83 research outputs found
Unconventional magnetism of non-uniform distribution of Co in TiO2 nanoparticles
High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), magnetic methods, and density-functional theory (DFT) calculations were applied for the investigations of Co-doped anatase TiO2 nanoparticles (∼20 nm). It was found that high-spin Co2+ ions prefer to occupy the interstitial positions in the TiO2 lattice which are the most energetically favourable in compare to the substitutional those. A quantum mechanical model which operates mainly on two types of Co2+ – Co2+ dimers with different negative exchange interactions and the non-interacting paramagnetic Co2+ ions provides a satisfactorily description of magnetic properties for the TiO2:Co system. © 2020 Elsevier B.V.Russian Foundation for Basic Research. Ministry of Science and Higher Education of the Russian Federatio
Ultraviolet singularities in classical brane theory
We construct for the first time an energy-momentum tensor for the
electromagnetic field of a p-brane in arbitrary dimensions, entailing finite
energy-momentum integrals. The construction relies on distribution theory and
is based on a Lorentz-invariant regularization, followed by the subtraction of
divergent and finite counterterms supported on the brane. The resulting
energy-momentum tensor turns out to be uniquely determined. We perform the
construction explicitly for a generic flat brane. For a brane in arbitrary
motion our approach provides a new paradigm for the derivation of the,
otherwise divergent, self-force of the brane. The so derived self-force is
automatically finite and guarantees, by construction, energy-momentum
conservation.Comment: 41 pages, no figures, minor change
Correlation Effects on Optical Conductivity of FeSi
Effects of electron correlation in FeSi are studied in terms of the two-band
Hubbard model with the density of states obtained from the band calculation.
Using the self-consistent second-order perturbation theory combined with the
local approximation, the correlation effects are investigated on the density of
states and the optical conductivity spectrum, which are found to reproduce the
experiments done by Damascelli et al. semiquantitatively. It is also found that
the peak at the gap edge shifts to lower energy region by correlation effects,
as is seen in the experiments.Comment: 4 pages, 3 figure
Thermodynamic Bethe Ansatz Equations for Minimal Surfaces in AdS_3
We study classical open string solutions with a null polygonal boundary in
AdS_3 in relation to gluon scattering amplitudes in N=4 super Yang-Mills at
strong coupling. We derive in full detail the set of integral equations
governing the decagonal and the dodecagonal solutions and identify them with
the thermodynamic Bethe ansatz equations of the homogeneous sine-Gordon models.
By evaluating the free energy in the conformal limit we compute the central
charges, from which we observe general correspondence between the polygonal
solutions in AdS_n and generalized parafermions.Comment: 25 pages, 4 figures, v2: a figure and references added, minor
corrections, v3: references added, minor corrections, to appear in JHE
T-functions and multi-gluon scattering amplitudes
We study gluon scattering amplitudes/Wilson loops in N=4 super Yang-Mills
theory at strong coupling which correspond to minimal surfaces with a
light-like polygonal boundary in AdS_3. We find a concise expression of the
remainder function in terms of the T-function of the associated thermodynamic
Bethe ansatz (TBA) system. Continuing our previous work on the analytic
expansion around the CFT/regular-polygonal limit, we derive a formula of the
leading-order expansion for the general 2n-point remainder function. The
T-system allows us to encode its momentum dependence in only one function of
the TBA mass parameters, which is obtained by conformal perturbation theory. We
compute its explicit form in the single mass cases. We also find that the
rescaled remainder functions at strong coupling and at two loops are close to
each other, and their ratio at the leading order approaches a constant near 0.9
for large n.Comment: 36 pages, 5 figures, v2: published version, v3: minor correction
On the metallic conductivity of the delafossites PdCoO2 and PtCoO2
The origin of the quasi two-dimensional behavior of PdCoO2 and PtCoO2 is
investigated by means of electronic structure calculations. They are performed
using density functional theory in the generalized gradient approximation as
well as the new full-potential augmented spherical wave method. We show that
the electric conductivity is carried almost exclusively by the in-plane Pd (Pt)
d orbitals. In contrast, the insulating CoO2 sandwich layers of octahedrally
coordinated Co atoms may be regarded as charge carrier reservoirs. This leads
to a weak electronic coupling of the Pd (Pt) layers. The obtained nearly
cylindrical Fermi surface causes the strong anisotropy of the electric
conductivity.Comment: 5 pages, 7 figures, more information at
http://www.physik.uni-augsburg.de/~eyert
S-duality as a beta-deformed Fourier transform
An attempt is made to formulate Gaiotto's S-duality relations in an explicit
quantitative form. Formally the problem is that of evaluation of the Racah
coefficients for the Virasoro algebra, and we approach it with the help of the
matrix model representation of the AGT-related conformal blocks and Nekrasov
functions. In the Seiberg-Witten limit, this S-duality reduces to the Legendre
transformation. In the simplest case, its lifting to the level of Nekrasov
functions is just the Fourier transform, while corrections are related to the
beta-deformation. We calculate them with the help of the matrix model approach
and observe that they vanish for beta=1. Explicit evaluation of the same
corrections from the U_q(sl(2)) infinite-dimensional representation formulas
due to B.Ponsot and J.Teshner remains an open problem.Comment: 21 page
- …