10,626 research outputs found
Enrollment Trends in Northwest Arkansas Charter Schools
Northwest Arkansas is home to nine public charter schools, with plans to open a new charter school for the 2020-21 school year. These schools, which serve unique missions, are some of the most highly ranked schools in the State of Arkansas. While critics argue that public charter schools segregate based on race or academic ability, national evidence finds that these claims are highly context specific. What conclusions can we draw about northwest Arkansas charter schools based on enrollment trends in recent years
Quantum frustration in organic Mott insulators: from spin liquids to unconventional superconductors
We review the interplay of frustration and strong electronic correlations in
quasi-two-dimensional organic charge transfer salts, such as k-(BEDT-TTF)_2X
and Et_nMe_{4-n}Pn[Pd(dmit)2]2. These two forces drive a range of exotic phases
including spin liquids, valence bond crystals, pseudogapped metals, and
unconventional superconductivity. Of particular interest is that in several
materials there is a direct transition as a function of pressure from a spin
liquid Mott insulating state to a superconducting state. Experiments on these
materials raise a number of profound questions about the quantum behaviour of
frustrated systems, particularly the intimate connection between spin liquids
and superconductivity. Insights into these questions have come from a wide
range of theoretical techniques including first principles electronic
structure, quantum many-body theory and quantum field theory. In this review we
introduce the basic ideas of the field by discussing a simple frustrated
Heisenberg model with four spins. We then describe the key experimental
results, emphasizing that for two materials, k-(BEDT-TTF)_2Cu_2(CN)_3 and
EtMe_3Sb[Pd(dmit)_2]_2, there is strong evidence for a spin liquid ground
state, and for EtMe_3P[Pd(dmit)_2]_2, a valence bond solid ground state. We
review theoretical attempts to explain these phenomena, arguing that this can
be captured by a Hubbard model on the anisotropic triangular lattice at half
filling, and that resonating valence bond wavefunctions can capture most of the
essential physics. We review evidence that this model can have a spin liquid
ground state for a range of parameters that are realistic for the relevant
materials. We conclude by summarising the progress made thus far and
identifying some of the key questions still to be answered.Comment: Major rewrite. New material added and many typos corrected. 67 pages,
41 figures. Thanks to those who commented on the previous versio
Symmetry of the superconducting order parameter in frustrated systems determined by the spatial anisotropy of spin correlations
We study the resonating valence bond (RVB) theory of the Hubbard-Heisenberg
model on the half-filled anisotropic triangular lattice. Varying the
frustration changes the wavevector of maximum spin correlation in the Mott
insulating phase. This, in turn, changes the symmetry of the superconducting
state, that occurs at the boundary of the Mott insulating phase. We propose
that this physics is realised in several families of quasi-two-dimensional
organic superconductors.Comment: To appear in Phys. Rev. Lett. - 5 pages, 4 fig
Interplay of frustration, magnetism, charge ordering, and covalency in a model of Na0.5CoO2
We investigate an effective Hamiltonian for Na0.5CoO2 that includes the
electrostatic potential due to the ordered Na ions and strong electronic
correlations. This model displays a subtle interplay between metallic and
insulating phases and between charge and magnetic order. For realistic
parameters, the model predicts an insulating phase with similarities to a
covalent insulator. We show that this interpretation gives a consistent
explanation of experiments on Na0.5CoO2, including the small degree of charge
ordering, the small charge gap, the large moment, and the optical conductivity.Comment: 5 pages, 4 figures. Text revised making more emphasis on model
properties. Figures compacte
Sensitivity of the photo-physical properties of organometallic complexes to small chemical changes
We investigate an effective model Hamiltonian for organometallic complexes
that are widely used in optoelectronic devices. The two most important
parameters in the model are , the effective exchange interaction between the
and orbitals of the ligands, and , the renormalized
energy gap between the highest occupied orbitals on the metal and on the
ligand. We find that the degree of metal-to-ligand charge transfer (MLCT)
character of the lowest triplet state is strongly dependent on the ratio
. is purely a property of the complex and can be
changed significantly by even small variations in the complex's chemistry, such
as replacing substituents on the ligands. We find that that small changes in
can cause large changes in the properties of the complex,
including the lifetime of the triplet state and the probability of injected
charges (electrons and holes) forming triplet excitations. These results give
some insight into the observed large changes in the photophysical properties of
organometallic complexes caused by small changes in the ligands.Comment: Accepted for publication in J. Chem. Phys. 14 pages, 9 figures,
Supplementary Info: 15 pages, 17 figure
Trans-sonic cusped shaped, periodic waves and solitary waves of the electrostatic ion-cyclotron type
By adopting an essentially fluid dynamic viewpoint we derive the wave structure equation for stationary, fully nonlinear, electrostatic, ion-cyclotron waves. The existence of two fundamental constants of the motion, namely, conservation of momentum flux parallel to the ambient magnetic field, and energy flux parallel to the direction of wave propagation, enables the wave structure equation to be reduced to a first order differential equation, which has solutions that are physically transparent. The analysis shows that sufficiently oblique waves, propagating at sub-ion acoustic speeds, form soliton pulse-like solutions whose amplitudes are greatest for perpendicular propagation. Waves that propagate supersonically have periodic cnoidal waveforms, which are asymmetric about the compressive and rarefactive phases of the wave. It is also shown that there exist critical driver fields for which the end point of the compressive phase goes sonic (in the wave frame), with the consequence that the wave form develops a cusp. It is possible that this trans-sonic, choked flow feature provides a mechanism for the 'spiky' waveforms observed in auroral electric field measurements
Presidential Note
Letter from the American Academy of Health Behavior President, Dr. Lara McKenzie
Alien Registration- Mckenzie, Louise B. (Portland, Cumberland County)
https://digitalmaine.com/alien_docs/26676/thumbnail.jp
Alien Registration- Mckenzie, Louise B. (Portland, Cumberland County)
https://digitalmaine.com/alien_docs/26676/thumbnail.jp
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