1,548 research outputs found
Anisotropic optical response of nanocrystalline v2O5 thin films and effects of oxygen vacancy formation
We report a high sensitivity to oxygen vacancy formation as well as the temperature dependence of anisotropic optical properties of a nanocrystalline V2O5 thin film. Spectroscopic ellipsometry data show that the film has a uniaxial optical anisotropy due to a highly ordered nanocrystalline layer structure. The anisotropic optical properties of the film begin to change at 400 K in a high vacuum and can be linked to the changes in the electronic structure of the film upon reduction. An increase in the refractive indices, anisotropic changes of the optical conductivities, and a decrease in film thickness are observed after vacuum heating and are recovered to original values by heating in air up to 480 K. The experimental results are supported by the previous theoretical studies on the stability of reduced V2O5 surface through the formation of oxygen vacancies. We discuss the formation of oxygen vacancies, vacancy-induced structural relaxations, and changes in the electronic structure of V2O5 in conjunction with the experimental results.1441sciescopu
Transition from Knudsen to molecular diffusion in activity of absorbing irregular interfaces
We investigate through molecular dynamics the transition from Knudsen to
molecular diffusion transport towards 2d absorbing interfaces with irregular
geometry. Our results indicate that the length of the active zone decreases
continuously with density from the Knudsen to the molecular diffusion regime.
In the limit where molecular diffusion dominates, we find that this length
approaches a constant value of the order of the system size, in agreement with
theoretical predictions for Laplacian transport in irregular geometries.
Finally, we show that all these features can be qualitatively described in
terms of a simple random-walk model of the diffusion process.Comment: 4 pages, 4 figure
Constructing the fermion-boson vertex in QED3
We derive perturbative constraints on the transverse part of the
fermion-boson vertex in massive QED3 through its one loop evaluation in an
arbitrary covariant gauge. Written in a particular form, these constraints
naturally lead us to the first non-perturbative construction of the vertex,
which is in complete agreement with its one loop expansion in all momentum
regimes. Without affecting its one-loop perturbative properties, we also
construct an effective vertex in such a way that the unknown functions defining
it have no dependence on the angle between the incoming and outgoing fermion
momenta. Such a vertex should be useful for the numerical study of dynamical
chiral symmetry breaking, leading to more reliable results.Comment: 13 pages, 2 figure
Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems
We present a theory of Andreev reflection in a
ferromagnet/superconductor/ferromagnet double junction system. The spin
polarized quasiparticles penetrate to the superconductor in the range of
penetration depth from the interface by the Andreev reflection. When the
thickness of the superconductor is comparable to or smaller than the
penetration depth, the spin polarized quasiparticles pass through the
superconductor and therefore the electric current depends on the relative
orientation of magnetizations of the ferromagnets. The dependences of the
magnetoresistance on the thickness of the superconductor, temperature, the
exchange field of the ferromagnets and the height of the interfacial barriers
are analyzed. Our theory explains recent experimental results well.Comment: 8 pages, 9 figures, submitted to Phys. Rev.
Landau-Khalatnikov-Fradkin Transformations and the Fermion Propagator in Quantum Electrodynamics
We study the gauge covariance of the massive fermion propagator in three as
well as four dimensional Quantum Electrodynamics (QED). Starting from its value
at the lowest order in perturbation theory, we evaluate a non-perturbative
expression for it by means of its Landau-Khalatnikov-Fradkin (LKF)
transformation. We compare the perturbative expansion of our findings with the
known one loop results and observe perfect agreement upto a gauge parameter
independent term, a difference permitted by the structure of the LKF
transformations.Comment: 9 pages, no figures, uses revte
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Momentum dependent dxz/yz band splitting in LaFeAsO
The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the dxz and dyz hole bands near the Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors
The computational fluid dynamics modelling of the autorotation of square, flat plates
This paper examines the use of a coupled Computational Fluid Dynamics (CFD) – Rigid Body Dynamics (RBD) model to study the fixed-axis autorotation of a square flat plate. The calibration of the model against existing wind tunnel data is described. During the calibration, the CFD models were able to identify complex period autoration rates, which were attributable to a mass eccentricity in the experimental plate. The predicted flow fields around the autorotating plates are found to be consistent with existing observations. In addition, the pressure coefficients from the wind tunnel and computational work were found to be in good agreement. By comparing these pressure distributions and the vortex shedding patterns at various stages through an autorotation cycle, it was possible to gain important insights into the flow structures that evolve around the plate. The CFD model is also compared against existing correlation functions that relate the mean tip speed ratio of the plate to the aspect ratio, thickness ratio and mass moment of inertia of the plate. Agreement is found to be good for aspect ratios of 1, but poor away from this value. However, other aspects of the numerical modelling are consistent with the correlations
Double-exchange model study of multiferroic MnO perovskites
In this proceeding, recent theoretical investigations by the authors on the
multiferroic MnO perovskites are briefly reviewed at first. Using the
double-exchange model, the realistic spiral spin order in undoped manganites
such as TbMnO and DyMnO is well reproduced by incorporating a weak
next-nearest neighbor superexchange ( of nearest neighbor
superexchange) and moderate Jahn-Teller distortion. The phase transitions from
the A-type antiferromagnet (as in LaMnO), to the spiral phase (as in
TbMnO), and finally to the E-type antiferromagnet (as in HoMnO), with
decreasing size of the ions, were also explained. Moreover, new results of
phase diagram of the three-dimensional lattice are also included. The
ferromagnetic tendency recently discovered in the LaMnO and TbMnO thin
films is explained by considering the substrate stress. Finally, the
relationship between our double-exchange model and a previously used
-- model is further discussed from the perspective of spin wave
excitations.Comment: 6 pages, 3 figures; Proceeding of the Workshop on Magnetoelectric
Interaction Phenomena in Crystals (MEIPIC-6); To be appeared in European
Physical Journal
Regularization-independent study of renormalized non-perturbative quenched QED
A recently proposed regularization-independent method is used for the first
time to solve the renormalized fermion Schwinger-Dyson equation numerically in
quenched QED. The Curtis-Pennington vertex is used to illustrate the
technique and to facilitate comparison with previous calculations which used
the alternative regularization schemes of modified ultraviolet cut-off and
dimensional regularization. Our new results are in excellent numerical
agreement with these, and so we can now conclude with confidence that there is
no residual regularization dependence in these results. Moreover, from a
computational point of view the regularization independent method has enormous
advantages, since all integrals are absolutely convergent by construction, and
so do not mix small and arbitrarily large momentum scales. We analytically
predict power law behaviour in the asymptotic region, which is confirmed
numerically with high precision. The successful demonstration of this efficient
new technique opens the way for studies of unquenched QED to be undertaken in
the near future.Comment: 20 pages,5 figure
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