Inter-grain tunneling in the half-metallic double-perovskites Sr2_2BB'O6_6 (BB'-- FeMo, FeRe, CrMo, CrW, CrRe

The zero-field conductivities ($\sigma$) of the polycrystaline title materials, are governed by inter-grain transport. In the majority of cases their $\sigma$(T) can be described by the "fluctuation induced tunneling" model. Analysis of the results in terms of this model reveals two remarkable features: 1. For \emph{all} Sr$_2$FeMoO$_6$ samples of various microstructures, the tunneling constant (barrier width $\times$ inverse decay-length of the wave-function) is $\sim$ 2, indicating the existence of an intrinsic insulating boundary layer with a well defined electronic (and magnetic) structure. 2. The tunneling constant for \emph{all} cold-pressed samples decreases linearly with increasing magnetic-moment/formula-unit.Comment: 10 pages, 2 tables, 3 figure

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, Pr_{2/3}Ca_{1/3}MnO_3, Pr_{1/2}Ca_{1/2}MnO_3, Bi_{1/2}Sr_{1/2}MnO_3 and of a double-perovskite Sr_2MnReO_6, in a temperature range where their ohmic resistivity obeys the Efros-Shklovskii variable range hopping relation. For all samples, the DC I(V) exhibits at high currents negative differential resistance and hysteresis, which mask a perfectly ohmic or a moderately nonohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used DC I-V measurements are usually misleading.Comment: 6 pages, 4 figures. Accepted for publication to AP

Absence of charge-density-wave sliding in epitaxial charge-ordered Pr0.48Ca0.52MnO3 films

For an epitaxial Pr0.48Ca0.52MnO3 film on NdGaO3, we use transmission electron microscopy to observe a "charge-ordered" superlattice along the in-plane direction a. The same film shows no electrical signatures of charge order. The in-plane electrical anisotropy (rho)a/(rho)c = 28 is constant, and there is no evidence of sliding charge density waves up to the large field of ~10^3 V/cm.Comment: 8 pages, 3 figure

Resistivity and Thermoelectric-Power Measurements of Pr_xY_1-xBa₂Cu₃O_7-δ up to 1200 K and an Electronic-Structure Analysis

We report measurements of absolute thermoelectric power and resistivity in PrxY1-xBa2Cu3O7-δ, as function of x and δ, over a wide temperature range. At high temperature, the substitution of Pr for Y does not affect the thermopower and resistivity; Pr behaves then as a trivalent ion: Pr3+. For small x and δ the effects of substitution of Y by Pr and of oxygen deficiency on the low-temperature resistivity are additive. A simple electronic-structure model is proposed to account for thermopower as function of temperature, x, and δ.</p

Electric-field effects in resistive oxides: facts and artifacts

Striking non-linear conductivity effects induced by surprisingly low electric-fields in charge-ordered oxides, were reported variously as dielectric breakdown, charge-order collapse, depinning of charge-density-waves or other electronic effects. Our pulsed and d.c. I-V measurements on resistive oxides show that non-linear conductivity of electronic origin at low electric-fields is a rare phenomenon. In the majority of cases we detected no deviations from linearity in pulsed I-V characteristics under fields up to E ~ 500 V/cm. Current-controlled negative-differential-resistance (NDR) and hysteresis were found in d.c. measurements at fields that decrease with increasing temperatures, a behavior typical of Joule heating in materials with negative temperature coefficient of resistivity. For the d.c. I-V characteristics of our samples exhibiting NDR, we found a rather unexpected correlation between ρ(Em) - the resistivity at maximum field (at the onset of NDR) and ρ(E=0) – the ohmic resistivity. The data points for ρ(Em) versus ρ(E=0) obtained from such characteristics of 13 samples (8 manganites, 4 nickelates and one multiferroic) at various ambient temperatures, plotted together on a log-log scale, follow closely a linear dependence with slope one that spans more than five orders of magnitude. This dependence is reproduced by several simple models

Inter-grain tunnelling in the half-metallic double-perovskites Sr2BB'O6 (BB'= FeMo, FeRe, CrMo, CrW, CrRe)

The zero-field conductivities (σ) of polycrystalline title materials, are governed by intergrain transport. In the majority of cases their σ(T) can be described by the "fluctuation induced tunnelling" model. Analysis of the results in terms of this model reveals two remarkable features: 1. For all Sr2FeMoO6 samples of various microstructures, the tunnelling constant (barrier width × inverse decay-length of the wave-function) is ~ 2, indicating the existence of an intrinsic insulating boundary layer with a well-defined electronic (and magnetic) structure. 2. The tunnelling constant for all cold-pressed samples decreases linearly with increasing magnetic-moment/formula-unit

I-V Characteristics of Resistive Oxides: DC versus Pulsed Measurements

We investigated pulsed and DC I-V characteristics of a variety of resistive oxides. Examples of pulsed, compared with DC characteristics, are shown for samples of La$\text{}_{1.2}$Sr$\text{}_{1.8}$Mn$\text{}_{2}$O$\text{}_{7}$ (a double layer manganite), Pr$\text{}_{2}\text{}_{/}\text{}_{3}$ Ca$\text{}_{1}\text{}_{/}\text{}_{3}$ MnO$\text{}_{3}$ and Bi$\text{}_{1}\text{}_{/}\text{}_{2}$ Sr$\text{}_{1}\text{}_{/}\text{}_{2}$ MnO$\text{}_{3}$ (charge-ordered manganites), Sr$\text{}_{2}$FeMoO$\text{}_{6}$, Sr$\text{}_{2}$CrWO$\text{}_{6}$ and Ba$\text{}_{2}$MnReO$\text{}_{6}$ (double perovskites). For pulsed measurements, single pulses in the ms range were applied. For short-rise-time square pulses, the Joule heating is negligible, as long as the response remains independent of time. The DC I-V characteristics were measured up to current runaway in the negative resistance regime;Δ T never exceeded a fraction of a degree. In most cases the DC characteristics mask a perfectly ohmic or moderately non-ohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used DC I-V measurements in the high current regime are frequently misleading