Low-temperature Conductivity Data Research Articles

Observation of power-law behavior in temperature dependent conductivity of multiwall carbon nanotube/polystyrene composites

For last two decades, a pronounced temperature dependence of electrical conductivity (σ) in the multiwall carbon nanotube (MWCNT) - based polymer composite (even above the percolation threshold) has been widely reported; the conduction mechanism has been understood by employing conventional models, namely variable range hopping and fluctuation induced tunneling. Herein, we report on the observation of a weak temperature dependence of σ in the MWCNT/polystyrene composites above percolation threshold (0.7–7 wt %) at temperatures down to 1.4 K, with a conductivity ratio smaller than 3. The low temperature conductivity data follow power-law exhibiting two slope behavior, with exponent values β1 ∼0.10–0.14 (at T > 5 K) and β2 ∼0.23–0.36 (at T < 5 K). The observation of weak temperature dependence of σ is attributed to high aspect ratio (more than 4000), achievement of high degree of dispersion, and excellent electrical properties of MWCNT as well as optimized composite processing. Further, all the samples exhibit negative magnetoresistance which can be explained within the framework of weak localization model.

Low Temperature Electrical Transport in Double Layered CMR Manganite La&amp;lt;sub&amp;gt;1.2&amp;lt;/sub&amp;gt;Sr&amp;lt;sub&amp;gt;1.4&amp;lt;/sub&amp;gt;Ba&amp;lt;sub&amp;gt;0.4&amp;lt;/sub&amp;gt;Mn&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;7&amp;lt;/sub&amp;gt;

The electrical transport behavior and magnetoresistance (MR) of a polycrystalline double layered manganite La1.2Sr1.4Ba0.4Mn2O7, synthesized by the sol-gel method, are investigated in the temperature range 4.2 K - 300 K. The sample exhibits an insulator-to-metal transition at 87 K (TIM) and the spin-glass (SG)-like behavior is observed below 50 K (TSG). The transport behavior is analyzed in the entire temperature range considering three different regions: paramagnetic insulating region (T>TIM), ferromagnetic metallic region (TSG IM) and antiferromagnetic insulating region (TSG) by fitting the temperature dependent resistivity data to the equations governing the conduction process in the respective temperature regions. The results show that the conduction at T>TIM follows Mott variable range hopping (VRH) process, while the two-magnon scattering process is evidenced at TSG IM which is suppressed with the applied magnetic field of 4 T. The low temperature conductivity data are also fitted with Mott VRH equation. The sample exhibits a large MR (≈45%) over a temperature range 5 K – 50 K and it shows ≈32% MR at 5 K with a magnetic field of 0.5 T.

Non-adiabatic small-polaron hopping conduction in SrTiO3−doped 90V2O5−10Bi2O3 glassy semiconductors

Abstract Dc electrical conductivities [sgrave]dc of 90V2O5−10Bi2O3 (VB) glasses doped with different SrTiO3 concentrations (0–40 wt%) are reported between 80 and 450 K. At higher temperatures, the Mott model of phonon-assisted small-polaron hopping between nearest neighbours is consistent with the conductivity data while, at low temperatures, the variable-range hopping model is found to be appropriate. The Holstein model indicates that, in the high-temperature regime, hopping occurs by a non-adiabatic process in contrast with the vanadate (VB) glasses. The percolation model of Triberis and Friedman applied to the small-polaron hopping regime is found to be consistent with the conductivity data. However, the consistency of the low-temperature conductivity data with the prediction of different theoretical models cannot be easily distinguished for these glasses. The estimated model parameters such as density of states, localization length and carrier mobility are found to be consistent with the formation of ...

The metal-insulator transition in ‘random’ Al−Ge films

Electronic transport properties have been measured on 3500 A ‘random’ Al−Ge films. The room temperature resistivity exhibits a sharp discontinuous jump at the metal-insulator transition at the critical metallic fraction, φc=8.8 vol% Al. A new procedure is described for extracting values for the zero temperature conductivity σ(0) from the metallic low temperature conductivity data. When σ(0) is extrapolated to zero as a function of Al content, the value obtained for the critical aluminum fraction φc is in good agreement with the room temperature value.

Electronic conduction in `random' Al - Ge films

Electronic transport properties have been measured for 3500 Å Al - Ge films with a random microstructure. The room temperature resistivity exhibits a sharp discontinuous jump at the metal - insulator transition, allowing for the direct determination of the critical metallic fraction, vol% Al. A new procedure is described for extracting values for the zero-temperature conductivity from the low-temperature conductivity data. When is extrapolated to zero as a function of Al content, the value obtained for the critical aluminium fraction is in excellent agreement with the value obtained from the room temperature data. The films exhibit two transition regions below 1.2 K as the Al content is decreased - a transition from the superconductivity state to the normal-metallic state, followed by a second transition from the normal-metallic state to the insulating, variable-range-hopping state. Superconducting fluctuation data taken above 1.2 K were well described using the 2D Aslamazov - Larkin and Maki - Thompson formulae; the `resistive tails' below 1.2 K are also discussed.

DC electrical conduction in Bi 4Sr 3Ca 3Cu yO z (3 ≤ y ≤ 6) glasses

The dc electrical conductivity of Bi 4Sr 3Ca 3Cu y O z glasses is measured in the temperature range 100–500 K. The small polaron hopping in the adiabatic approximation is found to be the most appropriate conduction mechanism in these glasses. The low temperature conductivity data could be fitted to Mott's Variable Range Hopping model.

Low temperature A.C. conductivity of oxide glasses

The electrical conductivity (σ) of various alkali silicate, borate and germanate glasses has been measured from 3 K to 300 K in the frequency range 10–100 kHz. Mostly the frequency (ω) and temperature ( T) dependence of conductivity below 150 K can be described as σ ~ T α ω β where 0 < α < 0.25 and 1.0 < β < 1.15. An abnormal decrease of conductivity with increasing temperature is observed at about 100–150 K in glasses with low alkali oxide concentration (< 1 mol%). A theoretical model based on thermally activated excitations of asymmetric double well potential (ADWP) configurations has been used to explain the low temperature conductivity data. From the alkali concentration and size dependence of the parameters in ADWP model, a physical description of the species responsible for low temperature a.c. conductivity is obtained.

Various methods for determining the critical metallic volume fraction phi c at the metal-insulator transition

The metal-insulator transition in composite Al-Ge films has been extensively studied experimentally. Using the superconducting properties of clusters composed of aluminium grains in the presence of a magnetic field, the critical metallic volume fraction phi cgp has been determined for the case of grain percolation (gp). Because the films investigated are composed of extremely small grains of Al partially coated with amorphous Ge, the critical volume fraction phi c takes on a comparably large value. Typically about 56 at.% Al. The low-temperature electrical conductivity data are examined using a number of criteria for phi c. Most of these criteria yield values phi c to within 1% of phi cgp. However, one widely used criterion involving the fitting of the low-temperature conductivity data to a T1/2 dependence, as suggested by the electron-electron interaction theory, was found to yield a significantly smaller value for phi c. Methods based on room-temperature data are also evaluated. These criteria yield phi c values that are within +or-1% of phi cgp.

Various methods for determining the critical metallic volume fraction φ c at the metal-insulator transition

The metal-insulator transition in composite Al-Ge films has been extensively studied experimentally. Using the superconducting properties of percolating clusters composed of small aluminum grains in the presence of a magnetic field, the critical metallic volume fraction φ c(sc) for the metal-insulator transition has been uniquely determined. The low temperature electrical conductivity data are examined using a number of criteria for φ c. Some of these criteria are quite successful, yielding φ c's to within one percent of φ c(sc), the definitive value. However, one widely used criterion involving the fitting of the low temperature conductivity data to a T 1 2 dependence, which results from the electron-electron interaction theory, was found to yield a very different value of φ c. Other methods and/or criteria, using room temperature conductivity data to determine φ c, are evaluated. These also yield φ c values that are within 1% of φ c(sc).

Low- and high-frequency conductivities of RbAg 4I 5 and Ag β″-alumina at low temperatures

The specific ionic conductivities, σ, of polycrystalline RbAg 4I 5 and Ag β″-alumina superionic conductors were determined by two- and four-probe impedance measurements at low frequencies (10 −3Hz≤ƒ≤10 2Hz) and waveguide measurements at high frequencies (18×10 9Hz≤ƒ≤26.5×10 9Hz≡:k-band) in the temperature range 15K≤ T≤298K. Above a transition temperature T t=(90±20)K, the conductivity data represent clearly a silver ion transport process with thermal activation. The activation energies of the α, β, and γ phases of RbAg 4I 5 were determined for the low- and high-frequency ranges. Below T t a nearly temperature-independent conductivity was observed in accordance with previous results. Around T t a long-time relaxation effect is observed in low-frequency EIS measurements, changing α and the dielectric constant ε. The low-temperature conductivity data can be explained neither in terms of the jump-relaxation nor in terms of the two-level tunneling system (TLS) model. A different mechanism which takes into account the influence of inhomogeneities on the ionic conductivity is proposed. This mechanism is obviously valid in a restricted temperature range located between the ion TLS-tunneling and the jump-relaxation regimes.

Low temperature conductivity of ultra thin deposits of Ag on Ge(100)

We present low temperature conductivity data on ultra thin (d<25Å) Ag layers grown on Ge(001)2×1 substrates under UHV conditions. Samples grown at 160K and subsequently annealed at room temperature appear to have an intermediate layer plus islands structure. The islands are elongated in one of two normal directions. After air exposure, these samples show a maximum in their electrical resistance at about 8 K and a minimum around 5 K. The current and magnetic field dependence of the resistance between 8K and 0.7K is consistent with an incomplete superconducting transition. Although all samples show an increase in overall resistance with time after air exposure, the transition temperature remains fairly constant. The increase in resistance below 5K is ascribed to normal regions within the intermediate layer.

Point defects in sodium chlorate crystals

D. c. electrical conductivity of single crystals of NaClO 3 with varying concentrations of divalent cation impurity are studied. The low-temperature conductivity data are analysed to obtain the association energy between the divalent cation impurity and the cation vacancy. By a suitable analysis developed for the extrinsic and intrinsic overlapping region the formation energy per defect pair has been determined. Almost all the defect parameters are determined by graphical treatment of the conductivity data. Finally these parameters are refined by a least square fit of the conductivity data. The values of the various para­meters obtained are: (1) impurity-vacancy association energy E a = 0.22 ± 0.01 eV. (2) jump activation energy of cation vacancy E 1j = 0.46 ± 0.01 eV. (3) pre exponential factor for vacancy mobility μ 01 = 0.004 cm 2 V ‒1 s -1 . (4) jump activation energy of interstitial cation E 2j = 0.85 ± 0.04 eV. (5) pre-exponential factor for interstitial mobility μ 02 = 14.7 cm 2 V ‒1 s ‒1 . (6) formation energy of a Frenkel defect pair E t = 1.98 ± 0.02 eV. (7) entropy of formation of a Frenkel defect pair S t = 3.0 x 10 ‒3 eV K ‒1 .