D. Laroze

A detailed analysis of dipolar interactions in arrays of bi-stable magnetic nanowires

The investigation of the role of interactions in magnetic wire arrays is complex and often involves substantial simplifications. In this paper analytical expressions taking into consideration the geometry of the wires and dipolar interactions between them have been obtained. An expansion of these terms, at first order, can be easily evaluated and shows a good agreement with the total expression for the energy. The extent of the interwire magnetostatic coupling has also been investigated, and it is shown that the number of wires required to reach a size independent magnetic state in the array strongly depends on the relative magnetic orientation of the wires.

Electronic and intraband optical properties of single quantum rings under intense laser field radiation

The influence of an intense laser field on one-electron states and intraband optical absorption coefficients is investigated in two-dimensional GaAs/Ga0.7Al0.3As quantum rings. An analytical expression of the effective lateral confining potential induced by the laser field is obtained. The one-electron energy spectrum and wave functions are found using the effective mass approximation and exact diagonalization technique. We have shown that changes in the incident light polarization lead to blue- or redshifts in the intraband optical absorption spectrum. Moreover, we found that only blueshift is obtained with increasing outer radius of the quantum ring.

The Solar Spectrum in the Atacama Desert

The Atacama Desert has been pointed out as one of the places on earth where the highest surface irradiance may occur. This area is characterized by its high altitude, prevalent cloudless conditions and relatively low columns of ozone and water vapor. Aimed at the characterization of the solar spectrum in the Atacama Desert, we carried out in February-March 2015 ground-based measurements of the spectral irradiance (from the ultraviolet to the near infrared) at seven locations that ranged from the city of Antofagasta (on the southern pacific coastline) to the Chajnantor Plateau (5,100 m altitude). Our spectral measurements allowed us to retrieve the total ozone column, the precipitable water, and the aerosol properties at each location. We found that changes in these parameters, as well as the shorter optical path length at high-altitude locations, lead to significant increases in the surface irradiance with the altitude. Our measurements show that, in the range 0–5100 m altitude, surface irradiance increases with the altitude by about 27% in the infrared range, 6% in the visible range, and 20% in the ultraviolet range. Spectral measurements carried out at the Izaña Observatory (Tenerife, Spain), in Hannover (Germany) and in Santiago (Chile), were used for further comparisons.

Magnetostatic interactions between two magnetic wires

The results of the magnetic dipolar field in a simple set of two amorphous ferromagnetic wires of composition Fe77.5Si12.5B15 placed side by side are presented. Owing to their peculiar domain structure, they could, in principle, be approximated by macroscopic magnetic dipoles, allowing the analysis of the magnetostatic field between these magnetic entities. Magnetization measurements as a function of the distance between the parallel wires were performed. Results can be explained considering the magnetostatic field created by one wire in the neighboring one. It is clearly shown that this field is responsible for changes of the reversal field of the wires, leading to the appearance of plateaux during the demagnetization process. Instead of pure dipolar model that does not fit experimental data, a multipolar model has been developed, showing a rather good agreement with the experimental results.

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 of 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. It is also shown that uniaxial anisotropy in a system of noninteracting particles can be misinterpreted as a ferromagnetic or antiferromagnetic coupling among magnetic particles, depending on the angle between the anisotropy axes and magnetic field.

NONVARIATIONAL ISING–BLOCH TRANSITION IN PARAMETRICALLY DRIVEN SYSTEMS

Transition from motionless to moving domain walls connecting two uniform oscillatory equivalent states in both a magnetic wire forced with a transversal oscillating magnetic field and a parametrically driven damped pendula chain are studied. These domain walls are not contained in the conventional approach to these systems — parametrically driven damped nonlinear Schrodinger equation. By adding in this model higher order terms, we are able to explain these solutions and the transition between resting and moving walls. Based on amended amplitude equation, we deduced a set of ordinary differential equations which describes the nonvariational Ising–Bloch transition in unified manner.

Quantum transport through aromatic molecules

In this paper, we study the electronic transport properties through aromatic molecules connected to two semi-infinite leads. The molecules are in different geometrical configurations including arrays. Using a nearest neighbor tight-binding approach, the transport properties are analyzed into a Greens function technique within a real-space renormalization scheme. We calculate the transmission probability and the Current-Voltage characteristics as a function of a molecule-leads coupling parameter. Our results show different transport regimes for these systems, exhibiting metal-semiconductor-insulator transitions and the possibility to employ them in molecular devices.

Characterization of the Chaotic Magnetic Particle Dynamics

In this paper, we study the deterministic spin dynamics of an anisotropic magnetic particle in the presence of a time-dependent magnetic field using the Landau-Lifshitz equation. In particular, we study the case when the magnetic field is homogeneous with a fixed direction perpendicular to the anisotropy direction and consists of a constant and a time-periodic part. We characterize the dynamical behavior of the system by monitoring the Lyapunov exponents and by bifurcation diagrams. We focus on the dependence of the largest Lyapunov exponent on the magnitude and frequency of the applied magnetic field as well as on the anisotropy parameter of the particle. We find rather complicated landscape of sometimes closely intermingled chaotic and nonchaotic areas in parameter space with rather fuzzy boundaries in-between. For actual experiments that means the system can exhibit multiple transitions between regular and chaotic behavior.

Monte Carlo-based uncertainties of surface UV estimates from models and from spectroradiometers

Although some of the adverse effects of the ultraviolet (UV) radiation may be strictly proportional to cumulative UV doses, others may relate to the frequency of extreme UV events. Therefore, an improved understanding of the UV global climate, including variability and trends, has become of great interest. Variability and trend analyses require quality-ensured surface UV series. The quality of surface UV data depends on their uncertainty. Building upon our prior efforts, we have used a Monte Carlo-based method to compute, under different conditions, the uncertainties affecting UV data rendered by models (1D radiative transfer models) and by spectroradiometers (double monochromator-based and CCD array-based). We found that the uncertainty of spectral UV measurements is driven by the signal-to-noise ratio of the detector, while the uncertainty of spectral UV calculations strongly depends on the uncertainty of the ozone input. The presented uncertainty figures allow comparison of the performance of modern UV gathering techniques (models and instruments), and provide a frame to assess the significance of differences when intercomparing.

Magnetization Dynamics Under a Quasiperiodic Magnetic Field

In the present work, we study the deterministic spin dynamics of an anisotropic magnetic particle in the presence of a time dependent magnetic field using the Landau-Lifshitz-Gilbert equation. In particular, we study the case when the magnetic field consists in two terms. One is perpendicular to the anisotropy direction and has quasiperiodic time dependence, while the other term is constant and parallel to the anisotropy direction. We numerically characterize the dynamical behavior of the system by monitoring the Lyapunov exponents, and by calculating Poincaré sections and Fourier spectra. In addition, we calculate analytically the corresponding Melnikov function which gives us the bifurcations of the homoclinic orbits. We find a rather complicated landscape of sometimes closely intermingled chaotic and non-chaotic areas in parameters space. Finally, we show that the system exhibits strange nonchaotic attractors.