99 research outputs found
Optical properties of pyrochlore oxide
We present optical conductivity spectra for
single crystal at different temperatures. Among reported pyrochlore ruthenates,
this compound exhibits metallic behavior in a wide temperature range and has
the least resistivity. At low frequencies, the optical spectra show typical
Drude responses, but with a knee feature around 1000 \cm. Above 20000 \cm, a
broad absorption feature is observed. Our analysis suggests that the low
frequency responses can be understood from two Drude components arising from
the partially filled Ru bands with different plasma frequencies and
scattering rates. The high frequency broad absorption may be contributed by two
interband transitions: from occupied Ru states to empty bands
and from the fully filled O 2p bands to unoccupied Ru states.Comment: 4 pages, 6 figure
Phase diagram of the extended Hubbard chain with charge-dipole interactions
We consider a modified extended Hubbard model (EHM) which, in addition to the
on-site repulsion U and nearest-neighbor repulsion V, includes polarization
effects in second-order perturbation theory. The model is equivalent to an EHM
with renormalized U plus a next-nearest-neighbor repulsion term. Using a method
based on topological quantum numbers (charge and spin Berry phases), we
generalize to finite hopping t the quantum phase diagram in one dimension
constructed by van den Brink et al. (Phys. Rev. Lett. 75, 4658 (1995)). At
hopping t=0 there are two charge density-wave phases, one spin density-wave
phase and one intermediate phase with charge and spin ordering, depending on
the parameter values. At t \neq 0 the nature of each phase is confirmed by
studying correlation functions. However, in addition to the strong-coupling
phases, a small region with bond ordering appears. The region occupied by the
intermediate phase first increases and then decreases with increasing t, until
it finally disappears for t of the order but larger than U. For small t, the
topological transitions agree with the results of second order perturbation
theory.Comment: 6 pages, 5 figures, two columns latex version. Accepted for
publication in Physical Review B. Mistaken reference 16 has been correcte
Neel Order and Electron Spectral Functions in the Two-Dimensional Hubbard Model: a Spin-Charge Rotating Frame Approach
Using recently developed quantum SU(2)xU(1) rotor approach, that provides a
self-consistent treatment of the antiferromagnetic state we have performed
electronic spectral function calculations for the Hubbard model on the square
lattice. The collective variables for charge and spin are isolated in the form
of the space-time fluctuating U(1) phase field and rotating spin quantization
axis governed by the SU(2) symmetry, respectively. As a result interacting
electrons appear as composite objects consisting of bare fermions with attached
U(1) and SU(2) gauge fields. This allows us to write the fermion Green's
function in the space-time domain as the product CP^1 propagator resulting from
the SU(2) gauge fields, U(1) phase propagator and the pseudo-fermion
correlation function. As a result the problem of calculating the spectral line
shapes now becomes one of performing the convolution of spin, charge and
pseudo-fermion Green's functions. The collective spin and charge fluctuations
are governed by the effective actions that are derived from the Hubbard model
for any value of the Coulomb interaction. The emergence of a sharp peak in the
electron spectral function in the antiferromagnetic state indicates the decay
of the electron into separate spin and charge carrying particle excitations.Comment: 16 pages, 5 figures, submitted to Phys. Rev.
Evolution of a Metal to Insulator Transition in CaNaCuOCl, as seen by ARPES
We present angle resolved photoemission (ARPES) data on Na-doped
CaCuOCl. We demonstrate that the chemical potential shifts upon
doping the system across the insulator to metal transition. The resulting low
energy spectra reveal a gap structure which appears to deviate from the
canonical form. To reconcile the measured
gap structure with d-wave superconductivity one can understand the data in
terms of two gaps, a very small one contributing to the nodal region and a very
large one dominating the anti-nodal region. The latter is a result of the
electronic structure observed in the undoped antiferromagnetic insulator.
Furthermore, the low energy electronic structure of the metallic sample
contains a two component structure in the nodal direction, and a change in
velocity of the dispersion in the nodal direction at roughly 50 meV. We discuss
these results in connection with photoemission data on other cuprate systems.Comment: 10 pages, 12 figures, accepted by PRB; a high quality pdf is
available at
http://helios.physics.utoronto.ca/~fronning/RonningNaCCOCResub.pdf (2.2MB
Viscoelastic sorption behavior of starch and gluten
The migration of gasses and liquids through (bio) polymers plays an important role in numerous applications and processes like, e.g., drying and (re)wetting of foods and food ingredients, packaging, and controlled release. A good understanding of the transport mechanisms is therefore of great importance to control these processes. The transport of solvents through (bio) polymers is a complex process and involves diffusion due to water concentration gradients, swelling, shrinkage, and relaxation of the polymeric matrix. Often also transitions between the glassy, rubbery, and/or viscous states of the polymer system are involved. Here we report on a theoretical and experimental study of water transport in biopolymer films of starch and gluten
Dynamic water vapour sorption in gluten and starch films
Water sorption of gluten and wheat starch films as a function of water activity was studied using gravimetric step-change sorption experiments. Films of different thicknesses were used with the aim to vary the characteristic diffusion time and to get insights in the contribution of the polymer-chain rearrangement in the sorption behaviour. It is shown that both starch and gluten are in the glassy state for a water activity aw below 0.9. From comparison of the dynamical sorption curves with a Fickian diffusion model, it is shown that water diffusion in gluten films seems Fickian for aw 0.7, while for starch films, non-Fickian sorption behaviour is observed for aw > 0.1. The results show that polymer-chain rearrangement and the stress built up in the matrix play an important role in the sorption dynamics of these films. Even when the material is in the glassy state matrix relaxation phenomena play a role in the sorption behaviour of starch and gluten
Proteins at air-water interfaces studied using external reflection circular dichroism
In this report we describe the first attempts to record external reflection circular dichroism (ERCD) spectra of beta -lactoglobulin solutions. It is shown that the accumulated proteins at and near the air–water interface can be detected using ERCD and that the signals obtained contain information on the conformational properties and concentration of the proteins residing at the interface. The local protein concentration and its conformation are in full agreement with previous observations using external reflection infrared spectroscopy. The ERCD signals are dominated by linear dichroism (LD) due to non-ideal behavior of the instrumental optics, but can be explained for using the theoretical description of chiral reflection. This allows the analysis of ERCD spectra of protein solutions. The measured ERCD signals are described accurately in the region between 190 and 220 nm, but poor resemblance is obtained at higher wavelengths. We are however confident that improvement of experimental conditions and theoretical description will allow that in the near future, external reflection circular dichroism (CD) can be a valuable tool that complements the application of external reflection infrared spectroscopy to study interfacial system
Modeling water sorption dynamics of cellular solid food systems using free volume theory
Water sorption and dynamical properties of bread crust were studied using gravimetric sorption experiments. Water uptake and loss were measured while relative humidity (RH) was step-wise in- or decreased. Experimental results were compared with Fickian diffusion models and empirical models like the exponential and power-law model. From comparison of experimental sorption curves and the power-law model for short times it followed for all bread crust that the diffusional coefficient n is close to one. It turned out that this is not due to so-called case II diffusion and water transport that is limited by relaxation of the solid material but due to the fact that RH did not instantaneously but gradually increased to the set value. Sorption curves of isotherm experiments could be best described by the Fickian diffusion model for low RH and by the exponential model for large RH. Transport rates depend on moisture content and show a maximum around RH = 0.7, corresponding to a water mass fraction ¿1 = 0.12. Diffusion rates could be well described by free volume theory up to the maximum, but this theory could not explain the strong decrease at higher ¿1. Indications for a local glass-rubber transition at room temperature were found near a water mass fraction ¿1 ˜ 0.09. This corresponds very well to the start of the crisp–non-crisp transition as measured by a sensory panel, but not to the glass-rubber transition at ¿1 ˜ 0.12 as measured by other techniques like Differential Scanning Calorimetry. So it seems that more than one glass-rubber like transitions may be important to describe the properties of heterogeneous cellular food systems
- …