insight into potential device performance as a function of the properties of superconducting materials. We present data on candidate low-temperature superconducting materials and the motivation to consider low-copper fraction, high Jc materials. Measured data on recent Nb Sn conductors is provided, together with wire and cable design issues that are tailored for undulator applications. Key design concerns are then addressed, in particular the quench protection system limitations and the system performance. Progress on the construction and testing of a prototype 30mm period device is described.
An Experimental and Semi-Empirical Method to Determine the Pauli-Limiting Field in Quasi 2D Superconductors as applied to -(BEDT-TTF)Cu(NCS): Strong Evidence of a FFLO State
We present upper critical field data for -(BEDT-TTF)Cu(NCS)
with the magnetic field close to parallel and parallel to the conducting
layers. We show that we can eliminate the effect of vortex dynamics in these
layered materials if the layers are oriented within 0.3 degrees of parallel to
the applied magnetic field. Eliminating vortex effects leaves one remaining
feature in the data that corresponds to the Pauli paramagnetic limit ().
We propose a semi-empirical method to calculate the in quasi 2D
superconductors. This method takes into account the energy gap of each of the
quasi 2D superconductors, which is calculated from specific heat data, and the
influence of many body effects. The calculated Pauli paramagnetic limits are
then compared to critical field data for the title compound and other organic
conductors. Many of the examined quasi 2D superconductors, including the above
organic superconductors and CeCoIn, exhibit upper critical fields that
exceed their calculated suggesting unconventional superconductivity. We
show that the high field low temperature state in
-(BEDT-TTF)Cu(NCS) is consistent with the Fulde Ferrell Larkin
Ovchinnikov state.Comment: 8 pages, 9 figures, 10 years of dat
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