G.L. Bhalla

Dissipation mechanism in a high-Tc Bi1.7Pb0.3Sr2Ca2Cu3Ox granular superconductor

Magneto-resistance studies in a high-Tc granular superconductor Bi1.7Pb0.3Sr2Ca2Cu3Ox in an applied magnetic field ranging from 0 to 0.6 T in the low current limits (1 mA) have been carried out to understand the dissipation mechanism in high-Tc superconducting materials. The Kosterlitz–Thouless (KT) as well as the Ambegaoker–Halperin (AH) widely employed models are invoked to interpret the results. The KT theory is found to account for dissipation in the temperature range TKT<T<TGL, i.e. between the Kosterlitz–Thouless temperature and the Ginzberg–Landau transition temperature. The dissipation around Tc cannot be explained adequately. It is suggested that dissipation in the entire range of temperature can be accounted for on the basis of a modified Ambegaokar–Halperin theory incorporating two mechanisms, viz. vortex-dynamics and the fluctuations of the order parameter and the two mechanisms crossover at a transition temperature (TBP).

Superconducting properties of adipic-acid-doped bulk MgB2 superconductor

We report the effect of adipic acid (C6H10O4) doping on lattice parameters, microstructure, critical temperature (Tc), current density (Jc) and irreversibility field (Hirr) for an MgB2 superconductor. Actual carbon (C) substitution level for boron (B) is estimated to be from 0.40 at.% to 2.95 at.% for different doping levels. A reduction in Tc from 38.43 to 34.93 K and in lattice parameter a from 3.084(3) A to 3.075(6) A is observed for the 10 wt% C6H10O4 doped sample in comparison to pristine MgB2. This is an indication of C substitution at boron sites, with the C coming from the decomposition of C6H10O4 at the time of reaction. Interestingly the doped samples have resulted in significant enhancement of Jc and Hirr. All the doped samples exhibit the Jc value of the order of 104 A cm−2 at 5 K and 8 T, which is higher by an order of magnitude as compared to the undoped sample. This result indicates that C6H10O4 is a promising material for MgB2 for obtaining the excellent Jc values under higher magnetic fields.

Stability estimation in YBCO by stepwise XRD and kinetic studies through TGA

Abstract The stability of the high-temperature superconductor YBa 2 Cu 3 O 7- δ on treatment with water has been examined and quantitatively determined by XRD, AC magnetic susceptibility measurements and TGA, and by calculating the lattice parameters through Cohens method. The kinetic parameters, viz. activation energy and order of reaction, have been determined from the TGA data. Progressively longer treatment results in the degradation of the material, as evidenced by corresponding changes in the values of lattice parameters, changes in orthorhombicity, and a continuous increase in the value of the activation energy. The reaction leading to the decomposition of the material is found to be of first order.

Low temperature specific heat of single phase samples of Bismuth and Thallium based high-Tc cuprates

Abstract In this paper we report the results of specific heat measurements carried out on single phase samples of Bi-based 2-1-2-2 and Tl-based 2-2-2-3 compounds. The data are compared with those obtained on Y-based 1-2-3 system. The samples have been simultaneously characterized for superconductivity using AC susceptibility and four-probe resistivity techniques and for their structural aspects using XRD. All samples studied unambiguously reveal the characteristic specific heat jump at T c . The data are used for estimation of various electronic parameters which are compared for the three systems. The parameter Δ C /(γƒ T c ) is found close to the BCS weak-coupling value.

On preventing the degradation in YBa2Cu3O7-δ superconductors caused by gamma irradiation

Abstract The stability of high-Tc YBa2Cu3O7−δ uncoated and coated with polymers viz. polymethyl methacrylate (PPMA), polyvinyl chloride (PVC) and polystyrene (PS) bulk samples irradiated by γ-rays of doses 3.024x105 and 1.210x106 rad (r) was investigated. The degradation was estimated by the change in critical temperature (Tc), thermogravimetric analysis (TGA) and oxygen content measurements. The activation energy was also determined to study the mechanism of degradation. The decrease in Tc (ΔT) was found to be different for samples coated with different materials. The results indicate that polymer coatings prevent degradation of the HTc superconductors. Though the extent of degradation depends on the nature of the polymer, plasma polymerized PMMA coating is found to be most effective in preventing the degradation of YBCO.

Anomalous thermoelectric power of the Mg1-xAlxB2 system with x = 0.0-1.0

The thermoelectric power, S(T), of the Mg1−xAlxB2 system has been measured for x = 0.0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0. X-ray diffraction, resistivity and magnetization measurements are also presented. It has been found that the thermoelectric power is positive for x≤0.4 and is negative for x≥0.6 over the entire temperature range studied up to 300 K. The thermoelectric power of x≤0.4 samples vanishes discontinuously below a certain temperature, implying the existence of superconductivity. In general, the magnitude of the thermoelectric power increases with temperature up to a certain temperature, and then it starts to decrease towards a zero baseline. In order to explain the observed behavior of the thermoelectric power, we have used a model in which diffusion and phonon drag processes are combined, by using a phenomenological interpolation between the low and high temperature behaviors of the thermoelectric power. The model considered provides an excellent fit to the observed data. It is further found that Al doping enhances the Debye temperature.

Laser-induced energy gap transitions in lead iodide crystals

Abstract Single crystals of the common polytype 2H of lead iodide were grown by zone melting and subjected to argon ion laser irradiation. Both time and intensity of radiation were systematically varied to assess their effect on the magnitude of band gap of the crystals. The band gap was determined by UV spectrophotometry. When the beam intensity was varied, the band gap value steadily decreased from 2.53 to 2.02 eV, but the value drastically decreased from 2.53 to 1.04 eV when the irradiation time was varied. The results are explained in terms of changes in polycrystallinity, surface roughness, amount of absorption, and anisotropic nature of thermal conductivity of the material.

Optimization of stability of YBa2−xSrxCu3O7−δ in water

Stability of YBa2−xSrxCu3O7−δ (0<x<0.6) in water has been investigated employing R–T measurements, oxygen content measurements, X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The Sr-doped samples are found to degrade slower than the undoped YBa2Cu3O7−δ samples, but rate of degradation increases with the dopant concentration. Optimum stability is observed for the dopant fraction x=0.2. The enhanced stability is attributed to a favourable redistribution of the oxygen ions and reorganisation of bonding in the unit cell.

Recovery of superconductivity in the water degraded YBCO samples

Abstract The recovery of superconductivity in water degraded YBa 2 Cu 3 O 7− δ samples has been studied by subjecting them to heat treatments. The possible recovery of superconductivity has been estimated through structural, electrical, magnetic and morphological investigations. The results reveal the transformation of secondary phases formed during degradation into YBCO phase. The presence of some additional phases of YBCO, possibly arising from deficiency of the constituent elements resulting from degradation/lack of proper reaction between the secondary phases, is also observed. The heat treatment carried out on a sample after pulverising it led to full recovery of superconductivity in it. A possible mechanism for the observed recovery of superconductivity has been suggested.

On the stability of YBa2Cu3−xAlxO7−δ in water

Abstract Diamagnetic Al 3+ ions have been substituted in YBa 2 Cu 3 O 7− δ sites. Comparative stability studies on 8 h and 16 h deionized water-treated samples (0 x x =0.2 is more resistant to water corrosion as compared to other compositions with x =0, 0.05, 0.1 and 0.3.