J. Genossar

A tilted‐plate capacitance displacement sensor

We have developed a capacitance displacement sensor whose novel design greatly simplifies its construction and use. It is small, compact, extremely simple, and capable of angstrom resolution. It can be used with samples of any shape, size, or dielectric constant. One variant of the design allows its use at high temperatures, where such resolution has been unattainable until now. It may also be used to detect small changes in angle.

Comparison of the ESR spectra in ceramic YBa2Cu3O7-y (1>y>0) and related phases

The authors report ESR measurements on YBa2Cu3O7-y(0<y<1), the 123 phase and related phases of the Y-Ba-Cu oxide system-Y2BaCuO5 (the green phase), Y2Cu2O5 (the blue phase) and BaCuO2 (the dark phase)-over a temperature range between 5 and 300 K. The ESR observed in the different phases are due to Cu ions. The temperature behaviour of the linewidths, Delta H, of the green and blue phases is in accordance with the magnetic phase transition. In the dark phase they observe a very large variation of the linewidth with temperature, d( Delta H)/dT=25 G K-1. The intensity of the 123 spectra varied by more than two orders of magnitude for different samples, indicating that its origin is due to impurity phases. Its lineshape, down to 80 K, resembles that of the green phase. On further cooling (below 60 K) the linewidth passes through a maximum at Tm, where Tm depends on y. Its signal intensity, as determined from the peak-to-peak intensity and linewidth, shows an exponential decrease with temperature. The authors show that the LT ESR line in the 123 samples originates from the interplay of the green and the dark impurity phases, and they present evidence of coupling between spins of these inclusions which is mediated by the conduction electrons of the 123 matrix. They also offer a qualitative explanation for the increase of the signal intensity of the LT line, and speculate about the rapid linear temperature dependence of the linewidth of the dark-phase ESR line.

Electronic transport and I-V characteristics of polycrystalline LuFe2O4

We report on electronic transport at low electric fields and I-V measurements on a polycrystalline sample of LuFe2O4. The range of temperatures of our measurements (limited by the high resistivity of the sample at low temperatures) includes the range where dramatic nonlinear conductivity and phase transitions were reported in the past. The temperature dependence of the absolute thermopower, measured up to 400 K, exhibits the characteristics of the 3-D charge-ordering and the magnetic-ordering transitions; the temperature dependence of the resistivity, measured only up to room temperature, indicates activated transport. Using single, short current pulses we obtained perfectly linear I-V characteristics up to fields of ∼500 V/cm. The large nonlinearity of the d.c. I-V characteristics at high fields is typical of self-heating in samples with activated conductivity.

Electronic transport under high electric fields in Sr2IrO4

We report measurements of conductivity under high electric fields of polycrystalline Sr2IrO4 at various temperatures, motivated by an earlier report on negative differential resistivity of a single crystal of this material. While our dc I‐V characteristics are strongly nonlinear, the pulsed characteristics show almost negligible nonlinearity up to fields of ∼500V∕cm. We report also on low-field electronic transport measurements (resistivity and thermopower) as function of temperature, which are in qualitative agreement with recently published results.

Metal-insulator transition and nonlinear conductivity in Mn-deficient (LaCa)MnO3

We report on linear and nonlinear electronic transport in the La0.65+3vCa0.35-3vMn1-vO3 system (0u2009≤u2009vu2009≤u20090.06), that exhibits a disorder driven metal-insulator (M-I) transition. This transition is compared with the compositionally induced M-I transition in the La1−xCaxMnO3 system (0.17u2009≤u2009xu2009≤u20090.22). Nonlinear conductivity of electronic origin, as measured using short, single current pulses, was detected only in the disordered non-metallic samples (vu2009=u20090.04−0.06), in electric fields E ≲500 V/cm. At low temperatures and high electric fields, the I-V characteristics exhibit power-law behavior. Conductivities two orders of magnitude higher than ohmic were obtained for vu2009=u20090.04.

Variable range hopping in A2MnReO6 (A=Ca, Sr, Ba)

We report on transport measurements on polycrystalline samples of A2MnReO6 (A=Ca,u2002Sr,u2002Ba), CMRO, SMRO, and BMRO, in the Ohmic and non-Ohmic regimes. The resistivity follows, over wide ranges of temperatures, the relation derived for variable range hopping of carriers localized in a parabolic gap, ρ=ρou2009exp(To/T)1/2. The values of To are unusually high for the Efros Shklovskii Coulomb gap interpretation. The thermopower is large and positive for SMRO and BMRO and even larger and negative for CMRO. The magnitude of the thermopower and its temperature dependence indicate that it may be unrelated to the hopping conductivity. The theoretical relations for hopping in the intermediate electric field regime provide reasonable approximations for the measured nonlinear conductivity of most samples (and for one sample also for the high-field regime). However, localization lengths derived from the non-Ohmic regime are much larger than those estimated from To. Clues to settle these inconsistencies are suggested.

Systematics in the metal-insulator transition temperatures in vanadium oxides

Abstract Nine of the known vanadium oxides, VO 2 − 1 / n (n – a positive or negative integer) with n = 2 − 6 , 8 , 9 , ∞ and −6, undergo metal-insulator transitions accompanied by structural transitions, at various temperatures TMIT (V7O13 is metallic above T=0). Among the persistent efforts to determine the driving force(s) of these transitions, electron–electron (Mott-like) and electron–phonon (Peierls-like) interactions, there were several attempts to find systematics in TMIT as function of n. Here we present an unexpectedly simple and illuminating systematics that holds for positive n: if TMIT is the absolute value of the difference between TM(n) and TP(n), which represent the contributions of electron–electron and electron–phonon interactions, respectively, all data points of TM-TP versus 1/n lie on, or close to, two simple straight lines; one is T M − T P = T ∞ ( 7 / n − 1 ) for V3O5, V4O7, V5O9, V7O13, V8O15, V9O17 and VO2 and the other is T M − T P = T ∞ ( 3 / n − 1 ) for V2O3, V6O11 and VO2.

Electronic transport in La1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 under high electric fields

We report high-field conductivity measurements of polycrystalline samples of La1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 at various temperatures in the stripe ordered state. Since their discovery, an important issue has been the response of the stripes to high electric fields. The main result of this work is that while the pulsed I−V characteristics, using single short current pulses, are ohmic for Nd1.67Sr0.33NiO4 up to fields of ∼550u2002V/cm and ohmic or only slightly non-ohmic for La1.67Sr0.33NiO4 in fields of the same order of magnitude, the corresponding dc I−V characteristics are strongly nonlinear. The conditions for four-probe pulsed I−V measurements that avoid self-heating were determined experimentally. The low-field resistivity and the thermopower of our samples are in satisfactory agreement with those reported in the literature for polycrystals and for single crystals of these materials.

Nonlinear conductivity of electronic origin in self-doped LaMnO3+δ

We report on electronic transport measurements on polycrystalline, nonmetallic samples of LaMnO3+δ, in the Ohmic and non-Ohmic regimes. The non-Ohmic regime has been investigated using single current pulses of short duration. The large nonlinearity of the pulsed I-V characteristics is probably of the same origin as that of the nonmetallicity of LaMnO3+δ, that is, disorder introduced by Mn vacancies. The dc I-V characteristics, under similar fields, show much larger nonlinearity and hysteresis, typical of self-heating in samples with activated conductivity.

The insulating-to-superconducting transition in europium high-temperature superconducting ceramics

Experiment resistivity data on high-temperature superconducting ceramics of fully oxygenated show that the insulating-to-superconducting transitions take place at liquid-helium temperature, provided that the cobalt fraction x exceeds 0.3. The resistivity follows a simple power-law dependence , attributed to electron - electron interactions. A model based upon intrinsic Josephson tunnelling junctions is suggested to explain the transition from insulating to superconducting states.