Alexei Bozhko

Premartensitic transition in Ni2+xMn1-xGa Heusler alloys

The temperature dependencies of the resistivity and magnetization of a series of Ni2+XMn1-XGa (X = 0 - 0.09) alloys were investigated. Along with the anomalies associated with ferromagnetic and martensitic transitions, well-defined anomalies were observed at the temperature of premartensitic transformation. The premartensitic phase existing in a temperature range 200 - 260 K in the stoichiometric Ni2MnGa is suppressed by the martensitic phase with increasing Ni content and vanishes in Ni2.09Mn0.91Ga composition.

Space-charge-limited current in hydrogenated amorphous carbon films containing silicon and oxygen

Abstract Evolution of the dark room-temperature current–voltage characteristics of hydrogenated amorphous carbon films containing silicon and oxygen with deposition energy growth was investigated at applied electric fields up to 6×10 5 V/cm. It was shown that the character of current voltage dependences is influenced by the deposition energy, which is determined by the value of the self-bias voltage, V sb , varied in the range from −100 to −1400 V, and is described in terms of the space-charge-limited current in the presence of bulk traps, presumably having an exponential energy distribution. In films deposited at moderate values of self-bias voltage (−400 to −800 V) the trap-filled limit mode of the Gaussian-distributed deep trap set in the electric fields 5×10 3 –10 5 V/cm was observed. At electric fields exceeding 3×10 5 V/cm, phonon-assisted tunneling through the reduced electric-field potential barrier of the trap enhances the space-charge-limited current. The absence of thermal activation of the carriers at the mobility threshold caused by the reduction in trap depth in the electric field suggests deviation of the trap potential from the Coulomb one.

Electroconductivity of amorphous carbon films containing silicon and tungsten

Abstract Data on the electron transport in insulating amorphous carbon films containing silicon and tungsten atoms are presented. The films were grown by plasma-assisted chemical vapour deposition. The tungsten atoms were introduced into the growing film from the hot filament. The structure of these films consists of an atomic-scale composite of carbon and silicon random networks. The carbon network is stabilized by hydrogen and the silicon network is stabilized by oxygen. Such self-stabilized C-Si amorphous structures form an ideal matrix for the introduction of metals, in particular transition metals. These metals are distributed as separate atoms or as separate disordered networks. The addition of metal atoms allows the electrical resistivity to be changed from 1014–1015 Ω cm down to 10−4 Ω cm in a controlled way. Various electron transport mechanisms and percolation phenomena are observed in the diamond-like nanocomposite. It was found that in the temperature range 150–350 K the electroconductivity is of thermo-activated Pool-Frenkel nature and is characterized by two values of activation energy, 0.32 and 0.2 eV. The observed saturation of the current vs. temperature on increasing the electric field is connected with the predominance of tunnelling effects (activationless hopping conductivity and direct tunnelling at the mobility threshold). The week magnetoresistance proportional to the square of the magnetic field can be explained by the small value of localization radius at the Fermi level.

Electron transport in W-containing amorphous carbon?silicon diamond-like nanocomposites

The electron transport in amorphous hydrogenated carbon?silicon diamond-like nanocomposite films containing tungsten over the concentration range 12?40?at.% was studied in the temperature range 80?400?K. The films were deposited onto polycrystalline substrates, placed on the RF-biased substrate holder, by the combination of two methods: PECVD of siloxane vapours in the stimulated dc discharge and dc magnetron sputtering of tungsten target. The experimental dependences of the conductivity on the temperature are well fitted by the power-law dependences over the entire temperature range. The results obtained are discussed in terms of the model of inelastic tunnelling of the electrons in amorphous dielectrics. The average number of localized states n in the conducting channels between metal clusters calculated in the framework of this model is characterized by the non-monotonic dependence on the tungsten concentration in the films. The qualitative explanation of the results on the basis of host carbon?silicon matrix structural modifications is proposed. The evolution of the carbon?silicon matrix microstructure by the increase in the tungsten concentration is confirmed by the Raman spectroscopy data.

Electron Transport in Amorphous Carbon–Silicon Nanocomposites Containing Nb

The evolution of the electron transport in Nb-containing hydrogenated amorphous carbon–silicon nanocomposite films with an increase in niobium concentration in the range of 13–33 at.% was studied in the wide temperature range of 80–400 K. The films were deposited onto ceramics substrates by the combination of DC magnetron sputtering of a Nb target and decomposition of siloxane vapors in a stimulated DC discharge. The model of inelastic tunneling of electrons in amorphous dielectrics was applied to analysis of the experimental conductivity–temperature dependences. It was shown that the average number of localized states in the potential barriers between metal clusters is not greater than 2 and nonmonotonically depends on Nb concentration in the investigated films. This dependence can be explained in terms of the modifications both of metal and carbon phase structures by increasing metal concentration. The Raman spectroscopy data demonstrate that these structural transformations take place in the carbon phase of the host matrix as the Nb concentration exceeds 23–25 at.%. An increase in the average size of sp2 clusters from 0.7–0.9 up to 1.1–1.3 nm is observed.

Cluster superconductivity in diamond-like carbon-silicon nanocomposites containing tungsten

It is shown that the nonhomogeneous structure of the superconducting phase in the diamond-like carbon-silicon nanocomposites containing tungsten with concentration close to the metal-insulator transition is responsible for the perculiarities in the character of the superconducting resistive response. The observed nontrivial quasireentrant resistive transitions can be explained in terms of the phase coherence destruction between the superconducting grains and by the renewal of superconducting phase generation at lower temperatures defined by the presence of the two scales of inhomogeneity in the investigated films. The resistance-current characteristics in the vicinity to the superconducting transition and magnetic measurements do not contradict the qualitative model proposed.

Superconducting critical fields of diamond-like films containing tungsten

The superconducting critical fields Hc2 of a some diamond-like films incorporating tungsten (WxC1−xSi, 0.22 <x< 0.28) having critical temperatures between 2 and 4 K have been determined from resistivity measurements. For the field perpendicular to the film Hc2 increases continuously as the temperature is decreased. At the lowest temperatures, the critical fields are greater than those expected for a normal type II superconductor. The probable origin of this effect lies in the amorphous nature of the films.

Superconducting property in tungsten containing amorphous carbon composite

Superconductivity in tungsten-containing carbon-oxide film was reported. The film with 500 nm thickness was deposited onto polycrystalline silicon-oxides using chemical vapor deposition and co-sputtering of tungsten metal target. The structure of the film was investigated by Raman spectroscopy and X-ray diffraction measurements and the results indicated that the structure of the film is amorphous. The temperature dependence on resistivity was measured in the temperature range of 2-300 K. At the temperature of around 4.2 K resistive superconducting transition was observed. In order that the tungsten oxide and tungsten carbide with which super-conductive transition temperature is different formed the finite cluster group, it can be understand by percolation theory that the superconducting phase of the total system appears. The diamagnetism was observed below 3.8 K, which is consistent with resistive superconducting transition.

Structure of the Superconducting Phase in Diamond‐Like Carbon Films Containing Tungsten

The structure of the superconducting phase in the diamond-like films containing tungsten is discussed. It is shown that when the tungsten concentration in the films approaches the metal-insulator transition from the metal side the superconducting resistive response takes a two-step form. It was found that there exists a sharp increase of the resistance in the immediate vicinity of the second step of the superconducting transition. The dependence of the resistivity on transport current across the films near the maximum of the resistance is described. A qualitative model of the superconducting phase structure which can lead to the observed resistive response based on the existence of two scales of inhomogeneity in the investigated films is proposed and discussed.

Transport in tungsten-containing diamond-like films

Abstract Electron transport in tungsten-containing diamond-like films (DLF) grown by combined plasma-enhanced chemical vapour deposition and magnetron sputtering was investigated. It was shown that at room temperature for dielectric films with a metal concentration of 1 × 10 16 –1 × 10 18 /cm 3 , the electron conductivity is in agreement with the Pool-Frenkel model and tends to be activationless following the Shklovskii mechanism or the Fowler-Nordheim one at low temperatures. Transition into a low-resistant state was observed, as an electric field of about E c = 1 × 10 5 V/cm was applied. As the field is more than the E c the resistance depends on neither temperature nor electric field. In the strong electric and magnetic fields the non-ohmic hopping conductivity of both Mott and Shklovskii types was observed in the DLF samples with tungsten concentration as low as 10 16 –10 18 /cm 3 . Using the experimental data the parameters of hopping conductivity theory were evaluated. The data of low-frequency capacitance measurements in insulating DLF are also presented. Possible mechanisms of obtained results are discussed.