7,662 research outputs found
First-principle density-functional calculation of the Raman spectra of BEDT-TTF
We present a first-principles density-functional calculation for the Raman
spectra of a neutral BEDT-TTF molecule. Our results are in excellent agreement
with experimental results. We show that a planar structure is not a stable
state of a neutral BEDT-TTF molecule. We consider three possible conformations
and discuss their relation to disorder in these systems.Comment: 3 pages, 2 figures, submitted to the proceedings of ISCOM 200
Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
We present an resonating valence bond (RVB) theory of superconductivity for
the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show
that these calculations are consistent with the observed phase diagram of the
half-filled layered organic superconductors, such as the beta, beta', kappa and
lambda phases of (BEDT-TTF)_2X [bis(ethylenedithio)tetrathiafulvalene] and
(BETS)_2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order
transition from a Mott insulator to a d_{x^2-y^2} superconductor with a small
superfluid stiffness and a pseudogap with d_{x^2-y^2} symmetry. The
Mott--Hubbard transition can be driven either by increasing the on-site Coulomb
repulsion, U, or by changing the anisotropy of the two hopping integrals, t'/t.
Our results suggest that the ratio t'/t plays an important role in determining
the phase diagram of the organic superconductors.Comment: 4 pages, 3 figur
Ferromagnetism, paramagnetism and a Curie-Weiss metal in an electron doped Hubbard model on a triangular lattice
Motivated by the unconventional properties and rich phase diagram of NaxCoO2
we consider the electronic and magnetic properties of a two-dimensional Hubbard
model on an isotropic triangular lattice doped with electrons away from
half-filling. Dynamical mean-field theory (DMFT) calculations predict that for
negative inter-site hopping amplitudes (t<0) and an on-site Coulomb repulsion,
U, comparable to the bandwidth, the system displays properties typical of a
weakly correlated metal. In contrast, for t>0 a large enhancement of the
effective mass, ferromagnetism and a Curie-Weiss magnetic susceptibility are
found in a broad electron doping range. Our observation of Nagaoka
ferromagnetism is consistent with the A-type antiferromagnetism (i.e.
ferromagnetic layers stacked antiferromagnetically) observed in neutron
scattering experiments on NaxCoO2. We propose that `Curie-Weiss metal' phase
observed in NaxCoO2 is a consequence of the crossover from ``bad metal'' with
incoherent quasiparticles at temperatures T>T* and Fermi liquid behavior with
enhanced parameters below T*, where T* is a low energy coherence scale induced
by strong local Coulomb electron correlations. We propose a model which
contains the charge ordering phenomena observed in the system which, we
propose, drives the system close to the Mott insulating phase even at large
dopings.Comment: 24 pages, 15 figures; accepted for publication in Phys. Rev.
The application of mechanical diagnosis and therapy in lateral epicondylalgia
Background: lateral epicondylalgia (LE) is a musculoskeletal diagnosis that causes pain and dysfunction in the lateral aspect of the elbow. Mechanical diagnosis and therapy (MDT) is an orthopaedic classification and treatment system based on mechanical and symptomatic response to repeated and sustained end-range movement. There has been no investigation of the association between MDT and patients diagnosed with LE. Case description: this report presents three patients matching the currently accepted diagnostic criteria for LE, two with a diagnosis of lateral epicondylitis (tennis elbow) from a medical doctor. These patients were classified and treated by a diplomat of MDT and two third-year doctoral students of physical therapy using MDT. Outcomes: short- and long-term (one year) outcomes were excellent, demonstrating rapid abolishment of symptoms and return to prior levels of function in 3–6 visits between 11–59 days. Patients demonstrated the ability to prevent and manage reoccurrence of symptoms independently without seeking further health care. Discussion: this case series raises questions about whether or not the pathologies traditionally associated with the aetiology of LE are actually at fault. Moreover, it raises questions about the utility of special tests typically utilized to identify those structures. The series provides preliminary evidence that MDT may be capable of providing effective short- and long-term outcomes in the management of LE. Level of Evidence: 4
Keywords:
Mechanical diagnosis and therapy, Lateral epicondylalgia, Case serie
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.
In Defense of Monopoly
"In Defense of Monopoly offers an unconventional but empirically grounded argument in favor of market monopolies. Authors McKenzie and Lee claim that conventional, static models exaggerate the harm done by real-world monopolies, and they show why some degree of monopoly presence is necessary to maximize the improvement of human welfare over time.
Inspired by Joseph Schumpeter's suggestion that market imperfections can drive an economy's long-term progress, In Defense of Monopoly defies conventional assumptions to show readers why an economic system's failure to efficiently allocate its resources is actually a necessary precondition for maximizing the system's long-term performance: the perfectly fluid, competitive economy idealized by most economists is decidedly inferior to one characterized by market entry and exit restrictions or costs.
An economy is not a board game in which players compete for a limited number of properties, nor is it much like the kind of blackboard games that economists use to develop their monopoly models. As McKenzie and Lee demonstrate, the creation of goods and services in the real world requires not only competition but the prospect of gains beyond a normal competitive rate of return.
Temperature Dependence of the Magnetic Susceptibility for Triangular-Lattice Antiferromagnets with spatially anisotropic exchange constants
We present the temperature dependence of the uniform susceptibility of
spin-half quantum antiferromagnets on spatially anisotropic
triangular-lattices, using high temperature series expansions. We consider a
model with two exchange constants, and on a lattice that
interpolates between the limits of a square-lattice (), a
triangular-lattice (), and decoupled linear chains (). In all
cases, the susceptibility which has a Curie-Weiss behavior at high
temperatures, rolls over and begins to decrease below a peak temperature,
. Scaling the exchange constants to get the same peak temperature, shows
that the susceptibilities for the square-lattice and linear chain limits have
similar magnitudes near the peak. Maximum deviation arises near the
triangular-lattice limit, where frustration leads to much smaller
susceptibility and with a flatter temperature dependence. We compare our
results to the inorganic materials CsCuCl and CsCuBr and to a
number of organic molecular crystals. We find that the former (CsCuCl
and CsCuBr) are weakly frustrated and their exchange parameters
determined through the temperature dependence of the susceptibility are in
agreement with neutron-scattering measurements. In contrast, the organic
materials are strongly frustrated with exchange parameters near the isotropic
triangular-lattice limit.Comment: 10 pages, 9 figures and 1 table, revised versio
Arkansas’s 9th Grade Course Failures and Building Configurations
In this brief, we examine the relationship between course failures among Arkansas’s 9th grade students and the grade levels served in their school. We find that in schools that terminate at 9th grade, 9th graders are less likely to fail one or more course during the year compared to failure rates in schools that terminate at 12th grade. We suggest an increase in awareness and examination of how we assess 9th graders in Arkansas
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