6,919 research outputs found
Derivation of the probability distribution function for the local density of states of a disordered quantum wire via the replica trick and supersymmetry
We consider the statistical properties of the local density of states of a
one-dimensional Dirac equation in the presence of various types of disorder
with Gaussian white-noise distribution. It is shown how either the replica
trick or supersymmetry can be used to calculate exactly all the moments of the
local density of states. Careful attention is paid to how the results change if
the local density of states is averaged over atomic length scales. For both the
replica trick and supersymmetry the problem is reduced to finding the ground
state of a zero-dimensional Hamiltonian which is written solely in terms of a
pair of coupled ``spins'' which are elements of u(1,1). This ground state is
explicitly found for the particular case of the Dirac equation corresponding to
an infinite metallic quantum wire with a single conduction channel. The
calculated moments of the local density of states agree with those found
previously by Al'tshuler and Prigodin [Sov. Phys. JETP 68 (1989) 198] using a
technique based on recursion relations for Feynman diagrams.Comment: 39 pages, 1 figur
Charge Distribution Near Oxygen Vacancies in Reduced Ceria
Understanding the electronic charge distribution around oxygen vacancies in
transition metal and rare earth oxides is a scientific challenge of
considerable technological importance. We show how significant information
about the charge distribution around vacancies in cerium oxide can be gained
from a study of high resolution crystal structures of higher order oxides which
exhibit ordering of oxygen vacancies. Specifically, we consider the
implications of a bond valence sum analysis of CeO and
CeO. To illuminate our analysis we show alternative
representations of the crystal structures in terms of orderly arrays of
co-ordination defects and in terms of flourite-type modules. We found that in
CeO, the excess charge resulting from removal of an oxygen atom
delocalizes among all three triclinic Ce sites closest to the O vacancy. In
CeO, the charge localizes on the next nearest neighbour Ce atoms.
Our main result is that the charge prefers to distribute itself so that it is
farthest away from the O vacancies. This contradicts \emph{the standard picture
of charge localisation} which assumes that each of the two excess electrons
localises on one of the cerium ions nearest to the vacancy. This standard
picture is assumed in most calculations based on density functional theory
(DFT). Based on the known crystal structure of PrO, we also
predict that the charge in CeO will be found in the second
coordination shell of the O vacancy. Although this review focuses on bulk
cerium oxides our approach to characterising electronic properties of oxygen
vacancies and the physical insights gained should also be relevant to surface
defects and to other rare earth and transition metal oxides.Comment: 20 pages, 23 figures. The replacement file has a new format for the
figures are the document layout but no change in content. v3 has the
following main changes: 1. The abstract and introduction were extensively
revised. 2. Sec. IV was removed. 3. The Conclusion was rewritte
Transport properties of the metallic state of overdoped cuprate superconductors from an anisotropic marginal Fermi liquid model
We consider the implications of a phenomenological model self-energy for the
charge transport properties of the metallic phase of the overdoped cuprate
superconductors. The self-energy is the sum of two terms with characteristic
dependencies on temperature, frequency, location on the Fermi surface, and
doping. The first term is isotropic over the Fermi surface, independent of
doping, and has the frequency and temperature dependence characteristic of a
Fermi liquid. The second term is anisotropic over the Fermi surface (vanishing
at the same points as the superconducting energy gap), strongly varies with
doping (scaling roughly with , the superconducting transition
temperature), and has the frequency and temperature dependence characteristic
of a marginal Fermi liquid. Previously it has been shown this self-energy can
describe a range of experimental data including angle-dependent
magnetoresistance (ADMR) and quasi-particle renormalisations determined from
specific heat, quantum oscillations, and angle-resolved photo-emission
spectroscopy (ARPES). Without introducing new parameters and neglecting vertex
corrections we show that this model self-energy can give a quantitative
description of the temperature and doping dependence of a range of reported
transport properties of Tl2201 samples. These include the intra-layer
resistivity, the frequency dependent optical conductivity, the intra-layer
magnetoresistance, and the Hall coefficient. The temperature dependence of the
latter two are particularly sensitive to the anisotropy of the scattering rate
and to the shape of the Fermi surface. In contrast, the temperature dependence
of the Hall angle is dominated by the Fermi liquid contribution to the
self-energy that determines the scattering rate in the nodal regions of the
Fermi surface.Comment: 17 pages, 16 figure
How linear features alter predator movement and the functional\ud response
In areas of oil and gas exploration, seismic lines have been reported to alter the movement patterns of wolves (Canis lupus). We developed a mechanistic first passage time model, based on an anisotropic elliptic partial differential equation, and used this to explore how wolf movement responses to seismic lines influence the encounter rate of the wolves with their prey. The model was parametrized using 5 min GPS location data. These data showed that wolves travelled faster on seismic lines and had a higher probability of staying on a seismic line once they were on it. We simulated wolf movement on a range of seismic line densities and drew implications for the rate of predator–prey interactions as described by the functional response. The functional response exhibited a more than linear increase with respect to prey density (type III) as well as interactions with seismic line density. Encounter rates were significantly higher in landscapes with high seismic line density and were most pronounced at low prey densities. This suggests that prey at low population densities are at higher risk in environments with a high seismic line density unless they learn to avoid them
The Late Time Light Curve of SN 1998bw Associated with GRB980425
We report 139 photometric observations through the B, V, and I filters of the
supernova SN 1998bw, an object which is associated with the Gamma-Ray Burst GRB
980425. Detailed light curves of this unique supernova can be compared to
theoretical models, so we report here our light curve for 123 days between 27
June 1998 and 28 October 1998. The light curve of SN 1988bw is consistent with
those of the Type Ic class. We find that the magnitude-versus-time relation for
this supernova is linear to within 0.05 mags in all colors over the entire
duration of our study. Our measured uniform decline rates are , , and magnitudes per day in the
B, V, and I bands. The linear decline and the rate of that decline suggest that
late time light curve is powered by the radioactive decay of cobalt with some
leakage of the gamma rays.Comment: 15 pages, 1 figure, 1 table, Accepted for publication in PAS
Photothermal Fluctuations as a Fundamental Limit to Low-Frequency Squeezing in a Degenerate Optical Parametric Amplifier
We study the effect of photothermal fluctuations on squeezed states of light
through the photo-refractive effect and thermal expansion in a degenerate
optical parametric amplifier (OPA). We also discuss the effect of the
photothermal noise in various cases and how to minimize its undesirable
consequences. We find that the photothermal noise in the OPA introduces a
significant amount of noise on phase squeezed beams, making them less than
ideal for low frequency applications such as gravitational wave (GW)
interferometers, whereas amplitude squeezed beams are relatively immune to the
photothermal noise and may represent the best choice for application in GW
interferometers
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