arXiv subjects

O. E. Omel'yanovskii

Publications and source records attributed to O. E. Omel'yanovskii.

8 recordsLinked to original sources

arXiv2011

Synthesis of novel rare earth - iron oxide chalcogenides with the La2Fe2O3Se2 structure

Our searches for new oxide chalcogenides of rare earths and Fe, Co, Ni, and Zn resulted in preparation of two new compounds Ce2Fe2O3S2 and Pr2Fe2O3S2 which are isostructural to La2Fe2O3Se2 and Sm2Ti2O3Sb2. Crystal structures of the new compounds Ce2Fe2O3S2 and Pr2Fe2O3S2 were determined from powder X-ray diffraction data. Magnetic measurements were performed for Ce2Fe2O3S2 and revealed behavior very similar to that of isostructural oxide chalcogenides of iron and pnictides of titanium. In particular, no superconductivity was observed down to 4 K. Crystal chemical factors determining the stability of the La2Fe2O3Se2 structure type are discussed.

cond-mat.mtrl-sci↗

arXiv2011

Synthesis, crystal structure, and properties of novel perovskite oxychalcogenides, Ca2CuFeO3Ch (Ch = S, Se)

Two new perovskite oxychalcogenides, Ca2CuFeO3S and Ca2CuFeO3Se, have been synthesized in evacuated quartz tubes. They crystallize in P4/nmm space group with lattice parameters a = 3.8271(1), c = 14.9485(2) Å and a = 3.8605(1), c = 15.3030(2) Å for Ca2CuFeO3S and Ca2CuFeO3Se, respectively. They appear to be the first layered chalcogenide perovskites involving calcium and are structural analogs of the corresponding Sr and Ba compounds. The new compounds exhibit semiconducting properties with energy gap decreasing from the oxysulfide to the oxyselenide. Possibility of introducing Ca2+ into structures of known layered oxychalcogenides and oxypnictides is discussed.

cond-mat.mtrl-sci↗

arXiv2011

Observation of Multi-Gap Superconductivity in GdO(F)FeAs by Andreev Spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO$_{0.88}$F$_{0.12}$FeAs samples with bulk critical temperature ${T_c}$ = (52.5 \pm 1)K using break-junction technique. The data obtained cannot be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2K are ΔL = 10.5 \pm 2 meV and ΔS = 2.3 \pm 0.4 meV, respectively.

cond-mat.supr-con↗

arXiv2008

Interaction Effects in Conductivity of a Two-Valley Electron System in High-Mobility Si Inversion Layers

We have measured the conductivity of high-mobility (001) Si metal-oxide-semiconductor field effect transistors (MOSFETs) over wide ranges of electron densities n=(1.8-15)x10^11cm^2, temperatures T=30mK-4.2K, and in-plane magnetic fields B=0-5T. The experimental data have been analyzed using the theory of interaction effects in the conductivity of disordered 2D systems. The parameters essential for comparison with the theory, such as the intervalley scattering time and valley splitting, have been measured or evaluated in independent experiments. The observed behavior of the conductivity, including its quasi-linear increase with decreasing T down to ~0.4K and its downturn at lower temperatures, is in agreement with the theory. The values of the Fermi- liquid parameter obtained from the comparison agree with the corresponding values extracted from the analysis of Shubnikov-de Haas oscillations based on the theory of magnetooscillations in interacting 2D systems.

cond-mat.dis-nn↗

arXiv2008

Metal-Insulator Transition in 2D: Experimental Test of the Two-Parameter Scaling

We report a detailed scaling analysis of resistivity ρ(T,n) measured for several high-mobility 2D electron systems in the vicinity of the 2D metal-insulator transition. We analyzed the data using the two parameter scaling approach and general scaling ideas. This enables us to determine the critical electron density, two critical indices, and temperature dependence for the separatrix in the self-consistent manner. In addition, we reconstruct the empirical scaling function describing a two-parameter surface which fits well the ρ(T,n) data.

cond-mat.str-el↗

arXiv1998

Giant transverse magnetoresistance in an asymmetric system of three GaAs/AlGaAs quantum wells in a strong magnetic field at room temperature

The giant transverse magnetoresistance is observed in the case of photoinduced nonequilibrium carriers in an asymmetric undoped system of three GaAs/AlGaAs quantum wells at room temperature. In a magnetic field of 75 kOe, the resistance of nanostructure being studied increases by a factor of 1.85. The magnetoresistance depends quadratically on the magnetic field in low fields and tends to saturation in high fields. This phenomenon is attributed to the rearrangement of the electron wave function in magnetic field. Using the fact that the incoherent part of the scattering probability for electron scattering on impurities and bulk defects is proportional to the integral of the forth power of the envelope wave function, the calculated field dependence of the magnetoresistance is shown to be similar to that observed experimentally.

cond-mat.mtrl-sci↗

arXiv1998

Temperature dependence of electric resistance and magnetoresistance of pressed nanocomposites of multilayer nanotubes with the structure of nested cones

Bulk samples of carbon multilayer nanotubes with the structure of nested cones (fishbone structure) suitable for transport measurements, were prepared by compressing under high pressure (~25 kbar) a nanotube precursor synthesized through thermal decomposition of polyethylene catalyzed by nickel. The structure of the initial nanotube material was studied using high-resolution transmission electron microscopy. In the low-temperature range (4.2 - 100 K) the electric resistance of the samples changes according to the law ln ρ~ (T_0/T)^{1/3}, where T_0 ~ 7 K. The measured magnetoresistance is quadratic in the magnetic field and linear in the reciprocal temperature. The measurements have been interpreted in terms of two-dimensional variable-range hopping conductivity. It is suggested that the space between the inside and outside walls of nanotubes acts as a two-dimensional conducting medium. Estimates suggest a high value of the density of electron states at the Fermi level of about 5 10^{21} eV^{-1} cm^{-3}.

cond-mat.mtrl-sci↗

arXiv1998

Field-asymmetric transverse magnetoresistance in a nonmagnetic quantum-size structure

A new phenomenon is observed experimentally in a heavily doped asymmetric quantum-size structure in a magnetic field parallel to the quantum-well layers - a transverse magnetoresistance which is asymmetric in the field (there can even be a change in sign) and is observed in the case that the structure has a built-in lateral electric field. A model of the effect is proposed. The observed asymmetry of the magnetoresistance is attributed to an additional current contribution that arises under nonequilibrium conditions and that is linear in the gradient of the electrochemical potential and proportional to the parameter characterizing the asymmetry of the spectrum with respect to the quasimomentum.

cond-mat.mtrl-sci↗