75,952 research outputs found
Impurity susceptibility and the fate of spin-flop transitions in lightly-doped La(2)CuO(4)
We investigate the occurrence of a two-step spin-flop transition and spin
reorientation when a longitudinal magnetic field is applied to lightly
hole-doped La(2)CuO(4). We find that for large and strongly frustrating
impurities, such as Sr in La(2-x)Sr(x)CuO(4), the huge enhancement of the
longitudinal susceptibility suppresses the intermediate flop and the
reorientation of spins is smooth and continuous. Contrary, for small and weakly
frustrating impurities, such as O in La(2)CuO(4+y), a discontinuous spin
reorientation (two-step spin-flop transition) takes place. Furthermore, we show
that for La(2-x)Sr(x)CuO(4) the field dependence of the magnon gaps differs
qualitatively from the La(2)CuO(4) case, a prediction to be verified with Raman
spectroscopy or neutron scattering.Comment: 4 pages, 3 figures, For the connection between spin-flops and
magnetoresistance, see cond-mat/061081
Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets: I. Theory
We consider theoretically the effects of an applied uniform magnetic field on
the magnetic spectrum of anisotropic two-dimensional and Dzyaloshinskii-Moriya
layered quantum Heisenberg antiferromagnets. The first case is relevant for
systems such as the two-dimensional square lattice antiferromagnet
Sr(2)CuO(2)Cl(2), while the later is known to be relevant to the physics of the
layered orthorhombic antiferromagnet La(2)CuO(4). We first establish the
correspondence betwenn the low-energy spectrum obtained within the anisotropic
non-linear sigma model and by means of the spin-wave approximation for a
standard easy-axis antiferromagent. Then, we focus on the field-theory approach
to calculate the magnetic field dependence of the magnon gaps and spectral
intensities for magnetic fields applied along the three possible
crystallographic directions. We discuss the various possible ground states and
their evolution with temperature for the different field orientations, and the
occurrence of spin-flop transitions for fields perpendicular to the layers
(transverse fields) as well as for fields along the easy axis (longitudinal
fields). Measurements of the one-magnon Raman spectrum in Sr(2)CuO(2)Cl(2) and
La(2)CuO(4) and a comparison between the experimental results and the
predictions of the present theory will be reported in part II of this research
work [L. Benfatto et al., cond-mat/0602664].Comment: 21 pages, 11 figures, final version. Part II of the present work is
presented in cond-mat/060266
Calibration of longwavelength exotech model 20-C spectroradiometer
A brief description of the Exotech model 20-C field spectroradiometer which measures the spectral radiance of a target in the wavelength ranges 0.37 to 2.5 microns (short wavelength unit), 2.8 to 5.6 microns and 7.0 to 14 microns (long wavelength unit) is given. Wavelength calibration of long wavelength unit was done by knowing the strong, sharp and accurately known absorption bands of polystyrene, atmospheric carbon dioxide and methyl cyclohexane (liquid) in the infrared wavelength region. The spectral radiance calibration was done by recording spectral scans of the hot and the cold blackbodies and assuming that spectral radiance varies linearly with the signal
Magnetically assisted self-injection and radiation generation for plasma based acceleration
It is shown through analytical modeling and numerical simulations that
external magnetic fields can relax the self-trapping thresholds in plasma based
accelerators. In addition, the transverse location where self-trapping occurs
can be selected by adequate choice of the spatial profile of the external
magnetic field. We also find that magnetic-field assisted self-injection can
lead to the emission of betatron radiation at well defined frequencies. This
controlled injection technique could be explored using state-of-the-art
magnetic fields in current/next generation plasma/laser wakefield accelerator
experiments.Comment: 7 pages, 4 figures, accepted for publication in Plasma Physics and
Controlled Fusio
Magnetic monopole and string excitations in a two-dimensional spin ice
We study the magnetic excitations of a square lattice spin-ice recently
produced in an artificial form, as an array of nanoscale magnets. Our analysis,
based upon the dipolar interaction between the nanomagnetic islands, correctly
reproduces the ground-state observed experimentally. In addition, we find
magnetic monopole-like excitations effectively interacting by means of the
usual Coulombic plus a linear confining potential, the latter being related to
a string-like excitation binding the monopoles pairs, what indicates that the
fractionalization of magnetic dipoles may not be so easy in two dimensions.
These findings contrast this material with the three-dimensional analogue,
where such monopoles experience only the Coulombic interaction. We discuss,
however, two entropic effects that affect the monopole interactions: firstly,
the string configurational entropy may loose the string tension and then, free
magnetic monopoles should also be found in lower dimensional spin ices;
secondly, in contrast to the string configurational entropy, an entropically
driven Coulomb force, which increases with temperature, has the opposite effect
of confining the magnetic defects.Comment: 8 pages. Accepted by Journal of Applied Physics (2009
Ion motion in the wake driven by long particle bunches in plasmas
We explore the role of the background plasma ion motion in self-modulated
plasma wakefield accelerators. We employ J. Dawson's plasma sheet model to
derive expressions for the transverse plasma electric field and ponderomotive
force in the narrow bunch limit. We use these results to determine the on-set
of the ion dynamics, and demonstrate that the ion motion could occur in
self-modulated plasma wakefield accelerators. Simulations show the motion of
the plasma ions can lead to the early suppression of the self-modulation
instability and of the accelerating fields. The background plasma ion motion
can nevertheless be fully mitigated by using plasmas with heavier plasmas.Comment: 23 pages, 6 figure
Cryopreservation of Byrsonima intermedia embryos followed by room temperature thawing
Byrsonima intermedia is a shrub from the Brazilian Cerrado with medicinal properties. The storage of biological material at ultra-low temperatures (-196°C) is termed cryopreservation and represents a promising technique for preserving plant diversity. Thawing is a crucial step that follows cryopreservation. The aim of this work was to cryopreserve B. intermedia zygotic embryos and subsequently thaw them at room temperature in a solution rich in sucrose. The embryos were decontaminated and desiccated in a laminar airflow hood for 0-4 hours prior to plunging into liquid nitrogen. The embryo moisture content (% MC) during dehydration was assessed. Cryopreserved embryos were thawed in a solution rich in sucrose at room temperature, inoculated in a germination medium and maintained in a growth chamber. After 30 days, the embryo germination was evaluated. No significant differences were observed between the different embryo dehydration times, where they were dehydrated for at least one hour. Embryos with a MC between 34.3 and 20.3% were germinated after cryopreservation. In the absence of dehydration, all embryos died following cryopreservation. We conclude that B. intermedia zygotic embryos can be successfully cryopreserved and thawed at room temperature after at least one hour of dehydration in a laminar airflow bench
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