Ajay Kumar Ghosh

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.

Pattern induced phase transition of vortex motion in high-Tc films

A micropattern induced transition in the mechanism of vortex motion and vortex mobility is demonstrated for high-Tc films. The competition between the anomalous Hall effect and the guidance of vortices by rows of microholes (antidots) leads to a sudden change in the direction of vortex motion that is accompanied by a change of the critical current density and microwave losses. The latter demonstrates the difference in vortex mobility in the different phases of vortex motion in between and within the rows of antidots.

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.

α-Irradiation induced changes in fluctuation-conductivity in (Bi, Pb)-2223 superconductors

Abstract We have irradiated (Bi, Pb)-2223 superconductors by 40 meV α-particles. The irradiation induces changes in resistivity as we increase the dose of irradiation. The various doses change the resistivity as a result of which the fluctuation induced conductivity also changes. We have analysed the experimental observations in the framework of Aslamazov-Larkin, Lawrence-Doniach and Maki-Thompson theory. We have extracted coherence lengths, dimensional crossover temperature and Josephson interlayer coupling strength. The role of disorders induced by irradiation has been discussed in this respect. The change in the fluctuation-conductivity is discussed in the light of the coherence length and the critical magnetic field H c .

Oxygen Knock-Out and Other Studies in α-Irradiated Polycrystalline Bi-2212 Superconductor

Bulk polycrystalline samples of Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) have been irradiated with 40 MeV α-particles. T c increases up to a certain dose. The increase in T c is correlated with the knock-out of oxygen, which has been verified by the determinations of the oxygen contents of the irradiated samples by iodometry. A model of the knock-out of oxygen is proposed on the basis of Monte-Carlo TRIM calculations. Resistivity versus temperature of the irradiated samples shows fairly metallic behaviour up to a certain dose. Excess conductivity analysis shows a cross-over from 2D to 3D behaviour in conductivity for the unirradiated sample. However, for irradiated samples, the critical fluctuation regime sets in. The interlayer coupling strengths decrease with the increase, in the irradiation dose. The sample with the highest dose shows a nonmetallic behaviour in resistivity. A detailed analysis shows a conductivity behaviour in the nonmetallic region characteristic of three-dimensional variable range hopping of charge carriers.

Fluctuation-induced conductivity of polycrystalline high-Tc superconductors

Abstract We have formulated the Aslamazov–Larkin equations for the fluctuation induced conductivity (FIC) of polycrystalline high- T c superconductors (HTSC) with the help of the effective medium theory (EMT) which may be used for the analysis of data of polycrystalline samples. The grains are assumed to be spherical in shape. As expected the temperature exponent in the three dimensional (3D) fluctuation remains unaltered whereas the temperature dependence of FIC in the 2D regime is different from that of a crystal and turns out to be more complicated in the polycrystalline sample. The change is controlled by the anisotropic nature of the sample. It is also interesting to find that the fluctuation induced conductivity in polycrystalline material is smaller corresponding to their counterparts in a single crystal for both the 3D and 2D regimes which is qualitatively corroborated by observations on HTSC.

Thermodynamic fluctuations in proton irradiated (Bi,Pb)-2223 superconductors

Abstract The paraconductivity of unirradiated and irradiated Pb doped Bi-2223 superconductors has been estimated. The energy of incident proton is 15 MeV. We want to study the effect of irradiation on Bi-2223 superconductors with respect to paraconductivity analysis. We tried to find out the dimensionality of conductivity of each sample in the framework of Aslamazov-Larkin and Lawrence-Doniach theory. We have observed the 2D to 3D transition in some cases and found out the crossover temperatures T 0 for various samples. We also calculated the smallest Josephson coupling strength J 2 and hopping probability ω between two CuO 2 layers for unirradiated and irradiated Bi-2223 superconductors.

Complex susceptibility of α-irradiated (Bi, Pb)-2 2 2 3 at low field amplitude: The role of intergranular pinning potential

We have measured the a.c. susceptibility of α-irradiated (Bi, Pb)-2 2 2 3 superconductors. The peak positions of the imaginary parts are shifted to the lower temperature with the irradiation. The critical temperatures and the peak temperatures are used to find out the intergranular pinning potentials of the unirradiated and irradiated samples. The critical state model has been used to find out the intergranular pinning potential for these samples. We discuss the role of the irradiation induced defects in this paper.

Change of phase coherence in proton irradiated (Bi, PB)-2 2 2 3 superconductors

Abstract We have irradiated (Bi, Pb)-2 2 2 3 superconductors by 15 MeV protons with various doses. The critical exponent and excess conductivity have been evaluated in the paracoberence region with the help of the resistivity data for the unirradiated and irradiated superconductors. The normalised Josephson coupling energies Jij have been evaluated by using excess conductivity and critical exponent γ. The destruction of phase coherence between the superconducting grains have been described in terms of the normalised Josephson coupling energy. The suppression of normalised Jij associated with the destruction of phase coherence have been analysed in the terms of the change of Tc and the grain volumes.