311 research outputs found
CaCu_3Ti_4O_12/CaTiO_3 Composite Dielectrics: A Ba/Pb-free Ceramics with High Dielectric Constants
We have measured dielectric properties of CaCuTiO
( = 0, 0.1, 0.5, 1, 1.5, 2, 2.9 and 3), and have found that
CaCuTiO (a composite of CaCuTiO and
CaTiO) exhibits a high dielectric constant of 1800 with a low dissipation
factor of 0.02 below 100 kHz from 220 to 300 K. These are comparable to (or
even better than) those of the Pb/Ba-based ceramics, which could be attributed
to a barrier layer of CaTiO on the surface of the CaCuTiO
grains. The composite dielectric ceramics reported here are environmentally
benign as they do not contain Ba/Pb.Comment: 4 pages, 4 figures, Appl. Phys. Lett. (scheduled on July 25, 2005
Magnetic properties of HO2 thin films
We report on the magnetic and transport studies of hafnium oxide thin films
grown by pulsed-laser deposition on sapphire substrates under different oxygen
pressures, ranging from 10-7 to 10-1 mbar. Some physical properties of these
thin films appear to depend on the oxygen pressure during growth: the film
grown at low oxygen pressure (P ~= 10-7 mbar) has a metallic aspect and is
conducting, with a positive Hall signal, while those grown under higher oxygen
pressures (7 x 10-5 <= P <= 0.4 mbar) are insulating. However, no intrinsic
ferromagnetic signal could be attributed to the HfO2 films, irrespective of the
oxygen pressure during the deposition.Comment: 1
Interatomic potentials for the vibrational properties of III-V semiconductor nanostructures
We derive interatomic potentials for zinc blende InAs, InP, GaAs and GaP
semiconductors with possible applications in the realm of nanostructures. The
potentials include bond stretching interaction between the nearest and
next-nearest neighbors, a three body term and a long-range Coulomb interaction.
The optimized potential parameters are obtained by (i) fitting to bulk phonon
dispersions and elastic properties and (ii) constraining the parameter space to
deliver well behaved potentials for the structural relaxation and vibrational
properties of nanostructure clusters. The targets are thereby calculated by
density functional theory for clusters of up to 633 atoms. We illustrate the
new capability by the calculation Kleinman and Gr\"uneisen parameters and of
the vibrational properties of nanostructures with 3 to 5.5 nm diameter.Comment: 22 pages, 5 figures; Phys. Rev. B 201
Carrier relaxation mechanisms in self-assembled (In,Ga)As/GaAs quantum dots: Efficient P -> S Auger relaxation of electrons
We calculate the P-shell--to-S-shell decay lifetime \tau(P->S) of electrons
in lens-shaped self-assembled (In,Ga)As/GaAs dots due to Auger electron-hole
scattering within an atomistic pseudopotential-based approach. We find that
this relaxation mechanism leads to fast decay of \tau(P->S)~1-7 ps for dots of
different sizes. Our calculated Auger-type P-shell--to-S-shell decay lifetimes
\tau(P->S) compare well to data in (In,Ga)As/GaAs dots, showing that as long as
holes are present there is no need for an alternative polaron mechanism.Comment: Version published in Phys. Rev.
Self-consistent model for ambipolar tunneling in quantum-well systems
We present a self-consistent approach to describe ambipolar tunneling in
asymmetrical double quantum wells under steady-state excitation and extend the
results to the case of tunneling from a near-surface quantum well to surface
states. The results of the model compare very well with the behavior observed
in photoluminescence experiments in asymmetric double quantum
wells and in near-surface single quantum wells.Comment: 10 pages, REVTeX 3.
Lattice diffusion and surface segregation of B during growth of SiGe heterostructures by molecular beam epitaxy: effect of Ge concentration and biaxial stress
Si1-xGex/Si1-yGey/Si(100) heterostructures grown by Molecular Beam Epitaxy
(MBE) were used in order to study B surface segregation during growth and B
lattice diffusion. Ge concentration and stress effects were separated. Analysis
of B segregation during growth shows that: i) for layers in epitaxy on
(100)Si), B segregation decreases with increasing Ge concentration, i.e. with
increased compressive stress, ii) for unstressed layers, B segregation
increases with Ge concentration, iii) at constant Ge concentration, B
segregation increases for layers in tension and decreases for layers in
compression. The contrasting behaviors observed as a function of Ge
concentration in compressively stressed and unstressed layers can be explained
by an increase of the equilibrium segregation driving force induced by Ge
additions and an increase of near-surface diffusion in compressively stressed
layers. Analysis of lattice diffusion shows that: i) in unstressed layers, B
lattice diffusion coefficient decreases with increasing Ge concentration, ii)
at constant Ge concentration, the diffusion coefficient of B decreases with
compressive biaxial stress and increases with tensile biaxial stress, iii) the
volume of activation of B diffusion () is positive for biaxial stress while it
is negative in the case of hydrostatic pressure. This confirms that under a
biaxial stress the activation volume is reduced to the relaxation volume
Kaon properties and cross sections in nuclear medium
Results for the reactions in nuclear matter
of Ref. nucl-th/0004011 are presented. To evaluate the in-medium modification
of the reaction amplitude as a function of the baryonic density we introduce
relativistic, mean-field potentials for the initial, final and intermediate
mesonic and baryonic states in the resonance model. These vector and scalar
potentials were calculated using the quark meson coupling model. Contrary to
earlier work which has not allowed for the change of the cross section in
medium, we find that the data for kaon production at SIS energies are
consistent with a repulsive -nucleus potential.Comment: 5 pages, 3 postscript figures included, uses iopart.cls and
iopart10.clo (included), presented by K.T. at the 5th International
Conference on Strangeness in Quark Matter, July 20 - 25, 2000, Berkeley,
California, to be published in the proceedings, J. Phys. G. An explanation
has been added in Sec. 3 with a new figur
Resonance model study of strangeness production in pp collisions
Results for the energy dependence of the elementary kaon production cross
sections in proton-proton collisions are reported. Calculations are performed
within an extended version of the resonance model which was used for the
previous studies of elementary kaon production in pion-nucleon and
pion- collisions. Although the model treatment is within the {\it
empirical} tree level (observed widths for the resonances are used), it is
fully relativistic, and includes all relevant baryon resonances up to 2 GeV.
One of the purposes of this study is to provide the results for the simulation
codes of subthreshold kaon production in heavy ion collisions. This is the
first, consistent study of the elementary kaon production reactions including
both and () collisions on the same footing.
Comparisons are made between the calculated results and the existing
semi-empirical parametrizations which are widely used for the simulation codes,
as well as the experimental data.Comment: 10 pages with 5 postscript figures, Latex, revised version for
publication in Phys. Lett.
Production of -mesons in pp and pn reactions
We investigate the cross section for the reaction near
threshold and at medium energies. An effective Lagrangian approach with
one-pion exchange is applied to analyze different contributions to the cross
section for different isospin channels. The Reggeon exchange mechanism is also
considered. The results are used to calculate the contribution of the
meson to the cross sections and invariant mass distributions of the
reactions and . It is found that the
experimental observation of mesons in the reaction is much more promising than the observation of mesons in the
reaction .Comment: 26 pages, including 11 eps figures, to be bublished in J. Phys.
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