1,736 research outputs found
Lattice Green functions in all dimensions
We give a systematic treatment of lattice Green functions (LGF) on the
-dimensional diamond, simple cubic, body-centred cubic and face-centred
cubic lattices for arbitrary dimensionality for the first three
lattices, and for for the hyper-fcc lattice. We show that there
is a close connection between the LGF of the -dimensional hypercubic lattice
and that of the -dimensional diamond lattice. We give constant-term
formulations of LGFs for all lattices and dimensions. Through a still
under-developed connection with Mahler measures, we point out an unexpected
connection between the coefficients of the s.c., b.c.c. and diamond LGFs and
some Ramanujan-type formulae for Comment: 30 page
Green's function of a finite chain and the discrete Fourier transform
A new expression for the Green's function of a finite one-dimensional lattice
with nearest neighbor interaction is derived via discrete Fourier transform.
Solution of the Heisenberg spin chain with periodic and open boundary
conditions is considered as an example. Comparison to Bethe ansatz clarifies
the relation between the two approaches.Comment: preprint of the paper published in Int. J. Modern Physics B Vol. 20,
No. 5 (2006) 593-60
RXTE monitoring observations of Markarian 3
We present Rossi X-ray Timing Explorer, monitoring observations of the
Seyfert 2 galaxy Markarian 3 spanning a 200 day period during which time the
source flux varied by a factor in the 4-20 keV bandpass. In broad
agreement with earlier Ginga results, the average spectrum can be represented
in terms of a simple spectral model consisting of a very hard power-law
continuum () modified below keV by a high
absorbing column ( \cunits) together with a high
equivalent width Fe-K emission feature at 6.4 keV. The abnormally flat spectral
index is probably the signature of a strong reflection component and we
consider two models incorporating such emission. In the first the reflected
signal suffers the same absorption as the intrinsic continuum, whereas in the
second the reflection is treated as an unabsorbed spectral component. In the
former case, we require a very strong reflection signal () in order to
match the data; in addition variability of both the intrinsic power-law and the
reflection component is required. The unabsorbed reflection model requires a
somewhat higher line-of-sight column density to the nuclear source ( \cunits), but in this case the reflected signal remains constant
whilst the level of the intrinsic continuum varies. The latter description is
consistent with the reflection originating from the illuminated far inner wall
of a molecular torus, the nearside of which screens our direct view of the
central continuum source.Comment: 7 pages, submitted to the MNRA
Scaling and Density of Lee-Yang Zeroes in the Four Dimensional Ising Model
The scaling behaviour of the edge of the Lee--Yang zeroes in the four
dimensional Ising model is analyzed. This model is believed to belong to the
same universality class as the model which plays a central role in
relativistic quantum field theory. While in the thermodynamic limit the scaling
of the Yang--Lee edge is not modified by multiplicative logarithmic
corrections, such corrections are manifest in the corresponding finite--size
formulae. The asymptotic form for the density of zeroes which recovers the
scaling behaviour of the susceptibility and the specific heat in the
thermodynamic limit is found to exhibit logarithmic corrections too. The
density of zeroes for a finite--size system is examined both analytically and
numerically.Comment: 17 pages (4 figures), LaTeX + POSTSCRIPT-file, preprint UNIGRAZ-UTP
20-11-9
Monitoring RXTE Observations of Markarian 348: the origin of the column density variations
We analyze 37 RXTE observations of the type 2 Seyfert galaxy Mrk348 obtained
during a period of 14 months. We confirm the spectral variability previous
reported by Smith et al., in the sense that thecolumn density decreases by a
factor of ~3 as the count rate increases. Column density variations could
possibly originate either due to the random drift of clouds within the
absorption screen, or due to photoionization processes. Our modeling of the
observed variations implies that the first scenario is more likely. These
clouds should lie in a distance of >2 light years from the source, having a
diameter of a few light days and a density of >10^7 cm^(-3), hence probably
residing outside the Broad Line Region.Comment: 6 pages, 3 figures, to appear in MNRA
FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS
We report theoretical electronic structure of Fibonacci superlattices of
narrow-gap III-V semiconductors. Electron dynamics is accurately described
within the envelope-function approximation in a two-band model.
Quasiperiodicity is introduced by considering two different III-V semiconductor
layers and arranging them according to the Fibonacci series along the growth
direction. The resulting energy spectrum is then found by solving exactly the
corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix
techniques. We find that a self-similar electronic spectrum can be seen in the
band structure. Electronic transport properties of samples are also studied and
related to the degree of spatial localization of electronic envelope-functions
via Landauer resistance and Lyapunov coefficient. As a working example, we
consider type II InAs/GaSb superlattices and discuss in detail our results in
this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in
Semiconductor Science and Technolog
Refinement of metabolite detection in cystic fibrosis sputum reveals heme correlates with lung function decline
The bacterial growth environment within cystic fibrosis (CF) sputum is complex, dynamic, and shaped by both host and microbial processes. Characterization of the chemical parameters within sputum that stimulate the in vivo growth of airway pathogens (e.g. Pseudomonas aeruginosa) and their associated virulence factors may lead to improved CF treatment strategies. Motivated by conflicting reports of the prevalence and abundance of P. aeruginosa-derived metabolites known as phenazines within CF airway secretions, we sought to quantify these metabolites in sputum using quadrupole time-of-flight mass spectrometry. In contrast to our previous work, all phenazines tested (pyocyanin (PYO), phenazine-1-carboxylic acid (PCA), phenazine-1-carboxamide, and 1-hydroxyphenazine) were below detection limits of the instrument (0.1 ÎŒM). Instead, we identified a late-eluting compound that shared retention time and absorbance characteristics with PCA, yet generated mass spectra and a fragmentation pattern consistent with ferriprotoporphyrin IX, otherwise known as heme B. These data suggested that UV-vis chromatographic peaks previously attributed to PCA and PYO in sputum were mis-assigned. Indeed, retrospective analysis of raw data from our prior study found that the heme B peak closely matched the peaks assigned to PCA, indicating that the previous study likely uncovered a positive correlation between pulmonary function (percent predicted forced expiratory volume in 1 second, or ppFEV1) and heme B, not PCA or any other phenazine. To independently test this observation, we performed a new tandem mass-spectrometry analysis of 71 additional samples provided by the Mountain West CF Consortium Sputum Biomarker study and revealed a positive correlation (Ï = â0.47, p<0.001) between sputum heme concentrations and ppFEV1. Given that hemoptysis is strongly associated with airway inflammation, pulmonary exacerbations and impaired lung function, these new data suggest that heme B may be a useful biomarker of CF pathophysiology
Exact results for some Madelung type constants in the finite-size scaling theory
A general formula is obtained from which the madelung type constant: extensively used in the finite-size
scaling theory is computed analytically for some particular cases of the
parameters and . By adjusting these parameters one can obtain
different physical situations corresponding to different geometries and
magnitudes of the interparticle interaction.Comment: IOP- macros, 5 pages, replaced with amended version (1 ref. added
Exact Zeros of the Partition Function for a Continuum System with Double Gaussian Peaks
We calculate the exact zeros of the partition function for a continuum system
where the probability distribution for the order parameter is given by two
asymmetric Gaussian peaks. When the positions of the two peaks coincide, the
two separate loci of zeros which used to give first-order transition touch each
other, with density of zeros vanishing at the contact point on the positive
real axis. Instead of the second-order transition of Ehrenfast classification
as one might naively expect, one finds a critical behavior in this limit.Comment: 13 pages, 6 figures, revtex, minor changes in fig.2, to be published
in Physical Review
Supersymmetric quantum mechanics with nonlocal potentials
We consider supersymmetric quantum mechanical models with both local and
nonlocal potentials. We present a nonlocal deformation of exactly solvable
local models. Its energy eigenfunctions and eigenvalues are determined exactly.
We observe that both our model Hamiltonian and its supersymmetric partner may
have normalizable zero-energy ground states, in contrast to local models with
nonperiodic or periodic potentials.Comment: 4 pages, REVTeX, Minor revisions for clarificatio
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