R. Madar

Structural and electronic transport properties of ReSi2−δ single crystals

We investigated some structural and transport properties of semiconducting ReSi2−δ . In the literature this silicides is reported to crystallize in an orthorhombic structure and to be stoichiometric ReSi2. Our investigations clearly show that the stable composition is ReSi1.75 crystallizing in the space group P1. Transport measurements show thermally activated behavior at high temperatures with one (or two) energy gap Eg=0.16 (0.30 eV). We also report Hall‐effect measurements on this material: we found that RH is positive between 30 and 660 K and at room temperature the Hall number nH=1/eRH is equal to 3.7×1018 cm−3. The Hall mobility at room temperature is relatively high (μH=370 cm2/V s) for a single crystal.

Resistivity of Ru2Si3 single crystals

Abstract Below around 1000°C the Nowotny chimney-ladder compound Ru 2 Si 3 crystallizes in orthorhombic structure (space group Pbcn). In this structure it is reported to be a semiconductor with a band gap of 0.7 eV. We obtained large single crystals of Ru 2 Si 3 by a modified Czochralski pulling technique from a levitated melt in a cold Hukin type crucible. We report here resistivity measurements from 4.2 K to room temperature for current flowing along the three main crystallographic directions. The resistivity is anisotropic within the whole temperature region with ϱ a /ϱ b ≈ 2 and ϱ c /ϱ b ≈ 1.5 and is roughly constant in the whole temperature region. At room temperature and below, the conduction is extrinsic and scattering is dominated by phonons. Below 100 K freezing-out of impurity electrons gradually occurs. Below 20 K a hopping-like regime is expected but no clear evidence for a known theoretical law is obtained in a significant temperature range. At room temperature Hall effect measurement leads to positive carriers with a concentration n = 1.0 × 10 18 cm -3 and a mobility μ = 3.5 cm 2 /V·s.

Organometallic chemical vapor deposition of superconducting YBa2Cu3O7 − δ films

Abstract YBa 2 Cu 3 O 7 − δ thin films have been deposited In a low pressure chemical vapor deposition reactor at a total pressure of 5 Torr. Yttrium, barium and copper precursors were β — diketone chelates and argon was used as a carrier gas. The films were deposited on single crystal MgO substrates, with (100) orientation at a temperature of 825 °C. Four point probe resistivity measurements indicate zero resistance at 81 K and a narrow transition width (of the order of 2 K). Films deposited in the same conditions on SrTiO 3 (100) exhibit zero resistance at 89 K and a ΔT c of 0.8 K.

CRYSTAL STRUCTURE AND PROPERTIES OF TETRAGONAL CEGE1.6

Abstract An X-ray study was performed on single crystals of the compound CeGe 1.6 . The structure is derived from the tetragonal α-ThSi 2 type. The superstructure is shown to be related to the order of vacancies. Magnetic measurements and neutron diffraction experiments showed a ferromagnetic structure with magnetization along the c axis, T C = 6 K and M = 1.5 μ B at T = 1.5 K. The crystal field level scheme is deduced from single-crystal susceptibility and inelastic neutron scattering.

Electron-phonon interaction spectra of TaSi2, NbSi2 and VSi2

Abstract Using point-contact spectroscopy, the spectral function for the electron-phonon interaction has been measured for the three silicides TaSi2, NbSi2, and VSi2. The difference in the energy-dependence of the phonon spectra between these isoelectronic compounds can be explained in terms of the standard mass dependence of the phonon frequencies.

Some physical properties of ReSi1.75 single crystals

We investigated the electronic transport properties and the magnetic susceptibility of the semiconducting silicide ReSi1.75. This compound crystallises in a monoclinic structure (space group P1). The resistivity of this silicide is anisotropic depending on the direction of the current flow. At high temperatures we observe thermally activated behaviour for the resistivity with one (or two) energy gap(s) Eg = 0.16 eV (0.30 eV). Hall effect measurements yield a positive Hall coefficient in the temperature range between 30 and 660 K. At room temperature we found a Hall carrier concentration of 3.7 × 1018 cm−3 and a quite high Hall mobility of 370 cm2/V · s. As the resistivity, the magnetic susceptibility of ReSi1.75 is anisotropic depending on the orientation of the magnetic field relative to the crystallographic axes. At room temperature χ is strongly diamagnetic. Below about 50 K, χ increases with decreasing temperature.

Low temperature specific heat of CoSi2

Abstract The specific heat of CoSi 2 has been measured between 0.5 and 6 K. The electronic specific heat was found to be enhanced, principally because of renormalization effects on the density of states. The value of the renormalization parameter of 0.4 is in agreement with values obtained by other methods. The low temperature Debye temperature is 520 ± 10 K.

Crystal structure of Rh12As7

The crystal structure of the rhodium-rich limit of the hexagonal phase Rh1+xAs (0.3 < x < 0.7) has been found to be isostructural with Th7S12like Cr12P7. Crystals of Rh12As7 are hexagonal (space group, P63/m) with a = 9.315(7) A and c = 3.659(4) A. This structure is closely related to those of the transition metal phosphides M2P and can be usefully described in the same way on the basis of the arsenic lattice.

Magnetoresistance and Hall effect of NbSi2 single crystals

We measured the transverse angular magnetoresistance and the Hall effect on high-quality NbSi2 single crystals at low temperatures (4.2 K ≤ T ≤ 160 K) and high magnetic fields (B ≤ 20 T). The material behaves like a compensated metal, i.e. the magnetoresistance grows generally proportionally to B2. For some current–field configurations, however, saturation of the magnetoresistance occurs, giving evidence for the presence of an open orbit on the Fermi surface of NbSi2 parallel to the c axis. The Hall coefficient shows, as does the magnetoresistance, a low-field–high-field transition. It varies between −3.5 and −4.5 × 1010 m3 C−1 in low magnetic field to −10.5 × 10−10 m3 C−1 in the high-field region. We compare our results with those obtained for NbSi2 thin films and discuss the validity of parameters deduced from transport data.

Magnetic susceptibility of semiconducting ReSi1.75

Abstract We measured the magnetic susceptibility of a single crystal of semiconducting ReSi 1.75 with the magnetic field aligned along the three main crystallographic directions. The susceptibility of ReSi 1.75 is strongly diamagnetic and anisotropic. Below 40 K, χ increases with decreasing temperature. We discuss the different contributions to the susceptibility of ReSi 1.75 and we explain the low temperature behaviour of χ in analogy to the behaviour of a doped semiconductor near the metal-insulator transition