585 research outputs found
AlumÃnio extraÃvel em solos. Determinação espectrofotométrica pelo alaranjado de xilenol.
bitstream/item/36130/1/Aluminio-extraivel.pd
LOCAL VARIABILITY IN THE ORBIT OF SATURN'S F RING
This work was supported by the Science and Technology Facilities Council (grant number ST/F007566/1)
Algebraic approach to a two-qubit quantum thermal machine
Algebraic methods for solving time dependent Hamiltonians are used to
investigate the performance of quantum thermal machines. We investigate the
thermodynamic properties of an engine formed by two coupled q-bits, performing
an Otto cycle. The thermal interaction occurs with two baths at different
temperatures, while work is associated with the interaction with an arbitrary
time-dependent magnetic field that varies in intensity and direction. For the
coupling, we consider the 1-d isotropic Heisenberg model, which allows us to
describe the system by means of the irreducible representation of the
Lie algebra within the triplet subspace. We inspect
different settings of the temperatures and frequencies of the cycle and
investigate the corresponding operation regimes of the engine. Finally, we
numerically investigate the engine efficiency under a time varying Rabi
frequency, interpolating the abrupt and adiabatic limits.Comment: 11 pages, 5 figures, submitted to Physical Review
Against the flow: unexpected migration movements over the open sea by inexperienced ospreys
As part of a long-term monitoring program, more than 80 Mediterranean ospreys Pandion haliaetus (both adults and juveniles) were tagged with GPS-GSM transmitters and tracked to study their spatiotemporal behaviour. Here we document the peculiar and unexpected migration movements performed by three inexperienced (juvenile/immature) individuals, who crossed the open sea "against the flow", in the opposite direction to that foreseen for the given season. Using a combination of GPS tracking data and weather information, we found that such movements were linked to particular meteorological conditions occurring over the Mediterranean Sea during migration. Mean values of wind gust of approximately 20 km/h and moderate tailwinds seem to have mediated the onset of the movements, facilitating the flight of ospreys over water. Our findings suggest that both weather conditions (sidewinds) and the inexperience of the birds explain these long migration movements performed towards unexpected directions over the open sea. We conclude that migratory capabilities and the ability to cope with external conditions may lead inexperienced birds to perform extensive and tortuous dispersal/explotrative movements during both first autumn and spring migration
Acidez extraivel do solo. Comparação entre as metodologias internacional e do Servico Nacional de Levantamento e Conservação de Solos (SNLCS).
bitstream/item/36128/1/Acidez-extraivel.pd
Quantum-based solution of time-dependent complex Riccati equations
Using the Wei-Norman theory we obtain a time-dependent complex Riccati
equation (TDCRE) as the solution of the time evolution operator (TEO) of
quantum systems described by time-dependent (TD) Hamiltonians that are linear
combinations of the generators of the ,
and Lie algebras. Using a recently developed solution for
the time evolution of these quantum systems we solve the TDCRE recursively as
generalized continued fractions, which are optimal for numerical
implementations, and establish the necessary and sufficient conditions for the
unitarity of the TEO in the factorized representation. The inherited symmetries
of quantum systems can be recognized by a simple inspection of the TDCRE,
allowing effective quantum Hamiltonians to be associated with it, as we show
for the Bloch-Riccati equation whose Hamiltonian corresponds to that of a
generic TD system of the Lie algebra . As an application, but
also as a consistency test, we compare our solution with the analytic one for
the Bloch-Riccati equation considering the Rabi frequency driven by a complex
hyperbolic secant pulse generating spin inversion, showing an excellent
agreement.Comment: 10 Pages, 1 Figur
Modeling and Real-Time Simulation of a Vascularized Liver Tissue
International audienceIn Europe only, about 100,000 deaths per year are related to cirrhosis or liver cancer. While surgery remains the option that offers the foremost success rate against such pathologies, several limitations still hinder its widespread development. Among the limiting factors is the lack of accurate planning systems, which has been a motivation for several recent works, aiming at better resection planning and training systems, relying on pre-operative imaging, anatomical and biomechanical modelling. While the vascular network in the liver plays a key role in defining the operative strategy, its influence at a biomechanical level has not been taken into account. In the paper we propose a real-time model of vascularized organs such as the liver. The model takes into account separate constitutive laws for the parenchyma and vessels, and defines a coupling mechanism between these two entities. In the evaluation section, we present results of in vitro porcine liver experiments that indicate a significant influence of vascular structures on the mechanical behaviour of tissue. We confirm the val- ues obtained in the experiments by computer simulation using standard FEM. Finally, we show that the conventional modelling approach can be efficiently approximated with the proposed composite model capable of real-time calculations
Learning the Tangent Space of Dynamical Instabilities from Data
For a large class of dynamical systems, the optimally time-dependent (OTD)
modes, a set of deformable orthonormal tangent vectors that track directions of
instabilities along any trajectory, are known to depend "pointwise" on the
state of the system on the attractor, and not on the history of the trajectory.
We leverage the power of neural networks to learn this "pointwise" mapping from
phase space to OTD space directly from data. The result of the learning process
is a cartography of directions associated with strongest instabilities in phase
space. Implications for data-driven prediction and control of dynamical
instabilities are discussed
Biomechanical Simulation of Electrode Migration for Deep Brain Stimulation
International audienceDeep Brain Stimulation is a modern surgical technique for treating patients who suffer from affective or motion disorders such as Parkinson's disease. The efficiency of the procedure relies heavily on the accuracy of the placement of a micro-electrode which sends electrical pulses to a specific part of the brain that controls motion and affective symptoms. However, targeting this small anatomical structure is rendered difficult due to a series of brain shifts that take place during and after the procedure. This paper introduces a biomechanical simulation of the intra and postoperative stages of the procedure in order to determine lead deformation and electrode migration due to brain shift. To achieve this goal, we propose a global approach, which accounts for brain deformation but also for the numerous interactions that take place during the procedure (contacts between the brain and the inner part of the skull and falx cerebri, effect of the cerebro-spinal fluid, and biomechanical interactions between the brain and the electrodes and cannula used during the procedure). Preliminary results show a good correlation between our simulations and various results reported in the literature
- …