B. K. Chaudhuri

The nature of the low-temperature minimum of resistivity in ceramic manganites

Abstract Electrical transport measurements were carried out on a ceramic samples of La 0.5 Pb 0.5 MnO 3 containing 10 at.% of Ag in a dispersed form and La 0.8 Sr 0.2 MnO 3 . Their resistivity at zero applied magnetic field exhibits a shallow minimum at the interval of T ∼25–30 K. This minimum shifts towards lower T upon applying a magnetic field ( H ) and disappears at a certain field H cr , while residual resistivity decreases notably with increasing of H . It is found that the bulk-scattering model is not adequate for describing of such behavior of ρ in the presence of H . It seems that the proposed model of charge-carrier tunneling between antiferromagnetically coupled grains accounts for the results obtained in spite of significant difference in the crystallinity (grain size) of the samples.

Electron magnetic resonance study of polycrystalline La0.5Pb0.5MnO3

Abstract Electron ferromagnetic (FMR)/paramagnetic (EPR) resonance spectra of La0.5Pb0.5MnO3 powder sample have been investigated in the temperature range, 120–500 K. The spectra exhibit a gradual narrowing of the asymmetric FMR signals, upon increasing the temperature and approaching the Curie temperature, Tc. The temperature dependence of linewidth (ΔHpp), g-factor and double integrated (DI) intensity of the resonance signals are discussed within the framework of existing models. Effects of non-resonant surface microwave absorption on the DI are also considered.

A photoacoustic spectroscopic investigation of the optical energy gap in Zn–Mn–Se type dilute magnetic semiconductors

Abstract Using photoacoustic spectroscopy (PAS) we have measured the composition dependent optical absorption coefficient and band gap of the Znl−xMnxSe (0≤x≤0.5) diluted magnetic semiconductors at room temperature. The band gap E0 estimated from the PAS spectra varies nonlinearly, showing a downwards bowing with the Mn-concentration. The exchange interaction of electrons in conduction and valence bands with the d-electrons of Mn ion interactions, effects of polytypes, micro-structures and mixed crystallization (zinc-blende and wurtzite structures) are considered for the analysis of the data. The observed exponential edge (Urbachs edge) can be considered as an internal Franz–Keldish effect arising from the charged impurity-generated and “frozen-in” optical phonon-generated fields. This can be described in the framework of the Tauc and Dow–Redfield model. The phonon-assisted indirect transition at the band tail regions for some samples are also observed from the present studies.

Effect of Fe doping on the transport properties of as-quenched Bi3.9Sr3.3Ca1.3Cu3Ox glasses showing superconducting behavior in the corresponding glass-ceramic phases

Abstract Transport properties of a series of Bi 3.9 Sr 3.3 Ca 1.3 (Cu 1− n Fe n ) 3 O x ( n = 0-0.05) glasses containing two different transition metal ions have been reported. The d.c. conductivity data of the as quenched glasses are explained by the “small polaron” hopping conduction mechanism. All these glasses become superconductors by annealing them at an optimum temperature 840°C for 40 h in air. The superconducting transition temperatures ( T c ) vary between 78 and 60 K depending on Fe concentration. The maximum zero resistance temperature T c 0 (= 40 K obtained for n = 0) consistently decreases with increase in the Fe content. This behavior is consistent with the pair breaking mechanism. The Seebeck coefficient ( S ) of the crystalline superconductors showed nonlinear thermal variation which was explained by phonon drag effect.

A simple method for the preparation of YBa2Cu3Ox superconducting wires and tapes using common polymers

Wires and tapes of YBa2Cu3Ox (Y-123) type high temperature superconducting oxides (HITSO) have been prepared by annealing the (Y-123)-polymer (polyethylene or polymethyl methacrylate) composites at (500-800°C) for eight hours. The (HITSO) wires and tapes of different porosity thus obtained are characterized from the studies of electrical and magnetic properties. Increase of porosity has little effect on the superconducting transition temperature. Importance of these composites for making non-porous (HITSO) wires or tapes has also been discussed.

Optical reflectivity of superconducting Bi3.9Pb0.1Sr3Ca3Cu4Ox obtained by glass-ceramic route in 0.6–6.2 eV range

Abstract The room temperature reflectance spectra in the UV-VIS-NIR region (0.6–6.2 eV) for the superconducting ceramic phase of Bi3.9Pb0.1Sr3Ca3Cu4Ox, obtained by glass-ceramic route have been studied by Kramers-Kronig (KK) analysis. From these analyses, the real (e′) and imaginary (e″) parts of the complex dielectric function, e = e′ + ie″, optical conductivity (σ′) and the absorption spectra (α) have been determined. These studies have indicated the presence of plasmon around 1 eV and have also shown that in these oxide systems, Biue5f8O and Srue5f8O layers have little role to play in the mechanism of superconductivity. The possibility of excitonic absorption in Bi-based high Tc superconductors has also been discussed from the knowledge of the absorption spectra.

Study of normalised resistance and thermopower of a hole doped Bi2-xCdxSr2CaCU2Oy (x = 0.0-0.8) superconducting system

Abstract Temperature variation of normalised resistances and thermopowers of the Cd doped superconducting Bi 2− x Cd x Sr 2 CaCu 2 O y ( x = 0–0.8) oxides have been thoroughly studied. The superconducting transition temperature is found to decrease with the increase of Cd content and for the sample with x = 0.8 no T c 0 (zero resistance temperature) is observed down to 10 K, although T c (onset) could be observed. Interestingly none of the samples (with varying Cd content) exhibit semiconductor-like normal state behaviour. Both negative and positive thermopower have been noted for the present Cd-doped Bi- 2212 superconductors, indicating that both types of carrier (electron and hole) are involved in conduction mechanism. Among the existing theoretical models, Two-Band model and Nagaosa-Lee model have been used to fit the normal state TEP data of those superconductors.

Preparation and characterization of superconducting (YBa2Cu3O x -Ag) composites obtained by sol-gel method

For the first time a simple sol-gel method has been developed for the preparation of [YBa2Cu3Ox-Ag] composites using nitrates of Y, Ba, Cu, and Ag. For the two composites (with 10 and 20 wt% Ag), the electrical resistivities and magnetic susceptibilities (both alternating and direct current) show critical temperatureTc (zero) around 90–91 K. The X-ray diffraction studies indicate no major change of lattice parameters for the orthorhombic phase. The critical current density,Jc, is found to be around 200 A cm−2 at zero field and 80 K. The degradation ofTc of the composites in presence of water is also very low, similar to the previous observations. This sol-gel method could be used for the large-scale production of superconducting composite powders necessary for drawing wires/tapes or targets for sputtering.

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi3.9Pb0.1Sr3Ca3Cu4Ox have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (Eg) has also been made from the linear fit ofα2 vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.

Superconductivity in a New Pb1—xZnxSr2LaCu2Oy (x = 0.1, 0.3 and 0.5) System: An Optical Reflectivity Study on Pb0.9Zn0.1Sr2LaCu2Oy Superconducting Oxide

A new superconducting series, viz., Pb 1-x Zn x Sr 2 LaCu 2 O y for x = 0.1, 0.3 and 0.5, has been synthesized by a solid state reaction method. The highest superconducting transition temperature (T C 23 K) is observed for x = 0.1. T C is found to decrease with higher Zn concentration, and metal-like normal state behaviour is exhibited for all concentrations of Zn. The X-ray diffraction lines could be fitted to a tetragonal structure, yielding the lattice parameters a and c. Room temperature optical reflectivity spectra in the UV-VIS-NIR range (0.6 to 6.2 eV) for the superconducting sample Pb 0.9 Zn 0.1 Sr 2 LaCu 2 O y have been analyzed by the Kramers-Kronig method. This analysis revealed a few interesting features like the absence of an absorption peak around 1.7 eV and the appearance of a hump-like feature around 4.1 eV, distinguishing this superconducting cuprate from other high-T C cuprates.