26,161 research outputs found
Sign Changes of Intrinsic Spin Hall Effect in Semiconductors and Simple Metals: First-Principles Calculations
First-principles calculations are applied to study spin Hall effect in
semiconductors and simple metals. We found that intrinsic spin Hall
conductivity (ISHC) in realistic materials shows rich sign changes, which may
be used to distinguish the effect from the extrinsic one. The calculated ISHC
in n-doped GaAs can be well compared with experiment, and it differs from the
sign obtained from the extrinsic effect. On the other hand, the ISHC in W and
Au, which shows opposite sign respectively, is robust and not sensitive to the
disorder.Comment: 4 pages, 4 figure
Study on QoS support in 802.11e-based multi-hop vehicular wireless ad hoc networks
Multimedia communications over vehicular ad hoc networks (VANET) will play an important role in the future intelligent transport system (ITS). QoS support for VANET therefore becomes an essential problem. In this paper, we first study the QoS performance in multi-hop VANET by using the standard IEEE 802.11e EDCA MAC and our proposed triple-constraint QoS routing protocol, Delay-Reliability-Hop (DeReHQ). In particular, we evaluate the DeReHQ protocol together with EDCA in highway and urban areas. Simulation results show that end-to-end delay performance can sometimes be achieved when both 802.11e EDCA and DeReHQ extended AODV are used. However, further studies on cross-layer optimization for QoS support in multi-hop environment are required
Neutrino emission from a GRB afterglow shock during an inner supernova shock breakout
The observations of a nearby low-luminosity gamma-ray burst (GRB) 060218
associated with supernova SN 2006aj may imply an interesting astronomical
picture where a supernova shock breakout locates behind a relativistic GRB jet.
Based on this picture, we study neutrino emission for early afterglows of GRB
060218-like GRBs, where neutrinos are expected to be produced from photopion
interactions in a GRB blast wave that propagates into a dense wind.
Relativistic protons for the interactions are accelerated by an external shock,
while target photons are basically provided by the incoming thermal emission
from the shock breakout and its inverse-Compton scattered component. Because of
a high estimated event rate of low-luminosity GRBs, we would have more
opportunities to detect afterglow neutrinos from a single nearby GRB event of
this type by IceCube. Such a possible detection could provide evidence for the
picture described above.Comment: 6 pages, 2 figures, accepted for publication in MNRA
A Unified and Complete Construction of All Finite Dimensional Irreducible Representations of gl(2|2)
Representations of the non-semisimple superalgebra in the standard
basis are investigated by means of the vector coherent state method and
boson-fermion realization. All finite-dimensional irreducible typical and
atypical representations and lowest weight (indecomposable) Kac modules of
are constructed explicitly through the explicit construction of all
particle states (multiplets) in terms of boson and fermion
creation operators in the super-Fock space. This gives a unified and complete
treatment of finite-dimensional representations of in explicit form,
essential for the construction of primary fields of the corresponding current
superalgebra at arbitrary level.Comment: LaTex file, 23 pages, two references and a comment added, to appear
in J. Math. Phy
A model of rotating hotspots for 3:2 frequency ratio of HFQPOs in black hole X-ray binaries
We propose a model to explain a puzzling 3:2 frequency ratio of high
frequency quasi-periodic oscillations (HFQPOs) in black hole (BH) X-ray
binaries, GRO J1655-40, GRS 1915+105 and XTE J1550-564. In our model a
non-axisymmetric magnetic coupling (MC) of a rotating black hole (BH) with its
surrounding accretion disc coexists with the Blandford-Znajek (BZ) process. The
upper frequency is fitted by a rotating hotspot near the inner edge of the
disc, which is produced by the energy transferred from the BH to the disc, and
the lower frequency is fitted by another rotating hotspot somewhere away from
the inner edge of the disc, which arises from the screw instability of the
magnetic field on the disc. It turns out that the 3:2 frequency ratio of HFQPOs
in these X-ray binaries could be well fitted to the observational data with a
much narrower range of the BH spin. In addition, the spectral properties of
HFQPOs are discussed. The correlation of HFQPOs with jets from microquasars is
contained naturally in our model.Comment: 8 pages, 4 figures. accepted by MNRA
Mass and width of the sigma
I report on recent work done in collaboration with Irinel Caprini and
Gilberto Colangelo. We observe that the Roy equations lead to a representation
of the pion pion scattering amplitude that exclusively involves observable
quantities, but is valid for complex values of s. At low energies, this
representation is dominated by the contributions from the two subtraction
constants, which are known to remarkable precision from the low energy theorems
of chiral perturbation theory. Evaluating the remaining contributions on the
basis of the available data, we demonstrate that the lowest resonance carries
the quantum numbers of the vacuum and occurs in the vicinity of the threshold.
Although the uncertainties in the data are substantial, the pole position can
be calculated quite accurately, because it occurs in the region where the
amplitude is dominated by the subtractions. The calculation neatly illustrates
the fact that the dynamics of the Goldstone bosons is governed by the
symmetries of QCD.Comment: Contribution to the proceedings of MESON 2006 (Krakow
Enhanced collectivity in neutron-deficient Sn isotopes in energy functional based collective Hamiltonian
The low-lying collective states in Sn isotopes are studied by a
five-dimensional collective Hamiltonian with parameters determined from the
triaxial relativistic mean-field calculations using the PC-PK1 energy density
functional. The systematics for both the excitation energies of states
and values are reproduced rather well, in particular,
the enhanced E2 transitions in the neutron-deficient Sn isotopes with N<66. We
show that the gradual degeneracy of neutron levels 1g7/2 and 2d5/2 around the
Fermi surface leads to the increase of level density and consequently the
enhanced paring correlations from N=66 to 58. It provokes a large quadrupole
shape fluctuation around the spherical shape, and leads to an enhanced
collectivity in the isotopes around N=58.Comment: 5 pages, 4 figures, accepted for publication in Physics Letters
A comparative study of the electronic and magnetic properties of BaFe_2As_2 and BaMn_2As_2 using the Gutzwiller approximation
To elucidate the role played by the transition metal ion in the pnictide
materials, we compare the electronic and magnetic properties of BaFe_{2}As_{2}
with BaMn_{2}As_{2}. To this end we employ the LDA+Gutzwiller method to analyze
the mass renormalizations and the size of the ordered magnetic moment of the
two systems. We study a model that contains all five transition metal 3d
orbitals together with the Ba-5d and As-4p states (ddp-model) and compare these
results with a downfolded model that consists of Fe/Mn d-states only (d-model).
Electronic correlations are treated using the multiband Gutzwiller
approximation. The paramagnetic phase has also been investigated using
LDA+Gutzwiller method with electron density self-consistency. The
renormalization factors for the correlated Mn 3d orbitals in the paramagnetic
phase of BaMn_{2}As_{2} are shown to be generally smaller than those of
BaFe_{2}As_{2}, which indicates that BaMn_{2}As_{2} has stronger electron
correlation effect than BaFe_{2}As_{2}. The screening effect of the main As 4p
electrons to the correlated Fe/Mn 3d electrons is evident by the systematic
shift of the results to larger Hund's rule coupling J side from the ddp-model
compared with those from the d-model. A gradual transition from paramagnetic
state to the antiferromagnetic ground state with increasing J is obtained for
the models of BaFe_{2}As_{2} which has a small experimental magnetic moment;
while a rather sharp jump occurs for the models of BaMn_{2}As_{2}, which has a
large experimental magnetic moment. The key difference between the two systems
is shown to be the d-level occupation. BaMn_{2}As_{2}, with approximately five
d-electrons per Mn atom, is for same values of the electron correlations closer
to the transition to a Mott insulating state than BaFe_{2}As_{2}. Here an
orbitally selective transition, required for a system with close to six
electrons only occurs at significantly larger values for the Coulomb
interactions
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