D.c. galvanomagnetic properties in a heavily compensated single crystal of n-type CdTe

Experimental data on d.c. galvanomagnetic properties, namely the Hall coefficientR _H, the low-electric-field d.c. conductivityσ and the Hall mobilityμ _H, in a heavily compensated single crystal of n-type CdTe in the temperature range 77 to 300 K have been analysed, firstly on the basis of a simple one-band model involving normal free-electron conduction, and secondly on the basis of a two-band model involving normal free-electron conduction along with impurity-band conduction. The analysis provides evidence for a significant contribution of impurityband conduction to the transport phenomena at temperatures below ～ 150 K. This conclusion is further substantiated by transverse magnetoresistance measurements and the observed dependence of d.c. conductivity on the electric field. From the analysis of these data, the average hopping distanceR in the impurity band is calculated, and it is found to increase with decrease of temperature. The relative contributions of normal free-electron conduction and impurity-band conduction to the d.c. galvanomagnetic properties are estimated at different temperatures.     

A new method for point-by-point determination of the electron-phonon scattering rate in metals

Experimental results on the temperature dependence of the amplitude of time-of-flight effect signals in silver are presented. It is shown that it is possible to determine the electron-phonon scattering rate in the form of, on the one hand, an average over a certain orbit and, on the other hand, a point-by-point determination on the Fermi surface of the metal considered. The anisotropy of the electron-phonon scattering rate v(k) is obtained for the {100} plane from measurements of the amplitude of time-of-flight signals for two silver samples with the normals n100 and n110, respectively, for the magnetic field direction B100. From the analysis of the data, combined with results obtained earlier by means of the radiofrequency size effect, it is suggested that the electron-phonon scattering rate is frequency dependent; however, more experiments are needed to verify this conclusion.Supported by Naturvidenskabelige Forskningsrad, Denmark.     

Structural and galvanomagnetic properties of thin antimony films

The structural and electrical properties of thin antimony films evaporated onto glass substrates were studied. The influence of the deposition conditions (in particular the substrate temperature and the evaporation rate) on the condensation mechanism and the shape, size and orientation of the crystallites was determined. The structure of the films was studied by transmission electron microscopy. The degree of texturization of the films was also determined. Measurements of the electrical resistivity were carried out over the temperature range 4.2–300 K; the Hall effect and the magnetoresistance were measured at 77 and 300 K. From these data the electrical conductivity σ, the temperature coefficient of resistivity β, the Hall coefficient R_H, the mobilities μ_e and μ_h of electrons and holes and their respective concentrations (n = p) were determined for films of various structures and thicknesses. The influence of the structure on the electrical properties of thin antimony films was observed.The results obtained were analysed on the basis of the Fuchs-Sondheimer and Mayadas-Schatzkes theories. The mean free path of the carriers as well as the scattering parameters p at the film surface and R at the grain boundaries were determined.     

Thermoelectric and galvanomagnetic properties of systems with mutually penetrating components

The effective coefficients of thermal conductivity, electrical conductivity, thermal emf, the effective Hall mobility, and the effective Hall coefficient are determined. The analytical dependences obtained are compared with experimental results for a Bi-Cd alloy.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 3, pp. 439–448, September, 1977.     

High-field superconducting properties of multifilamentary nbtihf superconductors

A Nb-61.7at%Ti-3at%Hf alloy was fabricated into multifilamentary wires and studied in terms of high field superconducting properties. The effects of precipitation heat treatment and cold working after the heat treatment on the superconducting properties were examined. To obtain high J_c in high fields, a process emerges where the heat treatment producing a maximum upper critical field, B_c2, is followed by severe cold working. The superconducting properties at 1.8 K were then investigated which indicated that the temperature scaling law may hold for this NbTiHf superconductor.     

Experimental verification and quantitative analysis of the temperature (phonon) breakdown phenomenon in the high-field magnetoresistivity of compensated metals

The transverse magnetoresistivity of tungsten single crystals with resistivity ratios of up to 80.000 was measured in the temperature range from 4.2 to 60 K and in magnetic fields up to 15 T. The experiments show that the temperature (phonon) breakdown effect occurs in compensated metals as a result of strong electron-phonon scattering. This leads to a transformation of closed electron orbits into quasi-open ones and, consequently, to anomalies of the galvanomagnetic properties. A large anisotropy of the magnetoresistivity (up to 83%), i.e. a dependence of resistivity on current direction, is observed in a cubic crystal. The power index n in the field dependence of the magnetoresistivity decreases and the characteristic intersheet gap temperatures T₀, which are determined from an exponential temperature dependence of the magnetoresistivity, formally depend on magnetic field. A detailed analysis of the experimental results allows us to explain all these anomalies by the temperature breakdown effect and to obtain new information on this phenomenon in metals at high magnetic fields.     

High-temperature mechanical properties of WC-Co hard metals (Review)

Experimental results are discussed of the laboratory investigations into high-temperature mechanical properties of WC-Co hard metals as functions of the microstructure parameters (cobalt content, average sizes of carbide grains and cobalt interlayers), binder composition (carbon and cubic carbide contents) and the thermal and force action found in the literature. The effect of high temperatures on deformation characteristics in bending, tension and compression has been analyzed. The problems of short-and long-time strengths (high-temperature strength) and thermomechanical fatigue are discussed. The revealed mechanisms of the high-temperature deformation are considered.     

Studies on photoelectrical,optical and galvanomagnetic properties of CdSe films

CdSe films were deposited onto glass substrates by the hot wall technique at a system pressure of 10^–6torr. The optimum deposition conditions for obtaining device grade films were determined. Photoelectrical and optical properties of the films deposited at the optimum conditions were studied. Information on the refractive index and absorption coefficient were derived from the above measurements and the data were analysed in the light of the existing theories. Direct transitions corresponding to band gaps of 1.7 and 1.92 eV were obtained. Measurement of mobility of CdSe films as a function of temperature indicated ionized impurity scattering to be predominant in our temperature range of measurements. The scattering parameter and the effective massm _e ^* of carriers was determined from thermoelectric power measurements.     

Morphology-tailored synthesis of tungsten trioxide (hydrate) thin films and their photocatalytic properties

Tungsten trioxide hydrate (3WO(3)·H(2)O) films with different morphologies were directly grown on fluorine doped tin oxide (FTO) substrate via a facile crystal-seed-assisted hydrothermal method. Scanning electron microscopy (SEM) analysis showed that 3WO(3)·H(2)O thin films composed of platelike, wedgelike, and sheetlike nanostructures could be selectively synthesized by adding Na(2)SO(4), (NH(4))(2)SO(4), and CH(3)COONH(4) as capping agents, respectively. X-ray diffraction (XRD) studies indicated that these films were of orthorhombic structure. The as-prepared thin films after dehydration showed obvious photocatalytic activities. The best film grown using CH(3)COONH(4) as a capping agent generated anodic photocurrents of 1.16 mA/cm(2) for oxidization of methanol and 0.5 mA/cm(2) for water splitting with the highest photoconversion efficiency of about 0.3% under simulated solar illumination.     

Aligned tungsten oxide nanowires on tungsten (100) substrates

WO₃ nanowires in body center cubic structure were grown on W (100) substrates by heating in an argon atmosphere. Scanning electron microscope and transmission electron microscope characterizations show the WO₃ NWs grew along the [100] crystallographic orientation and were aligned in three directions. The diameter of WO₃ NWs is in the range of several to 20 nm and the length is up to 1 µm. Field emission measurements show that the field emission current density can reach 1.8 mA/cm² under electrical field 10 V/µm and the turn-on field can be as low as 2.6 V/µm.     

Microstructure and properties of CVD tungsten carbide from tungsten hexafluoride and dimethyl ether

Tungsten carbide was deposited from tungsten hexafluoride, dimethyl ether, and hydrogen using a horizontal, cold-wall reactor. The effects of substrate temperature, reactor pressure, and reagent ratio on the coating growth rate, morphology, composition, and microhardness were studied. Under most conditions, the solid deposit was primarily W₃C with minor amounts of W. The tungsten carbide growth rate data fit an Arrhenius rate expression for temperatures from 425 to 550°C and had an activation energy of 24kcal/mol at 70mmHg total pressure and a WF₆/DME ratio of 6.3. A variety of surface morphologies and microstructures were observed. The microhardness of the coated substrates increased with coating thickness to a maximum value of 2400kg/mm².     

Thermophysical properties of solid and liquid tungsten

The thermophysical properties of solid and liquid tungsten have been measured up to an enthalpy ofH = 1.4 MJ · kg^–1 using an isobaric expansion technique. These measurements give the pressure, temperature, volume, enthalpy, electrical resistivity, and sound velocity as fundamental quantities. From these, other properties may be calculated, such as specific heat at constant volume and pressure, heat of fusion, isothermal and adiabatic bulk moduli and compressibilities, and thermodynamic. Results of these calculations are presented for liquid tungsten and compared with literature values where such data exist. These data will help in understanding liquid-metal phenomenology theoretically and in the design and modeling of exploding wires, foils, and fuses.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.     

Anisotropy of electron-phonon Umklapp scattering and the hall coefficient: Model results for an alkali metal

The large anisotropy in the scattering time and hence the anisotropy in the Hall coefficient at low temperatures is shown to be a consequence of the anisotropic nature of Umkalpp electron-phonon scattering.     

Electronic and magnetic properties of dilute transition clusters in noble (or transition) metals

Using a realistic band structure for the host (noble or transition metal), we present a detailed study of the electronic and magnetic properties of transitional impurity clusters without interactions between the clusters. For the calculation of the one-electron properties, the Hartree-Fock environment effects are self-consistently taken into account by Friedel's rule. The impurity potentials and densities of states are very sensitive to the impurity-impurity interactions inside the clusters. For the calculation of the magnetic properties, the electron-electron interactions are taken into account in the random phase approximation, which allows one to obtain simple expressions for the specific heat and the low-temperature-dependent resistivity. These properties are expressed in terms of three characteristic temperatures T _f(1), T _f(2) ⁺ , and T _f(2) ^– corresponding to three different modes of spin fluctuations for the considered model. We present numerical results from the study of the local magnetic susceptibility, which depends on local environments via several combined effects. Our conclusions on nearly magnetic copper-based alloys are in good agreement with the experimental data. In particular, our calculations confirm that a magnetic moment appears on a nickel atom when it is surrounded by approximately eight nearest-neighboring Ni atoms.This publication is a part of a thesis to be presented at the University Louis Pasteur.     

A small-angle neutron scattering (SANS) study of worm-like micelles under shear

The structure of a cationic worm-like cylindrical micelle was investigated by SANS (small-angle neutron scattering). Intensities from 0.1% by weight solutions in D₂O, at rest and under shear, were measured on the NIST Cold Neutron Research facility 30-m spectrometer in the wave vector range 0.03Q (nm¹)-2.0. Scattered intensity patterns from the solutions subjected to shears equal to or greater than 40s¹ showed pronounced anisotropy, but such anisotropy could not be detected below this apparent threshold shear. The threshold was characterized by a relaxation time since anisotropy was detected only after several minutes of shearing. In contrast, the anisotropy was apparent immediately the shear was applied at the higher shears. The data were analyzed based on the assumption that the micelles behave as rigid rods. Estimates of the radii and length under shear are given. Polydispersity in rod length is discussed, and we argue that it contributes significantly to the scattering patterns.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Charge transport and mechanisms of electron scattering in (HgSe)3(In2Se3) crystals doped with 3d transition metals

We have studied the transport properties of (HgSe)₃(In₂Se₃), (HgSe)₃(In₂Se₃)〈Mn〉, and (HgSe)₃(In₂Se₃)〈Fe〉 crystals and identified the predominant mechanisms of electron scattering in them. The transport properties of the (HgSe)₃(In₂Se₃) crystals, undoped and doped with 3d transition metals, have been studied by the four-probe technique. The results demonstrate that the Hall coefficient of the crystals is temperature-independent, their electrical conductivity shows metallic behavior and is an almost linear function of temperature, and their thermoelectric power increases with increasing temperature.     

Lattice dynamics and electron-phonon coupling in superconducting HgBa2CuO4 investigated by inelastic neutron scattering

The generalized phonon density of states (GDOS) is obtained from the measuring data and compared to model calculations. By means of the partial densities of states which are obtained from these calculations observed structures in the GDOS can be attributed to particular vibrations of atoms. Most significant are shifts of high frequency oxygen modes which are observed upon cooling. These findings seem to be connected with the transition into the superconducting state.