1,296 research outputs found
First-principles study of the atomic and electronic structure of the Si(111)-(5x2-Au surface reconstruction
We present a systematic study of the atomic and electronic structure of the
Si(111)-(5x2)-Au reconstruction using first-principles electronic structure
calculations based on the density functional theory. We analyze the structural
models proposed by Marks and Plass [Phys. Rev. Lett.75, 2172 (1995)], those
proposed recently by Erwin [Phys. Rev. Lett.91, 206101 (2003)], and a
completely new structure that was found during our structural optimizations. We
study in detail the energetics and the structural and electronic properties of
the different models. For the two most stable models, we also calculate the
change in the surface energy as a function of the content of silicon adatoms
for a realistic range of concentrations. Our new model is the energetically
most favorable in the range of low adatom concentrations, while Erwin's "5x2"
model becomes favorable for larger adatom concentrations. The crossing between
the surface energies of both structures is found close to 1/2 adatoms per 5x2
unit cell, i.e. near the maximum adatom coverage observed in the experiments.
Both models, the new structure and Erwin's "5x2" model, seem to provide a good
description of many of the available experimental data, particularly of the
angle-resolved photoemission measurements
Crossover between distinct mechanisms of microwave photoresistance in bilayer systems
We report on temperature-dependent magnetoresistance measurements in balanced
double quantum wells exposed to microwave irradiation for various frequencies.
We have found that the resistance oscillations are described by the
microwave-induced modification of electron distribution function limited by
inelastic scattering (inelastic mechanism), up to a temperature of T*~4 K. With
increasing temperature, a strong deviation of the oscillation amplitudes from
the behavior predicted by this mechanism is observed, presumably indicating a
crossover to another mechanism of microwave photoresistance, with similar
frequency dependence. Our analysis shows that this deviation cannot be fully
understood in terms of contribution from the mechanisms discussed in theory.Comment: 7 pages, 4 figure
Chandra and optical/IR observations of CXOJ1415.2+3610, a massive, newly discovered galaxy cluster at z~1.5
(Abridged) We report the discovery of CXO J1415.2+3610, a distant (z~1.5)
galaxy cluster serendipitously detected in a deep, high-resolution Chandra
observation targeted to study the cluster WARP J1415.1+3612 at z=1.03. This is
the highest-z cluster discovered with Chandra so far. Moreover, the total
exposure time of 280 ks with ACIS-S provides the deepest X-ray observation
currently achieved on a cluster at z>1.5. We perform an X-ray spectral fit of
the extended emission of the intracluster medium (ICM) with XSPEC, and we
detect at a 99.5% confidence level the rest frame 6.7-6.9 keV Iron K_\alpha
line complex, from which we obtain z_X=1.46\pm0.025. The analysis of the
z-3.6\mu m color-magnitude diagram shows a well defined sequence of red
galaxies within 1' from the cluster X-ray emission peak with a color range [5 <
z-3.6 \mu m < 6]. The photometric redshift obtained by spectral energy
distribution (SED) fitting is z_phot=1.47\pm 0.25. After fixing the redshift to
z=1.46, we perform the final spectral analysis and measure the average gas
temperature with a 20% error, kT=5.8^{+1.2}_{-1.0} keV, and the Fe abundance
Z_Fe = 1.3_{-0.5}^{+0.8}Z_\odot. We fit the background subtracted surface
brightness with a single beta--model out to 35" and derive the deprojected
electron density profile. The ICM mass is 1.09_{-0.2}^{+0.3}\times 10^{13}
M_\odot within 300 kpc. The total mass is M_{2500}= 8.6_{-1.7}^{+2.1} \times 10
^{13} M_\odot for R_{2500}=(220\pm 55) kpc. Extrapolating the profile at larger
radii we find M_{500}= 2.1_{-0.5}^{+0.7} \times 10 ^{14} M_\odot for R_{500} =
510_{-50}^{+55}$ kpc. This analysis establishes CXOJ1415.2+3610 as one of the
best characterized distant galaxy clusters based on X-ray data alone.Comment: 12 pages, 9 figures, A\&A in press, minor modifications in the tex
Structural models for the Si(553)-Au atomic chain reconstruction
Recent photoemission experiments on the Si(553)-Au reconstruction show a
one-dimensional band with a peculiar ~1/4 filling. This band could provide an
opportunity for observing large spin-charge separation if electron-electron
interactions could be increased. To this end, it is necessary to understand in
detail the origin of this surface band. A first step is the determination of
the structure of the reconstruction. We present here a study of several
structural models using first-principles density functional calculations. Our
models are based on a plausible analogy with the similar and better known
Si(557)-Au surface, and compared against the sole structure proposed to date
for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ].
Results for the energetics and the band structures are given. Lines for the
future investigation are also sketched
Magnetoresistance oscillations in multilayer systems - triple quantum wells
Magnetoresistance of two-dimensional electron systems with several occupied
subbands oscillates owing to periodic modulation of the probability of
intersubband transitions by the quantizing magnetic field. In addition to
previous investigations of these magneto-intersubband (MIS) oscillations in
two-subband systems, we report on both experimental and theoretical studies of
such a phenomenon in three-subband systems realized in triple quantum wells. We
show that the presence of more than two subbands leads to a qualitatively
different MIS oscillation picture, described as a superposition of several
oscillating contributions. Under a continuous microwave irradiation, the
magnetoresistance of triple-well systems exhibits an interference of MIS
oscillations and microwaveinduced resistance oscillations. The theory
explaining these phenomena is presented in the general form, valid for an
arbitrary number of subbands. A comparison of theory and experiment allows us
to extract temperature dependence of quantum lifetime of electrons and to
confirm the applicability of the inelastic mechanism of microwave
photoresistance for the description of magnetotransport in multilayer systems.Comment: 10 pages, 5 figure
Envelhecimento acelerado de aguardentes vÃnicas na presença de fragmentos de madeira. Influência nos compostos odorantes
A tecnologia de envelhecimento tradicional, que consiste na colocação da aguardente vÃnica
em vasilhas de madeira durante vários anos, é uma técnica morosa e onerosa. Assim, têm vindo a ser
introduzidas e desenvolvidas novas técnicas, com o intuito de optimizar o processo e reduzir os custos.
Uma dessas novas técnicas consiste em introduzir pedaços de madeira (aparas, toros ou outras formas)
na bebida a envelhecer, condicionada em depósitos de inox. Têm sido realizados muitos estudos em
vinhos, mas em aguardentes a experimentação é escassa [1, 2].
Assim, este trabalho teve como objectivo avaliar a influência da utilização de alternativas ao
envelhecimento de aguardentes em vasilhas de madeira, na composição quÃmica das aguardentes
obtidas, dando particular atenção aos compostos odorantes provenientes da madeira.
Para tal, uma mesma aguardente vÃnica da Lourinhã foi submetida a um processo de envelhecimento,
com três formas de madeira: aguardente colocada em vasilha de madeira (V), aguardente colocada em
vasilha de inox com introdução de madeira sob a forma de dominós (D) e aguardente colocada em
vasilha de inox com introdução de madeira sob a forma de tábuas (T), tendo sido colhidas amostras de
aguardente, ao fim de 180 dias de envelhecimento, para análise e quantificação dos compostos
odorantes.
Os resultados obtidos mostram que a forma da madeira teve um efeito altamente significativo na
maioria dos compostos analisados. No caso dos compostos odorantes derivados da lenhina da madeira
(fenois voláteis e vanilina), os teores mais elevados foram encontrados nas aguardentes envelhecidas
na presença de fragmentos (dominós ou tábuas), enquanto para os compostos odorantes derivados das
hemiceluloses da madeira (ácido acético e aldeÃdos furânicos) os teores mais elevados foram
determinados nas aguardentes envelhecidas em vasilha de madeira.
Os resultados deste trabalho sugerem a possibilidade de diferenciação quÃmica das aguardentes, em
função da tecnologia de envelhecimento
Double non-equivalent chain structure on vicinal Si(557)-Au surface
We study electronic and topographic properties of the vicinal Si(557)-Au
surface using scanning tunneling microscopy and reflection of high energy
electron diffraction technique. STM data reveal double wire structures along
terraces. Moreover behavior of the voltage dependent STM tip - surface distance
is different in different chains. While the one chain shows oscillations of the
distance which are sensitive to the sign of the voltage bias, the oscillations
in the other chain remain unchanged with respect to the positive/negative
biases. This suggests that one wire has metallic character while the other one
- semiconducting. The experimental results are supplemented by theoretical
calculations within tight binding model suggesting that the observed chains are
made of different materials, one is gold and the other one is silicon chain.Comment: 9 pages, 12 figures, accepted for publication in Phys. Rev.
Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N
The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is
extensively studied over a vast range of Mn concentrations. We calculate the
electronic structures of these materials using density functional theory in
both the local spin density approximation and the LDA+U scheme, that we have
now implemented in the code SIESTA.
For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the
ferromagnetism, with an exchange constant =~ -2.8 eV. This is smaller
than that obtained with LSDA, which overestimates the exchange coupling between
Mn ions and the As holes.
In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the
double-exchange mechanism, since a hole of strong character is found at the
Fermi level in both the LSDA and the LDA+U approaches. In this case the
coupling between the Mn ions decays rapidly with the Mn-Mn separation. This
suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a
robust ferromagnetic phase at large Mn concentration coexisting with a diluted
weak ferromagnetic phase.Comment: 12 pages, 11 figure
Thermally activated intersubband scattering and oscillating magnetoresistance in quantum wells
Experimental studies of magnetoresistance in high-mobility wide quantum wells
reveal oscillations which appear with an increase in temperature to 10 K and
whose period is close to that of Shubnikov-de Haas oscillations. The observed
phenomenon is identified as magnetointersubband oscillations caused by the
scattering of electrons between two occupied subbands and the third subband
which becomes occupied as a result of thermal activation. These small-period
oscillations are less sensitive to thermal suppression than the largeperiod
magnetointersubband oscillations caused by the scattering between the first and
the second subbands. Theoretical study, based on consideration of electron
scattering near the edge of the third subband, gives a reasonable explanation
of our experimental findings.Comment: 9 pages, 5 figure
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