2,877 research outputs found
Effect of ligand substitution on the exchange interactions in {Mn12}-type single-molecule magnets
We investigate how the ligand substitution affects the intra-molecular spin
exchange interactions, studying a prototypal family of single-molecule magnets
comprising dodecanuclear cluster molecules [Mn12O12(COOR)16]. We identify a
simple scheme based on accumulated Pauling electronegativity numbers (a.e.n.)
of the carboxylate ligand groups (R). The redistribution of the electron
density, controlled by a.e.n. of a ligand, changes the degree of hybridization
between 3d electrons of manganese and 2p electrons of oxygen atoms, thus
changing the exchange interactions. This scheme, despite its conceptual
simplicity, provides a strong correlation with the exchange energies associated
with carboxylate bridges, and is confirmed by the electronic structure
calculations taking into account the Coulomb correlations in magnetic
molecules.Comment: 18 pages, 1 table, 4 figures. Accepted to "Inorganic Chemistry
Hyperfine interaction induced decoherence of electron spins in quantum dots
We investigate in detail, using both analytical and numerical tools, the
decoherence of electron spins in quantum dots (QDs) coupled to a bath of
nuclear spins in magnetic fields or with various initial bath polarizations,
focusing on the longitudinal relaxation in low and moderate field/polarization
regimes. An increase of the initial polarization of nuclear spin bath has the
same effect on the decoherence process as an increase of the external magnetic
field, namely, the decoherence dynamics changes from smooth decay to damped
oscillations. This change can be observed experimentally for a single QD and
for a double-QD setup. Our results indicate that substantial increase of the
decoherence time requires very large bath polarizations, and the use of other
methods (dynamical decoupling or control of the nuclear spins distribution) may
be more practical for suppressing decoherence of QD-based qubits.Comment: Rev. Tex, 5 pages, 3 eps color figures, submitted to Phys. Rev.
Long-term source monitoring with BATSE
The uncollimated Burst and Transient Source Experiment (BATSE) large area detectors (LADs) are well suited to nearly continuous monitoring of the stronger hard x-ray sources, and time series analysis for pulsars. An overview of the analysis techniques presently being applied to the data are discussed, including representative observations of the Crab Nebula, Crab pulsar, and summaries of the sources detected to data. Results of a search for variability in the Crab Pulsar pulse profile are presented
Hard X-ray variability of the black-hole candidate GRO J0422+32 during its 1992 outburst
We have studied the hard X-ray variability of the soft X-ray transient GRO
J0422+32 with BATSE in the 20-100 keV energy band. Our analysis covers 180 days
following the first X-ray detection of the source on 1992 August 5, fully
covering its primary and secondary X-ray outburst. We computed power density
spectra (PDSs) in the 20-50, 50-100, and 20-100 keV energy bands, in the
frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately
flat up to a break frequency, and decay as a power law above, with index ~1.
The canonical anticorrelation between the break frequency and the power density
at the break, observed in Cyg X-1 and other BHCs in the low state, is not
observed in the PDSs of GRO J0422+32. We compare our results with those of
similar variability studies of Cyg X-1. The relation between the spectral slope
and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of
Cyg X-1; however, the relation between the hard X-ray flux and the amplitude of
its variation is opposite to what has been found in Cyg X-1. Phase lags between
the X-ray flux variations of GRO J0422+32 at high and low photon energies,
could only be derived during the first 30 days of its outburst. During this
period, the variations in the 50-100 keV lag those in the 20-50 keV energy band
by an approximately constant phase difference of 0.039(3) rad in the frequency
interval 0.02-0.20 Hz.Comment: 33 pages, including 14 postscript figures, AASTEX. To appear in ApJ
1999, March 1, vol. 513 #
Non-collinear magnetic structure and anisotropic magnetoelastic coupling in cobalt pyrovanadate Co2V2O7
The Co2V2O7 is recently reported to exhibit amazing magnetic field-induced
magnetization plateaus and ferroelectricity, but its magnetic ground state
remains ambiguous due to its structural complexity. Magnetometry measurements,
and time-of-flight neutron powder diffraction (NPD) have been employed to study
the structural and magnetic properties of Co2V2O7, which consists of two
non-equivalent Co sites. Upon cooling below the Ne\'el temperature TN = 6.3 K,
we observe magnetic Bragg peaks at 2K in NPD which indicated the formation of
long range magnetic order of Co2+ moments. After symmetry analysis and magnetic
structure refinement, we demonstrate that Co2V2O7 possesses a complicated
non-collinear magnetic ground state with Co moments mainly located in b-c plane
and forming a non-collinear spin-chain-like structure along the c-axis. The ab
initio calculations demonstrate that the non-collinear magnetic structure is
more stable than various ferromagnetic states at low temperature. The
non-collinear magnetic structure with canted up-up-down-down spin configuration
is considered as the origin of magnetoelectric coupling in Co2V2O7 because the
inequivalent exchange striction induced by the spin-exchange interaction
between the neighboring spins is the driving force of ferroelectricity.
Besides, it is found that the deviation of lattice parameters a and b is
opposite below TN, while the lattice parameter c and stay almost constant below
TN, evidencing the anisotropic magnetoelastic coupling in Co2V2O7.Comment: 9 pages, 8 figure
Diffuse Galactic Soft Gamma-Ray Emission
The Galactic diffuse soft gamma-ray (30-800 keV) emission has been measured
from the Galactic Center by the HIREGS balloon-borne germanium spectrometer to
determine the spectral characteristics and origin of the emission. The
resulting Galactic diffuse continuum is found to agree well with a single
power-law (plus positronium) over the entire energy range, consistent with RXTE
and COMPTEL/CGRO observations at lower and higher energies, respectively. We
find no evidence of spectral steepening below 200 keV, as has been reported in
previous observations. The spatial distribution along the Galactic ridge is
found to be nearly flat, with upper limits set on the longitudinal gradient,
and with no evidence of an edge in the observed region. The soft gamma-ray
diffuse spectrum is well modeled by inverse Compton scattering of interstellar
radiation off of cosmic-ray electrons, minimizing the need to invoke
inefficient nonthermal bremsstrahlung emission. The resulting power requirement
is well within that provided by Galactic supernovae. We speculate that the
measured spectrum provides the first direct constraints on the cosmic-ray
electron spectrum below 300 MeV.Comment: 26 pages, 7 figure, submitted to Ap
Occultation analysis of BATSE data: Operational aspects
The Burst and Transient Source Experiment (BATSE) large area detectors are being used to monitor hard x-ray/gamma ray sources on a daily basis for evidence of transient behavior. Flux measurements are performed using a simple earth occultation technique. Daily searches are also being performed to detect occultation steps of sources which are not being routinely monitored. Topics concerning the operational aspects of the occultation measurements are presented. Preliminary spectral results are also presented for several of the brighter sources
RXTE Observations of Cygnus X-3
In the period between May 1997 and August 1997 a series of pointed RXTE
observations were made of Cyg X-3. During this period Cyg X-3 made a transition
from a quiescent radio state to a flare state (including a major flare) and
then returned to a quiescent radio state. Analyses of the observations are made
in the context of concurrent observations in the hard X-ray (CGRO/BATSE), soft
X-ray (RXTE/ASM) and the radio (Green Bank Interferometer, Ryle Telescope, and
RATAN-600). Preliminary analyses of the observations are presented.Comment: 4 pages, 4 figures. newarcrc.sty included. To appear in 2nd Workshop
of Relativistic Jets from Galactic Sources, R.N. Ogley and S.J. Bell Burnell
eds, NewAR 42, in pres
- …