B. Ghosh

Excess conductivity and thermally activated dissipation studies in Bi2Sr2Ca1Cu2Ox single crystals

Abstract The resistivity of Bi 2 Sr 2 Ca 1 Cu 2 O x single crystals has been measured in the ab -plane and along the c -axis. In all the directions the resistivity shows metallic behavior in the normal state. Room temperature resistivity along the c -direction is about 64 times higher than that in the ab -plane. Departure from the linear behavior occurs at 96 K and the zero resistance is obtained at 76 K. Excess conductivity analysis shows that the system is 2-dimensional in the temperature range 78.7–96 K and a 3-dimensional one below 78.7 K. Thus, a crossover from 2D to 3D is observed at 78.7 K in the Bi-2212 single crystal. This crossover was not detected in Bi 2212 or 2223 polycrystalline samples. The width of the transition (Δ T ) varies with the application of magnetic field ( H ) as H 0.85 in Bi 2212 single crystal. The pinning potential ( U 0 ) varies as H -α where α =0.3 for the Bi-2212 single crystal and 0.47 for polycrystalline sample.

Electrical resistivity, magnetoresistance, magnetisation, hall coefficient and excess conductivity in Pb-doped Bi-Sr-Ca-Cu oxides

The electrical resistance, Meissner signal, magnetoresistance and Hall coefficient of Bi1.75Pb0.25Sr2Ca2Cu3Ox and Bi1.5Pb0.5Sr2Ca2Cu3Ox (nominal compositions) have been measured. Resistance of the Bi1.75Pb0.25 sample becomes zero for T≤106 K while the Bi1.5Pb0.5 sample shows a 65% drop in the resistance around 106 K and TR = 0c ≈ 73 K. Powder X-ray diffraction analysis reveals that the major phase in the Bi1.75Pb0.25 sample is the high-Tc phase and in the Bi1.5 Pb0.5 sample the low-Tc phase. The Hall coefficients (RH) of both samples are positive with R−1H> linear in temperature. The temperature dependence of RH is stronger in the Bi1.75Pb0.25 sample than in the Bi1.5Pb0.5 sample. The carrier concentration determined R−1H for the Bi1.75Pb0.25 and the Bi1.5Pb0.5 samples at 300 K are 1.59 × 1021 cm−3 and 3.66 × 1021 cm−3, respectively. The excess conductivity of both samples is analyzed using the Aslamazov-Larkin expression. The critical exponent λ obtained for the Bi1.75Pb0.25 sample is 0.78 and that for the Bi1.5Pb0.5 sample, in the temperature region where the high-Tc phase contributes only, is 0.77.

Scaling of the thermoelectric power in the and systems

The temperature-dependent thermoelectric power S of insulating and metallic Bi-2212 and Tl-2212 samples are scaled to universal master curves. For insulating samples, S passes through a peak at a characteristic temperature , which decreases with increasing carrier concentration. For metallic samples a scaling parameter (= dS/dT at high temperatures) decreases with increasing carrier density. It is argued that is related to the energy needed for activation conduction whereas the change in may be due to increase in with carrier density, where n is the concentration and is the effective mass of the carriers.

Excess conductivity in Tl1Ca3Ba1Cu3Oχ and Bi1Ca1Sr1Cu2Oχ systems due to thermodynamic fluctuations

The excess conductivity (Δσ) in Tl1Ca3Ba1Cu3Oχ and Bi1Ca3Sr1Cu2Oχ (nominal compositions) is determined using the linear extrapolation method and Anderson-Zou formula. Δσ for the TI-system shows a 2-dimensional character and is well described by the Aslamazov-Larkin (A-L) contribution for a layered superconductor. The interlayer coupling for the TI-system is found to be very small. The excess conductivity in a magnetic field perpendicular to the current direction Δσ⊥(H), for T≤Tc and H≥62; Hc2 shows a 3-dimensional character. Similar behavior is also observed in the Bi-system.

A new mercury-based high-Tc cuprate Hg0.7V0.3Sr2−xLaxCuO4+δ

Abstract A new barium-free mercury-based superconductor Hg 0.7 V 0.3 Sr 2− x La x CuO 4+δ ( x = 0.1–0.8) has been synthesized. T c onset of this compound is ∼50 K and does not vary appreciably with x . While the lanthanum-free ( x = 0) sample contains predominantly various impurity phases, samples with x = 0.2 and 0.3 are almost single phase. X-ray diffraction analysis reveals that the crystal structure has a tetragonal symmetry with lattice parameters a=3.8133–3.8266 A and c = 8.7361–8.7944 A . A neutron powder diffraction (NPD) study on the x = 0.2 sample shows that the O(3) site is four-fold disordered in the HgO plane having a total occupancy ( δ ) ∼ 0.8. The variation of the resistivity with temperature shows a metallic character for all the samples. The as-prepared samples are in the overdoped region which has been established by thermoelectric-power measurements and annealing treatments. T c is found to increase with CuO(1) and CuO(2) bond distances whereas it is inversely related to (1/ ϱ 300 ) d ϱ /d T .

Studies of transport properties of MgB2 superconductor

Abstract We present the results of electrical resistivity, thermoelectric power and thermal conductivity on polycrystalline MgB 2 samples. From the analysis of the results we find the Fermi energy E F =0.8 eV and the electron–phonon coupling constant, λ tr , estimated from transport properties, lies in the range 1.25–2.25.

Destruction of ferromagnetism in Cu-doped ZnO upon thermal annealing: role of oxygen vacancy

We report on the role that oxygen vacancies have in determining the magnetic properties of Cu-doped ZnO. The observed magnetic hysteresis and the temperature dependence (irreversibility) of magnetization behave like supermoments blocked along random directions for a low-temperature annealed sample. The magnetic hysteresis (finite coercivity and low field saturation) and the irreversibility of temperature-dependent magnetization are destroyed upon thermal annealing. We propose that oxygen vacancies play an important role in inducing supermoments to explain the observations.

Impurity substitution in bismuth and thallium cuprates: suppression of Tc and estimation of pseudogap

Abstract Suppression of T c in bilayer bismuth and thallium cuprates by substitution of Co impurities at Cu sites were taken for examination. T c suppression data on differently doped Bi2212 and Tl2212 were analysed within the unitary pair-breaking formalism due to Abrikosov and Gorkov, by fitting data points to a phenomenological relation valid for weak coupling d-wave superconductors. Values of the pseudogap magnitude at each doping were thereby estimated within a “Fermi-level density of states suppression” picture. Pseudogap magnitude from our estimation was observed to have a correspondence with a related characteristic temperature T * obtained by thermoelectric power measurements. Effects of pseudogap on the density of states were studied by calculating the susceptibility, which shows a broad peak at high temperature. This peak feature in susceptibility is indicative of an unusual metallic state, which could be explored further by other systematic measurements.

Infrared studies of Bi2Sr2Ca1-xYxCu2O8+δ and correlation between Cu-O distance derived from X-ray and IR

Abstract Absorption spectra of well-characterized Bi2Sr2Ca1-xYxCu2O8+δ ceramic samples have been studied in the middle and far infrared region. Assignment of different vibrational frequencies have been revisited in the framework of Y substitution in the Ca site which creates screening and a shift of different vibrations apart from the metal-to-insulator transition. The estimated values of the Cu-O distance (and thus the a parameter of the unit cell) from the Cu-O stretching vibration frequency and from X-ray diffraction data have been compared.

Superconductivity, antiferromagnetism, hall coefficient and the thermoelectric power for a single-band hubbard model

Abstract The superconductivity, antiferromagnetism, Hall coefficient and thermoelectric power of a system described by a single-band Hubbard model with strong on-site correlation have been studied using mean-field approximations. In one approach, the strong correlation reduces the bandwidth, the band remains half-filled for n =1 (one electron per site) and the chemical potential lies at the centre of the band. This approach describes the antiferromagnetic transition satisfactorily but not the electronic transport properties. The other approach follows the Hubbard method in which for n ⩽1 and strong correlation only the states of the lower Hubbard subband are occupied. This approach can describe superconductivity in the region of hole concentration in which high-temperature superconductivity has been observed in the oxide systems and also explains the Hall coefficient and thermoelectric power. Comparison of the theoretical results with the observed superconductivity and the thermoelectric power of the high- T c systems suggests that the effective bandwidths in these systems are small.