S. Vinu

Doping controlled metal to insulator transition in the (Bi, Pb)-2212 system

We report on the details of the metal?insulator transition in the Bi1.7Pb0.4Sr2?xPrxCa1.1Cu2.1O8+? system, in the concentration range of x = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.3 and 1.5. In addition to the electrical resistivity measurements at 6?300?K, the phase, microstructural and elemental analyses were also carried out to assess the relative performance of the samples. A detailed analysis of measurement results reveals that Pr atoms are successfully substituted at the strontium site of the (Bi, Pb)-2212 system. The system is found to be superconducting in the doping range of x?0.6 and semiconducting for x?0.7. A metal to insulator transition is found to occur in the doping range 0.6<x?0.7, and the results reveal that a disorder induced transition leads to the localization of states in the vicinity of the Fermi level and to the said phenomenon in the Pr substituted (Bi, Pb)-2212 system. Resistivity in the insulating regime of samples was analyzed using a generalized hopping approach and the transport mechanism is found to be governed by the VRH mechanism. Superconductivity in conjunction with 2D VRH is observed near the superconductor?insulator boundary and is in agreement with a theoretical model that considers the competition between superconductivity and localization in a disordered system.

Enhancement of critical current density and flux pinning properties of Gd-doped (Bi, Pb)-2212 superconductor

The enhanced critical current density (JC) and flux pinning properties of Bi1.7Pb0.4Sr2-xGdxCa1.1Cu2.1O8+δ (where x=0.0, 0.1, 0.2, and 0.3) system prepared by solid-state synthesis in bulk polycrystalline form were studied. Phase analysis, microstructural investigation, and superconducting characterization were performed to evaluate the relative performance of the samples. The x-ray diffraction and energy dispersive spectroscopy analyses show that Gd atoms are successfully doped in place of Sr in the system. It is found that Gd in (Bi, Pb)-2212 enhances the critical temperature (TC), JC, and flux pinning strength of the system. The flux pinning force (FP) calculated from the field dependant JC values shows that the irreversibility line of the Gd-doped (Bi, Pb)-2212 shift toward higher fields to different extents depending on the value of x. The samples with x=0.2 show maximum FP of 645 kN m−3 and the peak position of FP shifts to higher fields (0.68 T) as against 26 kN m−3 and 0.12 T for the pure sample. ...

Metal-insulator transition and conduction mechanism in dysprosium doped Bi1.7Pb0.4Sr2Ca1.1Cu2.1O8+δ system

To explore the mechanisms behind the metal-insulator transition and charge transport in high temperature superconducting cuprates, a systematic study on the Bi1.7Pb0.4Sr2Ca1.1Cu2.1O8+δ compound was made by adding the rare-earth dysprosium at stoichiometric amounts (x) of 0.5≤x≤1. Phase analysis, determination of lattice parameters, microstructure analysis, and elemental analysis were carried out to evaluate the relative performance of the samples prepared by the solid state synthesis route. Charge transport in the insulating and superconducting samples were analyzed through resistivity measurements at (64–300 K). It is found that the x=0.5 sample is superconducting with a critical transition temperature of 94.8 K while for the samples with x>0.5 superconductivity is suppressed, along with an increase in their normal state resistivities. A metal-insulator transition is found to take place around 0.5<x≤6. A detailed analysis of the experimental data shows that the hole filling and disorder, induced by chang...

The influence of sintering temperature on the microstructure and superconducting properties of Bi1.7Pb0.4Sr1.8Nd0.2Ca1.1Cu2.1O8+δ superconductor

The effect of sintering temperature on the microstructure and superconducting properties of Bi1.7Pb0.4Sr1.8Nd0.2Ca1.1Cu2.1O8+? has been investigated. The samples are prepared by the solid-state route. Remarkable variations in microstructure, critical current density (JC) and flux pinning properties have been observed for Nd-substituted samples, sintered at different temperatures in the range 846?852??C. The flux pinning force (FP) calculated from the field-dependent JC values show that the irreversibility lines (IL) of Nd-substituted samples shift towards higher fields to different extents depending on the sintering temperature. The samples sintered at 846??C show maximum flux pinning force (FP) of 631?kN?m?3 and the peak position of FP shifts to higher fields (0.8?T) as against 262?kN?m?3 and 0.44?T for the samples sintered at 852??C and 18?kN?m?3 and 0.08?T for the undoped sample sintered at its optimum sintering temperature, respectively. But the self-field JC value of the samples sintered at 846??C is lower than that of the samples sintered at 852??C. The samples sintered at 852??C show maximum self-field JC due to improved microstructure. The changes in microstructure followed by very high enhancement of self-field JC, JC?B characteristic and pinning force density (FP) due to Nd?substitution within a narrow temperature range, is of great scientific and technological significance.

Critical current density and flux pinning in a Bi1.7Pb0.4Sr2−xLaxCa1.1Cu2.1Oy system

The critical current density of Bi2Sr2CaCu2O8+δ (Bi-2212) superconductors can be enhanced either by changing the charge carrier concentration (holes) to the optimum value, from the overdoped state, or by creating flux pinning centres to inhibit the flux flow in magnetic fields. In this work we substitute the rare earth La at the Sr site of Pb-doped Bi-2212 [(Bi, Pb)-2212]. La3+ ions at the Sr2+ site decrease the hole concentration to the optimum value and enhance the superconducting properties. Also La at the Sr site creates point defects which act as flux pinning centres. These dual effects of La at the Sr site highly enhance the transport JC in self and applied magnetic fields.

Refinement of microstructure and highly improved electrical properties of Bi1.6Pb0.5Sr1.925Ho0.075Ca1.1Cu2.1O8+δ superconductor

The importance of rare-earth (RE) doping and precise tuning of sintering temperature on the microstructural features, electrical properties, and magnetic performance of a (Bi,Pb)-2212 superconductor sintered between 846 and 860 °C has been investigated. The results show that the RE (holmium) atoms enter into the crystal structure and induce significant changes in the hole concentration, lattice parameter values, microstructure, normal state conductivity, self- and in-field critical current densities, and flux pinning properties of the system. The microstructural analysis shows that the samples sintered at a comparatively lower sintering temperature (846 °C) have more grain boundaries with smaller grains, while those sintered at a higher sintering temperature (856 °C) contain larger grains with good texturing. Also, the irreversibility lines are shifted to higher temperatures and magnetic fields to different extents depending on the sintering temperature. The flux pinning force density (FP) curves are theo...

Transport properties near the metal to insulator transition in samarium substituted (Bi,Pb)-2212 system

The electrical transport properties of insulating and superconducting samples of Bi1.7Pb0.4Sr2−xSmxCa1.1Cu2.1O8+δ system is studied across the metal to insulator transition (MIT) region by varying the x values from 0.5 to 1.0 in steps of 0.1. X-ray diffraction analysis, scanning electron microscopy, energy dispersive x-ray analysis (EDAX), and electrical resistivity measurements have been employed for the characterization of all samples. The x-ray and EDAX analyses indicate that samarium (Sm) atoms are incorporated into the crystalline structure of Bi1.7Pb0.4Sr2Ca1Cu2O8+δ [(Bi,Pb)-2212]. Samples with x≤0.6 undergo superconducting transitions while those with 0.7≤x≤1.0 exhibit semiconducting behavior. The MIT is observed at 0.6<x≤0.7 with a suppression in superconductivity. The results show that with increase in temperature, the transport mechanism of the semiconducting samples transits from two dimensional variable range hopping to thermally activated conduction. A scaling model is presented for the MIT a...

Structural and Transport Properties of the

The effects of Lu-doping on phase evolution, microstructure, electrical, and superconducting properties of Bi-2212 have been studied. The results show that Lu+3 ions enter into the crystal structure in place of Sr+2 ions and, hence, reduce the hole concentration in the electronic structure of the system. Lu-doping results in the reduction of c-axis length and increases the Josephson coupling between the Cu-O2 layers at lower Lu-contents. The critical temperature Tc and the critical current density Jc are significantly enhanced for optimum Lu concentration. The variations in the electrical and superconducting properties are due to the structural and electronic inhomogeneities and changes in charge carrier concentration due to Lu-doping in the (Bi, Pb)-2212 superconductor.

( \hbox{Bi}_{1.6}\hbox{Pb}_{0.5})(\hbox{Sr}_{2-x}\hbox{Lu}_{x})\hbox{Ca}_{1.1}\hbox{Cu}_{2.1}\hbox{O}_{8 + \delta}

The vortex-liquid resistivity close to the vortex glass to liquid transition region is analyzed in detail for Ho doped (Bi, Pb)-2212 superconductor. A large broadening of resistive transition in magnetic field is observed, which is a direct evidence for the thermal fluctuation in the vortex system, and hence the modified vortex glass to liquid transition theory is used to calculate the temperature and magnetic field dependent activation energy U0(B,T). The resistivity shows a glassy behavior even at higher temperature and magnetic field for the Ho stoichiometry x=0.075, which is believed to come mainly from the point defect brought about by the doped Ho atoms in the (Bi, Pb)-2212 system. The results also show that the glass transition temperature (Tg), magnetic field dependent activation energy U0(B), and U0(B,T) values are maximum for x=0.075, which shows that the flux lines are effectively pinned and the vortices are in glassy state for this sample, and the enhancement due to Ho doping has both scientif...

Superconductor

Knowledge of the electric field versus current density (E-J) characteristics is very important for the development of superconducting high-field magnets particularly for operation in the persistent mode. A systematic study of the E-J characteristics and the associated n-indices has been carried out both in self and applied fields in respect of Tb-doped (Bi,Pb)-2212, a promising superconductor for high-field application. The results on n-indices and pinning energy (Uc) show that the flux-lines in (Bi,Pb)-2212 doped with an optimum concentration of Tb (x=0.075–0.100) are in glass-state and hence the material allows maximum current to flow with minimum dissipation of energy. Further, it has been shown that the modified material is a promising candidate for persistent mode superconducting magnets.