5,388 research outputs found
First principles theory of the EPR g-tensor in solids: defects in quartz
A theory for the reliable prediction of the EPR g-tensor for paramagnetic
defects in solids is presented. It is based on density functional theory and on
the gauge including projector augmented wave (GIPAW) approach to the
calculation of all-electron magnetic response. The method is validated by
comparison with existing quantum chemical and experimental data for a selection
of diatomic radicals. We thenperform the first prediction of EPR -tensors in the solid state and find the results to be in excellent agreement
with experiment for the and substitutional P defect centers in quartz.Comment: 5 pages, 4 table
On the Arnold Conjecture and the Atiyah-Patodi-Singer Index Theorem
The Arnold conjecture yields a lower bound to the number of periodic
classical trajectories in a Hamiltonian system. Here we count these
trajectories with the help of a path integral, which we inspect using
properties of the spectral flow of a Dirac operator in the background of a
\Sp(2N) valued gauge field. We compute the spectral flow from the
Atiyah-Patodi-Singer index theorem, and apply the results to evaluate the path
integral using localization methods. In this manner we find a lower bound to
the number of periodic classical trajectories which is consistent with the
Arnold conjecture.Comment: 12 pages, references correcte
Wannier interpolation of the electron-phonon matrix elements in polar semiconductors: Polar-optical coupling in GaAs
We generalize the Wannier interpolation of the electron-phonon matrix
elements to the case of polar-optical coupling in polar semiconductors. We
verify our methodological developments against experiments, by calculating the
widths of the electronic bands due to electron-phonon scattering in GaAs, the
prototype polar semiconductor. The calculated widths are then used to estimate
the broadenings of excitons at critical points in GaAs and the electron-phonon
relaxation times of hot electrons. Our findings are in good agreement with
available experimental data. Finally, we demonstrate that while the Fr\"ohlich
interaction is the dominant scattering process for electrons/holes close to the
valley minima, in agreement with low-field transport results, at higher
energies, the intervalley scattering dominates the relaxation dynamics of hot
electrons or holes. The capability of interpolating the polar-optical coupling
opens new perspectives in the calculation of optical absorption and transport
properties in semiconductors and thermoelectrics.Comment: To appear on Phys. Rev.
Electron Transport and Hot Phonons in Carbon Nanotubes
We demonstrate the key role of phonon occupation in limiting the high-field
ballistic transport in metallic carbon nanotubes. In particular, we provide a
simple analytic formula for the electron transport scattering length, that we
validate by accurate first principles calculations on (6,6) and (11,11)
nanotubes. The comparison of our results with the scattering lengths fitted
from experimental I-V curves indicates the presence of a non-equilibrium
optical phonon heating induced by electron transport. We predict an effective
temperature for optical phonons of thousands Kelvin.Comment: 4 pages, 1 figur
Hygrothermal damage mechanisms in graphite-epoxy composites
T300/5209 and T300/5208 graphite epoxy laminates were studied experimentally and analytically in order to: (1) determine the coupling between applied stress, internal residual stress, and moisture sorption kinetics; (2) examine the microscopic damage mechanisms due to hygrothermal cycling; (3) evaluate the effect of absorbed moisture and hygrothermal cycling on inplane shear response; (4) determine the permanent loss of interfacial bond strength after moisture absorption and drying; and (5) evaluate the three dimensional stress state in laminates under a combination of hygroscopic, thermal, and mechanical loads. Specimens were conditioned to equilibrium moisture content under steady exposure to 55% or 95% RH at 70 C or 93 C. Some specimens were tested subsequent to moisture conditioning and 100 cycles between -54 C and either 70 C or 93 C
Nonlocal pseudopotentials and magnetic fields
We show how to describe the coupling of electrons to non-uniform magnetic
fields in the framework of the widely used norm-conserving pseudopotential
appro ximation for electronic structure calculations. Our derivation applies to
magnetic fields that are smooth on the scale of the core region. The method is
validated by application to the calculation of the magnetic susceptibility of
molecules. Our results are compared with high quality all electron quantum
chemical results, and another recently proposed formalism.Comment: 4 pages, submitted to Physical Review Letter
Aspects of Duality and Confining Strings
We inspect the excitation energy spectrum of a confining string in terms of
solitons in an effective field theory model. The spectrum can be characterized
by a spectral function, and twisting and bending of the string is manifested by
the invariance of this function under a duality transformation. Both general
considerations and numerical simulations reveal that the spectral function can
be approximated by a simple rational form, which we propose becomes exact in
the Yang-Mills theory.Comment: refinement of certain argument
Star product and the general Leigh-Strassler deformation
We extend the definition of the star product introduced by Lunin and
Maldacena to study marginal deformations of N=4 SYM. The essential difference
from the latter is that instead of considering U(1)xU(1) non-R-symmetry, with
charges in a corresponding diagonal matrix, we consider two Z_3-symmetries
followed by an SU(3) transformation, with resulting off-diagonal elements. From
this procedure we obtain a more general Leigh-Strassler deformation, including
cubic terms with the same index, for specific values of the coupling constants.
We argue that the conformal property of N=4 SYM is preserved, in both beta-
(one-parameter) and gamma_{i}-deformed (three-parameters) theories, since the
deformation for each amplitude can be extracted in a prefactor. We also
conclude that the obtained amplitudes should follow the iterative structure of
MHV amplitudes found by Bern, Dixon and Smirnov.Comment: 21 pages, no figures, JHEP3, v2: references added, v3: appendix A
added, v4: clarification in section 3.
Kohn Anomalies and Electron-Phonon Interaction in Graphite
We demonstrate that graphite phonon dispersions have two Kohn anomalies at
the Gamma-E_2g and K-A'1 modes. The anomalies are revealed by two sharp kinks.
By an exact analytic derivation, we show that the slope of these kinks is
proportional to the square of the electron-phonon coupling (EPC). Thus, we can
directly measure the EPC from the experimental dispersions. The Gamma-E_2g and
K-A'1 EPCs are particularly large, whilst they are negligible for all the other
modes at Gamma and K.Comment: 4 pages, 2 figure
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