Ab initio study of the influence of nanoscale doping inhomogeneities in the phase separated state of La1-xCaxMnO3.

The chemical influence in the phase separation phenomenon that occurs in perovskite manganites is discussed by means of ab initio calculations. Supercells have been used to simulate a phase separated state, that occurs at Ca concentrations close to the localized itinerant crossover. We have first considered a model with two types of magnetic ordering coexisting within the same compound. This is not stable. However, a non-isotropic distribution of chemical dopants is found to be the ground state. This leads to regions in the system with different effective concentrations, that would always accompany the magnetic phase separation at the same nanometric scale, with hole-rich regions being more ferromagnetic in character and hole-poor regions being in the antiferromagnetic region of the phase diagram, as long as the system is close to a phase crossover.

A first-principles study of the influence of helium atoms on the optical response of small silver clusters.

Optical excitation spectra of Ag(n) and Ag(n)@He(60) (n = 2, 8) clusters are investigated in the framework of the time-dependent density functional theory (TDDFT) within the linear response regime. We have performed the ab initio calculations for two different exact exchange functionals (GGA-exact and LDA-exact). The computed spectra of Ag(n)@He(60) clusters with the GGA-exact functional accounting for exchange-correlation effects are found to be generally in a relatively good agreement with the experiment. A strategy is proposed to obtain the ground-state structures of the Ag(n)@He(60) clusters and in the initial process of the geometry optimization, the He environment is simulated with buckyballs. A redshift of the silver clusters spectra is observed in the He environment with respect to the ones of bare silver clusters. This observation is discussed and explained in terms of a contraction of the Ag-He bonding length and a consequent confinement of the s valence electrons in silver clusters. Likewise, the Mie-Gans predictions combined with our TDDFT calculations also show that the dielectric effect produced by the He matrix is considerably less important in explaining the redshifting observed in the optical spectra of Ag(n)@He(60) clusters.

Density functional study of the magnetic properties of Bi4Mn clusters: discrepancy between theory and experiment.

We have performed collinear and noncollinear calculations on neutral Bi(4)Mn and collinear ones on ionized Bi(4)Mn with charges +1 and -1 to find out why theoretical calculations will not predict the magnetic state found in the experiment. We have used the density functional theory to find a fit between the theoretical prediction of the magnetic moment and the experimental value. Our calculations have consisted in a structural search of local energy minima, and the lowest energy magnetic state for each resulting isomer. The geometry optimization found three local minima whose fundamental state is the doublet spin state. These isomers could not be found in previous theoretical works, but they are higher in energy than the lowest-lying isomer by ≈1.75 eV. This magnetic state could help understand the experiment. Calculations of noncollinear magnetic states for the Bi(4)Mn do not lower the total magnetic moment. We conclude arguing how the three isomers with doublet state could actually be the ones measured in the experiment.

Magnetocaloric effect in magnetic nanoparticle systems: how to choose the best magnetic material?

Magnetic nanoparticles with controlled magnetocaloric properties are a good candidate to lower the temperature of nanosized systems: they are easy to manipulate and to distribute into different geometries, as wires or planes. Using a Monte Carlo technique we study the entropy change and refrigerant capacity of an assembly of fine magnetic particles as a function of their anisotropy and magnetization, key-parameters of the magnetic behavior of the system. We focus our attention on the anisotropy energy/dipolar energy ratio by means of the related parameter c0 = 2K/M(S)2, where K is the anisotropy constant and M(S) is the saturation magnetization of the nanoparticles. Making to vary the value of co parameter by choosing different K-M(S) combinations, allows us to discuss how the magnetocaloric response of an assembly of magnetic nanoparticles may be tuned by an appropriate choice of the magnetic material composition.

Density functional study of the oxidation of small neutral and charged silver clusters.

We have studied the energetic and structural stability of the interaction of molecular oxygen with small neutral, anionic and cationic silver clusters, Ag(n) (3 < or = n < or = < 8). The calculations have been carried out using a linear combination of atomic Gaussian-type orbitals within the density functional theory as it is implemented in the demon-ks3.5 code. The O2 molecule has been placed in different positions surrounding the cluster, in order to increase the configurational space of the structural minima. We have found that the oxidized cation and neutral clusters undergo a 2D-3D structural transition even before than the nonoxidized counterparts. Moreover, our results show that the adsorption energies on the cationic and neutral silver oxide clusters manifest an odd-even alternation pattern. Likewise, the average magnetic moment of the O2 radical in the charged and neutral silver environment tends to be greater than the charged and neutral bare diatomic oxygen molecule.

Role of the dipolar interaction on the macroscopic state of magnetic nanoparticle systems: a Monte Carlo study.

We have performed Monte Carlo simulations to treat the effect of the dipolar interaction in assemblies of superparamagnetic nanoparticles. Our simulations reproduce correctly the increase of the blocking temperature (T(B)) as the concentration increases, as observed experimentally. Interestingly, we have observed a progressive displacement of the M2 versus H/M isotherms (Arrott plots) from the origin as the concentration of nanoparticles increases. Moreover, the curvature of the isotherms at T > T(B) changes from positive to negative slope at high sample concentrations, resembling the shape of a first order phase transition. These results are surprisingly similar to that found in a conventional magnetic phase transition under the effect of a random anisotropy or a random field.

Small Au clusters oxidation: an ab initio study.

We have performed ab initio calculations in the Density Functional Theory framework on unsupported small gold clusters with size ranging from three to seven atoms. In our calculations we have introduced a single O2 molecule on different places around the cluster surface, and in both parallel and perpendicular position with respect to the cluster surface. We have found that the oxygen molecule bonds in-plane with the bidimensional Au cluster when the number of Au atoms is even, and it will be adsorbed off-plane if the number of Au atoms is odd. The latter case, despite not presenting a true chemical bonding, has great stability due to spin pairing and electrostatic interactions, and the structures will be distorted respect to the geometry of their pure Au cluster equivalents.

Real time monitoring and analysis via the medical information bus, Part I.

Because of the complexity of monitored data in modern intensive care units (ICUs), and the risk of information being overlooked if medical staff have to pay attention to a multiplicity of monitoring apparatuses and alarm signals, the data for each patient may well be best presented on a single bedside screen after digestion by expert system techniques. Such central units should be able to deal with data from any monitoring apparatus, not just a predefined set. Furthermore, relay of the information from each bed to a central control station (one per ICU) is desirable for the purposes of permanent storage and for in-depth analysis. The paper describes a comprehensive system for ICU monitoring management and patient data analysis that integrates multiple expert systems and computers. The basic difficulties in applying expert system techniques to monitoring are overcome with the shell OPS/83, which allows calls to sequential C routines and allows time-driven reasoning through appropriate design of the inference engine and rules. Flexibility as regards connectable monitoring apparatus is afforded by basing data acquisition mainly, though not exclusively, on the IEEE Medical Information Bus.

Real time monitoring and analysis via the medical information bus, Part II.

The complexity of the monitored data available in modern intensive care units suggests that they may be best processed, for presentation to medical staff, by expert system techniques. However, standard expert system shells are ill-fitted to either the basic sequential monitoring tasks of data acquisition and storage, or to handling the temporal considerations inherent in monitoring and in the recognition and processing of sporadic alarm signals. A solution to this dilemma is described: an expert system that has an appropriately designed inference engine and manages data acquisition via a medical information bus (MIB). Use of this MIB standard allows great flexibility as regards the monitoring apparatuses employed; in particular, the connection or disconnection of any apparatus is recognised and triggers the automatic reconfiguration of the network.

Syntactic pattern recognition of fetal stress.

At present, units designed to diagnose the state of the foetus during labour base their decisions on relatively short segments of the monitored foetal heart rate and uterine pressure signals. This article describes a new method which takes a sequence of such local diagnoses of individual contractions, extracts the most relevant pathological features and predicts the neuro-physiological performance of the child at birth.