52,676 research outputs found
Optical response of high- cuprates: possible role of scattering rate saturation and in-plane anisotropy
We present a generalized Drude analysis of the in-plane optical conductivity
(,) in cuprates taking into account the effects of
in-plane anisotropy. A simple ansatz for the scattering rate
(,), that includes anisotropy, a quadratic frequency
dependence and saturation at the Mott-Ioffe-Regel limit, is able to reproduce
recent normal state data on an optimally doped cuprate over a wide frequency
range. We highlight the potential importance of including anisotropy in the
full expression for (,) and challenge previous
determinations of () in which anisotropy was neglected and
() was indicated to be strictly linear in frequency over a wide
frequency range. Possible implications of our findings for understanding
thermodynamic properties and self-energy effects in high- cuprates will
also be discussed.Comment: 8 pages, 7 figures. To be published in Physical Review
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Material Morphology and energy barriers to electrical ageing
A distribution of the parameters representing activation energy of the ageing process within the Dissado-Montanari-Mazzanti (DMM) lifetime model has been shown to model experimental lifetime distributions of PET films well. The results imply small differences in the local environments of the moieties involved in the ageing process. Very small changes in the minimum activation energy values have a pronounced effect on the resultant lifetimes of polymer specimens. Changes in the distributions of activation energies with field and temperature can be explained by assuming the ageing process to be one whereby polymer segments on lamella surfaces crystallise to create free volume within the polymer
Mechanism-based strategies for the management of autoimmunity and immune dysregulation in primary immunodeficiencies
A broad spectrum of autoimmunity is now well described in patients with primary immunodeficiencies (PIDs). Management of autoimmune disease in the background of PID is particularly challenging given the seemingly discordant goals of immune support and immune suppression. Our growing ability to define the molecular underpinnings of immune dysregulation has facilitated novel targeted therapeutics. This review focuses on mechanism-based treatment strategies for the most common autoimmune and inflammatory complications of PID including autoimmune cytopenias, rheumatologic disease, and gastrointestinal disease. We aim to provide guidance regarding the rational use of these agents in the complex PID patient population
Gamma-Ray Burst and Relativistic Shells: The Surface Filling Factor
The variability observed in many complex gamma-ray bursts (GRBs) is
inconsistent with causally connected variations in a single, symmetric,
relativistic shell interacting with the ambient material ("external shocks").
Rather, the symmetry of the shell must be broken on an angular scale much
smaller than Gamma^{-1} where Gamma is the bulk Lorentz factor for the shell.
The observed variability in the external shock models arises from the number of
causally connected regions that (randomly) become active. We define the
"surface filling factor" to be the ratio of the area of causally connected
regions that become active to the observable area of the shell. From the
observed variability in 52 BATSE bursts, we estimate the surface filling factor
to be typically 0.005 although some values are near unity. We find that the
surface filling factor is about 0.1 Delta T/T in both the constant Gamma phase
(which probably produces the GRB) and the deaccelerating phase (which probably
produces the x-ray afterglows). Here, \Delta T is a typical time scale of
variability and T is the time since the initial signal. We analyze the 2 hr
flare seen by ASCA 36 hr after the GRB and conclude that the surface filling
factor must be small (0.001) in the x-ray afterglow phase as well. Explanations
for low surface filling factor can either require more or less energy (by a
factor of about 1000) compared to that expected for a symmetric shell.Comment: 26 pages, 5 embedded figures, Latex, revised version as in press,
ApJ, added figure to show the possible expanding shell geometries that can
give low filling facto
Stable Topological Superfluid Phase of Ultracold Polar Fermionic Molecules
We show that single-component fermionic polar molecules confined to a 2D
geometry and dressed by a microwave field, may acquire an attractive
dipole-dipole interaction leading to superfluid p-wave pairing at sufficiently
low temperatures even in the BCS regime. The emerging state is the topological
phase promising for topologically protected quantum information
processing. The main decay channel is via collisional transitions to dressed
states with lower energies and is rather slow, setting a lifetime of the order
of seconds at 2D densities cm
Exact and approximate dynamics of the quantum mechanical O(N) model
We study a quantum dynamical system of N, O(N) symmetric, nonlinear
oscillators as a toy model to investigate the systematics of a 1/N expansion.
The closed time path (CTP) formalism melded with an expansion in 1/N is used to
derive time evolution equations valid to order 1/N (next-to-leading order). The
effective potential is also obtained to this order and its properties
areelucidated. In order to compare theoretical predictions against numerical
solutions of the time-dependent Schrodinger equation, we consider two initial
conditions consistent with O(N) symmetry, one of them a quantum roll, the other
a wave packet initially to one side of the potential minimum, whose center has
all coordinates equal. For the case of the quantum roll we map out the domain
of validity of the large-N expansion. We discuss unitarity violation in the 1/N
expansion; a well-known problem faced by moment truncation techniques. The 1/N
results, both static and dynamic, are also compared to those given by the
Hartree variational ansatz at given values of N. We conclude that late-time
behavior, where nonlinear effects are significant, is not well-described by
either approximation.Comment: 16 pages, 12 figrures, revte
Spin 1 inversion: a Majorana tensor force for deuteron alpha scattering
We demonstrate, for the first time, successful S-matrix to potential
inversion for spin one projectiles with non-diagonal yielding a
interaction. The method is a generalization of the
iterative-perturbative, IP, method. We present a test case indicating the
degree of uniqueness of the potential. The method is adapted, using established
procedures, into direct observable to potential inversion, fitting ,
, , and for d + alpha scattering over
a range of energies near 10 MeV. The interaction which we find is
very different from that proposed elsewhere, both real and imaginary parts
being very different for odd and even parity channels.Comment: 7 pages Revtex, 4 ps figure
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