Magnetoelectric response of multiferroic BiFeO3 and related materials

We present a first-principles scheme for computing the magnetoelectric response of multiferroics. We apply our method to BiFeO3 (BFO) and related compounds in which Fe is substituted by other magnetic species. We show that under certain relevant conditions -- i.e., in absence of incommensurate spin modulation, as in BFO thin films and some BFO-based solid solutions -- these materials display a large linear magnetoelectric response. Our calculations reveal the atomistic origin of the coupling and allow us to identify the most promising strategies to enhance it.Comment: 4 pages with 1 figure embedded. More information at http://www.icmab.es/dmmis/leem/jorg

Negative high-frequency differential conductivity in semiconductor superlattices

We examine the high-frequency differential conductivity response properties of semiconductor superlattices having various miniband dispersion laws. Our analysis shows that the anharmonicity of Bloch oscillations (beyond tight-binding approximation) leads to the occurrence of negative high-frequency differential conductivity at frequency multiples of the Bloch frequency. This effect can arise even in regions of positive static differential conductivity. The influence of strong electron scattering by optic phonons is analyzed. We propose an optimal superlattice miniband dispersion law to achieve high-frequency field amplification

Superlattice with hot electron injection: an approach to a Bloch oscillator

A semiconductor superlattice with hot electron injection into the miniband is considered. The injection changes the stationary distribution function and results in a qualitative change of the frequency behaviour of the differential conductivity. In the regime with Bloch oscillating electrons and injection into the upper part of the miniband the region of negative differential conductivity is shifted from low frequencies to higher frequencies. We find that the dc differential conductivity can be made positive and thus the domain instability can be suppressed. At the same time the high-frequency differential conductivity is negative above the Bloch frequency. This opens a new way to make a Bloch oscillator operating at THz frequencies.Comment: RevTeX, 8 pages, 2 figures, to be published in Phys. Rev. B, 15 Januar 200

Pseudorandom number generator influence on the genetic algorithm performance to minimize maritime cargo delivery route length

We consider a problem of minimizing the maritime cargo delivery route length to reduce the delivery cost. In our model, the cost is equivalent to the sum of tour lengths of feeders used for the delivery to cover the route. Formulated as a multiple traveling salesman problem, we solve it with a genetic algorithm. The algorithm performance is dramatically influenced by the stream of pseudorandom numbers used for randomly generating the starting population and accomplishing random mutations. As the number of ports increases from 10 to 80, the route length variation intensifies from 3.5% to 22.5% on average. However, we increase the route length minimization accuracy by re-running the algorithm to solve the same problem until closely the best solution is obtained. The number of reruns is about 3 to 6 for up to 20 ports. For more than 20 ports the required number of algorithm reruns abruptly increases from 28 reruns for 30 ports to about 51 reruns within the range of 40 to 80 ports

Field dependence of the magnetocaloric effect in materials with a second order phase transition: A master curve for the magnetic entropy change

The field dependence of the magnetic entropy change can be expressed as SM Hn. For soft magnetic amorphous alloys n=1 well below the Curie temperature TC , n=2 in the paramagnetic range, and n 0.75 for T=TC. The first value can be explained with simple arguments, n=2 is a consequence of the Curie-Weiss law, but n TC deviates from mean field predictions. From the Arrott-Noakes equation of state, a relation between n TC and the critical exponents has been obtained, showing remarkable agreement with experimental data for an example alloy, predicted n=0.72 versus experimental n=0.73 . A master curve behavior for the temperature dependence of SM measured for different maximum fields is proposed

Continuous Neel to Bloch Transition as Thickness Increases: Statics and Dynamics

We analyze the properties of Neel and Bloch domain walls as a function of film thickness h, for systems where, in addition to exchange, the dipole-dipole interaction must be included. The Neel to Bloch phase transition is found to be a second order transition at hc, mediated by a single unstable mode that corresponds to oscillatory motion of the domain wall center. A uniform out-of-plane rf-field couples strongly to this critical mode only in the Neel phase. An analytical Landau theory shows that the critical mode frequency varies as the square root of (hc - h) just below the transition, as found numerically.Comment: 4 pages, 4 figure

Sorption-desorption kinetics for powdered and non-powered coal

Diffusion through macro- and meso-pores with the subsequent filling of open micropores is a relatively fast process and the manometric measurements with fifteen minute pressure stabilization steps provide a good estimate of excess (ad)sorption. This can be followed by a much slower processes of the penetrant diffusion into the macromolecular network, accompanied by its structural relaxation, with or without free volume changes. The “free volume” effect is a change of the sample’s excluded volume because of the penetrant molecules mixing within the formerly excluded volume of the network or because of contraction (either reversible or semi-permanent) due to external pressure. If the resulting swelling of the sample leads to the mixture’s volume equal to the sum of the initial volumes of the components, there is no apparent change to the void volume and no pressure relaxation is observed (b). On the other hand, if the external forces (either macroscopic pressure or microscopic molecular interaction forces) change the excluded volume of the network (its density), without exchange of the sorbent molecules between the sample and the free fluid phase, such change causes a corresponding change in the void volume, which constitutes a pure free volume effect (c). In this case, no post-decompression exodus of the sorbent out of the sample is observed. As a special case, we consider an incorporation of the penetrant molecules into the network without any change in the volume of the mixture (d), though mathematically this can be written as a superposition of the previous two case