Nonquasiparticle states in half-metallic ferromagnets

Anomalous magnetic and electronic properties of the half-metallic ferromagnets (HMF) have been discussed. The general conception of the HMF electronic structure which take into account the most important correlation effects from electron-magnon interactions, in particular, the spin-polaron effects, is presented. Special attention is paid to the so called non-quasiparticle (NQP) or incoherent states which are present in the gap near the Fermi level and can give considerable contributions to thermodynamic and transport properties. Prospects of experimental observation of the NQP states in core-level spectroscopy is discussed. Special features of transport properties of the HMF which are connected with the absence of one-magnon spin-flip scattering processes are investigated. The temperature and magnetic field dependences of resistivity in various regimes are calculated. It is shown that the NQP states can give a dominate contribution to the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity. First principle calculations of the NQP-states for the prototype half-metallic material NiMnSb within the local-density approximation plus dynamical mean field theory (LDA+DMFT) are presented.Comment: 27 pages, 9 figures, Proceedings of Berlin/Wandlitz workshop 2004; Local-Moment Ferromagnets. Unique Properties for Moder Applications, ed. M. Donath, W.Nolting, Springer, Berlin, 200

An equation of state of the Heisenberg ferromagnet

The thermodynamic properties of the classical Heisenberg model on 3-d lattices are calculated in the approximation of the first order in 1/z (z is the number of nearest neighbours) which takes into account the two-point spin correlations resulting in the Gaussian fluctuations of the molecular field components. The numerical solution for the magnetization, susceptibility and the internal energy is valid in the whole plane (H, T) excluding the close vicinity of the critical point (| τ |≲ 1/z2) and agrees well with available series results. The extension of the method to the quantum case is proposed.Nous calculons les propriétés thermodynamiques du modèle de Heisenberg classique à trois dimensions au premier ordre en 1/z (z est le nombre de voisins immédiats sur le même réseau). Nous tenons compte ainsi des corrélations à deux spins qui donnent aux fluctuations gaussiennes des composantes du champ moléculaire. Nous obtenons numériquement des valeurs pour l'aimantation, la susceptibilité magnétique et l'énergie libre qui sont valables dans tout le plan (H, T) sauf dans un petit intervalle au voisinage du point critique (|τ |≲ 1/z2) et sont en bon accord avec les résultats disponibles venant des séries. Nous proposons une généralisation de la méthode au modèle quantique

Continuous stochastic theory of birth and death processes with long-range interaction. Application to electrolytes

The problem of stochastic description of the spatially distributed birth and death processes with long-range interaction is considered. It is shown how the problem can be formulated in terms of the functional master equation for the probability functional. The theory is applied to the calculation of the relaxational correction to the mobility of ions in the electrolytes. The influence of recombination — dissociation processes on the Debye screening of moving ions and their relative contributions to the mobility are discussed. The particular example concerns the system of a two-component electrolyte in the limit of small fluctuations of the concentrations ( e2/ελD kB T <<; 1, where λD is the Debye radius and T is the absolute temperature).L'article porte sur la description stochastique des processus de naissance et de mort dans des systèmes spatialement distribués et sièges d'interactions à longue portée. On montre comment on peut formuler le problème à l'aide d'une équation maîtresse régissant l'évolution d'une fonctionnelle « probabilité ». La théorie est appliquée au calcul d'une « correction relaxationnelle » à la mobilité des ions dans un électrolyte. On montre en particulier que les processus de dissociation et recombinaison s'ajoutent à l'effet d'écran des ions mobiles (Debye screening). L'importance relative de ces deux phénomènes sur la mobilité est examinée

The influence of a magnetic field on dielectric relaxation processes

The influence of a magnetic field on the dielectric losses in weak-polar dielectric compounds is investigated. By the solution of the Fokker-Planck equation describing the rotational diffusion of the polar group over a potential barrier it is shown that with the increasing of a magnetic field the maximum of the dielectric loss curve tg δ is shifted to the higher temperatures and diminished, in accordance with the experimental data [3, 4].Nous étudions l'influence du champ magnétique sur les pertes diélectriques dans des substances peu polaires. En utilisant la solution de l'équation de Fokker-Planck décrivant la diffusion de rotation des groupes polaires à travers une barrière de potentiel, nous montrons qu'avec l'augmentation du champ magnétique le maximum de la courbe des pertes diélectriques tg δ se déplace vers les hautes températures et diminue, en accord avec les résultats expérimentaux [3, 4]

XIPE

XIPE, the X-ray Imaging Polarimetry Explorer, is a mission dedicated to X-ray Astronomy. At the time of writing XIPE is in a competitive phase A as fourth medium size mission of ESA (M4). It promises to reopen the polarimetry window in high energy Astrophysics after more than 4 decades thanks to a detector that efficiently exploits the photoelectric effect and to X-ray optics with large effective area. XIPE uniqueness is time-spectrally-spatially-resolved X-ray polarimetry as a breakthrough in high energy astrophysics and fundamental physics. Indeed the payload consists of three Gas Pixel Detectors at the focus of three X-ray optics with a total effective area larger than one XMM mirror but with a low weight. The payload is compatible with the fairing of the Vega launcher. XIPE is designed as an observatory for X-ray astronomers with 75 % of the time dedicated to a Guest Observer competitive program and it is organized as a consortium across Europe with main contributions from Italy, Germany, Spain, United Kingdom, Poland, Sweden.Peer reviewe

Multi-messenger Observations of a Binary Neutron Star Merger

International audienceOn 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of $\sim 1.7\,{\rm{s}}$ with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg(2) at a luminosity distance of ${40}_{-8}^{+8}$ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 $\,{M}_{\odot }$. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at $\sim 40\,{\rm{Mpc}}$) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position $\sim 9$ and $\sim 16$ days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta