Yu. A. Ossipyan

Direct observation of the lattice of Abrikosov vortices in high-Tc superconductor YBa2Cu3Ox single crystals

Abstract The magnetic structure of the YBa2Cu3Ox single crystal has been investigated using the technique of decorating the sample with small ferromagnetic particles. The lattice of the Abrikosov vortices and the effects caused by pinning on the twin boundaries have been observed in both the electron and optical microscopes. The twin structure of the single crystals has been investigated using the polarized light in the optical microscope. The penetration depth for the magnetic field parallel to the c axis has been evaluated as ≲ 0.3 μm at 4.2 K for the YBa2Cu3Ox single crystal.

Magnetic ordering in hydrofullerite C60H24

Abstract Hydrofullerites C 60 H x synthesised at hydrogen pressures of 0.6 and 3 GPa were found to possess ferromagnetic properties at room temperature. The magnitude of magnetisation varied from sample to sample and reached 0.001–0.16 Bohr magnetons per C 60 molecule at H =10 kOe. The coercivity of all the samples was about 100 Oe. The hydrofullerites had either an fcc or bcc lattice formed of C 60 H x units. The maximum values of magnetisation were observed for the fcc hydrofullerites with x ≈24.

Spin and Orbital Ground State of Co in Cobalt Phthalocyanine

The 3d orbital ground state of transition-metal ions that are incorporated in a molecular matrix determines the total spin of the transition-metal ion as well as the spin anisotropy and thus the essential magnetic properties of the corresponding molecule. However, there is little known to date on the exact 3d ground state of many molecular systems, including quite complex molecular magnets as well as relatively simple systems such as, for instance, cobalt phthalocyanine (CoPc). For the latter, there are contradictory theoretical predictions with respect to the occupation of the various Co 3d electronic levels. We demonstrate that polarization-dependent X-ray absorption spectroscopy in combination with a simulation of the spectra is able to shed a brighter light on the spin and orbital ground state of the transition-metal ion in CoPc. Our results reveal a temperature-dependent ground state and emphasize the importance of taking 3d correlation effects properly into account.

Electronic structure of the organic semiconductor copper phthalocyanine: experiment and theory.

The electronic structure of the organic semiconductor copper-phthalocyanine (CuPc) has been determined by a combination of conventional and resonant photoemission, near-edge x-ray absorption, as well as by the first-principles calculations. The experimentally obtained electronic valence band structure of CuPc is in very good agreement with the calculated density of states results, allowing the derivation of detailed site specific information.

Advantages and Problems of Nanocrystalline Scintillators

Our experiments with nanocrystalline scintillating rare earth oxides and rare earth fluorides have shown that in some cases nanoscopic dimensions provide essential improvement of the most important scintillation parameters: light yield, kinetics of scintillations, radiation hardness, etc. We found that in the range from 20 to 100-nm of the oxide and fluoride particles there are 3 types of layered structures: with expanded exterior layer, with changed phase structure, and with changed chemical composition. These layered structures can strongly influence scintillation parameters: cause an increase or decrease in the light yield, vary scintillation kinetics, modify radiation hardness, etc. Control of dimensions and structures of nanoscintillators can be used for significant modifications of parameters of radiation detectors (radical acceleration of kinetics, enhancement of light yield, increase in radiation hardness, etc.). Radiation detectors based on nanoscintillators have promising prospects for applications in new generations of devices for medical diagnostics, security inspection, radiation monitoring of nuclear reactors.

Thermally stable hydrogen compounds obtained under high pressure on the basis of carbon nanotubes and nanofibers

Compounds containing 6.3–6.5 wt % H and thermally stable in vacuum up to 500°C were obtained by annealing graphite nanofibers and single-walled carbon nanotubes in hydrogen atmosphere under a pressure of 9 GPa at temperatures up to 45°C. A change in the X-ray diffraction patterns indicates that the crystal lattice of graphite nanofibers swells upon hydrogenation and that the structure is recovered after the removal of hydrogen. It was established by IR spectroscopy that hydrogenation enhances light transmission by nanomaterials in the energy range studied (400–5000 cm−1) and results in the appearance of absorption bands at 2860–2920 cm−1 that are characteristic of the C–H stretching vibrations. The removal of about 40% of hydrogen absorbed under pressure fully suppresses the C–H vibrational peaks. The experimental results are evidence of two hydrogen states in the materials at room temperature; a noticeable portion of hydrogen forms C–H bonds, but the most of the hydrogen is situated between the graphene layers or inside the nanotubes.

Physical properties of YBa2Cu3O7−x single crystals

Abstract Some recent results concerning the real crystal structure and anisotropic properties of YBa 2 Cu 3 O 7−x single crystals are reviewed.

Structure of Ge(111) surfaces obtained by cleavage in superhigh vacuum at low temperatures

Abstract Early low-temperature LEED results for Ge(111) surfaces obtained by cleavages in vacuum ∼10 −10 Torr are reported. The atomic structure of Ge(111) surfaces is found to be essentially dependent on cleavage temperature, T c . It is established that changes in temperature (after cleavage) have no effect on the surface structure over the range 12–300 K. At low-temperature cleavages, T c ≲20–40 K, there appears the Ge(111)-1 × 1 LEED pattern; at high-temperature cleavages, T c ≳40–60 K, the Ge(111)-2 × 1 pattern is found.

Magnetoresonant hardening of silicon single crystals

A microwave magnetic field crossed with a static field was found to exert a resonance effect on the dislocation mobility in single crystals of p-type silicon. The frequency of alternating field and the magnitude of static magnetic field corresponding to the maximal crystal hardening satisfy conditions for EPR of structural defects. This is evidence that the primary elementary processes observed previously in magnetoplasticity effects (influence of a static magnetic field on plasticity) are spin-dependent in silicon crystals. The dislocation path detected EPR spectrum was found to be anisotropic.

Superconductivity and structure of YBa2Cu3O6 single crystals treated in halogen vapours

Abstract Dielectric tetragonal samples of Y-Ba-Cu-O single crystals have been heat treated in an atmosphere of iodine or bromine gas. After halogenation the single crystals exhibited the transition to the superconducting state with T c ≈ 60 K and 85 K, respectively.