Melt nonstoichiometry and defect structure of ZnGeP<Subscript>2</Subscript> crystals

The defect structure of ZnGeP₂ crystals grown from a melt by the vertical Bridgman method has been investigated. A deviation of the melt composition from stoichiometric leads to the formation of striations and the inclusions of other phases which are observed as structures (chains) oriented parallel to the growth axis. According to the microanalysis data, the inclusion composition corresponds to a mixture of ZnGeP₂, Zn₃P₂, and Ge. Nanoinclusions of germanium phosphide are detected by transmission electron microscopy. X-ray topography reveals defects of four types. The main defects in the central part of an ingot are related to the composition fluctuations, and the newly formed dislocations are basically single ones. Most dislocations are formed at the crystal periphery.     

Phase equilibria and electrical and magnetic properties of a eutectic in the GaSb-MnSb system

A set of physicochemical analysis methods showed that GaSb and MnSb form a eutectic at 41 mol % MnSb and T _melt = 632°C. The eutectic is of the platelet type and has metallic conduction. The eutectic GaSb + MnSb is a composite ferromagnetic with a Curie temperature of ～600 K. The behavior of the electrical resistance in a magnetic field is complex. At low magnetic fields up to 0.8 T, the resistance abruptly drops, and at high magnetic fields, it slowly increases. Such behavior occurs both at low temperatures (5 K) and at 300 K. The change of the trend in the resistance takes place at the magnetic field at which there is magnetization saturation in the magnetic field dependences. Such a dependence of the resistance on the magnetic field is explained by a change of the mechanism of scattering of charge carriers.     

Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

Measuring the distribution of particles according to their velocity in accelerator beams on the basis of Cherenkov radiation in the optical and microwave range

Methods for measuring the energy spectra of powerful beams of charged particles and their bunches according to the dependence of the intensity of Cherenkov radiation on the refractive index of the radiator (with and without using of dispersion) are considered. The methods are practically nondestructive. In the case of special demands on the Cherenkov radiation collection system, beam spectra with unknown angular distributions can be measured. Uncharged particle spectra are determined from the secondary charged particles spectra.     

Approximation algorithms for finding and partitioning unit-disk graphs into co-<Emphasis Type="Italic">k</Emphasis>-plexes

This article studies a degree-bounded generalization of independent sets called co-k-plexes. Constant factor approximation algorithms are developed for the maximum co-k-plex problem on unit-disk graphs. The related problem of minimum co-k-plex coloring that generalizes classical vertex coloring is also studied in the context of unit-disk graphs. We extend several classical approximation results for independent sets in UDGs to co-k-plexes, and settle a recent conjecture on the approximability of co-k-plex coloring in UDGs.     

Spatially Localized Photoelectric Effect in Ambipolar Organic Field-Effect Phototransistors

JETP Letters - A numerical simulation has demonstrated that the conversion of incident radiation into the photocurrent in ambipolar phototransistors based on organic semiconductors at certain...     

Cation ordering and magnetic properties of neodymium-barium manganites

Nd_0.70Ba_0.30MnO_3+δ manganites with an ordered arrangement of Nd³⁺ and Ba²⁺ cations are fabricated. The initial stoichiometric Ba-disordered Nd_0.70Ba_0.30MnO₃ solid solution is synthesized in air using a traditional ceramic technology, is characterized by an orthorhombic unit cell (space group Imma, Z = 4), and is ferromagnetic with Curie temperature T _C ≈ 151 K. The average crystallite size in the initial sample is 〈D〉 ≈ 2.169 μm. Annealing of the initial sample in a reducing atmosphere at a pressure P[O₂] ≈ 10^?4 Pa and then in air at T = 800°C leads to the formation of a material with a crystallite size 〈D〉 ≈ 440 nm. This material consists of the following two perovskite phases: a Ba-ordered stoichiometric NdBaMn₂O₆ phase with a tetragonal unit cell (P4/mmm, Z = 2) and a Curie temperature of ～301 K and a Ba-disordered hyperstoichiometric Nd_0.90Ba_0.10MnO_3+δ phase with an orthorhombic unit cell (Imma, Z = 4) and T _C ～ 121 K. Significant changes in the magnetic properties are explained in terms of chemical phase separation with allowance for cation ordering.     

Study of crystal and magnetic structures of manganite Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of manganite Pr_0.7Ba_0.3MnO₃ have been studied at high pressures of up to 5.1 GPa and temperatures from 10 to 300 K by means of the neutron diffraction. At normal pressure and a temperature T _C = 200 K, a ferromagnetic state forms in Pr_0.7Ba_0.3MnO₃. At high pressures P ≥ 1.9 GPa and T < T _N ≈ 153 K, a new antiferromagnetic state of A-type have been observed. Under high pressure, the Curie temperature T _C increases with the characteristic quantity dT _C/dP ≈ 2.4 K/GPa. A possible reason for the appearance of an A-type antiferromagnetic phase in Pr_0.7Ba_0.3MnO₃ at high pressures may be anisotropic uniaxial compression of oxygen octahedra along the b axis of the orthorhombic structure.     

Preparation and magnetic and electrical properties of EuBaMn<Subscript>2</Subscript>O<Subscript>6−δ</Subscript> (δ=0,1)

The EuBaMn₂O₆ composition prepared in air at T=1500°C is shown to be a cubic perovskite exhibiting spin-glass properties below T_f=40 K. The reduced composition, EuBaMn₂O₅, crystallizes in a perovskite-like, YBaCuFeO₅-type layered structure with a tetragonal unit cell. EuBaMn₂O₅ with crystallographically ordered Eu³⁺ and Ba²⁺ ions is a ferromagnet with T_N=160 K. The anomalous behavior of the paramagnetic susceptibility is assumed to be due to a partial ordering of the Mn²⁺ and Mn³⁺ ions. EuBaMn₂O₅ oxidized in air at 900°C to EuBaMn₂O₆ has a magnetic ordering temperature T_M=260 K, near which the magnetoresistance reaches a peak value. X-ray diffraction measurements show the long-range order in the Eu³⁺ and Ba²⁺ ion arrangement to persist in the oxidized EuBaMn₂O₆ sample.     

Magnetoresistive effect in RCu<Subscript>3</Subscript>Mn<Subscript>4</Subscript>O<Subscript>12</Subscript> perovskite-like oxides

The electrical properties of and the magnetoresistive effect in RCu₃Mn₄O₁₂ (R=rare-earth ion or Th) are studied. In all compounds of this series, the magnetoresistive effect amounts to 20% at liquid nitrogen temperature in the presence of a field of 0.9 T. An increase in the magnetoresistance with decreasing temperature and a high sensitivity to weak magnetic fields at low temperatures point to the intergranular nature of the effect. The magnetoresistance shows a peak in the vicinity of the Curie temperature T_C. Based on the dependences of the magnetoresistance on an external magnetic field, it is assumed that the magnetoresistance peak near T_C is related to the charge carrier scattering by magnetic inhomogeneities as in substituted orthomanganites. We believe that the magnetoresistance value near the magnetic ordering temperature depends on the synthesis conditions and the effect of the intergranular spacer on the transport properties of these compounds.