We report on the observation of magnon thermal conductivity κm ∼ 70 W/mK near 5 K in the helimagnetic insulator Cu2OSeO3, exceeding that measured in any other ferromagnet by almost two orders of magnitude. Ballistic, boundary-limited transport for both magnons and phonons is established below 1 K, and Poiseuille flow of magnons is proposed to explain a magnon mean-free path substantially exceeding the specimen width for the least defective specimens in the range 2K < T < 10 K. These observations establish Cu2OSeO3 as a model system for studying longwavelength magnon dynamics.

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An analytical expression is received for the effective interaction potential of a fast charged particle with the ionic crystal CsCl near the direction of axis h100i as a function of the temperature of the medium. By numerical analysis it is shown that the effective potential of axial channeling of positrons along the axis h100i of negatively charged Cl À ions practically doesn't depend on temperature of the media. The wavefunction and the energy spectrum of the localized state are investigated, and the possibility is proved for the formation of metastable, two-dimensional relativistic positron systems or the positron atoms (PA), i.e. the bound state of a positron in the present case with a negatively charged axis of ions. The problem of one-photon decay of PA is investigated. The possibilities of stimulation of resonant transitions between quantum states of PA by the external hypersound are investigated in detail.

Spatial carrier concentration distributions with a region of very rapid variation in the carrier density were investigated in germanium samples under the contact exclusion effect. The studies were in a range of sample temperatures from299 to 320 K, under sample illumination with a strongly absorbed light and under various treatments of the sample's 1ateralfaas.The resultsobtainedareingoodagreementwithresultsofananalytical theory.

We report studies of thermal conductivity as functions of magnetic field and temperature in the helimagnetic insulator Cu2OSeO3 that reveal novel features of the spin-phase transitions as probed by magnon heat conduction. The tilted conical spiral and low-temperature skyrmion phases, recently identified in small-angle neutron scattering studies, are clearly identified by sharp signatures in the magnon thermal conductivity. Magnon scattering associated with the presence of domain boundaries in the tilted conical phase and regions of skyrmion and conical-phase coexistence are identified.The cubic chiral magnets (MnSi, FeGe, Cu 2 OSeO 3 ) have attracted considerable attention for their complex variety of non-collinear spin phases that include spin modulations with long periods (many lattice spacings) and topological skyrmion phases. Their similar and rich magnetic phase diagrams are dictated by their common noncentrosymmetric cubic lattice symmetry and a hierarchy of competing energy scales (e.g. exchange, Dzyaloshinsky-Moriya, magnetocrystalline anisotropy, Zeeman). Recently two new spin phases, low-temperature skyrmion and "titled conical spiral," were identified in the insulating compound Cu 2 OSeO 3 by small-angle neutron scattering (SANS) [1,2]. These novel phases, arising at low temperature and relatively high magnetic field, reflect competing Zeeman and anisotropy energies that lead to surprising spin textures and a re-orientation of the long-period spin modulation direction.Here we report that field dependent thermal conductivity measurements are a particularly sensitive probe of the spin phase transitions in Cu 2 OSeO 3 because of the compound's unprecedentedly large magnon heat conductivity [3]. Rather little is known experimentally about magnons in ferro-or ferri-magnets from heat transport, their scattering or its dependency on spin textures, whether longrange ordered or not. Such information has increased in its importance and relevance with the surging interest in spintronic and magnonic device applications [4,5]. Cu 2 OSeO 3 is an ideal material for investigating these characteristics because the spin and lattice systems are weakly coupled as evidenced by very low spin-lattice damping [6,7] and by mean-free paths for both magnons and phonons as large as 0.3 mm below 2K [3]. These conditions ensure that energy (e.g. from a heater in thermal conductivity measurements) is transferred from the lattice to the spin system, but is weak enough so the contributions from phonons (κ L ) and magnons (κ m ) are approximately additive [3,8], κ ≃ κ L + κ m .Our measurements reveal a complete suppression of the magnon heat flux in the tilted conical phase along the 110 directions that we attribute to strong magnon scattering by tilt domain boundaries. This observation raises the prospect of exploiting this configuration of heat flux and applied field in a field-controllable spin-current switch. The low-temperature skyrmion phase, characterized by long-range skyrmion lattice order, supports maximum magnon heat conduct...