Physical properties of n-CdGeAs2 single crystals prepared by low-temperature crystallization

The first results obtained in studies of the temperature dependences of electrical conductivity and Hall constant of n-CdGeAs₂ single crystals prepared by low-temperature crystallization are reported. It has been established that the method developed permits growing single crystals with a free-electron concentration ⋍(1−2)×10¹⁸ cm⁻³ and a Hall mobility ⋍10000 cm²/(Vs) at T=300 K. It is shown that the temperature dependence of Hall mobility exhibits a behavior characteristic of electron scattering by lattice vibrations, whereas below 150 K a deviation from this law is observed to occur evidencing an increasing contribution of static lattice defects to scattering. The Hall mobility in the crystals prepared was found to reach ⋍36000 cm²/(Vs) at 77 K. Photosensitive heterojunctions based on n-CdGeAs₂ single crystals were prepared. The spectral response of the photosensitivity of these structures is analyzed. It is concluded that this method is promising for preparation of perfect CdGeAs₂ crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1190–1193 (July 1999)     

Galvanomagnetic properties of Hg1−x MnxTe1−y Sey semimagnetic semiconductors

The galvanomagnetic properties of single crystals of the semimagnetic semiconductors Hg_1−x Mn_xTe_1−y Se_y with 0.01<y<0.1 and x=0.05 and 0.14 in the temperature range 4.2–300 K are investigated. The features of the temperature dependence of the Hall coefficient R _H and the complicated behavior of R _H in a magnetic field are attributed quantitatively to the existence of three groups of current carriers, viz., electrons and two types of holes, for which the temperature dependences of the densities and mobilities are obtained. A transition from p-type to n-type conductivity is observed as the Se content is increased, and the negative magnetoresistance simultaneously gives way to positive magnetoresistance. Zh. éksp. Teor. Fiz. 112, 1809–1815 (November 1997)     

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.     

Propagation of nonequilibrium phonons in ferroelectric ceramics and single crystals

Phonon transport in ferroelectric ceramics and single crystals has been experimentally investigated. Our measurements indicate that, in the temperature range 1.7–3.8 K studied, the effective phonon diffusion coefficient behaves as D _eff∞T ⁻⁵ in ferroelectrics with a broadened phase transition. This experimental dependence is in accord with the presence of a plateau in the thermal conductivity of such materials. The scattering by domain walls in BaTiO₃ single crystals has been identified, and our results are in quantitative agreement with calculations. Zh. éksp. Teor. Fiz. 115, 624–631 (February 1999)     

Thermal conductivity of single crystals of Ca1 ? xErxF2 + x and Ca1 ? xTmxF2 + x solid solutions

The thermal conductivity of single crystals of Ca_{1 − x} Er _x F_{2 + x} (x = 0.01, 0.05, 0.07, and 0.10) and Ca_{1 − x} Tm _x F_{2 + x} (x = 0.02, 0.04, and 0.06) solid solutions is studied in the temperature ranges 50–300 and 298–673 K. With increasing content of rare-earth elements, the behavior of thermal conductivity in these solid solutions changes from the characteristic of defect single crystals to glasslike. The concentration dependences of thermal conductivity for the two systems differ insignificantly.     

Thermal conductivity of single-crystal ZrO2-Y2O3 solid solutions in the temperature range 50–300 K

The thermal conductivity of ZrO_2−x Y₂O₃ single crystals (x = 0.5, 1.5, 2.0, 2.5, 3.0, 8.0 mol %) has been studied experimentally in the temperature range 50–300 K. The influence of high-temperature annealings on the thermal conductivity has been analyzed.     

Mechanisms of charge transfer and magnetoresistance in CuInSe<Subscript>2</Subscript>

The electrical conductivity and magnetoresistance effect of n and p types of CuInSe₂ single crystals were studied within the temperature range of 4.2–300 K and within magnetic fields of up to 6 T. It was found that the hopping mechanism of conductivity dominates in the temperature range of 4.2–100 K. A peculiarity of electrical conductivity behavior, accompanied by a change in the magnetoresistance sign in the vicinity of T ≈ 60 K, was revealed.     

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)     

Proposal on the measurements of d–p elastic scattering analyzing powers at 0.3–2.0 GeV at the Internal Target Station of the Nuclotron

A new high-energy beam polarimeter is proposed for Nuclotron, which uses the Internal Target Station (ITS). This polarimeter based on a measurement of asymmetry for the d–p elastic scattering will allow one to measure simultaneously both vector and tensor components of deuteron beam polarization. For this purpose an analyzing powers measurement for the d–p elastic scattering at energies T _d = 0.88–2 GeV is proposed. The precise measurements of the deuteron analyzing powers over the energy range T _d = 300–2000 MeV can give an irreplaceable clue to the study of the spin dependence of three nucleon forces. The text was submitted by the authors in English.     

Manganese energy level in <Emphasis Type="Italic">p</Emphasis>-InAs〈Mn〉 under pressure

Resistivity ρ and the Hall coefficient R _H at atmospheric pressure in the temperature range of 77–400 K and the dependences of these parameters (ρ(P) and R _H(P)) and magnetic susceptibility (χ(P)) on hydrostatic pressures of up to P ≤ 7 GPa at 300 K in p-InAs〈Mn〉 single crystals was investigated. The baric coefficients of the ionization energy of Mn impurity centers and the pressure dependence of the dielectric constant ɛ(P) were determined.     

Ion transport in the anion-deficient nonstoichiometric phases La0.95(Ba1−x Srx)0.05F2.95 (0⩽x⩽1)

The fluoride-ion conductivity of the nonstoichiometric tysonite phases La_0.95(Ba_1−x Sr_x)_0.05Fe_2.95 (0⩽x⩽1) is investigated by impedance spectroscopy. Electrophysical measurements are performed in the frequency range 5–5×10⁵ Hz and temperature range 300–700 K. A discontinuity is observed in the temperature dependence of the conductivity at T _c=410–430 K. The behavior of the temperature dependence of the electrical conductivity is explained within a transport model taking into account the migration of fluoride ions between different inequivalent structural sites. The maximum value of the conductivity at room temperature (293 K) is 2×10⁻⁴ Ω⁻¹ cm⁻¹ for the solid solution La_0.95Sr_0.05F_2.95. The fluorine-ion conductivity in La_0.95(Ba_1−x Sr_x)_0.05F_2.95 single crystals is almost an order of magnitude larger than the value for the commercial solid electrolyte La_0.992Eu_0.008F_2.992 (a fluorine-selective membrane) having a tysonite structure. Fiz. Tverd. Tela (St. Petersburg) 40, 658–661 (April 1998)     

Thermal conductivity of single crystals with a fluorite structure: Cadmium fluoride

The thermal conductivity of Ca, Sr, Ba, and Cd difluoride single crystals and the CdF₂ samples doped by 3 mol % NdF₃, 15 mol % HoF₃, and 10 mol % ErF₃ has been studied using the method of steady longitudinal heat flow in the temperature range 50–300 K. The thermal conductivity of the matrices of these compounds decreases in the order CaF₂-SrF₂-BaF₂-CdF₂. The temperature dependences of the phonon mean free path for these crystals have been calculated from experimental data and exhibit different behaviors. It has been assumed that the intense phonon scattering observed in the undoped CdF₂ sample is caused by the specific features of the processes of phonon-phonon scattering. The formation of heterovalent solid solutions of cadmium difluoride and rare-earth trifluorides is accompanied by a drastic decrease in the thermal conductivity and a change in its character from that typical of dielectric single crystals to that typical of glassy materials.     

Photoelectric properties of Al/In<Subscript>2</Subscript>Se<Subscript>3</Subscript> Schottky barriers

We used directional solidification of the melt to grow single crystals of the binary compound In2Se3 and then determined the composition of the crystals obtained and their structure. From Hall effect measurements, we determined the type of conductivity, the concentration, and the Hall mobility of the free electrons in the single crystals obtained, on which we developed photosensitive Al/In₂Se₃ Schottky barriers for the first time and determined their photoelectric properties. We established that the indicated barriers can be used to design broadband optical photoconverters based on In₂Se₃ single crystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 425–427, May–June, 2008.     

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C₆₀ crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C₆₀, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ _p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ _p∼d=1.4 nm at temperatures T>200 K and reaches values λ_p∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)     

Processing and properties of some II–IV–V2 amorphous and crystalline semiconductors

Experimental investigations on the preparation, characterization, and properties of several bulk and thin-film ternary alloys based on the chalcopyrite II–IV–V₂ semiconductors are presented. Rapid melt solidification in vacuum-sealed fused-silica tubes resulted in amorphous alloy formation in almost all compositions in the system CdGeAs₂-CdSiAs₂. ZnGeAs₂-CdGeAs₂ alloys showed very limited tendency toward amorphous phase formation. Phase separation, crystallization and electrical properties were studied for amorphous Cd-Ge-Si-As alloys by thermal analysis, transmission electron microscopy, X-ray diffraction, and Hall measurements. Rapid crystallization resulted in a reversal of conductivity type (p-to-n or vice versa). Crystallized glassy alloys showed room-temperature mobility of 64 cm²/V s, and a hole concentration of 1020 cm^–3. The p-to-n change in conductivity type upon amorphous-to-crystal transformation suggests that these alloys can be used to fabricate p-n junction devices by surface crystallization of the amorphous phase.     

Crystal growth,electrical and photophysical properties of Tl<Subscript>2</Subscript>S layered single crystals

The Tl₂S compound was prepared in a single crystal form using a special local technique, and the obtained crystals were analysed by X-ray diffraction. For the resultant crystals, the electrical properties (electrical conductivity and Hall effect) and steady-state photoconductivity were elucidated in this work. The electrical measurements extend from 170 to 430 K, where it was found that σ _⊥ = 8.82 × 10⁻⁵ Sm⁻¹ when current flow direction makes right angle to the cleavage plane of the crystals. In the same range of temperatures, it was found that σ _‖ = 4.73 × 10⁻⁵ Sm⁻¹ when the current flow is parallel to the cleavage plane. In line with the investigated range of temperatures, the widths of the band gaps were calculated and discussed as also the results of the electrical conductivity and Hall effect measurements. In addition, the anisotropy of the electrical conductivity (σ _⊥/σ _‖) for the obtained crystals was also studied in this work. Finally the photosensitivity was calculated for different levels of illumination as a result of the photoconductivity measurements, which showed that the recombination process in Tl₂S single crystals is a monomolecular process.      

Energy filtration of charge carriers in a nanostructured material based on bismuth telluride

The dependences of the electrical conductivity and thermopower on the size of grains in a nanocrystalline material based on Bi₂Te₃-Sb₂Te₃ solid solutions of the p type have been investigated theoretically and experimentally. The relaxation time in the case of hole scattering by nanograin boundaries in an isotropic polycrystal has been calculated taking into account the energy dependence of the probability of tunneling of charge carriers and the dependence of the scattering intensity on the nanograin size L _n . A decrease in the probability of boundary scattering with an increase in the energy of charge carriers leads to an increase in the thermopower. The dependences of the thermopower and electrical conductivity on the nanograin size, which have been obtained taking into account the boundary scattering and scattering by acoustic phonons, are in good agreement with experimental data. For the material under consideration, the thermopower coefficient increases by 10–20% compared to the initial solid solution at L _n = 20–30 nm. This can lead to an increase in the thermoelectric figure of merit by 20–40%, provided that the decrease in the electrical conductivity and the decrease in the lattice thermal conductivity compensate each other. Despite the absence of a complete compensation, it has been possible to increase the thermoelectric figure of merit for the samples under investigation to ZT = 1.10–1.12.     

Magnetic and thermal properties of CuFeS<Subscript>2</Subscript> at low temperatures

The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS₂, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS₂ sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.     

Effect of intercalation by silver on the charge-density-wave state in TiSe2

The temperature dependence of electrical conductivity σ (77–300 K) and magnetic susceptibility χ (2–300 K) of Ag_xTiSe₂ in the Ag_1/4TiSe₂ phase has been studied in order to determine the possibility of preserving the charge-density-wave state in silver-intercalated TiSe₂. The behavior of χ and σ in this phase is compared with that of the starting compound TiSe₂. Fiz. Tverd. Tela (St. Petersburg) 40, 2165–2167 (December 1998)     

Hall effect and negative magnetoresistance in thin crystals of NbSe<Subscript>3</Subscript>

We have measured the Hall effect and the transverse magnetoresistance in NbSe₃ single crystals. In the liquid helium temperature range we observed an absolute negative magnetoresistance (NMR) — the value of the resistance under magnetic field being much lower than that at zero field — in NbSe₃ single crystals with a thickness less than 5 μm with the magnetic field oriented in the (b, c) plane. We show that this NMR effect is observed in the magnetic field range in which the Hall constant changes its sign. The results are qualitatively explained by the change of the surface scattering contribution to the magnetoconductance in the magnetic field range near the Hall voltage zero crossing.