12,217 research outputs found
High-spin low-spin transition
Temperature dependent nuclear inelastic-scattering (NIS) of synchrotron
radiation was applied to investigate both spin states of the spin-crossover
complex [Fe(tpa)(NCS)(2)] (tpa = tris(2-pyridylmethyl)amine). A remarkable
increase of the iron-ligand bond stretching upon spin crossover has
unambiguously been identified by comparing the measured NIS spectra with
theoretical simulations based on density-functional calculations
First results of observations of transient pulsar SAXJ2103.5+4545 with the INTEGRAL observatory
We present preliminary results of observations of X-ray pulsar SAX
J2103.5+4545 with INTEGRAL observatory in Dec 2002. Maps of this sky region in
energy bands 3-10, 15-40, 40-100 and 100-200 keV are presented. The source is
significantly detected up to energies of keV. The hard X-ray flux in
the 15-100 energy band is variable, that could be connected with the orbital
phase of the binary system. We roughly reconstructed the source spectrum using
its comparison to that of Crab nebula. It is shown that the parameters of the
source spectrum in 18-150 keV energy range are compatible with that obtained
earlier by RXTE observatoryComment: 5 pages, 4 figures, accepted for publication in the Astronomy Letter
Side-jumps in the spin-Hall effect: construction of the Boltzmann collision integral
We present a systematic derivation of the side-jump contribution to the
spin-Hall current in systems without band structure spin-orbit interactions,
focusing on the construction of the collision integral for the Boltzmann
equation. Starting from the quantum Liouville equation for the density operator
we derive an equation describing the dynamics of the density matrix in the
first Born approximation and to first order in the driving electric field.
Elastic scattering requires conservation of the total energy, including the
spin-orbit interaction energy with the electric field: this results in a first
correction to the customary collision integral found in the Born approximation.
A second correction is due to the change in the carrier position during
collisions. It stems from the part of the density matrix off-diagonal in wave
vector. The two corrections to the collision integral add up and are
responsible for the total side-jump contribution to the spin-Hall current. The
spin-orbit-induced correction to the velocity operator also contains terms
diagonal and off-diagonal in momentum space, which together involve the total
force acting on the system. This force is explicitly shown to vanish (on the
average) in the steady state: thus the total contribution to the spin-Hall
current due to the additional terms in the velocity operator is zero.Comment: Added references, expanded discussion, revised introductio
Pressure-induced changes of the vibrational modes of spin-crossover complexes studied by nuclear resonance scattering of synchrotron radiation
Nuclear inelastic scattering (NIS) spectra were recorded for the
spin-crossover complexes STP and ETP (STP =
[Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg- ethane)](ClO4)2 and
ETP =
[Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg-butane)](ClO4)2) at
30 K and at room temperature and also at ambient pressure and applied pressure
(up to 2.6 GPa). Spin transition from the high-spin (HS) to the low-spin (LS)
state was observed by lowering temperature and also by applying pressure at
room temperature and has been assigned to the hardening of iron-bond stretching
modes due to the smaller volume in the LS isomer
Spin relaxation in an InAs quantum dot in the presence of terahertz driving fields
The spin relaxation in a 1D InAs quantum dot with the Rashba spin-orbit
coupling under driving THz magnetic fields is investigated by developing the
kinetic equation with the help of the Floquet-Markov theory, which is
generalized to the system with the spin-orbit coupling, to include both the
strong driving field and the electron-phonon scattering. The spin relaxation
time can be effectively prolonged or shortened by the terahertz magnetic field
depending on the frequency and strength of the terahertz magnetic field. The
effect can be understood as the sideband-modulated spin-phonon scattering. This
offers an additional way to manipulate the spin relaxation time.Comment: 8 pages, 1 figure, to be published in PR
Ultrafast Magnetization Dynamics in Diluted Magnetic Semiconductors
We present a dynamical model that successfully explains the observed time
evolution of the magnetization in diluted magnetic semiconductor quantum wells
after weak laser excitation. Based on the pseudo-fermion formalism and a second
order many-particle expansion of the exact p-d exchange interaction, our
approach goes beyond the usual mean-field approximation. It includes both the
sub-picosecond demagnetization dynamics and the slower relaxation processes
which restore the initial ferromagnetic order in a nanosecond time scale. In
agreement with experimental results, our numerical simulations show that,
depending on the value of the initial lattice temperature, a subsequent
enhancement of the total magnetization may be observed within a time scale of
few hundreds of picoseconds.Comment: Submitted to PR
The AGN nature of 11 out of 12 Swift/RXTE unidentified sources through optical and X-ray spectroscopy
The Swift Burst Alert Telescope (BAT) is performing a high Galactic latitude
survey in the 14-195 keV band at a flux limit of ~10^{-11} erg cm^{-2} s^{-1},
leading to the discovery of new high energy sources, most of which have not so
far been properly classified. A similar work has also been performed with the
RXTE slew survey leading to the discovery of 68 sources detected above 8 keV,
many of which are still unclassified. Follow-up observations with the Swift
X-ray Telescope (XRT) provide, for many of these objects, source localization
with a positional accuracy of few arcsec, thus allowing the search for optical
counterparts to be more efficient and reliable. We present the results of
optical/X-ray follow-up studies of 11 Swift BAT detections and one AGN detected
in the RXTE Slew Survey, aimed at identifying their counterparts and at
assessing their nature. These data allowed, for the first time, the optical
classification of 8 objects and a distance determination for 3 of them. For
another object, a more refined optical classification than that available in
the literature is also provided. For the remaining sources, optical
spectroscopy provides a characterization of the source near in time to the
X-ray measurement. The sample consists of 6 Seyfert 2 galaxies, 5 Seyferts of
intermediate type 1.2-1.8, and one object of Galactic nature - an Intermediate
Polar (i.e., magnetic) Cataclysmic Variable. Out of the 11 AGNs, 8 (~70%)
including 2 Seyferts of type 1.2 and 1.5, are absorbed with NH > 10^{22}
cm^{-2}. Up to 3 objects could be Compton thick (i.e. NH > 1.5 x 10^{24}
cm^{-2}), but only in one case (Swift J0609.1-8636) does all the observational
evidence strongly suggests this possibility.Comment: 50 pages, including 16 figures and 7 tables. Accepted for publication
in Ap
Coulomb-induced Rashba spin-orbit coupling in semiconductor quantum wells
In the absence of an external field, the Rashba spin-orbit interaction (SOI)
in a two-dimensional electron gas in a semiconductor quantum well arises
entirely from the screened electrostatic potential of ionized donors. We adjust
the wave functions of a quantum well so that electrons occupying the first
(lowest) subband conserve their spin projection along the growth axis (Sz),
while the electrons occupying the second subband precess due to Rashba SOI.
Such a specially designed quantum well may be used as a spin relaxation
trigger: electrons conserve Sz when the applied voltage (or current) is lower
than a certain threshold V*; higher voltage switches on the Dyakonov-Perel spin
relaxation.Comment: 4+ pages, 6 figure
Semiclassical states for quantum cosmology
In a metric variable based Hamiltonian quantization, we give a prescription
for constructing semiclassical matter-geometry states for homogeneous and
isotropic cosmological models. These "collective" states arise as infinite
linear combinations of fundamental excitations in an unconventional "polymer"
quantization. They satisfy a number of properties characteristic of
semiclassicality, such as peaking on classical phase space configurations. We
describe how these states can be used to determine quantum corrections to the
classical evolution equations, and to compute the initial state of the universe
by a backward time evolution.Comment: 13 page
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