375 research outputs found
Direct probing of band-structure Berry phase in diluted magnetic semiconductors
We report on experimental evidence of the Berry phase accumulated by the
charge carrier wave function in single-domain nanowires made from a
(Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the
mesoscopic transport measurements is revealed as unusual patterns in the
magnetoconductance, that are clearly distinguished from the universal
conductance fluctuations. We show that these patterns appear in a magnetic
field region where the magnetization rotates coherently and are related to a
change in the band-structure Berry phase as the magnetization direction
changes. They should be thus considered as a band structure Berry phase
fingerprint of the effective magnetic monopoles in the momentum space. We argue
that this is an efficient method to vary the band structure in a controlled way
and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting
test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure
Angular Momentum Loss Rates in Be Stars Determined by the Viscous Decretion Disc Model
Circumstellar discs around Be stars are formed by the material ejected by the
central star. This process removes excess angular momentum from the star as
viscosity facilitates the mass and angular momentum transfer within the disc
and its growth. The angular momentum loss rates (AMLR) of Be stars is a subject
of debate in the literature. Through the modelling of the disc formation and
dissipation phases observed from Be stars, their average AMLR can be determined
and this is the goal of this work. We use the viscous decretion disc (VDD)
model to provide a range of the average AMLR for Be stars and compare these
rates with predicted values from the literature. We explore the reasons for
discrepancies between the predicted values of average AMLR using the VDD and
Geneva stellar evolution (GSE) models that were previously reported in
literature and find that the largest differences occur when Be stars are
rotating below their critical speeds. We show that the time over which the mass
reservoir builds up is inversely proportional to the average AMLR. Also, we
determine a revised value of the average AMLR for the Galactic Be star omega
CMa of 4.7x10^36 g cm^2/s^2, which is in better agreement with the values
expected for a typical B2 type star. Finally, the effect of disc truncation due
to the presence of a companion star is investigated and we find that this has a
minimal effect on the average AMLR
Formalism for obtaining nuclear momentum distributions by the Deep Inelastic Neutron Scattering technique
We present a new formalism to obtain momentum distributions in condensed
matter from Neutron Compton Profiles measured by the Deep Inelastic Neutron
Scattering technique. The formalism describes exactly the Neutron Compton
Profiles as an integral in the momentum variable . As a result we obtain a
Volterra equation of the first kind that relates the experimentally measured
magnitude with the momentum distributions of the nuclei in the sample. The
integration kernel is related with the incident neutron spectrum, the total
cross section of the filter analyzer and the detectors efficiency function. A
comparison of the present formalism with the customarily employed approximation
based on a convolution of the momentum distribution with a resolution function
is presented. We describe the inaccuracies that the use of this approximation
produces, and propose a new data treatment procedure based on the present
formalism.Comment: 11 pages, 8 figure
Localized magnetoplasmon modes arising from broken translational symmetry in semiconductor superlattices
The electromagnetic propagator associated with the localized collective
magnetoplasmon excitations in a semiconductor superlattice with broken
translational symmetry, is calculated analytically within linear response
theory. We discuss the properties of these collective excitations in both
radiative and non-radiative regimes of the electromagnetic spectra. We find
that low frequency retarded modes arise when the surface density of carriers at
the symmetry breaking layer is lower than the density at the remaining layers.
Otherwise a doublet of localized, high-frequency magnetoplasmon-like modes
occurs.Comment: Revtex file + separate pdf figure
Simultaneous J-, H-, K- and L-band spectroscopic observations of galactic Be stars. I. IR atlas
It is already accepted that Be stars are surrounded by circumstellar
envelopes, which are mostly compatible with a disc geometry in Keplerian
rotation. We aim to obtain a more complete characterisation of the properties
of the circumstellar environment of Be stars that helps to constrain the
theoretical models of the Be phenomenon. We present near-infrared,
medium-resolution spectra of a sample of galactic Be stars with different
spectral subtypes and luminosity classes. We measure different parameters of
the hydrogen recombination lines from the Paschen, Brackett, Pfund, and
Humphreys series, and use them to diagnose physical conditions in the
circumstellar environment. We analysed the equivalent-width (EW) ratio between
Br and Br lines and different diagrams of flux ratios. We also
identify lines from He I, C I, N I, O I, Na I, Mg I, Mg II, Si I, Fe I, and Fe
II. Analysing the EW measurements of particular He I, Mg II, Fe I, Fe II and O
I lines, we find that for some lines they correlate with the spectral type of
the star. Particularly, the emission of the O I~m line
decreases towards the later spectral types. We present an atlas of 22 Be stars,
that covers a wide infrared (IR) spectral range with quasi-simultaneous
observations. From a detailed analysis, we define new complementary criteria to
Mennickent's classification of Be stars according to their disc opacity. Some
objects in our sample present compact thick envelopes, while in others the
envelope is extended and optically thin. The correlation between the full
widths at half maximum (FWHM) and the peak separation ()
versus for the Br10, Br, and Hu14 lines reveals that the
broadening mechanism is rotational. The Ly fluorescence is a key
mechanism to explain the intensity of the emission of Mg II and O I lines.Comment: 27 pages, 14 figures. Accepted for publication in A&
Avaliação in vivo da eficácia anti-helmíntica de extratos vegetais sobre Haemanchus contortus em ovinos experimentalmente infectados.
Os nematoides gastrintestinais (NGI) acarretam perdas econômicas aos rebanhos ovinos, sendo o Haemonchus contortus o principal e mais patogênico NGI.Editora técnica Claudia Cristina Gulias Gomes
Efeito in vitro de extratos vegetais sobre a inibição da migração de larvas infectantes de Haemonchus contortus.
O presente trabalho objetivou avaliar o efeito anti-helmíntico in vitro de extratos provenientes de plantas do bioma Pampa sobre larvas infectantes (L3) de H. contortus.Editora técnica Claudia Cristina Gulias Gomes
How Coupling Determines the Entrainment of Circadian Clocks
Autonomous circadian clocks drive daily rhythms in physiology and behaviour.
A network of coupled neurons, the suprachiasmatic nucleus (SCN), serves as a
robust self-sustained circadian pacemaker. Synchronization of this timer to the
environmental light-dark cycle is crucial for an organism's fitness. In a
recent theoretical and experimental study it was shown that coupling governs
the entrainment range of circadian clocks. We apply the theory of coupled
oscillators to analyse how diffusive and mean-field coupling affects the
entrainment range of interacting cells. Mean-field coupling leads to amplitude
expansion of weak oscillators and, as a result, reduces the entrainment range.
We also show that coupling determines the rigidity of the synchronized SCN
network, i.e. the relaxation rates upon perturbation. %(Floquet exponents). Our
simulations and analytical calculations using generic oscillator models help to
elucidate how coupling determines the entrainment of the SCN. Our theoretical
framework helps to interpret experimental data
- …