A. Barman

Hopping transport in HCl doped conducting polyaniline

Abstract The present work includes a study on the transport property based on the measurement of the electrical conductivity of HCl doped conducting polyaniline in the presence as well as in the absence of a magnetic field in the temperature range 1.8 K≤ T ≤300 K. It has been observed that the conductivity of the samples increases with increasing temperature and the conductivity ratio [ r = σ (300 K)/ σ (1.8 K)] is large for the sample having higher dopant concentration. A crossover from Mott ( T −1/4 ) to Efros–Shklovskii (ES) ( T −1/2 ) in temperature dependent conductivity of the samples is observed at 10 K. In the presence of a magnetic field the electrical conductivity varies as T −3/4 . From the plot of conductivity versus temperature, different physical quantities like density of states, molecular vibrational frequency, hopping distance and localization length have been determined.

Transport properties of HCl doped polyaniline and polyaniline–methyl cellulose dispersion

Electrical resistance and magnetoresistance of the HCl doped polyaniline (PANI) in aqueous ethanol medium and polyaniline–methyl cellulose dispersion (PANI–MC) has been investigated at low temperature down to 1.8 K and in magnetic field up to 8 T. The weaker temperature dependence of resistivity characterized by the resistivity ratio ρr=ρ(1.8 K)/ρ(300 K) indicates that a better homogeneity and less disorder can be obtained by protonation with HCl in ethanol media. The temperature dependence of resistivity for both PANI and PANI–MC has been analyzed by three-dimensional variable range hopping (VRH) type of conduction. The samples with the resistivity ratio lying in the range of 102⩽ρr⩽103 exhibit a crossover from Mott to Efros–Shklovskii VRH conduction below 10 K. A large positive magnetoresistance at low temperature has been obtained.

Electrical transport in paratoluene sulfonate doped polypyrrole films at low temperature

Transport data for paratoluene sulfonate dope polypyrrole films in the insulating regime and near the metal-insulator (M-I) boundary are presented and analyzed. Samples in the insulating region show a crossover from Mott to Efros–Shklovskii variable range hopping conduction at T=5 K and magnetoconductance of these samples is also explained by variable range hopping theory. The power law dependence of conductivity σ(T)∝Tβ is observed for the sample close to metallic side of the M-I transition with β=0.83 for 20 K<T<300 K and β=1/2 for 1.8 K⩽T⩽20 K. The magnetoconductance of this sample is analyzed by three dimensional electron-electron interaction and weak electron localization theory. The inelastic scattering length Lin obeys a power law temperature dependence, Lin∝T−p/2 with p=1.

Crossover from Mott to Efros-Shklovskii variable-range-hopping conductivity in conducting polyaniline

Abstract Electrical resistance and magnetoresistance of the HCl-doped polyaniline (PANI) in aqueous ethanol have been investigated at low temperature down to 1.8 K and in magnetic field up to 8 T. The weaker temperature dependence of resistivity characterized by the ratio, ϱ r = ϱ(1.8 K) / ϱ(300 K) indicates that a better homogeneity and less disorder can be obtained by protonation with HCl in ethanol media. The samples with resistivity ratio lying in the range 10 2 ≤ϱ r ≤10 3 exhibit a crossover from Mott to Efros-Shklovskii variable-range-hopping (VRH) conduction below 10 K. The Coulomb gap energy has been calculated and is small (0.22–0.04 meV).

Magnetocaloric properties of the La0.7−xYxSr0.3MnO3 giant magnetoresistance ceramics

Abstract Giant magnetoresistance (GMR), observed in several La 0.7 x A 0.3 MnO 3 ceramics, where A is a two-valent addition, is the result of an interaction of conduction electrons and intrinsic magnetization, which is likely to result in unusual magnetocaloric effects. On this basis, magnetization, magnetic entropy change and adiabatic temperature change were measured as a function of Y content in La 0.7− x Y x Sr 0.3 MnO 3 ceramics. It is found that doping influences only the transition temperature, while the magnetic entropy change remains constant and displays a saturation tendency in high fields.

Low temperature electrical conductivity of polyaniline-polyvinyl alcohol blends

Abstract Experimental results on electrical conductivity of polyaniline-hydrochloric acid (PANI-HCl) in polyvinyl alcohol (PVA) are investigated with PANI-HCl content above the percolation threshold. The negative temperature co-efficient of conductivity near the room temperature indicates the presence of intrinsic metallic character in the dispersed medium. The conductivity at low temperature has been analyzed on the basis of the superlocalisation of electronic states in the fractal structure of polyblends.

Giant magnetoresistance in La0.8Sr0.2FexCo1−xO3 (0.025⩽X⩽0.3)

The electrical resistance and magnetoresistance of La0.8Sr0.2FexCo1−xO3 (0.025⩽X⩽0.3) have been studied at low temperatures down to 1.5 K and magnetic fields up to 7.5 T. The large magnetoresistance at high and low temperature has been found. The magnetoresistance exhibits a broad minima in the temperature interval between 150 and 50 K for all the compositions. The large magnetoresistance and its broad minima may be interpreted as the interplay of spin state transition, Jahn–Teller distortion, and orbital ordering of Co ions.

Structural features of the La-Sr-Fe-Co-O system

Abstract:A structural study has been performed on the La0.8Sr0.2FexCo1-xO3 (x = 0.025 to 0.3) system displaying large magnetoresistance (MR) at room temperature. A detailed analysis of the crystal structure and microstructure was done by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM). The atomic resolution TEM images and the appearing superreflections in the corresponding SAED patterns revealed that a superstructure is formed due to the presence of iron. The correlation between the ordered microstructure and the observed large MR ratio is discussed. 57Fe Mössbauer spectroscopy was utilized to gain information on the valence state of iron in the sample with x = 0.3. The lattice parameters of Fe- doped La0.8Sr0.2FexCo1-xO3 compounds were found to increase monotonously with increasing Fe content. The valence state of iron was found to be Fe3+.

Electrical and magnetic properties of perovskite at low temperature

Electrical and magnetic properties of have been extensively investigated at low temperature down to 1.8 K. The irreversibility of magnetic susceptibility between zero field cooling and field cooling increases with increasing Y content. The metal-insulator transition temperature shifts towards lower temperature and the peak resistivity and magnetoresistance increase with the increase of x. Electrical conduction above is dominated by the variable range hopping mechanism. The increase in resistivity at low temperature has been interpreted in terms of electron-electron interaction. The enhancement of magnetoresistance with the increase of Y content is due to the reduction in Mn-O-Mn bond angle and the mobility of charge carriers.

ELECTRICAL TRANSPORT PROPERTIES OF BULK LA1-XBAXCOO3 AT LOW TEMPERATURE

Abstract The resistivity and magnetoresistance of La 1− x Ba x CoO 3 (0.1 ≤ x ≤ 0.4) systems have been investigated in the temperature range 1.8–300 K and magnetic fields up to 8 T. Electrical conduction is dominated by the spin state transition of Co ions and thermally activated electrons above 50 K (30 K) for x = 0.1 (≥ 0.2) and hopping electrons below 30 K for x ≥ 0.2. A large magnetoresistance has been observed for low doping concentration and it has been interpreted by the double exchange model.