S.K. Bandyopadhyay

Generalization of fluctuation induced conductivity in polycrystalline Y1−xCaxBa2Cu3Oy and Bi2Sr2Ca1Cu2O8+δ superconductors

We have investigated the fluctuation induced conductivity (FIC) of two high-Tc superconductors (HTSC) namely Bi2Sr2Ca1Cu2O8+δ and Y1−xCaxBa2Cu3Oy. The effective medium theory is used to formulate the Aslamazov–Larkin type equations for these polycrystalline superconductors. For the three dimensional case the temperature exponent remains unaltered since both the exponents for the ab plane and c direction FICs in the crystal are same. But the situation is more complex in the two dimensional case where the counterparts in the crystal have different temperature dependencies. The FIC is evaluated and compared with the theoretically obtained values from the model proposed. We have also evaluated the exponents, the coherence lengths, and the crossover temperature. The analysis based on the present model reveals that these two superconductors are similar concerning their nature of fluctuations. The extent of a particular fluctuation region of course differs from one HTSC to another.

Fluctuation-induced conductivity of polycrystalline Y1−xCaxBa2Cu3O7−δ superconductors

Abstract We report the fluctuation-induced conductivity of Ca doped Y 1 Ba 2 Ca 3 O 7−δ samples. The analysis has been done with partial substitution of Ca(II) in place of Y(III). The Ca contents are 0.02, 0.05, 0.1 and 0.2. In each case excess conductivity has been analysed and we tried to make a fit with the Aslamazov-Larkin and Lawrence-Doniach equations. It is observed that our data fit well with the 3D AL equation and a cross-over from 2D to 3D has been found in our samples. For the first time we have employed this transition to estimate the Josephson coupling strength in the multiperiodic layered defect superconductor. This interlayer coupling strenght J that controls the transition decreases with increase in Ca content. The variation of the Josephson coupling strength and T c with x have been discussed. The Ginzburg-Landau coherence lengths ξ c (0) for all cases have also been calculated and plotted.

Excess-conductivity analysis of α irradiated polycrystalline Bi-2212 superconductor

Abstract Excess conductivities of polycrystalline samples irradiated with alpha particles at various doses and of unirradiated ones have been studied from the data of resistivity versus temperature. The range of temperature chosen was from around 2 T c to T c onset taken as T c mf , the mean-field-transition temperature phenomenologically. The logarithmic plots of excess conductivity and reduced temperatures [( T - T c mf ] reveal two cross-over temperatures with three exponents. Except for the unirradiated sample, which shows a distinct cross-over from two-dimensional to three-dimensional conductivity, all the irradiated samples fit only with the two-dimensional Aslamazov-Larkin equation above the cross-over temperature T 0 . Apparently, there is an onset of critical fluctuation around T 0 in the case of the irradiated samples. Interlayer-coupling strengths estimated show a decreasing trend with the increase in dose, corresponding to a decrease in carrier concentration.

Scaling behavior of the Portevin-Le Chatelier effect in an Al-2.5%Mg alloy

The scaling behavior of the Portevin-Le Chatelier (PLC) effect was studied by deforming an Al-2.5%Mg alloy for a wide range of strain rates. To reveal the exact scaling nature, the time series data of true stress versus time, obtained during deformation, were analyzed by two complementary methods: the finite variance scaling method and the diffusion entropy analysis. From these analyses we could establish that, in the entire span of strain rates, the PLC effect showed the Levy-walk property.

Characterisation of microstructural parameters in oxygen-irradiated Zr–1.0%Nb–1.0%Sn–0.1%Fe

Abstract We have characterised the microstructural parameters like domain size, microstrain within the domain, dislocation density and stacking fault probabilities in Zr–1.0%Nb–1.0%Sn–0.1%Fe alloy, irradiated with 116 MeV O 5+ ion at different doses, by X-ray diffraction line profile analysis using modified Rietveld technique. The analysis revealed that there is a significant decrease in domain size but increase in microstrain and dislocation density with dose of irradiation. These microstructural parameters have been compared with our earlier observations on the same alloy irradiated at different doses with 15 MeV proton. We find significant changes of these parameters in oxygen-irradiated samples as compared to the proton-irradiated samples. The variations of microstructures in these samples have been explained in the light of the mechanism of defect evolution for light and heavy ion irradiation.

The study of microstructural defects and mechanical properties in proton-irradiated Zr–1.0%Nb–1.0%Sn–0.1%Fe

Abstract We had earlier analysed point defects in irradiated Zr–1.0%Nb–1.0% Sn–0.1%Fe samples by positron annihilation lifetime spectroscopy (PALS). The studies revealed the generation of irradiation induced di- and tri-vacancy clusters. In the present work, we have carried out X-ray diffraction line profile analysis (XRDLPA) and studies of mechanical properties like ultimate tensile strength (UTS), yield strength (YS) and percentage elongation (%E) on the same irradiated samples at different doses. XRDLPA reveals that order of dislocation density remains almost unchanged up to the experimental limit of irradiation dose. Attempts have also been made to correlate the changes in mechanical properties with irradiation-induced defects. YS of the alloy increases with irradiation due to locking, or pinning of dislocations with vacancy clusters. However, there is an anomaly in YS at a dose of 5×10 20 protons/m 2 which has been explained in the light of interaction of vacancies with solute atoms. Percentage elongation on the other hand, shows a monotonic fall with increasing dose.

Proton irradiation effects in Zr-1.0 Nb-1.0 Sn-0.1 Fe probed by positron annihilation

Samples of a new zirconium-based alloy (zirlo) were proton-irradiated and the structural defects were characterised by positron annihilation. The lifetimes of positrons trapped in the defects generated at different doses of irradiation appeared to suggest significant interaction of the defects with the substitutional solute atoms. The isochronal recovery of the sample irradiated at the highest dose showed three distinct stages. The initial annealing of free vacancy clusters was followed by a solute atom-vacancy dissociation stage above 673 K. The vacancies thus released annealed out at 973 K.

The study of defects, transport properties and dissipative flux motion in proton irradiated textured polycrystalline Bi2Sr2CaCu2O8+δ and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+δ superconductors

Abstract Textured polycrystalline Bi2Sr2CaCu2O8+δ (Bi-2212) and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+δ (Bi-2223) samples have been irradiated with 15 MeV protons at different fluences (1×1015 protons/cm2 to 1×1016 protons/cm2). Positron annihilation spectroscopy studies show that divacancies and monovacancies are the predominant trapping sites of positrons in unirradiated Bi-2212 and Bi-2223, respectively. In Bi-2212, divacancies agglomerate to trivacancies at moderate doses, while at higher doses, vacancies reduce to monovacancy and defect concentration increases. There is no appreciable change in size of vacancies in Bi-2223 with dose. Resistivity under magnetic field (up to 7 T) oriented in the direction of irradiation, has been studied to analyze the dissipative flux motion based on thermally activated flux creep model. The sharp fall in activation energy U(0,H) in Bi-2223 at high dose has been observed compared to that of Bi-2212.

The study of texturing of Bi2Sr2CaCu2O8+δ and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+δ superconductors as a function of pelletisation pressure

Abstract Crystallographic texture evolution in polycrystalline Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) and Bi 1.84 Pb 0.34 Sr 1.91 Ca 2.03 Cu 3.06 O 10+δ (Bi-2223) phases under axisymmetric compression has been investigated. The degree of texturing in these samples is ascertained by X-ray diffraction. The variation of microstructure and electrical with temperature as a function of pelletization pressure of these samples are studied. T c onset changes with pressure in Bi-2212, but not in Bi-2223. The degree of texturing in Bi-2212 increases almost linearly with pressure during cold compression but in annealed samples it reaches a maximum at a pressure of 600 MPa. The optimised pressure needed for single-phase formation of Bi-2223 has been evaluated. Low- T c phase (Bi-2212) segregation in Bi-2223 occurs at 800 MPa, higher than the pressure (400 MPa) responsible for attaining maximum texturing. Texturing is achieved presumably through two different mechanisms,i.e. by deformation texturing through slip plane during cold compression and by recrystallisation texturing through oriented grain growth during annealing.

A STUDY OF SUPERCONDUCTING (Y1-XCAX)BA2CU3OY

Abstract We have investigated the effects of hole doping in the YBaCuO system by substituting calcium(II) in the Y(III)-site. We have analysed the resistivity, paraconductivity, interlayer coupling strength ( J ) as a function of dopant concentration. T c and J are found to decrease with increasing calcium content. We have also studied surface binding energies of various components in a Ca doped sample through secondary ion mass spectrometry (SIMS). The binding energy of CuO in a CuO 2 sheet does not increase as compared to that in undoped YBCO, indicating that the extra hole doped in the Y site does not bring positive charge to the conducting CuO 2 layer. Doping of calcium perhaps destroys the ordering of oxygen in the CuOCu chain, which affects superconductivity.