1,143 research outputs found
Orbital Ordering and Spin-Ladder Formation in La2RuO5
The semiconductor-semiconductor transition of La2RuO5 is studied by means of
augmented spherical wave (ASW) electronic structure calculations as based on
density functional theory and the local density approximation. This transition
has lately been reported to lead to orbital ordering and a quenching of the
local spin magnetic moment. Our results hint towards an orbital ordering
scenario which, markedly different from the previously proposed scheme,
preserves the local S = 1 moment at the Ru sites in the low-temperature phase.
The unusual magnetic behaviour is interpreted by the formation of spin-ladders,
which result from the structural changes occurring at the transition and are
characterized by antiferromagnetic coupling along the rungs.Comment: 5 pages, 4 figures, for more information see
http://www.physik.uni-augsburg.de/~eyert
Particle interactions and lattice dynamics: Scenarios for efficient bidirectional stochastic transport?
Intracellular transport processes driven by molecular motors can be described
by stochastic lattice models of self-driven particles. Here we focus on
bidirectional transport models excluding the exchange of particles on the same
track. We explore the possibility to have efficient transport in these systems.
One possibility would be to have appropriate interactions between the various
motors' species, so as to form lanes. However, we show that the lane formation
mechanism based on modified attachment/detachment rates as it was proposed
previously is not necessarily connected to an efficient transport state and is
suppressed when the diffusivity of unbound particles is finite. We propose
another interaction mechanism based on obstacle avoidance that allows to have
lane formation for limited diffusion. Besides, we had shown in a separate paper
that the dynamics of the lattice itself could be a key ingredient for the
efficiency of bidirectional transport. Here we show that lattice dynamics and
interactions can both contribute in a cooperative way to the efficiency of
transport. In particular, lattice dynamics can decrease the interaction
threshold beyond which lanes form. Lattice dynamics may also enhance the
transport capacity of the system even when lane formation is suppressed.Comment: 25 pages, 17 figures, 2 table
Non-intrinsic origin of the Colossal Dielectric Constants in CaCu3Ti4O12
The dielectric properties of CaCu3Ti4O12, a material showing colossal values
of the dielectric constant, were investigated in a broad temperature and
frequency range extending up to 1.3 GHz. A detailed equivalent circuit analysis
of the results and two crucial experiments, employing different types of
contacts and varying sample thickness, provide clear evidence that the
apparently high values of the dielectric constant in CaCu3Ti4O12 are
non-intrinsic and due to electrode polarization effects. The intrinsic
properties of CaCu3Ti4O12 are characterized by charge transport via hopping of
localized charge carriers and a relatively high dielectric constant of the
order of 100.Comment: 4 pages, 4 figure
Regular subspaces of a quaternionic Hilbert space from quaternionic Hermite polynomials and associated coherent states
We define quaternionic Hermite polynomials by analogy with two families of
complex Hermite polynomials. As in the complex case, these polynomials
consatitute orthogonal families of vectors in ambient quaternionic
-spaces. Using these polynomials, we then define regular and anti-regular
subspaces of these -spaces, the associated reproducing kernels and the
ensuing quaternionic coherent states
Broadband dielectric response of CaCu3Ti4O12: From dc to the electronic transition regime
We report on phonon properties and electronic transitions in CaCu3Ti4O12, a
material which reveals a colossal dielectric constant at room temperature
without any ferroelectric transition. The results of far- and mid-infrared
measurements are compared to those obtained by broadband dielectric and
millimeter-wave spectroscopy on the same single crystal. The unusual
temperature dependence of phonon eigenfrequencies, dampings and ionic plasma
frequencies of low lying phonon modes are analyzed and discussed in detail.
Electronic excitations below 4 eV are identified as transitions between full
and empty hybridized oxygen-copper bands and between oxygen-copper and
unoccupied Ti 3d bands. The unusually small band gap determined from the
dc-conductivity (~200 meV) compares well with the optical results.Comment: 7 pages, 8 figure
Magnetoelectric and HR-STEM investigations on eutectic CoFe<sub>2</sub>O<sub>4</sub>-Ba<sub>1-x</sub>Sr<sub>x</sub>TiO<sub>3</sub> composites
Multiferroic Ba1–xSrxTiO3–CoFe2O4 (x = 0.03, 0.05) composites with rarely investigated 3-3 connectivity were prepared by eutectic crystallization in an optical floating zone furnace. High-resolution scanning transmission electron microscopy investigations of the CoFe2O4–BaTiO3 interface revealed an almost perfect connection between both components. These micrographs also showed that the impact of post-annealing in air was much larger than expected and resulted in formation of small BaTiO3 inclusions in the CoFe2O4 phase. The magnetoelectric coefficient αME was studied in detail with respect to its dependence on the static magnetic field, the frequency of the driving AC-field and temperature. Furthermore, the influence of different growth rates (5, 10 and 20 mm h-1), chemical composition, sample thickness and the alignment of electrical polarization and magnetic field (collinear or vertical) on the magnetoelectric properties were studied. The largest value of αME = 1.3 mV Oe-1 cm-1 was found for a sample grown at 5 mm h-1. For even slower growth rates, a higher Sr content was required to avoid the formation of impurity phases leading to a decrease of αME
Broadband dielectric spectroscopy on single-crystalline and ceramic CaCu3Ti4O12
We present dielectric measurements of the colossal dielectric constant
material CaCu3Ti4O12 extending up to 1.3 GHz also covering so far only rarely
investigated single crystalline samples. Special emphasis is put on the second
relaxation reported in several works on polycrystals, which we detect also in
single crystals. For polycrystalline samples we provide a recipe to achieve
values of the dielectric constant as high as in single crystals.Comment: 3 pages, 3 figure
Dielectric behavior of Copper Tantalum Oxide
A thorough investigation of the dielectric properties of Cu2Ta4O12, a
material crystallizing in a pseudo-cubic, perovskite-derived structure is
presented. We measured the dielectric constant and conductivity of single
crystals in an exceptionally broad frequency range up to GHz frequencies and at
temperatures from 25 - 500 K. The detected dielectric constant is unusually
high (reaching values up to 105) and almost constant in a broad frequency and
temperature range. Cu2Ta4O12 possesses a crystal structure similar to
CaCu3Ti4O12, the compound for which such an unusually high dielectric constant
was first observed. An analysis of the results using a simple equivalent
circuit and measurements with different types of contact revealed that
extrinsic interfacial polarization effects, derived from surface barrier
capacitors are the origin of the observed giant dielectric constants. The
intrinsic properties of Cu2Ta4O12 are characterized by a (still relatively
high) dielectric constant in the order of 100 and by charge transport via
hopping conduction of Anderson-localized charge carriers.Comment: 18 pages, 6 figures, submitted to Jouranl of Physical Chemestr
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