544 research outputs found
Spin-Orbital Entanglement and Violation of the Goodenough-Kanamori Rules
We point out that large composite spin-orbital fluctuations in Mott
insulators with orbital degeneracy are a manifestation of quantum
entanglement of spin and orbital variables. This results in a dynamical nature
of the spin superexchange interactions, which fluctuate over positive and
negative values, and leads to an apparent violation of the Goodenough-Kanamori
rules. [{\it Published in Phys. Rev. Lett. {\bf 96}, 147205 (2006).}]Comment: 4 pages, 2 figure
Making it real: exploring the potential of Augmented Reality for teaching primary school science
The use of Augmented Reality (AR) in formal education could prove a key component in future learning environments that are richly populated with a blend of hardware and software applications. However, relatively little is known about the potential of this technology to support teaching and learning with groups of young children in the classroom. Analysis of teacher-child dialogue in a comparative study between use of an AR virtual mirror interface and more traditional science teaching methods for 10-year-old children, revealed that the children using AR were less engaged than those using traditional resources. We suggest four design requirements that need to be considered if AR is to be successfully adopted into classroom practice. These requirements are: flexible content that teachers can adapt to the needs of their children, guided exploration so learning opportunities can be maximised, in a limited time, and attention to the needs of institutional and curricular requirements
Suppression of static stripe formation by next-neighbor hopping
We show from real-space Hartree-Fock calculations within the extended Hubbard
model that next-nearest neighbor (t') hopping processes act to suppress the
formation of static charge stripes. This result is confirmed by investigating
the evolution of charge-inhomogeneous corral and stripe phases with increasing
t' of both signs. We propose that large t' values in YBCO prevent static stripe
formation, while anomalously small t' in LSCO provides an additional reason for
the appearance of static stripes only in these systems.Comment: 4 pages, 5 figure
Damped orbital excitations in the titanates
A possible mechanism for the removal of the orbital degeneracy in RTiO3
(where R=La, Y, ...) is considered. The calculation is based on the
Kugel-Khomskii Hamiltonian for electrons residing in the t2g orbitals of the Ti
ions, and uses a self-consistent pe rturbation expansion in the interaction
between the orbital and the spin degrees of freedom. The latter are assumed to
be ordered in a Neel state, brought about by delicate interactions that are not
included in the Kugel-Khomskii Hamiltonian. Within our model calculations, each
of the t2g bands is found to acquire a finite, temperature-dependent
dispersion, that lifts the orbital degeneracy. The orbital excitations are
found to be heavily damped over a rather wide band. Consequently, they do not
participate as a separate branch of excitations in the low-temperature
thermodynamics.eComment: 6 pages, 3 figure
Elementary excitations of the symmetric spin-orbital model: The XY limit
The elementary excitations of the 1D, symmetric, spin-orbital model are
investigated by studying two anisotropic versions of the model, the pure XY and
the dimerized XXZ case, with analytical and numerical methods. While they
preserve the symmetry between spin and orbital degrees of freedom, these models
allow for a simple and transparent picture of the low--lying excitations: In
the pure XY case, a phase separation takes place between two phases with
free--fermion like, gapless excitations, while in the dimerized case, the
low-energy effective Hamiltonian reduces to the 1D Ising model with gapped
excitations. In both cases, all the elementary excitations involve simultaneous
flips of the spin and orbital degrees of freedom, a clear indication of the
breakdown of the traditional mean-field theory.Comment: Revtex, two figure
Orbital dynamics in ferromagnetic transition metal oxides
We consider a model of strongly correlated electrons interacting by
superexchange orbital interactions in the ferromagnetic phase of LaMnO. It
is found that the classical orbital order with alternating occupied
orbitals has a full rotational symmetry at orbital degeneracy, and the
excitation spectrum derived using the linear spin-wave theory is gapless. The
quantum (fluctuation) corrections to the order parameter and to the ground
state energy restore the cubic symmetry of the model. By applying a uniaxial
pressure orbital degeneracy is lifted in a tetragonal field and one finds an
orbital-flop phase with a gap in the excitation spectrum. In two dimensions the
classical order is more robust near the orbital degeneracy point and quantum
effects are suppressed. The orbital excitation spectra obtained using finite
temperature diagonalization of two-dimensional clusters consist of a
quasiparticle accompanied by satellite structures. The orbital waves found
within the linear spin-wave theory provide an excellent description of the
dominant pole of these spectra.Comment: 13 pages, 12 figures, to appear in Phys. Rev.
Orbital and spin physics in LiNiO2 and NaNiO2
We derive a spin-orbital Hamiltonian for a triangular lattice of e_g orbital
degenerate (Ni^{3+}) transition metal ions interacting via 90 degree
superexchange involving (O^{2-}) anions, taking into account the on-site
Coulomb interactions on both the anions and the transition metal ions. The
derived interactions in the spin-orbital model are strongly frustrated, with
the strongest orbital interactions selecting different orbitals for pairs of Ni
ions along the three different lattice directions. In the orbital ordered
phase, favoured in mean field theory, the spin-orbital interaction can play an
important role by breaking the U(1) symmetry generated by the much stronger
orbital interaction and restoring the threefold symmetry of the lattice. As a
result the effective magnetic exchange is non-uniform and includes both
ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic
interactions still dominate, this offers yet insufficient explanation for the
absence of magnetic order and the low-temperature behaviour of the magnetic
susceptibility of stoichiometric LiNiO_2. The scenario proposed to explain the
observed difference in the physical properties of LiNiO_2 and NaNiO_2 includes
small covalency of Ni-O-Li-O-Ni bonds inducing weaker interplane superexchange
in LiNiO_2, insufficient to stabilize orbital long-range order in the presence
of stronger intraplane competition between superexchange and Jahn-Teller
coupling.Comment: 33 pages, 12 postscript figures, uses iopams.sty . This article
features in New Journal of Physics as part of a Focus Issue on Orbital
Physics - all contributions may be freely accessed at
(http://stacks.iop.org/1367-2630/6/i=1/a=E05). The published version of this
article may be found at http://stacks.iop.org/1367-2630/7/12
Order from disorder: Quantum spin gap in magnon spectra of LaTiO_3
A theory of the anisotropic superexchange and low energy spin excitations in
a Mott insulator with t_{2g} orbital degeneracy is presented. We observe that
the spin-orbit coupling induces frustrating Ising-like anisotropy terms in the
spin Hamiltonian, which invalidate noninteracting spin wave theory. The
frustration of classical states is resolved by an order from disorder
mechanism, which selects a particular direction of the staggered moment and
generates a quantum spin gap. The theory explains well the observed magnon gaps
in LaTiO_3. As a test case, a specific prediction is made on the splitting of
magnon branches at certain momentum directions.Comment: 5 pages, 2 figures, final versio
Fishing regulations, sexual dimorphism, and the life history of harvest
Freshwater recreational fisheries regulations are a vital tool for achieving social and ecological fisheries objectives. However, angler behavior and fish biology may interact to influence regulation efficacy in unexpected ways. We combined models of fish growth and angler behavior to explore how angler behavior interacts with fish life history to shape the probability of fish harvest given capture across ages, life stages, and sexes of walleye (Sander vitreus). Compared to females, males grew more quickly as juveniles, matured earlier, and reached smaller maximum sizes. Male walleye were therefore vulnerable to harvest for more of their reproductive lives than females because males spent more time at sizes where anglers were very likely to harvest them. We suggest that restricting harvest of large individuals in sexually dimorphic species may favor the survival of large, reproductive-aged females. Moreover, we show that combining models of fish growth and harvester behavior can provide insights into how harvest affects fish with complex life histories over the course of their lives.
La réglementation relative aux pêches sportives en eau douce constitue un outil d’importance capitale pour l’atteinte des objectifs sociaux et écologiques des pêches. Les interactions des comportements des pêcheurs et de la biologie des poissons peuvent toutefois influencer l’efficacité de la réglementation de manière imprévue. Nous combinons des modèles de croissance des poissons et de comportement des pêcheurs afin d’examiner l’effet de l’interaction du comportement des pêcheurs et du cycle biologique des poissons sur la probabilité de récolte de poissons au vu des prises selon l’âge, de l’étape du cycle de vie et du sexe de dorés jaunes (Sander vitreus). Comparativement aux femelles, les mâles croissent plus vite quand ils sont juvéniles, arrivent à maturité plus tôt et atteignent des tailles maximums plus petites. Les dorés mâles sont donc plus vulnérables à la récolte pour une plus grande partie de leur vie reproductive que les femelles parce qu’ils passent plus de temps à des tailles qui les rendent plus susceptibles d’être récoltés par les pêcheurs. Nous suggérons que le fait de restreindre la récolte aux grands individus pour des espèces qui présentent un dimorphisme sexuel pourrait favoriser la survie des grandes femelles en âge de reproduction. Nous démontrons en outre que le jumelage de modèles de croissance des poissons et de comportement des pêcheurs peut fournir de l’information utile sur l’effet de la récolte sur les poissons aux cycles biologiques complexes au fil de leur vie
Just-In-Place Information for Mobile Device Interfaces
Abstract. This paper addresses the potentials of context sensitivity for making mobile device interfaces less complex and easier to interact with. Based on a semiotic approach to information representation, it is argued that the design of mobile device interfaces can benefit from spatial and temporal indexicality, reducing information complexity and interaction space of the device while focusing on information and functionality relevant here and now. Illustrating this approach, a series of design sketches show the possible redesign of an existing web and wap-based information service.
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