175 research outputs found
Effect of pulsed electric field pretreatment on shrinkage, rehydration capacity and texture of freeze-dried plant materials
Abstract For this study, strawberries and red bell peppers were pre-treated with pulsed electric fields (PEF) to reduce the negative effects on physical properties of the products after freeze-drying. PEF treatment was carried out at constant electric field strength of E = 1.0 kV/cm and specific energy input was varied between 0.3 and 6.0 kJ/kg (treatment time 2.0–28.6 ms). Additionally, the impact of different pre-freezing temperatures (−4 and −40 °C) on the final product quality was described. Investigations showed that due to PEF treatment a significant reduction of the shrinkage phenomena for both bell peppers and strawberries was detected compared to untreated samples with 30% and 50% lower volume losses, respectively. The rehydration capacity of PEF pre-treated freeze-dried samples increased for both matrices up to 50%. Furthermore, the mechanical properties of the final product were improved for both matrices with a significant firmness reduction up to 60%. The results of this study suggest that PEF can be an effective pretreatment with low energy requirements to improve the quality of freeze-dried fruits and vegetables
Influence of pulsed electric field and ohmic heating pretreatments on enzyme and antioxidant activity of fruit and vegetable juices
The objective of this work was to optimize pulsed electric field (PEF) or ohmic heating (OH) application for carrot and apple mashes treatment at different preheating temperatures (40, 60 or 80 \ub0C). The effect of tissue disintegration on the properties of recovered juices was quantified, taking into account the colour change, the antioxidant activity and the enzyme activity of peroxidase (POD) in both carrot and apple juice and polyphenol oxidase (PPO) in apple juice. Lower \u394E and an increase of the antioxidant activity were obtained for juice samples treated with temperature at 80 \ub0C with or without PEF and OH pretreatment compared with those of untreated samples. The inactivation by 90% for POD and PPO was achieved when a temperature of 80 \ub0C was applied for both carrot and apple mash. A better retention of plant secondary metabolites from carrot and apple mashes could be achieved by additional PEF or OH application. Obtained results are the basis for the development of targeted processing concepts considering the release, inactivation and retention of ingredients
Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3
Using femtosecond time- and angle- resolved photoemission spectroscopy, we
investigated the nonequilibrium dynamics of the topological insulator Bi2Se3.
We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk
conduction bands are unoccupied. Optical excitation leads to a meta-stable
population at the bulk conduction band edge, which feeds a nonequilibrium
population of the surface state persisting for >10ps. This unusually long-lived
population of a metallic Dirac surface state with spin texture may present a
channel in which to drive transient spin-polarized currents
Unoccupied Topological States on Bismuth Chalcogenides
The unoccupied part of the band structure of topological insulators
BiTeSe () is studied by angle-resolved two-photon
photoemission and density functional theory. For all surfaces
linearly-dispersing surface states are found at the center of the surface
Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical
analysis shows that this feature appears in a spin-orbit-interaction induced
and inverted local energy gap. This inversion is insensitive to variation of
electronic and structural parameters in BiSe and BiTeSe. In
BiTe small structural variations can change the character of the local
energy gap depending on which an unoccupied Dirac state does or does not exist.
Circular dichroism measurements confirm the expected spin texture. From these
findings we assign the observed state to an unoccupied topological surface
state
Adiabatic-Connection-Fluctuation-Dissipation approach to the long-range behavior of the exchange-correlation energy at metal surfaces: A numerical study for jellium slabs
A still open issue in many-body theory is the asymptotic behavior of the
exchange-correlation energy and potential in the vacuum region of a metal
surface. Here we report a numerical study of the position-dependent
exchange-correlation energy for jellium slabs, as obtained by combining the
formally exact adiabatic-connection-fluctuation-dissipation theorem with either
time-dependent density-functional theory or an inhomogeneous
Singwi-Tosi-Land-Sj\"olander approach. We find that the inclusion of
correlation allows to obtain well-converged semi-infinite-jellium results
(independent of the slab thickness) that exhibit an image-like asymptotic
behavior close to the classical image potential .Comment: 6 pages, 4 Figure
Electronic structure of superconducting graphite intercalate compounds: The role of the interlayer state
Although not an intrinsic superconductor, it has been long--known that, when
intercalated with certain dopants, graphite is capable of exhibiting
superconductivity. Of the family of graphite--based materials which are known
to superconduct, perhaps the most well--studied are the alkali metal--graphite
intercalation compounds (GIC) and, of these, the most easily fabricated is the
CK system which exhibits a transition temperature K. By increasing the alkali metal concentration (through high pressure
fabrication techniques), the transition temperature has been shown to increase
to as much as K in CNa. Lately, in an important recent
development, Weller \emph{et al.} have shown that, at ambient conditions, the
intercalated compounds \cyb and \cca exhibit superconductivity with transition
temperatures K and K respectively, in excess
of that presently reported for other graphite--based compounds. We explore the
architecture of the states near the Fermi level and identify characteristics of
the electronic band structure generic to GICs. As expected, we find that charge
transfer from the intercalant atoms to the graphene sheets results in the
occupation of the --bands. Yet, remarkably, in all those -- and only
those -- compounds that superconduct, we find that an interlayer state, which
is well separated from the carbon sheets, also becomes occupied. We show that
the energy of the interlayer band is controlled by a combination of its
occupancy and the separation between the carbon layers.Comment: 4 Figures. Please see accompanying experimental manuscript
"Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca" by
Weller et a
Electronic and structural properties of superconducting MgB, CaSi and related compounds
We report a detailed study of the electronic and structural properties of the
39K superconductor \mgbtwo and of several related systems of the same family,
namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which
include zone-center phonon frequencies and transport properties, are performed
within the local density approximation to the density functional theory, using
the full-potential linearized augmented plane wave (FLAPW) and the
norm-conserving pseudopotential methods. Our results indicate essentially
three-dimensional properties for these compounds; however, strongly
two-dimensional -bonding bands contribute significantly at the Fermi
level. Similarities and differences between \mgbtwo and \bebtwo (whose
superconducting properties have not been yet investigated) are analyzed in
detail. Our calculations for \mgalbtwo show that metal substitution cannot be
fully described in a rigid band model. \casitwo is studied as a function of
pressure, and Be substitution in the Si planes leads to a stable compound
similar in many aspects to diborides.Comment: Revised version, Phys.Rev.B in pres
Ab initio simulation of photoemission spectroscopy in solids: Plane-wave pseudopotential approach, with applications to normal-emission spectra of Cu(001) and Cu(111)
We introduce a new method for simulating photoemission spectra from bulk
crystals in the ultra-violet energy range, within a three-step model. Our
method explicitly accounts for transmission and matrix-element effects, as
calculated from state-of-the-art plane-wave pseudopotential techniques within
density-functional theory. Transmission effects, in particular, are included by
extending to the present problem a technique previously employed with success
to deal with ballistic conductance in metal nanowires. The spectra calculated
for normal emission in Cu(001) and Cu(111) are in fair agreement with previous
theoretical results and with experiments, including a newly determined
spectrum. The residual discrepancies between our results and the latter are
mainly due to the well-known deficiencies of density-functional theory in
accounting for correlation effects in quasi-particle spectra. A significant
improvement is obtained by the LDA+U method. Further improvements are obtained
by including surface-optics corrections, as described by Snell's law and
Fresnel's equations.Comment: 25 pages, 7 figures, accepted in PR
Lifetimes of image-potential states on copper surfaces
The lifetime of image states, which represent a key quantity to probe the
coupling of surface electronic states with the solid substrate, have been
recently determined for quantum numbers on Cu(100) by using
time-resolved two-photon photoemission in combination with the coherent
excitation of several states (U. H\"ofer et al, Science 277, 1480 (1997)). We
here report theoretical investigations of the lifetime of image states on
copper surfaces. We evaluate the lifetimes from the knowledge of the
self-energy of the excited quasiparticle, which we compute within the GW
approximation of many-body theory. Single-particle wave functions are obtained
by solving the Schr\"odinger equation with a realistic one-dimensional model
potential, and the screened interaction is evaluated in the random-phase
approximation (RPA). Our results are in good agreement with the experimentally
determined decay times.Comment: 4 pages, 1 figure, to appear in Phys. Rev. Let
Quantum point contact on graphite surface
The conductance through a quantum point contact created by a sharp and hard
metal tip on the graphite surface has features which to our knowledge have not
been encountered so far in metal contacts or in nanowires. In this paper we
first investigate these features which emerge from the strongly directional
bonding and electronic structure of graphite, and provide a theoretical
understanding for the electronic conduction through quantum point contacts. Our
study involves the molecular-dynamics simulations to reveal the variation of
interlayer distances and atomic structure at the proximity of the contact that
evolves by the tip pressing toward the surface. The effects of the elastic
deformation on the electronic structure, state density at the Fermi level, and
crystal potential are analyzed by performing self-consistent-field
pseudopotential calculations within the local-density approximation. It is
found that the metallicity of graphite increases under the uniaxial compressive
strain perpendicular to the basal plane. The quantum point contact is modeled
by a constriction with a realistic potential. The conductance is calculated by
representing the current transporting states in Laue representation, and the
variation of conductance with the evolution of contact is explained by taking
the characteristic features of graphite into account. It is shown that the
sequential puncturing of the layers characterizes the conductance.Comment: LaTeX, 11 pages, 9 figures (included), to be published in Phys. Rev.
B, tentatively scheduled for 15 September 1998 (Volume 58, Number 12
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