5,726 research outputs found
Spectral methods for modeling supersonic chemically reacting flow fields
A numerical algorithm was developed for solving the equations describing chemically reacting supersonic flows. The algorithm employs a two-stage Runge-Kutta method for integrating the equations in time and a Chebyshev spectral method for integrating the equations in space. The accuracy and efficiency of the technique were assessed by comparison with an existing implicit finite-difference procedure for modeling chemically reacting flows. The comparison showed that the procedure presented yields equivalent accuracy on much coarser grids as compared to the finite-difference procedure with resultant significant gains in computational efficiency
Binding energy of shallow donors in a quantum well in the presence of a tilted magnetic field
We present results of variational calculations of the binding energy of a
neutral donor in a quantum well in the presence of a magnetic field tilted
relative to the QW plane. Assuming that the donor is located in the center of
the QW, we perform calculations for parameters typical of a II-VI wide-gap
semiconductor heterostructure, using as an example the case of a rectangular
CdTe quantum well with CdMgTe barriers. We present the dependence of the
binding energy of a neutral donor on the tilt angle and on the magnitude of the
applied magnetic filed. As a key result, we show that measurement of the
binding energy of a donor at two angles of the magnetic field with respect to
the quantum well plane can be used to unambiguously determined the conduction
band offset of the materials building up heterostructure.Comment: 6 pages, 5 figure
Molecular-field approach to the spin-Peierls transition in CuGeO_3
We present a theory for the spin-Peierls transition in CuGeO_3. We map the
elementary excitations of the dimerized chain (solitons) on an effective Ising
model. Inter-chain coupling (or phonons) then introduce a linear binding
potential between a pair of soliton and anti-soliton, leading to a finite
transition temperature. We evaluate, as a function of temperature, the order
parameter, the singlet-triplet gap, the specific heat, and the susceptibility
and compare with experimental data on CuGeO_3. We find that CuGeO_3 is close to
a first-order phase transition. We point out, that the famous scaling law
\sim\delta^{2/3} of the triplet gap is a simple consequence of the linear
binding potential between pairs of solitons and anti-solitons in dimerized spin
chains.Comment: 7.1 pages, figures include
Mean-field theory of the spin-Peierls systems: Application to CuGeO3
A mean-field theory of the spin Peierls systems based on the two dimensional
dimerized Heisenberg model is proposed by introducing an alternating bond order
parameter. Improvements with respect to previous mean-field results are found
in the one-dimensional limit for the ground state and the gap energies. In two
dimensions, the analysis of the competition between antiferromagnetic long
range order and the spin-Peierls ordering is given as a function of the
coupling constants. We show that the lowest energy gap to be observed does not
have a singlet-triplet character in agreement with the low temperature
thermodynamic properties of CuGeO3.Comment: 3 Revtex pages. Submitted to Rapid Comm. Figures available upon
reques
Disordered Electrical Potential Observed on the Surface of SiO by Electric Field Microscopy
The electrical potential on the surface of nm thick SiO grown
on single crystalline Si substrates has been characterized at ambient
conditions using electric field microscopy. Our results show an inhomogeneous
potential distribution with fluctuations up to V within regions of
m. The potential fluctuations observed at the surface of these usual
dielectric holders of graphene sheets should induce strong variations in the
graphene charge densities and provide a simple explanation for some of the
anomalous behaviors of the transport properties of graphene.Comment: 4 pages and 4 figure
Preference of the aphid Myzus persicae (Hemiptera: Aphididae) for tobacco plants at specific stages of potato virus Y infection
Potato virus Y (PVY) is a common pathogen affecting agricultural production worldwide and is mainly transmitted by Myzus persicae in a non-persistent manner. Insect-borne plant viruses can modify the abundance, performance, and behavior of their vectors by altering host plant features; however, most studies have overlooked the fact that the dynamic progression of virus infection in plants can have variable effects on their vectors. We addressed this point in the present study by dividing the PVY infection process in tobacco into three stages (early state, steady state and late state); delineated by viral copy number. We then compared the differential effects of PVY-infected tobacco (Nicotiana tabacum) plants on the host selection and feeding behavior of M. persicae. We used Y-shaped olfactory apparatus and electrical penetration graph (EPG) methods to evaluate host selection and feeding behavior, respectively. Interestingly, we found that PVY-infected plants at the steady state attracted more aphids than healthy plants, whereas no differences were observed for those at the early and late states. In terms of feeding behavior, intracellular punctures (closely related to PVY acquisition and transmission) were more abundant on PVY-infected tobacco plants at the early and steady states of infection than in uninfected plants. These results indicate that PVY-infected host plants can alter the host selection and feeding behavior of aphids in a stage-dependent manner, which is an important consideration when studying the interactions among host plants, viruses, and insect vectors. © 2019, Springer-Verlag GmbH Austria, part of Springer Nature
Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice Model : A Qualitative Approach to Oxide Manganites
The transport properties of the ferromagnetic Kondo lattice model in one
dimension are studied via bosonization methods. The antiferromagnetic
fluctuations, which normally appear because of the RKKY interactions, are
explicitly taken into account as a direct exchange between the ``core'' spins.
It is shown that in the paramagnetic regime with the local antiferromagnetic
fluctuations, the resistivity decays exponentially as the temperature increases
while in the ferromagnetic regime the system is an almost perfect conductor. %A
non-perturbative description of localized spin polarons %in the paramagnetic
region is obtained.
The effect of a weak applied field is discussed to be reduced to the case of
the ferromagnetic state leading to band splitting. The qualitative relevance of
the results for the problem of the Oxide Manganites is emphasized.Comment: 4 pages, REVTe
Phase separation in double exchange systems
Ferromagnetic systems described by the double exchange model are
investigated. At temperatures close to the Curie temperature, and for a wide
range of doping levels, the system is unstable toward phase separation. The
chemical potential decreases upon increasing doping, due to the significant
dependence of the bandwidth on the number of carriers. The reduction of the
electronic bandwidth by spin disorder leads to an enormously enhanced
thermopower which peaks near T_c, with a sign opposite that predicted by a
rigid band model.Comment: 4 pages, 2 encapsulated PostScript figure
Many-body CPA for the Holstein-DE model
A many-body coherent potential approximation (CPA) previously developed for
the double exchange (DE) model is extended to include coupling to local quantum
phonons. The Holstein-DE model studied (equal to the Holstein model for zero
Hund coupling) is considered to be a simple model for the colossal
magnetoresistance manganites. We concentrate on effects due to the quantisation
of the phonons, such as the formation of polaron subbands. The electronic
spectrum and resistivity are investigated for a range of temperature and
electron-phonon coupling strengths. Good agreement with experiment is found for
the Curie temperature and resistivity with intermediate electron-phonon
coupling strength, but phonon quantisation is found not to have a significant
effect in this coupling regime.Comment: 19 pages, 7 figure
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