M. Chandra Sekhar

Bio: M. Chandra Sekhar is an academic researcher from Osmania University. The author has contributed to research in topics: Dielectric & Atmospheric temperature range. The author has an hindex of 5, co-authored 9 publications receiving 53 citations.

Thermoelectric power in Bi2Sr2−xKxCaCu2Oy

Abstract: The superconducting and normal state transport properties of ceramic samples with composition Bi 2 Sr 2− x K x CaCu 2 O y ( x = 0, 0.1, 0.2, 0.3, 0.4) have been investigated by measuring thermoelectric power. The samples were prepared by the solid state route, and single phase Bi-2212 was found for all compositions. The T c (0) and thermoelectric power data were collected from 50 to 300 K. The thermoelectric power changes slightly with increasing doping level and increases linearly with decreasing temperature reaching a maximum value at T c (0). The T c (0), hole concentration and thermoelectric data were correlated. The results obtained are analysed in terms of phenomenological theoretical models that makes it possible to determine the band spectrum parameters in the normal state and to trace their changes with varying composition. Following models applied such as (a) Hubbard model, (b) two-band model, (c) Fermi-liquid behaviour. Applicability of these models to the present HTSC system is discussed and the parameters related to the models have been evaluated.

Elastic anomalies in sodium-doped Bi-2212 single-phase HTSC materials

Abstract: A series of sodium-doped Bi-2212 single-phase high- superconductors have been prepared by the solid state reaction method. After the usual characterization by XRD, SEM, bulk density porosity etc, ultrasonic longitudinal velocity studies were undertaken over the temperature range 80 - 300 K using the pulse transmission technique. In the temperature range 300 - 230 K, in contrast to normal solids, the velocity of Bi - Na(0) and Bi - Na(3) samples is found to decrease with decreasing temperature (elastic softening) followed by a velocity maximum at around 150 K for all the four samples, signifying the presence of lattice instabilities. Plausible explanations are given for both the elastic softening and the velocity maxima on the basis of microstructure and ordering readjustments of the oxygen atoms among the Cu - O planes of the samples.

Effect of lithium doping in the Bi-2212 superconducting system

Abstract: Samples with the nominal composition Bi 2 Sr 2 Ca 1−x Li x Cu 2 O y (x = 0.2, 0.4, and 0.6) and Bi 2 Sr 2 CaCu 2−x Li x O z (x = 0.4, 0.6, and 1.0) were prepared by the solid state reaction method. The role of Li at both Ca-and Cu sites in the Bi-2212 composition were studied. From the X-ray diffraction data it was found that the Li-doped at the Ca site increases the c-axis and that doped at the Cu site decreases the c-axis. From the D.C. four-probe resistivity data it was found that Li-doped at the Ca site reduces the T c and the Li-doped at the Cu site gives T c (0) above the liquid nitrogen temperature. It was observed that the Li doping reduces the melting point of Bi-2212 composition. The presence of Lithium was confirmed by inductive the coupled plasma method

Superconductivity in the Bi - Sr/K - Ca/Na - Cu - O system

Abstract: Samples with the stoichiometric composition with x = 0.1 and y = 0.1, 0.2, 0.3 and 0.4 were prepared by the solid state reaction method. The role of Na and K doped at the Ca and Sr sites simultaneously in the Bi-2212 composition was studied. From the x-ray diffraction data it was found that the structure is single-phase Bi-2212. From the resistivity data the varies from 63 to 67 K for different dopant levels. The diamagnetic nature of the samples was confirmed by ac susceptibility measurements. The oxygen content was determined by the two-step iodometric titration technique. The present results on the effect of double doping are compared with those of single dopants published earlier by our group. The microstructures of the samples were examined by scanning electron microscopy.

High Dielectric Constant and Variable Range Hopping in Bi2Sr2YCu2Oy

Abstract: The sample Bi2Sr2YCu2Oy prepared by solid state reaction method showed a single phase Bi-2212. DC resistivity measured in the temperature range 77.5–300 K showed M–I like transition. AC conductivity measurements were performed in the temperature range 80–473 K at different frequencies. The conduction is by Mott's Variable Range Hopping. Various parameters pertaining to the conduction process are evaluated. The value of dielectric constant e increases with temperature and shows a very high value. For a given temperature the value is found to decrease with increase in frequency.

Giant Faraday rotation in Bi(x)Ce(3-x)Fe5O12 epitaxial garnet films.

Abstract: Thin films of Bi(x)Ce(3-x)Fe(5)O(12) with x = 0.7 and 0.8 compositions were prepared by using pulsed laser deposition. We investigated the effects of processing parameters used to fabricate these films by measuring various physical properties such as X-ray diffraction, transmittance, magnetization and Faraday rotation. In this study, we propose a phase diagram which provides a suitable window for the deposition of Bi(x)Ce(3-x)Fe(5)O(12) epitaxial films. We have also observed a giant Faraday rotation of 1-1.10 degree/µm in our optimized films. The measured Faraday rotation value is 1.6 and 50 times larger than that of CeYIG and YIG respectively. A theoretical model has been proposed for Faraday rotation based on density matrix method and an excellent agreement between experiment and theory is found.

Dielectric, Impedance, Magnetic and Magnetoelectric Measurements on YMnO3

Abstract: Polycrystalline hexagonal Yttrium Manganites (YMnO 3 ) were prepared by solid-state route. X-ray studies revealed the formation of homogenous and single phase material in hexagonal structure. Dielectric, impedance, electrical, magnetic and magnetoelectric measurements were performed on the YMnO 3 ceramics. Dielectric and impedance studies show relaxations near 200°C. Complex spectroscopic analysis, in the temperature range of 30–300°C from 1 kHz to 1 MHz, showed non-Debye relaxations. Ac and dc electrical measurements were extracted to understand the transport mechanism: Ac conductivity data is found to obey ‘Universal’ power law with exponent 0.8. Low temperature magnetization measurements were performed to understand the magnetic nature of the sample. Room temperature magnetoelectric (ME) measurements were also performed and corroborated with the magnetization data.

Influence of sintering temperature on thermopower and hardness of RuSr2GdCu2O8

Abstract: The structural, mechanical and transport properties of RuSr2GdCu2O8 sintered between 1000 and 1065 ◦ Ci n O 2 atmosphere have been studied by XRD, microhardness and Seebeck coefficient measurements. The pellets have a tetragonal perovskite-type structure. Microhardness and bulk density of the considered samples increase with increasing sintering temperature. TEP is positive for all samples and decreases with increasing sintering temperature. The normal state TEP data are discussed in view of some theoretical models, namely a two-band model with an additional linear T term and variable range hopping models. (Some figures in this article are in colour only in the electronic version)