196,795 research outputs found
Common Fermi-liquid origin of T-squared resistivity and superconductivity in n-type SrTiO3
A detailed analysis is given of the T^2 term in the resistivity observed in
electron-doped SrTiO3. Novel bandstructure data are presented, which provide
values for the bare mass, density of states, and plasma frequency of the
quasiparticles as a function of doping. It is shown that these values are
renormalized by approximately a factor 2 due to electron-phonon interaction. It
is argued that the quasiparticles are in the anti-adiabatic limit with respect
to electron-phonon interaction. The condition of anti-adiabatic coupling
renders the interaction mediated through phonons effectively non-retarded. We
apply Fermi-liquid theory developed in the 70's for the T^2 term in the
resistivity of common metals, and combine this with expressions for Tc and with
the Brinkman-Platzman-Rice (BPR) sum-rule to obtain Landau parameters of n-type
SrTiO3. These parameters are comparable to those of liquid 3He, indicating
interesting parallels between these Fermi-liquids despite the differences
between the composite fermions from which they are formed.Comment: 12 pages, 9 figure
rPICARD: A CASA-based Calibration Pipeline for VLBI Data
Currently, HOPS and AIPS are the primary choices for the time-consuming
process of (millimeter) Very Long Baseline Interferometry (VLBI) data
calibration. However, for a full end-to-end pipeline, they either lack the
ability to perform easily scriptable incremental calibration or do not provide
full control over the workflow with the ability to manipulate and edit
calibration solutions directly. The Common Astronomy Software Application
(CASA) offers all these abilities, together with a secure development future
and an intuitive Python interface, which is very attractive for young radio
astronomers. Inspired by the recent addition of a global fringe-fitter, the
capability to convert FITS-IDI files to measurement sets, and amplitude
calibration routines based on ANTAB metadata, we have developed the the
CASA-based Radboud PIpeline for the Calibration of high Angular Resolution Data
(rPICARD). The pipeline will be able to handle data from multiple arrays: EHT,
GMVA, VLBA and the EVN in the first release. Polarization and phase-referencing
calibration are supported and a spectral line mode will be added in the future.
The large bandwidths of future radio observatories ask for a scalable reduction
software. Within CASA, a message passing interface (MPI) implementation is used
for parallelization, reducing the total time needed for processing. The most
significant gain is obtained for the time-consuming fringe-fitting task where
each scan be processed in parallel.Comment: 6 pages, 1 figure, EVN 2018 symposium proceeding
Representations for the extreme zeros of orthogonal polynomials
We establish some representations for the smallest and largest zeros of orthogonal polynomials in terms of the parameters in the three-terms recurrence relation. As a corollary we obtain representations for the endpoints of the true interval of orthogonality. Implications of these results for the decay parameter of a birth death process (with killing) are displayed
Long range Coulomb forces and the behaviour of the chemical potential of electrons in metals at a second order phase transition
We give a general thermodynamic analyzis of the behaviour of the chemical
potential of electrons in metals at a second order phase transition, including
in our analysis the effect of long range Coulomb forces. It is shown, that this
chemical potential can have a kink at T, both for fixed sample volume and
fixed external pressure. The Coulomb term transfers the changes in chemical
potential of the electrons into an experimentally observable shift of the
surface potential if the sample is electrically connected to a ground
potential. VSGD.93.9.th1Comment: 6 pages, no figures. Revtex, version 2, Materials Science Center
Internal Report Number VSGD.93.9.th
Representations for the extreme zeros of orthogonal polynomials
We establish some representations for the smallest and largest zeros of orthogonal polynomials in terms of the parameters in the three-terms recurrence relation. As a corollary we obtain representations for the endpoints of the true interval of orthogonality. Implications of these results for the decay parameter of a birth death process (with killing) are displayed
Hyperfine-interaction- and magnetic-field-induced Bose-Einstein-statistics suppressed two-photon transitions
Two-photon transitions between atomic states of total electronic angular
momentum and are forbidden when the photons are of the same
energy. This selection rule is analogous to the Landau-Yang theorem in particle
physics that forbids decays of vector particle into two photons. It arises
because it is impossible to construct a total angular momentum
quantum-mechanical state of two photons that is permutation symmetric, as
required by Bose-Einstein statistics. In atoms with non-zero nuclear spin, the
selection rule can be violated due to hyperfine interactions. Two distinct
mechanisms responsible for the hyperfine-induced two-photon transitions are
identified, and the hyperfine structure of the induced transitions is
evaluated. The selection rule is also relaxed, even for zero-nuclear-spin
atoms, by application of an external magnetic field. Once again, there are two
similar mechanisms at play: Zeeman splitting of the intermediate-state
sublevels, and off-diagonal mixing of states with different total electronic
angular momentum in the final state. The present theoretical treatment is
relevant to the ongoing experimental search for a possible
Bose-Einstein-statistics violation using two-photon transitions in barium,
where the hyperfine-induced transitions have been recently observed, and the
magnetic-field-induced transitions are being considered both as a possible
systematic effect, and as a way to calibrate the measurement
Parafermions, parabosons and representations of so(\infty) and osp(1|\infty)
The goal of this paper is to give an explicit construction of the Fock spaces
of the parafermion and the paraboson algebra, for an infinite set of
generators. This is equivalent to constructing certain unitary irreducible
lowest weight representations of the (infinite rank) Lie algebra so(\infty) and
of the Lie superalgebra osp(1|\infty). A complete solution to the problem is
presented, in which the Fock spaces have basis vectors labelled by certain
infinite but stable Gelfand-Zetlin patterns, and the transformation of the
basis is given explicitly. We also present expressions for the character of the
Fock space representations
Reply to 'Comment on "Extending Hirshfeld-I to bulk and periodic materials" '
The issues raised in the comment by T.A. Manz are addressed through the
presentation of calculated atomic charges for NaF, NaCl, MgO, SrTiO and
LaCeO, using our previously presented method for calculating
Hirshfeld-I charges in Solids [J. Comput. Chem.. doi: 10.1002/jcc.23088]. It is
shown that the use of pseudo-valence charges is sufficient to retrieve the full
all-electron Hirshfeld-I charges to good accuracy. Furthermore, we present
timing results of different systems, containing up to over atoms,
underlining the relatively low cost for large systems. A number of theoretical
issues is formulated, pointing out mainly that care must be taken when deriving
new atoms in molecules methods based on "expectations" for atomic charges.Comment: 7 pages, 2 Tables, 2 figure
Extending Hirshfeld-I to bulk and periodic materials
In this work, a method is described to extend the iterative Hirshfeld-I
method, generally used for molecules, to periodic systems. The implementation
makes use of precalculated pseudo-potential based charge density distributions,
and it is shown that high quality results are obtained for both molecules and
solids, such as ceria, diamond, and graphite. The use of such grids makes the
implementation independent of the solid state or quantum chemical code used for
studying the system. The extension described here allows for easy calculation
of atomic charges and charge transfer in periodic and bulk systems.Comment: 11 pages, 4 Tables, 5 Figures, pre-referee draft only, much extended
post referee version only available at publishe
The Airy fibre: an optical fibre that guides light diffracted by a circular aperture
We have designed and made an optical fibre that guides an approximate Airy
pattern as one of its guided modes. The fibre's attenuation was 11.0 dB/km at
1550 nm wavelength, the match between the fibre's mode and the ideal infinite
Airy pattern was 93.7%, and the far field resembled a top-hat beam. The
guidance mechanism has strong similarities to photonic bandgap guidance.Comment: 11 page
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