5,034 research outputs found
Antiferromagnetic Spin Fluctuations in the Metallic Phase of Quasi-Two-Dimensional Organic Superconductors
We give a quantitative analysis of the previously published nuclear magnetic
resonance (NMR) experiments in the k-(ET)2X family of organic charge transfer
salts by using the phenomenological spin fluctuation model of Moriya, and
Millis, Monien and Pines (M-MMP). For temperatures above T_nmr ~ 50 K, the
model gives a good quantitative description of the data in the metallic phases
of several k-(ET)2X materials. These materials display antiferromagnetic
correlation lengths which increase with decreasing temperature and grow to
several lattice constants by T_nmr. It is shown that the fact that the
dimensionless Korringa ratio is much larger than unity is inconsistent with a
broad class of theoretical models (such as dynamical mean-field theory) which
neglects spatial correlations and/or vertex corrections. For materials close to
the Mott insulating phase the nuclear spin relaxation rate, the Knight shift
and the Korringa ratio all decrease significantly with decreasing temperature
below T_nmr. This cannot be described by the M-MMP model and the most natural
explanation is that a pseudogap, similar to that observed in the underdoped
cuprate superconductors, opens up in the density of states below T_nmr. Such a
pseudogap has recently been predicted to occur in the dimerised organic charge
transfer salts materials by the resonating valence bond (RVB) theory. We
propose specific new experiments on organic superconductors to elucidate these
issues. For example, measurements to see if high magnetic fields or high
pressures can be used to close the pseudogap would be extremely valuable.Comment: 11 pages, 2 figures. Accepted for publication in Phys. Rev.
Economic evaluation of the treatment of grade II haemorrhoids : a comparison of stapled haemorrhoidopexy and rubber band ligation.
The definitive version is available at www3.interscience.wiley.comPeer reviewedPostprin
Sensitivity of the interlayer magnetoresistance of layered metals to intralayer anisotropies
Many of the most interesting and technologically important electronic
materials discovered in the past two decades have two common features: a
layered crystal structure and strong interactions between electrons. Two of the
most fundamental questions about such layered metals concern the origin of
intralayer anisotropies and the coherence of interlayer charge transport. We
show that angle dependent magnetoresistance oscillations (AMRO) are sensitive
to anisotropies around an intralayer Fermi surface. Hence, AMRO can be a probe
of intralayer anisotropies that is complementary to angle-resolved
photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM).
However, AMRO are not very sensitive to the coherence of the interlayer
transport. We illustrate this with comparisons to recent AMRO experiments on an
overdoped cuprate.Comment: 7 pages, 3 figure
Evidence for nonlinear diffusive shock acceleration of cosmic-rays in the 2006 outburst of the recurrent nova RS Ophiuchi
Spectroscopic observations of the 2006 outburst of the recurrent nova RS
Ophiuchi at both infrared (IR) and X-ray wavelengths have shown that the blast
wave has decelerated at a higher rate than predicted by the standard
test-particle adiabatic shock-wave model. Here we show that the observed
evolution of the nova remnant can be explained by the diffusive shock
acceleration of particles at the blast wave and the subsequent escape of the
highest energy ions from the shock region. Nonlinear particle acceleration can
also account for the difference of shock velocities deduced from the IR and
X-ray data. The maximum energy that accelerated electrons and protons can have
achieved in few days after outburst is found to be as high as a few TeV. Using
the semi-analytic model of nonlinear diffusive shock acceleration developed by
Berezhko & Ellison, we show that the postshock temperature of the shocked gas
measured with RXTE/PCA and Swift/XRT imply a relatively moderate acceleration
efficiency.Comment: Accepted for publication in ApJ
Progress of the National Bobwhite Conservation Initiative
Northern bobwhites (Colinus virginianus) have declined precipitously over 5 decades because of a decline in habitat, largely a result of agricultural intensification and inadequate management of natural plant succession. In response, quail biologists developed strategic and operational plans, and formed a national partnership of state and federal agencies, bobwhite institutions, non-government organizations, universities and private citizens. The early history of these efforts was reviewed in 2006 at the Sixth National Quail Symposium. Over the past 10 years, exponential growth occurred, including establishment of a home for national bobwhite conservation at the University of Tennessee, and funding from the National Fish and Wildlife Foundation, the United States Fish Wildlife Service Pitmann-Robertson (Federal Aid in Wildlife Restoration Program), individual state agencies and citizens. The result in 2016 is the National Bobwhite Conservation Initiative (NBCI), a 25-state consortium of state wildlife agencies and partners, led by the National Bobwhite Technical Committee and NBCI Management Board. In 2011, NBCI published an updated strategic restoration plan, and spatially-explicit planning tool, NBCI 2.0, followed in 2014 by an implementation plan, the NBCI Coordinated Implementation Program (CIP). We update the history of the NBCI, including changes in funding mechanisms, leadership, administration, and technical programs, and we assess current opportunities and the future of bobwhite conservation
Modeling Bell's Non-resonant Cosmic Ray Instability
We have studied the non-resonant streaming instability of charged energetic
particles moving through a background plasma, discovered by Bell (2004). We
confirm his numerical results regarding a significant magnetic field
amplification in the system. A detailed physical picture of the instability
development and of the magnetic field evolution is given.Comment: 12 pages, 4 figures, accepted to Ap
Fast simulation of a quantum phase transition in an ion-trap realisable unitary map
We demonstrate a method of exploring the quantum critical point of the Ising
universality class using unitary maps that have recently been demonstrated in
ion trap quantum gates. We reverse the idea with which Feynman conceived
quantum computing, and ask whether a realisable simulation corresponds to a
physical system. We proceed to show that a specific simulation (a unitary map)
is physically equivalent to a Hamiltonian that belongs to the same universality
class as the transverse Ising Hamiltonian. We present experimental signatures,
and numerical simulation for these in the six-qubit case.Comment: 12 pages, 6 figure
Ginzburg-Landau theory of phase transitions in quasi-one-dimensional systems
A wide range of quasi-one-dimensional materials, consisting of weakly coupled
chains, undergo three-dimensional phase transitions that can be described by a
complex order parameter. A Ginzburg-Landau theory is derived for such a
transition. It is shown that intrachain fluctuations in the order parameter
play a crucial role and must be treated exactly. The effect of these
fluctuations is determined by a single dimensionless parameter. The
three-dimensional transition temperature, the associated specific heat jump,
coherence lengths, and width of the critical region, are computed assuming that
the single chain Ginzburg-Landau coefficients are independent of temperature.
The width of the critical region, estimated from the Ginzburg criterion, is
virtually parameter independent, being about 5-8 per cent of the transition
temperature. To appear in {\it Physical Review B,} March 1, 1995.Comment: 15 pages, RevTeX, 5 figures in uuencoded compressed tar file
Quantum Transition State Theory for proton transfer reactions in enzymes
We consider the role of quantum effects in the transfer of hyrogen-like
species in enzyme-catalysed reactions. This study is stimulated by claims that
the observed magnitude and temperature dependence of kinetic isotope effects
imply that quantum tunneling below the energy barrier associated with the
transition state significantly enhances the reaction rate in many enzymes. We
use a path integral approach which provides a general framework to understand
tunneling in a quantum system which interacts with an environment at non-zero
temperature. Here the quantum system is the active site of the enzyme and the
environment is the surrounding protein and water. Tunneling well below the
barrier only occurs for temperatures less than a temperature which is
determined by the curvature of potential energy surface near the top of the
barrier. We argue that for most enzymes this temperature is less than room
temperature. For physically reasonable parameters quantum transition state
theory gives a quantitative description of the temperature dependence and
magnitude of kinetic isotope effects for two classes of enzymes which have been
claimed to exhibit signatures of quantum tunneling. The only quantum effects
are those associated with the transition state, both reflection at the barrier
top and tunneling just below the barrier. We establish that the friction due to
the environment is weak and only slightly modifies the reaction rate.
Furthermore, at room temperature and for typical energy barriers environmental
degrees of freedom with frequencies much less than 1000 cm do not have a
significant effect on quantum corrections to the reaction rate.Comment: Aspects of the article are discussed at
condensedconcepts.blogspot.co
Exact solution, scaling behaviour and quantum dynamics of a model of an atom-molecule Bose-Einstein condensate
We study the exact solution for a two-mode model describing coherent coupling
between atomic and molecular Bose-Einstein condensates (BEC), in the context of
the Bethe ansatz. By combining an asymptotic and numerical analysis, we
identify the scaling behaviour of the model and determine the zero temperature
expectation value for the coherence and average atomic occupation. The
threshold coupling for production of the molecular BEC is identified as the
point at which the energy gap is minimum. Our numerical results indicate a
parity effect for the energy gap between ground and first excited state
depending on whether the total atomic number is odd or even. The numerical
calculations for the quantum dynamics reveals a smooth transition from the
atomic to the molecular BEC.Comment: 5 pages, 4 figure
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