B. Fisher

Electrical Properties of Cobalt Monoxide

The electrical conductivity of polycrystalline CoO1+x was measured as a function of temperature and partial pressure of oxygen, P(O2). The range of temperature was 900°—1450°C and the range of partial pressure was 1 − 10−12 atm. The plot of the conductivity against partial pressure of oxygen gave two distinct regions: Region A, where the conductivity is proportional to P(O2)¼, and Region B, where the conductivity is proportional to P(O2)⅙. From these results, together with considerations of ionic radii, we concluded that the dominant defects in CoO1+x at high temperatures are cation vacancies, with the majority being singly ionized in Region A and doubly ionized in Region B.A combination of gravimetric and electrical measurements showed that the mobility of the holes contributed by these vacancies rises exponentially with temperature. The activation energy of mobility is Eu=0.3 eV. At 1350°C, the mobility is about 0.4 cm2 V−1·sec−1. These values are discussed in terms of polaron theory.The sum of the enth...

Highly oriented as‐deposited superconducting laser ablated thin films of Y1Ba2Cu3O7−δ on SrTiO3, zirconia, and Si substrates

KrF excimer laser ablation of an Y1Ba2Cu3O7−δ pellet in 0.1–0.2 Torr of O2 ambient was used to deposit thin superconducting films onto SrTiO3, yttria‐stabilized zirconia (YSZ), and silicon substrates at 600–700 °C. The as‐deposited 1‐μm‐thick films at 650–700 °C substrate temperature were superconducting, without further high‐temperature annealing. All films had a similar Tc onset of ∼92 K but different zero‐resistance Tc of 90, 85, and 70 K for the films on SrTiO3, YSZ, and Si substrates, respectively. Angular x‐ray diffraction analysis showed that all the films were highly oriented with the c axis perpendicular to their surface. Critical current densities at 77 K were about 40 000 and 10 000 A/cm2 for the films on SrTiO3 and YSZ, respectively. Smooth surface morphology was observed in all films, with occasional defects and cracks in the films on YSZ, which seems to explain the lower critical current in these films.

Pulsed versus dc I-V characteristics of resistive manganites

We report on pulsed and dc I-V characteristics of polycrystalline samples of three charge-ordered manganites, Pr2∕3Ca1∕3MnO3, Pr1∕2Ca1∕2MnO3, and Bi1∕2Sr1∕2MnO3, and of a double perovskite, Sr2MnReO6, in a temperature range where their Ohmic resistivity obeys the Efros-Shklovskii variable range hopping relation [J. Phys. C 8, L49 (1975)]. For all samples, the dc I(V) exhibits at high currents negative differential resistance and hysteresis, which mask a perfectly Ohmic or a moderately non-Ohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used dc I-V measurements are usually misleading.

Electrical transport measurements in TiS3

Results of measurements of conductivity and Hall coefficient in the temperature range 15–300K and of thermal emf in the temperature range 80–400K, carried out on TiS3 samples, are reported. The results indicate that these crystals are semiconducting with extrinsic n-type conductivity. The mobility of the carriers is about 30 cm2/V sec at room temperature, increases up to about 100 cm2/V sec at 100K and drops at lower temperatures. The Seebeck coefficient is in qualitative agreement with these findings but its detailed temperature dependence is not yet understood.

Voltage oscillations in switching VO2 needles

Voltage oscillations observed in switching VO2 needles are associated with traveling semiconductor domains driven by the Peltier effect at their boundaries. The model is supported by the dependence on current of the two time constants characterizing the rectangular synchronous oscillations.

Electronic transport in MgB2, AlB2 and ZrB2--a comparative study

Abstract We report on the temperature dependence of the resistivity ( ρ ) and the absolute thermopower ( S ) of the polycrystalline title materials and of AlB 2 single crystals. For all samples ρ ( T ) exhibits a Bloch–Gruneisen-like temperature dependence, with large characteristic temperatures θ R (≈ θ D ––the Debye temperature). At high temperatures the thermopower S ( T ) for ZrB 2 (n-type) is almost the mirror image of S ( T ) for MgB 2 (p-type) while S (AlB 2 ) is very small for all temperatures. The density of states distribution N ( E ) around E F seems to play a dominant role in determining S ( T ) of these materials. ln( T ) terms in the low-temperature ρ ( T ) and S ( T ) of ZrB 2 samples bear evidence for weak localization in 2D.

Metal-semiconductor domain configurations during switching of VO2 single crystals

The geometry of stationary or mobile metallic and semiconducting domain configurations and their formation during the switching of a VO2 single crystal are discussed. It is shown that the motion of narrow semiconducting domains in the sense of the electric current is driven by the Peltier effect at the two oppositely biased interfaces. The Peltier heat may be partially or totally compensated for by thermal fluxes along temperature gradients in the supercooled metal and in the superheated semiconductor.

The resistivity of Y1Ba2Cu3O7 between 80 K and 1240 K

Abstract Resistivity measurements between 80–1240 K and dilatometric measurements between 300–1200 K in YBa 2 Cu 3 O 7 are reported. We found that the measurements increases exponentially with temperature, with different exponents below and above the tetragonal-orthorhombic transition. The thermal expansion coefficient also changes at this transition. We suggest that conduction occurs by tunneling of electrons through potential barriers in the lattice, the width of which increases with temperature.

The thermoelectric power of TaS3

Abstract The thermoelectric power of TaS 3 filaments was measured as a function of temperature between 90–400K. Its positive sign in the whole temperature range and its linear dependence on reciprocal temperature between 120–200K are discussed in connection with the temperature dependence of the resistance.

Fully oxygenated RBa2Cu3−xCoxOy, (R=Y, Eu, Pr, and 0≤x≤1)— from high‐temperature superconductors to high‐resistivity nonmetals

Reported here are the results of measurements of the resistivity (ρ) up to 300 K and of the thermoelectric power (TEP) up to 400 K of ceramic samples of the title materials. We determined also their room‐temperature lattice parameters and oxygen content as functions of Co concentrations. The metal–nonmetal transition in YBa2Cu3−xCoxOy and in EuBa2Cu3−xCoxOy is marked by the onset of the deviation of the maximum absolute TEP from a value calculated from a simple narrow‐band formula. The results indicate that the effective valency of Pr in PrBa2Cu3−xCoxOy varies from ∼3.5 at x=0 to ∼3 for x=0.5. For x=0 this material is close to the metal–nonmetal transition. In the nonmetallic regime the electrical transport is by activated hopping. In certain ranges of Co content the results are consistent with two‐band hopping conductivity with two branches, a low‐temperature and a high‐temperature branch of variable‐range‐hopping (VRH) conductivity. In the VRH regime there is a remarkable correlation between the two res...