307 research outputs found
Poynting's theorem and energy conservation in the propagation of light in bounded media
Starting from the Maxwell-Lorentz equations, Poynting's theorem is
reconsidered. The energy flux vector is introduced as S_e=(E x B)/mu_0 instead
of E x H, because only by this choice the energy dissipation can be related to
the balance of the kinetic energy of the matter subsystem. Conservation of the
total energy as the sum of kinetic and electromagnetic energy follows. In our
discussion, media and their microscopic nature are represented exactly by their
susceptibility functions, which do not necessarily have to be known. On this
footing, it can be shown that energy conservation in the propagation of light
through bounded media is ensured by Maxwell's boundary conditions alone, even
for some frequently used approximations. This is demonstrated for approaches
using additional boundary conditions and the dielectric approximation in
detail, the latter of which suspected to violate energy conservation for
decades.Comment: 5 pages, RevTeX4, changes: complete rewrit
Correlation of optical conductivity and ARPES spectra of strong-coupling large polarons and its display in cuprates
Common approach is used to calculate band due to strong-coupling large
polaron (SCLP) photodissociation in ARPES and in optical conductivity (OC)
spectra. It is based on using the coherent-states representation for the phonon
field in SCLP. The calculated positions of both band maximums are universal
functions of one parameter - the SCLP binding energy Ep: ARPES band maximum
lies at binding energy about 3.2Ep; the OC band maximum is at the photon energy
about 4.2Ep. The half-widths of the bands are mainly determined by Ep and
slightly depend on Frohlich electron-phonon coupling constant: for its value
6-8 the ARPES band half-width is 1.7-1.3Ep and the OC band half-width is
2.8-2.2Ep. Using these results one can predict approximate position of ARPES
band maximum and half-width from the maximum of mid-IR OC band and vice versa.
Comparison of the results with experiments leads to a conclusion that
underdoped cuprates contain SCLPs with Ep=0.1-0.2 eV that is in good conformity
with the medium parameters in cuprates. The values of the polaron binding
energy determined from experimental ARPES and OC spectra of the same material
are in good conformity too: the difference between them is within 10 percent.Comment: 17 pages, 6 figure
Electron-phonon interaction via Pekar mechanism in nanostructures
We consider an electron-acoustic phonon coupling mechanism associated with
the dependence of crystal dielectric permittivity on the strain (the so-called
Pekar mechanism) in nanostructures characterized by strong confining electric
fields. The efficiency of Pekar coupling is a function of both the absolute
value and the spatial distribution of the electric field. It is demonstrated
that this mechanism exhibits a phonon wavevector dependence similar to that of
piezoelectricity and must be taken into account for electron transport
calculations in an extended field distribution. In particular, we analyze the
role of Pekar coupling in energy relaxation in silicon inversion layers.
Comparison with the recent experimental results is provided to illustrate its
potential significance
A Variational Approach to Nonlocal Exciton-Phonon Coupling
In this paper we apply variational energy band theory to a form of the
Holstein Hamiltonian in which the influence of lattice vibrations (optical
phonons) on both local site energies (local coupling) and transfers of
electronic excitations between neighboring sites (nonlocal coupling) is taken
into account. A flexible spanning set of orthonormal eigenfunctions of the
joint exciton-phonon crystal momentum is used to arrive at a variational
estimate (bound) of the ground state energy for every value of the joint
crystal momentum, yielding a variational estimate of the lowest polaron energy
band across the entire Brillouin zone, as well as the complete set of polaron
Bloch functions associated with this band. The variation is implemented
numerically, avoiding restrictive assumptions that have limited the scope of
previous assaults on the same and similar problems. Polaron energy bands and
the structure of the associated Bloch states are studied at general points in
the three-dimensional parameter space of the model Hamiltonian (electronic
tunneling, local coupling, nonlocal coupling), though our principal emphasis
lay in under-studied area of nonlocal coupling and its interplay with
electronic tunneling; a phase diagram summarizing the latter is presented. The
common notion of a "self-trapping transition" is addressed and generalized.Comment: 33 pages, 11 figure
Coherent Propagation of Polaritons in Semiconductor Heterostructures: Nonlinear Pulse Transmission in Theory and Experiment
The influence of coherent optical nonlinearities on polariton propagation
effects is studied within a theory-experiment comparison. A novel approach that
combines a microscopic treatment of the boundary problem in a sample of finite
thickness with excitonic and biexcitonic nonlinearities is introduced.
Light-polarization dependent spectral changes are analyzed for single-pulse
transmission and pump-probe excitation
A Theoretical Model for Gas Separation in a Glow Discharge: Cataphoresis
A theoretical model for transient and steady-state cataphoresis is developed starting with the macroscopic equations of continuity. After a brief breakdown period, the impurity ions are assumed to be closely coupled with their neutral counterparts. The basic assumptions in the model are that after breakdown, the level of ionization of the impurity, and the axial electric field remain constant; it is demonstrated that under these conditions a system involving rapid ionization-recombination reactions is equivalent to a system in which no reaction occurs, but in which the effective\u27\u27 ion mobility is a product of the true ion mobility and the fraction of impurity ionization. The influence of endbulbs commonly employed in experiments is analyzed and found to influence greatly the characteristic time required to reach steady state. Agreement is found between the model and available experimental data. Particular emphasis is placed upon mass spectrometer data reported by Matveeva, and by Beckey, Groth, and Welge; these data are for mixtures of rare gases and for mixtures of hydrogen and deuterium, and involve endbulbs. The ordinary diffusion case, associated with the collapse of the steady-state cataphoretic profile, is also analyzed for a system containing endbulbs
Theory of Electric Dipole Spin Resonance in a Parabolic Quantum Well
A theory of Electric Dipole Spin Resonance (EDSR), that is caused by various
mechanisms of spin-orbit coupling, is developed as applied to free electrons in
a parabolic quantum well. Choosing a parabolic shape of the well has allowed us
to find explicit expressions for the EDSR intensity and its dependence on the
magnetic field direction in terms of the basic parameters of the Hamiltonian.
By using these expressions, we have investigated and compared the effect of
specific mechanisms of spin orbit (SO) coupling and different polarizations of
ac electric field on the intensity of EDSR. Angular dependences of the EDSR
intensity are indicative of the relative contributions of the competing
mechanisms of SO coupling. Our results show that electrical manipulating
electron spins in quantum wells is generally highly efficient, especially by an
in-plane ac electric field.Comment: 45 pages 6 figur
Surface plasmons at composite surfaces with diffusive charges
Metal surfaces with disorder or with nanostructure modifications are studied,
allowing for a localized charge layer (CL) in addition to continuous charges
(CC) in the bulk, both charges having a compressional or diffusive non-local
response. The notorious problem of "additional boundary conditions" is resolved
with the help of a Boltzmann equation that involves the scattering between the
two charge types. Depending on the strength of this scattering, the oscillating
charges can be dominantly CC or CL; the surface plasmon (SP) resonance acquires
then a relatively small linewidth, in agreement with a large set of data. With
a few parameters our model describes a large variety of SP dispersions
corresponding to observed data.Comment: 6 pages, 2 figure
Electron-lattice interaction and its impact on high Tc superconductivity
In this Colloquium, the main features of the electron-lattice interaction are
discussed and high values of the critical temperature up to room temperature
could be provided. While the issue of the mechanism of superconductivity in the
high Tc cuprates continues to be controversial, one can state that there have
been many experimental results demonstrating that the lattice makes a strong
impact on the pairing of electrons. The polaronic nature of the carriers is
also a manifestation of strong electron-lattice interaction. One can propose an
experiment that allows an unambiguous determination of the intermediate boson
(phonon, magnon, exciton, etc.) which provides the pairing. The
electron-lattice interaction increases for nanosystems, and this is due to an
effective increase in the density of states
Random-effects substitution models for phylogenetics via scalable gradient approximations
Phylogenetic and discrete-trait evolutionary inference depend heavily on an
appropriate characterization of the underlying character substitution process.
In this paper, we present random-effects substitution models that extend common
continuous-time Markov chain models into a richer class of processes capable of
capturing a wider variety of substitution dynamics. As these random-effects
substitution models often require many more parameters than their usual
counterparts, inference can be both statistically and computationally
challenging. Thus, we also propose an efficient approach to compute an
approximation to the gradient of the data likelihood with respect to all
unknown substitution model parameters. We demonstrate that this approximate
gradient enables scaling of sampling-based inference, namely Bayesian inference
via Hamiltonian Monte Carlo, under random-effects substitution models across
large trees and state-spaces. Applied to a dataset of 583 SARS-CoV-2 sequences,
an HKY model with random-effects shows strong signals of nonreversibility in
the substitution process, and posterior predictive model checks clearly show
that it is a more adequate model than a reversible model. When analyzing the
pattern of phylogeographic spread of 1441 influenza A virus (H3N2) sequences
between 14 regions, a random-effects phylogeographic substitution model infers
that air travel volume adequately predicts almost all dispersal rates. A
random-effects state-dependent substitution model reveals no evidence for an
effect of arboreality on the swimming mode in the tree frog subfamily Hylinae.
Simulations reveal that random-effects substitution models can accommodate both
negligible and radical departures from the underlying base substitution model.
We show that our gradient-based inference approach is over an order of
magnitude more time efficient than conventional approaches
- …