948 research outputs found
Monte Carlo Study of the Spin Transport in Magnetic Materials
The resistivity in magnetic materials has been theoretically shown to depend
on the spin-spin correlation function which in turn depends on the
magnetic-field, the density of conduction electron, the magnetic ordering
stability, etc. However, these theories involved a lot of approximations, so
their validity remained to be confirmed. The purpose of this work is to show by
extensive Monte Carlo (MC) simulation the resistivity of the spin current from
low- ordered phase to high- paramagnetic phase in a ferromagnetic film.
We take into account the interaction between the itinerant spins and the
localized lattice spins as well as the interaction between itinerant spins
themselves. We show that the resistivity undergoes an anomalous behavior at the
magnetic phase transition in agreement with previous theories in spite of their
numerous approximations. The origin of the resistivity peak near the phase
transition in ferromagnets is interpreted here as stemming from the existence
of magnetic domains in the critical region. This interpretation is shown to be
in consistence with previous theoretical pictures. Resistivity in a simple
cubic antiferromagnet is also shown. The absence of a peak in this case is
explained
Theory and Simulation of Spin Transport in Antiferromagnetic Semiconductors: Application to MnTe
We study in this paper the parallel spin current in an antiferromagnetic
semiconductor thin film where we take into account the interaction between
itinerant spins and lattice spins. The spin model is an anisotropic Heisenberg
model. We use here the Boltzmann's equation with numerical data on cluster
distribution obtained by Monte Carlo simulations and cluster-construction
algorithms. We study the cases of degenerate and non-degenerate semiconductors.
The spin resistivity in both cases is shown to depend on the temperature with a
broad maximum at the transition temperature of the lattice spin system. The
shape of the maximum depends on the spin anisotropy and on the magnetic field.
It shows however no sharp peak in contrast to ferromagnetic materials. Our
method is applied to MnTe. Comparison to experimental data is given
Estimating the Leverage Parameter of Continuous-time Stochastic Volatility Models Using High Frequency S&P 500 VIX
This paper proposes a new method for estimating continuous-time stochastic volatility (SV) models for the S&P 500 stock index process using intraday high-frequency observations of both the S&P 500 index and the Chicago Board of Exchange (CBOE) implied (or expected) volatility index (VIX). Intraday high-frequency observations data have become readily available for an increasing number of financial assets and their derivatives in recent years, but it is well known that attempts to estimate the parameters of popular continuous-time models can lead to nonsensical estimates due to severe intraday seasonality. A primary purpose of the paper is to estimate the leverage parameter, , that is, the correlation between the two Brownian motions driving the diffusive components of the price process and its spot variance process, respectively. We show that, under the special case of Heston’s (1993) square-root SV model without measurement errors, the “realized leverage”, or the realized covariation of the price and VIX processes divided by the product of the realized volatilities of the two processes, converges to in probability as the time intervals between observations shrink to zero, even if the length of the whole sample period is fixed. Finite sample simulation results show that the proposed estimator delivers accurate estimates of the leverage parameter, unlike existing methods
Neutron scattering study of magnetic ordering and excitations in the ternary rare-earth diborocarbide Ce^{11}B_2C_2
Neutron scattering experiments have been performed on the ternary rare-earth
diborocarbide CeBC. The powder diffraction experiment confirms
formation of a long-range magnetic order at K, where a
sinusoidally modulated structure is realized with the modulation vector . Inelastic excitation spectra in the
paramagnetic phase comprise significantly broad quasielastic and inelastic
peaks centered at and 65 meV.
Crystalline-electric-field (CEF) analysis satisfactorily reproduces the
observed spectra, confirming their CEF origin. The broadness of the
quasielastic peak indicates strong spin fluctuations due to coupling between
localized spins and conduction electrons in the paramagnetic phase. A
prominent feature is suppression of the quasielastic fluctuations, and
concomitant growth of a sharp inelastic peak in a low energy region below
. This suggests dissociation of the conduction and localized
electrons on ordering, and contrasts the presently observed incommensurate
phase with spin-density-wave order frequently seen in heavy fermion compounds,
such as Ce(RuLa)Si.Comment: accepted for publication in Phys. Rev.
Theorems on ground-state phase transitions in Kohn-Sham models given by the Coulomb density functional
Some theorems on derivatives of the Coulomb density functional with respect
to the coupling constant are given. Consider an electron density
given by a ground state. A model Fermion system with the
reduced coupling constant, , is defined to reproduce and the ground state energy. Fixing the charge density, possible phase
transitions as level crossings detected in a value of the reduced density
functional happen only at discrete points along the axis. If the
density is -representable also for , accumulation of phase
transition points is forbidden when . Relevance of the
theorems for the multi-reference density functional theory is discussed.Comment: 19 page
Early Duplication of a Single MHC IIB Locus Prior to the Passerine Radiations.
A key characteristic of MHC genes is the persistence of allelic lineages over macroevolutionary periods, often through multiple speciation events. This phenomenon, known as trans-species polymorphism (TSP), is well documented in several major taxonomic groups, but has less frequently been observed in birds. The order Passeriformes is arguably the most successful terrestrial vertebrate order in terms of diversity of species and ecological range, but the reasons for this success remain unclear. Passerines exhibit the most highly duplicated MHC genes of any major vertebrate taxonomic group, which may generate increased immune response relative to other avian orders with fewer MHC loci. Here, we describe phylogenetic patterns of the MHC IIB in the passerine family Corvidae. Our results indicate wide-spread TSP within this family, with at least four supported MHC IIB allelic lineages that predate speciation by many millions of years. Markov chain Monte Carlo simulations indicate that divergence of these lineages occurred near the time of the divergence of the Passeriformes and other avian orders. We suggest that the current MHC diversity observed in passerines is due in part to the multiple duplication of a single MHC locus, DAB1, early in passerine evolution and that subsequent duplications of these paralogues have contributed to the enormous success of this order by increasing their ability to recognize and mount immune responses to novel pathogens
Analytical Tachyonic Lump Solutions in Open Superstring Field Theory
We construct a classical solution in the GSO(-) sector in the framework of a
Wess-Zumino-Witten-like open superstring field theory on a non-BPS D-brane. We
use an su(2) supercurrent, which is obtained by compactifying a direction to a
circle with the critical radius, in order to get analytical tachyonic lump
solutions to the equation of motion. By investigating the action expanded
around a solution we find that it represents a deformation from a non-BPS
D-brane to a D-brane-anti-D-brane system at the critical value of a parameter
which is contained in classical solutions. Although such a process was
discussed in terms of boundary conformal field theory before, our study is
based on open superstring field theory including interaction terms.Comment: 17 pages, references adde
Investigation of the molecular causes underlying physical abnormalities in Diamond‐Blackfan anemia patients with RPL5 haploinsufficiency
Diamond-Blackfan anemia (DBA) is a genetic disorder caused by mutations in genes encoding ribosomal proteins and characterized by erythroid aplasia and various physical abnormalities. Although accumulating evidence suggests that defective ribosome biogenesis leads to p53-mediated apoptosis in erythroid progenitor cells, little is known regarding the underlying causes of the physical abnormalities. In this study, we established induced pluripotent stem cells from a DBA patient with RPL5 haploinsufficiency. These cells retained the ability to differentiate into osteoblasts and chondrocytes. However, RPL5 haploinsufficiency impaired the production of mucins and increased apoptosis in differentiated chondrocytes. Increased expression of the pro-apoptotic genes BAX and CASP9 further indicated that RPL5 haploinsufficiency triggered p53-mediated apoptosis in chondrocytes. MDM2, the primary negative regulator of p53, plays a crucial role in erythroid aplasia in DBA patient. We found the phosphorylation level of MDM2 was significantly decreased in RPL5 haploinsufficient chondrocytes. In stark contrast, we found no evidence that RPL5 haploinsufficiency impaired osteogenesis. Collectively, our data support a model in which RPL5 haploinsufficiency specifically induces p53-mediated apoptosis in chondrocytes through MDM2 inhibition, which leads to physical abnormalities in DBA patients
Fermionic Ghosts in Moyal String Field Theory
We complete the construction of the Moyal star formulation of bosonic open
string field theory (MSFT) by providing a detailed study of the fermionic ghost
sector. In particular, as in the case of the matter sector, (1) we construct a
map from Witten's star product to the Moyal product, (2) we propose a
regularization scheme which is consistent with the matter sector and (3) as a
check of the formalism, we derive the ghost Neumann coefficients algebraically
directly from the Moyal product. The latter satisfy the Gross-Jevicki nonlinear
relations even in the presence of the regulator, and when the regulator is
removed they coincide numerically with the expression derived from conformal
field theory. After this basic construction, we derive a regularized action of
string field theory in the Siegel gauge and define the Feynman rules. We give
explicitly the analytic expression of the off-shell four point function for
tachyons, including the ghost contribution. Some of the results in this paper
have already been used in our previous publications. This paper provides the
technical details of the computations which were omitted there.Comment: 65 pages, typos correcte
Left-right olfactory asymmetry results from antagonistic functions of voltage-activated calcium channels and the Raw repeat protein OLRN-1 in C. elegans
<p>Abstract</p> <p>Background</p> <p>The left and right AWC olfactory neurons in <it>Caenorhabditis elegans </it>differ in their functions and in their expression of chemosensory receptor genes; in each animal, one AWC randomly takes on one identity, designated AWC<sup>OFF</sup>, and the contralateral AWC becomes AWC<sup>ON</sup>. Signaling between AWC neurons induces left-right asymmetry through a gap junction network and a claudin-related protein, which inhibit a calcium-regulated MAP kinase pathway in the neuron that becomes AWC<sup>ON</sup>.</p> <p>Results</p> <p>We show here that the asymmetry gene <it>olrn-1 </it>acts downstream of the gap junction and claudin genes to inhibit the calcium-MAP kinase pathway in AWC<sup>ON</sup>. OLRN-1, a protein with potential membrane-association domains, is related to the <it>Drosophila </it>Raw protein, a negative regulator of JNK mitogen-activated protein (MAP) kinase signaling. <it>olrn-1 </it>opposes the action of two voltage-activated calcium channel homologs, <it>unc-2 </it>(CaV2) and <it>egl-19 </it>(CaV1), which act together to stimulate the calcium/calmodulin-dependent kinase CaMKII and the MAP kinase pathway. Calcium channel activity is essential in AWC<sup>OFF</sup>, and the two AWC neurons coordinate left-right asymmetry using signals from the calcium channels and signals from <it>olrn-1</it>.</p> <p>Conclusion</p> <p><it>olrn-1 </it>and voltage-activated calcium channels are mediators and targets of AWC signaling that act at the transition between a multicellular signaling network and cell-autonomous execution of the decision. We suggest that the asymmetry decision in AWC results from the intercellular coupling of voltage-regulated channels, whose cross-regulation generates distinct calcium signals in the left and right AWC neurons. The interpretation of these signals by the kinase cascade initiates the sustained difference between the two cells.</p
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