Effect of γ irradiation on the low-temperature electrical conductivity and dielectric properties of TlGaSe2 crystals

Electrical conductivity and dielectric properties of single-crystal TlGaSe₂ have been studied as a function of γ irradiation dose in the 100–280 K range including the existence of an incommensurate phase. Anomalies in the form of maxima have been observed in the σ=f(T), tan δ=f(T), and ɛ=f(T) curves at the points of transition from the paraphase to incommensurate (IC) phase, T _i, and from the IC to commensurate phase, T _c. The increase in the quantities σ, tan δ, and ɛ observed initially with increasing irradiation dose is followed by their strong decrease and disappearance of the anomalies. It has been established that γ irradiation does not affect the phase transition temperatures T _i and T _c. Fiz. Tverd. Tela (St. Petersburg) 40, 1328–1331 (July 1998)     

Electrical and magnetic properties of titanium diselenide intercalated with cobalt

This paper discusses experimental studies of the temperature dependence of the magnetic susceptibility, electrical conductivity, and Seebeck coefficient of the compounds Co_xTiSe₂ (0.1⩽x⩽0.5), along with the structural behavior exhibited by these compounds. For Co_0.5TiSe₂, the observed temperature dependence of the magnetic susceptibility is characteristic of an antiferromagnet with a Néel temperature of 510 K. The data obtained on the structural characteristics and physical properties of the intercalated phases are interpreted using a model that assumes the formation of bands of localized states near the Fermi level. Fiz. Tverd. Tela (St. Petersburg) 39, 1618–1621 (Sepember 1997)     

Electronic conductivity of mechanochemically synthesized nanocrystalline Ag1−x Cu x I system using DC polarization technique

A study of electronic conductivity using the DC polarization technique has been carried out for AgI and Ag_1−x Cu _x I (where x=0.05, 0.15, 0.25) solid solutions over a range of temperatures from 300 K to 473 K. A diode-like current-voltage characteristics arises from microscopic p-n junctions an enhanced electronic conductivity of the order of 10⁻³A is observed for undoped AgI and Cu-doped AgI. Activation energies (E _a) for electronic conductivity obtained from log σ (Ω⁻¹ cm⁻¹) vs. 1000/T (K⁻¹) were 0.48, 0.6, 0.74 and 1.01 eV for AgI, Ag_0.95Cu_0.05I, Ag_0.85Cu_0.15I and Ag_0.75Cu_0.25I solid solutions respectively. The near-twofold increase in activation energy (1.01 eV) observed upon 25% Cu doping is due to the substantial concentration of current carriers/holes injected by Cu while replacing Ag⁺ in AgI. Based in part on the paper presented at first National Conference on Nanoscience and Technology, National Chemical Laboratory, Pune, 7–8 March 2005.     

Calorimetric study of phase transitions in the perovskite BaCeO3

Differential scanning calorimetry was used to study phase transitions (PT) in the perovskite BaCeO₃. It is shown that its phase state is determined by a second-order λ transition at T _tr=520–540 K and a first-order δ transition at T _tr=600–670 K. Differences in PT parameters between ceramic and fused BaCeO₃ have been established. Fiz. Tverd. Tela (St. Petersburg) 40, 2109–2112 (November 1998)     

A systematic study of J/ψ suppression in cold nuclear matter

Understanding the effects of cold nuclear matter on J/ψ production is a key requirement for interpreting the J/ψ suppression reported in heavy-ion collisions. Based on a Glauber model, the J/ψ–nucleon inelastic cross section is determined from a statistical analysis of the J/ψ world data on nuclear targets. The global fit of all data gives σ_J/ψN = 3.4 ± 0.2 mb, significantly smaller than previous estimates, yet the χ² of the fit is pretty large, χ²/ndf=1.5. A similar value, σ_J/ψN = 3.5 ± 0.2 ± 1.7 mb, is obtained when the De Florian–Sassot modifications of the nuclear parton densities are included in the analysis, although we emphasise that the present uncertainties on gluon (anti-) shadowing do not allow for a precise determination of σ_J/ψN. Finally, the possible energy dependence of the J/ψ–nucleon inelastic cross section is investigated within this framework. No significant energy dependence of the J/ψ–N interaction is observed. PACS  25.75.-q; 14.40.Gx; 24.85.+p     

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρ _xx and transverse ρ _xy resistivities and magnetic susceptibility χ _ac of La_0.8Sr_0.2MnO₃ single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρ _xy ∼ρ _xx , which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χ _ac. As a result, the curve of ρ _xy versus ρ _xx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρ _xx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽T _C . The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials. Zh. éksp. Teor. Fiz. 116, 671–683 (August 1999)     

Thermal conductivity of (VO)2P2O7 single crystals

Thermal conductivity ϰ of single-crystal (VO)₂P₂O₇ has been studied within the 4–300 K range. A break was found in the ϰ(T) relation about 200 K, in the region of the transition from diffuse antiferromagnetic ordering (200–4 K) to a classical paramagnet (T=200–300 K). In the low-temperature domain (4–200 K), one may expect an additional contribution to ϰ(T) from the magnon component of thermal conductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 2093–2094 (November 1998)     

Electrical properties of amorphous (Co45Fe45Zr10)x(Al2O3)1?x nanocomposites

The electrical properties of (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_1−x granular nanocomposites have been studied. The concentration dependences of electrical resistivity are S-shaped (in accordance with the percolation theory of conduction) with a threshold at a metallic component concentration of ∼41 at. %. An analysis of the temperature behavior carried out in the range 300–973 K revealed that structural relaxation and crystallization of the amorphous phase are accompanied by a decrease in the electrical resistivity of the composites above the percolation threshold and by its increase below the percolation threshold. For metallic phase concentrations x<41 at. %, variable range hopping conduction over localized states near the Fermi level was found to be dominant at low temperatures (77–180 K). A further increase in temperature brings about a crossover of the conduction mechanism from Mott’s law ln(σ) ∝ (1/T)^1/4 to ln(σ) ∝ (1/T)^1/2. A model of inelastic resonance tunneling over a chain of localized states of the dielectric matrix was used to find the average number of localized states involved in the charge transport between metallic grains. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 11, 2004, pp. 2076–2082. Original Russian Text Copyright ? 2004 by Kalinin, Remizov, Sitnikov.     

Structural and magnetic phase transitions in La0.9Sr0.1MnO3

A study is made of phase transitions in doped La_0.9Sr_0.1MnO₃ compounds using combined x-ray, electrical, and magnetic measurements. Structural phase transitions are observed accompanied by a change in the cell volume at temperatures of 100–110 K and 300–340 K. These structural changes are found to be related to different contributions of the rhombic Jahn-Teller Q ₂ mode to the formation of the crystal lattice. The structural transition at 100–110 K is accompanied by distinctive magnetic and electrical properties. The data are analyzed in detail. Fiz. Tverd. Tela (St. Petersburg) 41, 1064–1069 (June 1999)     

Electrical transport properties of CuWO4

The temperature dependence of the electrical conductivity, thermoelectric power and dielectric constant of the antiferromagnetic CuWO₄ have been studied in the temperature range 300–1000 K. The conductivity results can be summarised by the equations σ_I=6.31 × 10⁻³ exp (−0.29 eV/kT) ohm⁻¹ cm⁻¹ in the temperature range 300–600 K and σ_II=3.16 × 10⁵ exp (−1.48 eV/kT) ohm⁻¹ cm⁻¹ between 600 K and 1000 K. The thermoelectric power can be expressed byθ=[− 1.25 (10³/T) + 3.9] mV/K. Initially dielectric constant increases slowly but for high temperatures its increase is fast.     

Influence of impurities on the low-temperature deformation of nanocrystalline copper

The influence of ZrO₂ particles on the low-temperature deformation of nanocrystalline copper produced by strong plastic deformation is investigated using equichannel angular pressing. A comparison is made between the deformation characteristics in tension and compression in the temperature range 4.2–400 K, measured for copper and the composite Cu:0.3 vol. % ZrO₂. It is shown that within 4.2–200 K the yield point σ _sm of the composite is higher than that for copper, attaining 680 MPa at 4.2 K, then the yield points are close in value up to room temperature, and diverge again as the temperature is raised. Possible causes of the dissimilar influence of an impurity on the strength and plasticity characteristics of nanocrystalline copper in various temperature intervals are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1639–1641 (September 1998)     

Resistance and magnetic susceptibility of superconducting lead embedded in nanopores of glass

This paper reports on a study of the resistance and differential magnetic susceptibility χ _ac of lead embedded in nanosized glass pores with a diameter of ∼7 mm, which was performed at temperatures of 6–300 K and magnetic fields of up to 6 T. The field dependence of the resistance R(H) and the temperature dependences of the real, χ″(T), and imaginary, χ″(T), parts of magnetic susceptibility reveal indications of superconducting phase transitions associated with the volume and surface superconductivity of Pb nanopar ticles. The measurements of the field dependence of resistance have been used to set up the H _c -T _c phase diagram and to carry out a comparison with the study of the heat capacity performed on the same samples.     

On the nature of the Faraday effect in the rare-earth ortho-aluminate TbAlO3

The magnetic susceptibility χ, the absorption and luminescence spectra, and the Verde constant V along different crystallographic axes of the rhombic crystal TbAlO₃ have been investigated experimentally in the temperature interval 78–300 K. It is found that, in contrast to the magnetic susceptibility χ measured along the [110] axis, the Verde constant V varies in inverse proportion to the temperature in the given range. It is shown that this temperature dependence of the Verde constant measured along the [110] axis is connected with the absence of a contribution to the Faraday rotation of the van Vleck mechanism (in an external field H) of “mixing” of the states of the rare-earth ion Tb³⁺. From a comparison of the optical and magnetic measurements, we have determined the wave functions and magnitudes of the Stark intervals between the lower Stark sublevels for the ⁷ F ₆ ground multiplet of the Tb³⁺ ion in the ortho-aluminate TbAlO₃. Fiz. Tverd. Tela (St. Petersburg) 41, 2047–2052 (November 1999)     

The invar effect and phase transitions in Cs2ZnI4 crystals

An in situ x-ray diffraction study of Cs₂ZnI₄ crystals performed in the 4.2–300 K temperature range is reported. The lattice parameter measurements have revealed three anomalies corresponding to phase transitions. The thermal expansion coefficient along the c axis was found to vanish in the region of incommensurate and commensurate modulated phases, 120–96 K (the invar effect). A possible crystallographic model relating modulated atomic displacements to the invar effect is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 137–142 (January 1999)     

Vibrational wave packet dynamics in the silver tetramer probed by NeNePo femtosecond pump-probe spectroscopy

We present results on the ultrafast dynamics of mass-selected neutral Ag₄ clusters using NeNePo (negative ion - neutral - positive ion) femtosecond pump-probe spectroscopy. One-color pump-probe spectra of the Ag₄ ^-/Ag₄/Ag₄ ⁺ system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory structure is attributed to vibrational wave packet dynamics in an excited “dark" state of neutral Ag₄. A dominant 740 fs wave packet period as well as wave packet dephasing and rephasing are observed in the spectra. Fourier analysis of the spectra yields a group of frequencies centered around 45 cm^-1 and an anharmonicity χ _e2νχ _eχ _e of 2.65 cm^-1 for the active vibrational mode. Received 30 November 2000     

Ionic switching in mehanochemically synthesized Ag2S-Sb2S3 amorphous superionic conductor

Highly ionically conducting amorphous samples of Ag₂S·xSb₂S₃ (x=0.15−0.40) are mechanochemically synthesized and characterized by X-ray diffraction (XRD), DTA/DSC, electrical and electrochemical methods. The electrical conductivity measurements are carried out as a function of frequency (5 Hz – 13 MHz) and temperature (300 – 380 K). While the ac conductivity of x=0.40 composition obeys the well-known power law σ (ω)=σ (0) + A ωⁿ over the whole frequency range, the x=0.30 sample obeys the above relation only at lower temperatures. The compositions x=0.20 and 0.15 do not follow the famous dispersion relation at higher (MHz) frequencies. The conductivity isotherms of these a-SICs do not superimpose upon application of Summerfield scaling. The scaling behaviour and the variation of ac conductivity with frequency for x=0.20 and 0.15 are explained in terms of switching from the predominantly ionic (at lower frequencies) to the predominantly electronic conduction (at higher frequencies). Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Channai, India, Nov. 28–30, 2003.     

Magnetic susceptibility anisotropy and low-dimensional antiferromagnetism of CuO

The anisotropy of the magnetic susceptibility χ and the influence of oxygen vacancies in CuO single crystals on it are investigated. The temperature dependences of χ(T) along the a, b, and c axes in the range 60<T<600 K and the behavior of the field dependence of the magnetization σ(H) above and below the Néel temperature T _N are plotted for a crystal before and after heat treatment. The χ(T) curves have the form characteristic of low-dimensional systems, which become three-dimensional when the temperature is lowered. The character of the χ(T) curves remains unchanged after annealing. Oxygen vacancies have practically no influence on the a-axis magnetic susceptibility, but they alter the absolute values of the b-and c-axis susceptibilities. The significant effects of reducing the oxygen concentration include a decrease in the magnitude of the low-temperature anomaly (increase) in χ and an increase in the minimum value of χ. The results of the calculations of the exchange parameter I/k and the g factor are discussed in terms of the Heisenberg and Ising models for a one-dimensional system. Zh. éksp. Teor. Fiz. 113, 1026–1035 (March 1998)     

Magnetic and band parameters of (3HgS)1–x(Al2S3)x (x = 0.5) crystals doped with manganese

Investigations of magnetic susceptibility (χ) of (3HgS)_1–x (Al₂S₃) _x (x = 0.5) crystals doped with manganese investigated by the Faraday method in the range of Т = 77–300 K and Н = 0.25–4 kOe demonstrate that peculiar magnetic properties can be due to the presence of clusters of the Mn–S–Mn–S type in these crystals. Based on the investigation of kinetic coefficients of the crystals performed for Т = 77–300 K and Н = 0.5–5 kOe, it is established that the crystals demonstrate conductivity of the n-type, and their electric conduction only very slightly depends on the temperature and shows a maximum, the Hall coefficient is independent of the temperature, while the temperature dependences of mobility exhibit a maximum. Using the concentration dependence of the effective electron mass at the Fermi level, the width of the forbidden band, the matrix element of interatomic interaction, and the effective mass on the conduction-band bottom are obtained. Based on investigations by optical methods, the presence of direct optical interband transitions in the crystals is established and the value of the optical forbidden band (at Т = 300 K) is determined.