414 research outputs found
Thermal convection in a nonlinear non-Newtonian magnetic fluid
We report theoretical and numerical results on thermal convection of a magnetic fluid in a viscoelastic carrier liquid. The viscoelastic properties are described by a general nonlinear viscoelastic model that contains as special cases the standard phenomenological constitutive equations for the stress tensor. In order to explore numerically the system we perform a truncated Galerkin expansion obtaining a generalized Lorenz system with ten modes. We find numerically that the system has stationary, periodic and chaotic regimes. We establish phase diagrams to identify the different dynamical regimes as a function of the Rayleigh number and the viscoelastic material parameters
Correlations and synchronization in a Bose-Fermi mixture
We study a Bose-Fermi mixture within the framework of the mean-field theory,
including three possible regimes for the fermionic species: fully polarized,
BCS, and unitarity. Starting from the 3D description and using the variational
approximation (VA), we derive 1D and 2D systems of equations, under the
corresponding confining potentials. This method produces a pair of nonlinear
Schr\"{o}dinger (NLS) equations coupled to algebraic equations for the
transverse widths of the confined state. The equations incorporate interactions
between atoms of the same species and between the species, assuming that the
latter can be manipulated by means of the Feshbach resonance (FR). As an
application, we explore spatial density correlations in the ground state (GS)
between the species, concluding that they strongly depend on the sign and
strength of the inter-species interaction. Also studied are the dynamics of the
mixture in a vicinity of the GS and the corresponding spatiotemporal
inter-species correlation. The correlations are strongly affected by the
fermionic component, featuring the greatest variation in the unitary regime.
Results produced by the VA are verified by comparison with full numerical
solutions.Comment: Journal of Physics B: Atomic, Molecular and Optical Physics, in pres
Advantage of four-electrode over two-electrode defibrillators
Defibrillation is the standard clinical treatment used to stop ventricular fibrillation. An electrical device delivers a controlled amount of electrical energy via a pair of electrodes in order to reestablish the normal heart rate. We propose a new technique that is a combination of biphasic shocks applied with a four-electrode system rather than the standard two-electrode system. We use a numerical model of a one-dimensional ring of cardiac tissue in order to test and evaluate the benefit of such a new technique. We compare three different shock protocols, namely, a monophasic and two types of biphasic shocks. The results obtained by using a four-electrode system are compared quantitatively with those obtained with the standard two-electrode system. We find that a huge reduction in defibrillation threshold is achieved with the four-electrode system. For the most efficient protocol (asymmetric biphasic), we obtain a reduction in excess of 80 % in the energy required for a defibrillation success rate of 90 %. The mechanisms of successful defibrillation are also analyzed. This reveals that the advantage of asymmetric biphasic shocks with four electrodes lies in the duration of the cathodal and anodal phase of the shock
Thermodynamics of Two Dimensional Magnetic Nanoparticles
A two dimensional magnetic particle in the presence of an external magnetic
field is studied. Equilibrium thermodynamical properties are derived by
evaluating analytically the partition function. When the external field is
applied perpendicular to the anisotropy axis the system exhibits a second order
phase transition with order parameter being the magnetization parallel to the
field. In this case the system is isomorph to a mechanical system consisting in
a particle moving without friction in a circle rotating about its vertical
diameter. Contrary to a paramagnetic particle, equilibrium magnetization shows
a maximum at finite temperature. We also show that uniaxial anisotropy in a
system of noninteracting particles can be missinterpreted as a ferromagnetic or
antiferromagnetic coupling among the magnetic particles depending on the angle
between anisotropy axis and magnetic field.Comment: 4 pages 6 figures 19 reference
Impurity-related intraband absorption in coupled quantum dot-ring structure under lateral electric field
The effects of a lateral electric field on intraband absorption in GaAs/GaAlAs two-dimensional coupled quantum dot-ring structure with an on-centre hydrogenic donor impurity is investigated. The confining potential of the system consists of two parabolas with various confinement energies. The calculations are made using the exact diagonalization technique. A selection rule for intraband transitions was found for x-polarized incident light. The absorption spectrum mainly exhibits a redshift with the increment of electric field strength. On the other hand, the absorption spectrum can exhibit either a blue- or redshift depending on the values of confinement energies of dot and ring. Additionally, electric field changes the energetic shift direction influenced by the variation of barrier thickness of the structure
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