1,301 research outputs found
Neutrinos and Energetics of the Earth
We estimate terrestrial antineutrino and neutrino fluxes according to
different models of Earth composition. We find large variations, corresponding
to uncertainties on the estimated , and abundances in the mantle.
Information on the mantle composition can be derived from antineutrino flux
measurements after subtracting the crust contribution. This requires a good
description of the crust composition in the region of the detector site.
Measurements of terrestrial antineutrinos will provide a direct insight on the
main sources of Earth's heat flow.Comment: 11 pages, RevTeX file. To appear on Phys. Lett. B. Addendum contains
comparison with KamLAND result
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
Phonons in the beta-tin, Imma, and sh phases of Silicon from ab initio calculations
We present a new interpretation of measured Raman frequencies of a
high-pressure structure of Silicon which was assigned previously to the
beta-tin phase. Our results show that the beta-tin->Imma->sh phase transitions
have been already indicated in this experiment which was performed before the
discovery of the Imma phase. We have calculated phonon-dispersion curves for
the beta-tin, Imma, and sh phases of silicon using the plane-wave
pseudopotential approach to the density-functional theory and the
density-functional perturbation theory within the local density approximation.
With the new assignment, the calculated phonon frequencies display an excellent
agreement with the experimental data, and can be also used to determine
precisely the transition pressure for the Imma->beta-tin phase transition. The
sh->Imma transition is accompanied by soft modes.Comment: 4 pages, 5 figure
Huge Seebeck coefficients in non-aqueous electrolytes
The Seeebeck coefficients of the non-aqueous electrolytes tetrabutylammonium
nitrate, tetraoctylphosphonium bromide and tetradodecylammonium nitrate in
1-octanol, 1-dodecanol and ethylene-glycol are measured in a temperature range
from T=30 to T=45 C. The Seebeck coefficient is generally of the order of a few
hundreds of microvolts per Kelvin for aqueous solution of inorganic ions. Here
we report huge values of 7 mV/K at 0.1M concentration for tetrabutylammonium
nitrate in 1-dodecanol. These striking results open the question of
unexpectedly large kosmotrope or "structure making" effects of
tetraalkylammonium ions on the structure of alcohols.Comment: Submitted to J. Chem. Phy
Photoluminescence pressure coefficients of InAs/GaAs quantum dots
We have investigated the band-gap pressure coefficients of self-assembled
InAs/GaAs quantum dots by calculating 17 systems with different quantum dot
shape, size, and alloying profile using atomistic empirical pseudopotential
method within the ``strained linear combination of bulk bands'' approach. Our
results confirm the experimentally observed significant reductions of the band
gap pressure coefficients from the bulk values. We show that the nonlinear
pressure coefficients of the bulk InAs and GaAs are responsible for these
reductions. We also find a rough universal pressure coefficient versus band gap
relationship which agrees quantitatively with the experimental results. We find
linear relationships between the percentage of electron wavefunction on the
GaAs and the quantum dot band gaps and pressure coefficients. These linear
relationships can be used to get the information of the electron wavefunctions.Comment: 8 pages, 2 tables, 4 figure
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
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.
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.
Electron-phonon-scattering dynamics in ferromagnetic metals and its influence on ultrafast demagnetization processes
We theoretically investigate spin-dependent carrier dynamics due to the
electron-phonon interaction after ultrafast optical excitation in ferromagnetic
metals. We calculate the electron-phonon matrix elements including the
spin-orbit interaction in the electronic wave functions and the interaction
potential. Using the matrix elements in Boltzmann scattering integrals, the
momentum-resolved carrier distributions are obtained by solving their equation
of motion numerically. We find that the optical excitation with realistic laser
intensities alone leads to a negligible magnetization change, and that the
demagnetization due to electron-phonon interaction is mostly due to hole
scattering. Importantly, the calculated demagnetization quenching due to this
Elliot-Yafet type depolarization mechanism is not large enough to explain the
experimentally observed result. We argue that the ultrafast demagnetization of
ferromagnets does not occur exclusively via an Elliott-Yafet type process,
i.e., scattering in the presence of the spin-orbit interaction, but is
influenced to a large degree by a dynamical change of the band structure, i.e.,
the exchange splitting
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