5,498 research outputs found
Orbital Ordering and Unfrustrated Magnetism from Degenerate Double Exchange in the Iron Pnictides
The magnetic excitations of the iron pnictides are explained within a
degenerate double-exchange model. The local-moment spins are coupled by
superexchanges and between nearest and next-nearest neighbors,
respectively, and interact with the itinerant electrons of the degenerate
and orbitals via a ferromagnetic Hund exchange. The latter
stabilizes stripe antiferromagnetism due to emergent ferro-orbital
order and the resulting kinetic energy gain by hopping preferably along the
ferromagnetic spin direction. Taking the quantum nature of the spins into
account, we calculate the magnetic excitation spectra in the presence of both,
super- and double-exchange. A dramatic increase of the spin-wave energies at
the competing N\'eel ordering wave vector is found, in agreement with recent
neutron scattering data. The spectra are fitted to a spin-only model with a
strong spatial anisotropy and additional longer ranged couplings along the
ferromagnetic chains. Over a realistic parameter range, the effective couplings
along the chains are negative corresponding to unfrustrated stripe
antiferromagnetism.Comment: 11 pages, 6 figures. Version accepted in PR
The disappearing January blip and other state employment mysteries
Employment (Economic theory) ; Statistics
Evaluation of battery models for prediction of electric vehicle range
Three analytical models for predicting electric vehicle battery output and the corresponding electric vehicle range for various driving cycles were evaluated. The models were used to predict output and range, and then compared with experimentally determined values determined by laboratory tests on batteries using discharge cycles identical to those encountered by an actual electric vehicle while on SAE cycles. Results indicate that the modified Hoxie model gave the best predictions with an accuracy of about 97 to 98% in the best cases and 86% in the worst case. A computer program was written to perform the lengthy iterative calculations required. The program and hardware used to automatically discharge the battery are described
Two distinct Mott-Insulator to Bose-glass transitions and breakdown of self averaging in the disordered Bose-Hubbard model
We investigate the instabilities of the Mott-insulating phase of the weakly
disordered Bose-Hubbard model within a renormalization group analysis of the
replica field theory obtained by a strong-coupling expansion around the atomic
limit. We identify a new order parameter and associated correlation length
scale that is capable of capturing the transition from a state with zero
compressibility, the Mott insulator, to one in which the compressibility is
finite, the Bose glass. The order parameter is the relative variance of the
disorder-induced mass distribution. In the Mott insulator, the relative
variance renormalizes to zero, whereas it diverges in the Bose glass. The
divergence of the relative variance signals the breakdown of self-averaging.
The length scale governing the breakdown of self-averaging is the distance
between rare regions. This length scale is finite in the Bose glass but
diverges at the transition to the Mott insulator with an exponent of
for incommensurate fillings. Likewise, the compressibility vanishes with an
exponent of at the transition. At commensurate fillings, the
transition is controlled by a different fixed point at which both the disorder
and interaction vertices are relevant.Comment: Extended, published versio
First Record of a Barred Owl, Strix varia, in Labrador
A Barred Owl (Strix varia) was heard calling in central Labrador, Canada in 2001, and a dead owl was found in the same area in 2004. These are the first confirmed records for the Province of Newfoundland and Labrador
Anomalous suppression of the Bose glass at commensurate fillings in the disordered Bose-Hubbard model
We study the weakly disordered Bose-Hubbard model on a cubic lattice through
a one-loop renormalization group analysis of the corresponding effective field
theory which is explicitly derived by combining a strong-coupling expansion
with a replica average over the disorder. The method is applied not only to
generic uncorrelated on-site disorder but also to simultaneous hopping disorder
correlated with the differences of adjacent disorder potentials. Such
correlations are inherent in fine-grained optical speckle potentials used as a
source of disorder in optical lattice experiments. As a result of strong
coupling, the strength of the replica mixing disorder vertex, responsible for
the emergence of a Bose glass, crucially depends on the chemical potential and
the Hubbard repulsion and vanishes to leading order in the disorder at
commensurate boson fillings. As a consequence, at such fillings a direct
transition between the Mott-insulator and the superfluid in the presence of
disorder cannot be excluded on the basis of a one-loop calculation. At
incommensurate fillings, at a certain length scale, the Mott insulator will
eventually become unstable towards the formation of a Boss glass. Phase
diagrams as a function of the microscopic parameters are presented and the
finite-size crossover between the Mott-insulating state and the Bose glass is
analyzed.Comment: 13 pages, 6 figures, extended versio
Applying rotorcraft modelling technology to renewable energy research
The perceived need to reduce mankind's impact on the global climate motivates towards a future society in which a significant proportion of its energy needs will be extracted from the winds and the tides of the planet. This paper shows several examples of the application of Brown's Vorticity Transport Model, originally developed to perform simulations of helicopter aeromechanics and wake dynamics, to the analysis of the performance of renewable energy devices and their possible impact on the environment. Prediction of the loading on wind turbines introduces significant additional challenges to such a model, including the need to account fully for the effects of radial flow on blade stall. The wake-mediated aerodynamic interactions that occur within a wind farm can reduce its power output significantly, but this problem is very similar to that where the aerodynamic unsteadiness of the coupled wake of the main and tail rotors of a helicopter can result in significantly increased pilot workload. The helicopter-related problem of brownout, encountered during operations in desert conditions, has its analogue in the entrainment of sediment into the wakes of tidal turbines. In both cases it may be possible to ameliorate the influence of the rotor on its environment by careful and well-informed design. Finally, calculations of the distortion and dispersal of the exhaust plumes of a helicopter by the wake of its rotor allow insight into how wind turbines might interfere with the dispersal of pollutants from nearby industrial sites. These examples show how cross-disciplinary information transfer between the rotorcraft field and the renewable energy community is helping to develop the technologies that will be required by our future society, as well as helping to understand the environmental issues that might need to be faced as these technologies become more prevalent
Winter Occurrences of Ivory Gulls, Pagophila eburnea, in Inland Labrador
Ivory Gulls were observed during winter on three occasions up to 325 km inland from open coastal water, feeding on Caribou carcasses in inland Labrador. Other recent observations in central Labrador are also noted
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