5,531 research outputs found
Chiral properties of hematite ({\alpha}-Fe2O3) inferred from resonant Bragg diffraction using circularly polarized x-rays
Chiral properties of the two phases - collinear motif (below Morin transition
temperature, TM=250 K) and canted motif (above TM) - of magnetically ordered
hematite ({\alpha}-Fe2O3) have been identified in single crystal resonant x-ray
Bragg diffraction, using circular polarized incident x-rays tuned near the iron
K-edge. Magneto-electric multipoles, including an anapole, fully characterize
the high-temperature canted phase, whereas the low-temperature collinear phase
supports both parity-odd and parity-even multipoles that are time-odd. Orbital
angular momentum accompanies the collinear motif, while it is conspicuously
absent with the canted motif. Intensities have been successfully confronted
with analytic expressions derived from an atomic model fully compliant with
chemical and magnetic structures. Values of Fe atomic multipoles previously
derived from independent experimental data, are shown to be completely
trustworthy
Dense Ionized and Neutral Gas Surrounding Sgr A*
We present high resolution H41a hydrogen recombination line observations of
the 1.2' (3 pc) region surrounding Sgr A* at 92 GHz using the OVRO Millimeter
Array with an angular resolution of 7" x 3" and velocity resolution of 13 km/s.
New observations of H31a, H35a, H41a, and H44a lines were obtained using the
NRAO 12-m telescope, and their relative line strengths are interpreted in terms
of various emission mechanisms. These are the most extensive and most sensitive
observations of recombination line to date. Observations of HCO+ (1 - 0)
transition at 89 GHz are also obtained simultaneously with a 40% improved
angular resolution and 4-15 times improved sensitivity over previous
observations, and the distribution and kinematics of the dense molecular gas in
the circumnuclear disk (CND) are mapped and compared with those of the ionized
gas. The line brightness ratios of the hydrogen recombination lines are
consistent with purely spontaneous emission from 7000 K gas with n_e = 20,000
cm near LTE condition. A virial analysis suggests that the most
prominent molecular gas clumps in the CND have mean densities of 10^7 cm^{-3},
sufficient to withstand the tidal shear in the Galactic Center region.
Therefore, these clumps may survive over several dynamical times, and the CND
may be a dynamically stable structure. We estimate a total gas mass of 3 x 10^5
solar mass for the CND. \Comment: 34 pages including 11 figures (4 jpgs), Latex, uses aastex. The full
pdf format file including high resolution figures is available at
http://www.astro.umass.edu/~myun/papers/SgrA.pdf . To appear in the 20
November 2004 (V616) issue of the Astrophysical Journa
Decay of spin coherences in one-dimensional spin systems
Strategies to protect multi-qubit states against decoherence are difficult to formulate because of their complex many-body dynamics. A better knowledge of the decay dynamics would help in the construction of dynamical decoupling control schemes. Here we use solid-state nuclear magnetic resonance techniques to experimentally investigate decay of coherent multi-spin states in linear spin chains. Leveraging on the quasi-one-dimension geometry of fluorapatite crystal spin systems, we can gain a deeper insight on the multi-spin states created by the coherent evolution, and their subsequent decay, than it is possible in three-dimensional (3D) systems. We are then able to formulate an analytical model that captures the key features of the decay. We can thus compare the decoherence behavior for different initial states of the spin chain and link their decay rate to the state characteristics, in particular their coherence and long-range correlation among spins. Our experimental and theoretical study shows that the spin chains undergo a rich dynamics, with a slower decay rate than for the 3D case, and thus might be more amenable to decoupling techniques.National Science Foundation (U.S.) (Grant DMG-1005926)United States. Air Force Office of Scientific Research. Young Investigator Progra
High Isolation Quad-Element SWB-MIMO Antenna with Dual Band-Notch for ISM and WLAN Band Wireless Applications
A quad-element super-wideband (2-20GHz) MIMO antenna including dual notched-band response at WiMAX (3.30-3.70GHz) and satellite-band (6.99-8.09GHz) is designed on RO3035 with total dimension of 118mm×86mm×1.67mm. Unique decoupling structure has been deployed to enhance the isolation (˃20dB) between two antenna elements. The fundamental properties of MIMO antennas like bandwidth ratio (10:1), isolation (>18dB), gain (4.14dB), Envelop Correlation-Coefficient (<0.0065), Total Active Reflection-Coefficient (< 0dB), Channel Capacity Loss (<0.25bps/Hz) and radiation patterns are also investigated in order to determine their practicality. Measurement and simulation results of the proposed SWB-MIMO antenna from 2 to 20GHz indicate that it will be the suitable candidate for wireless and biomedical applications
High Isolation Quad-Element SWB-MIMO Antenna with Dual Band-Notch for ISM and WLAN Band Wireless Applications
A quad-element super-wideband (2-20GHz) MIMO antenna including dual notched-band response at WiMAX (3.30-3.70GHz) and satellite-band (6.99-8.09GHz) is designed on RO3035 with total dimension of 118mm×86mm×1.67mm. Unique decoupling structure has been deployed to enhance the isolation (˃20dB) between two antenna elements. The fundamental properties of MIMO antennas like bandwidth ratio (10:1), isolation (>18dB), gain (4.14dB), Envelop Correlation-Coefficient (<0.0065), Total Active Reflection-Coefficient (< 0dB), Channel Capacity Loss (<0.25bps/Hz) and radiation patterns are also investigated in order to determine their practicality. Measurement and simulation results of the proposed SWB-MIMO antenna from 2 to 20GHz indicate that it will be the suitable candidate for wireless and biomedical applications
Spin kinetic theory - quantum kinetic theory in extended phase space
The concept of phase space distribution functions and their evolution is used
in the case of en enlarged phase space. In particular, we include the intrinsic
spin of particles and present a quantum kinetic evolution equation for a scalar
quasi-distribution function. In contrast to the proper Wigner transformation
technique, for which we expect the corresponding quasi-distribution function to
be a complex matrix, we introduce a spin projection operator for the density
matrix in order to obtain the aforementioned scalar quasi-distribution
function. There is a close correspondence between this projection operator and
the Husimi (or Q) function used extensively in quantum optics. Such a function
is based on a Gaussian smearing of a Wigner function, giving a positive
definite distribution function. Thus, our approach gives a Wigner-Husimi
quasi-distribution function in extended phase space, for which the reduced
distribution function on the Bloch sphere is strictly positive. We also discuss
the gauge issue and the fluid moment hierarchy based on such a quantum kinetic
theory.Comment: 10 pages, to appear in Transport Theory and Statistical Physics,
proceedings of Vlasovia III, 200
Onset of magnetism in B2 transition metals aluminides
Ab initio calculation results for the electronic structure of disordered bcc
Fe(x)Al(1-x) (0.4<x<0.75), Co(x)Al(1-x) and Ni(x)Al(1-x) (x=0.4; 0.5; 0.6)
alloys near the 1:1 stoichiometry, as well as of the ordered B2 (FeAl, CoAl,
NiAl) phases with point defects are presented. The calculations were performed
using the coherent potential approximation within the Korringa-Kohn-Rostoker
method (KKR-CPA) for the disordered case and the tight-binding linear
muffin-tin orbital (TB-LMTO) method for the intermetallic compounds. We studied
in particular the onset of magnetism in Fe-Al and Co-Al systems as a function
of the defect structure. We found the appearance of large local magnetic
moments associated with the transition metal (TM) antisite defect in FeAl and
CoAl compounds, in agreement with the experimental findings. Moreover, we found
that any vacancies on both sublattices enhance the magnetic moments via
reducing the charge transfer to a TM atom. Disordered Fe-Al alloys are
ferromagnetically ordered for the whole range of composition studied, whereas
Co-Al becomes magnetic only for Co concentration >0.5.Comment: 11 pages with 9 embedded postscript figures, to be published in
Phys.Rev.
Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals
The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography
Effect of temperature anisotropy on various modes and instabilities for a magnetized non-relativistic bi-Maxwellian plasma
Using kinetic theory for homogeneous collisionless magnetized plasmas, we
present an extended review of the plasma waves and instabilities and discuss
the anisotropic response of generalized relativistic dielectric tensor and
Onsager symmetry properties for arbitrary distribution functions. In general,
we observe that for such plasmas only those electromagnetic modes whose
magnetic field perturbations are perpendicular to the ambient magneticeld,
i.e.,B1 \perp B0, are effected by the anisotropy. However, in oblique
propagation all modes do show such anisotropic effects. Considering the
non-relativistic bi-Maxwellian distribution and studying the relevant
components of the general dielectric tensor under appropriate conditions, we
derive the dispersion relations for various modes and instabilities. We show
that only the electromagnetic R- and L- waves, those derived from them and the
O-mode are affected by thermal anisotropies, since they satisfy the required
condition B1\perpB0. By contrast, the perpendicularly propagating X-mode and
the modes derived from it (the pure transverse X-mode and Bernstein mode) show
no such effect. In general, we note that the thermal anisotropy modifies the
parallel propagating modes via the parallel acoustic effect, while it modifies
the perpendicular propagating modes via the Larmor-radius effect. In oblique
propagation for kinetic Alfven waves, the thermal anisotropy affects the
kinetic regime more than it affects the inertial regime. The generalized fast
mode exhibits two distinct acoustic effects, one in the direction parallel to
the ambient magnetic field and the other in the direction perpendicular to it.
In the fast-mode instability, the magneto-sonic wave causes suppression of the
firehose instability. We discuss all these propagation characteristics and
present graphic illustrations
- …