http://nopr.niscair.res.in/handle/123456789/6872 Search the Channel search http://nopr.niscair.res.in/simple-search http://nopr.niscair.res.in/handle/123456789/6896 Title: Analog multiplier using operational amplifiers<b style=""></b> <br/> <br/>Authors: Riewruja, Vanchai; Rerkratn, Apinai <br/> <br/>Abstract: Simple circuit technique for implementing four-quadrant analog multiplier has been presented. The proposed circuit requires only operational amplifier (opamp) as the active element. The realization method is based on the quarter-square technique where a square is provided from the inherent quadratic behaviour of class-AB output stage of opamp. Experimental results showing the circuit performance are described. The wore-case linearity error and total harmonic distortion for maximum operating range are about 0.23 and 1.02%, respectively. <br/> <br/>Page(s): 67-70 Tue, 29 Dec 2009 22:58:59 GMT http://nopr.niscair.res.in/handle/123456789/6895 Title: Pseudopotential approach to superconductivity in MgB₂ <br/> <br/>Authors: Sharma, K S; Bhargava, Nidhi; Jain, Ritu; Goyal, Varsha; Sharma, Ritu; Sharma, Smita <br/> <br/>Abstract: Superconductivity in MgB₂ has been re-examined in BCS-Eliashberg framework by employing Mc-Millan’s [<i style="">Phys Rev</i>, 167 (1968) 331] <i style="">T</i>_c-equation and form factors of MgB₂ computed from the form factors of component metals (Model-I). The empty core model pseudopotential due to Ashcroft [<i style="">Phys Lett</i>, 23 (1966) 48] and random phase approximation form of dielectric screening due to Gellmann and Brueckner [<i style="">Phys Rev</i>, 106 (1958) 364] are used in the present work. An excellent agreement between the present values and other theoretically computed values of <i style="">T</i>_c and with the relevant experimental data for MgB₂confirms the validity of the present approach.The explicit dependence of λ and <i style="">T</i>_c on the isotopic masses of Mg and B, as revealed from the present work, confirms the role of lattice vibrations in the superconducting behaviour of MgB₂and the high value of <i style="">T</i>_c in it may be attributed to the phonon mediated e-e interaction coupled with higher values of phonon frequencies due to light mass of B atoms. It has also been observed that the pseudo-atom model (Model-II) with appropriate choice of the potential parameter <i style="">r</i>_c successfully explains high value of <i style="">T</i>_c and isotope effect in MgB₂, confirming the prominent role played by electron-phonon interaction in the high-<i style="">T</i>_c<i style=""> </i>superconductivity observed in MgB₂. The isotope effect exponent <img src='/image/spc_char/alpha.gif' border=0>-values obtained from the two models are in complete agreement with each other and the present value <img src='/image/spc_char/alpha.gif' border=0> = 0.46 is also much closer to the BCS value of 0.5. Interaction strength <i style="">N</i>₀<i style="">V</i> values obtained from the two models are also in perfect agreement with each other and the present value <i style="">N</i>₀<i style="">V</i> = 0.48 suggests that MgB₂ is a strong coupling superconductor. <br/> <br/>Page(s): 59-66 Tue, 29 Dec 2009 22:58:59 GMT http://nopr.niscair.res.in/handle/123456789/6894 Title: Role of bilayer chain coupling on junction voltage in layered high temperature cuprate superconductors <br/> <br/>Authors: Singh, Mahipal <br/> <br/>Abstract: The junction voltage as a function of hopping matrix element for CuO-chains, hopping of the single particle from bilayer to chain, Josephson like Cooper pair tunneling and other microscopic interactions that exit in bilayer high temperature cuprate superconductors has been studied. For this purpose, a tight binding model Hamiltonian that includes various intra and inter-bilayer interactions alongwith contributions of Josephson-like tunneling of Cooper pairs from bilayer of CuO₂ planes to CuO-chains and vice-versa as well as single particle hopping between CuO₂ bilayer to CuO-chains in the form of bilayer chain interaction within a unit cell has been considered. The situation considered here is equivalent to a Josephson’s coupled SNS junction. In the superconducting state, the CuO₂ bilayer acts as superconducting electrode and CuO-chains as one dimensional metal under fully oxygenated overdoped state sandwiched between these two electrodes as in the case of YBa₂Cu₃O₇ bilayer system. There is always a possibility of Cooper pairs to tunnel from one bilayer CuO₂ planes to other bilayer via CuO-chains. Using Green’s function technique, we have obtained expressions for superconducting order parameter, carrier density and junction voltage within BCS formalism. The numerical analysis shows that in bilayer cuprates, the junction voltage depends on the interlayer pair tunneling between the CuO₂ planes, hopping matrix element (<i>t</i>_ch) for chains and the hopping of single particle from bilayer to chains. Finally, we have compared our theoretical results on junction voltage with the existing experimental results. <br/> <br/>Page(s): 52-58 Tue, 29 Dec 2009 22:58:59 GMT http://nopr.niscair.res.in/handle/123456789/6893 Title: Field dependent dielectric study of PbTiO₃ and KNbO₃ ferroelectric perovskites <br/> <br/>Authors: Singh, Talvinder; Kumar, Ashok; Naithani, U C <br/> <br/>Abstract: An expression has been obtained for the complex dielectric constant in anharmonic pure ferroelectric crystals using double time temperature dependent Green’s function technique along with a modified Hamiltonian which includes anharmonicity up to the fourth order due to the interactions of the soft mode. The electric field, temperature and frequency dependent dielectric constant of PbTiO₃ and KNbO₃ crystals have been studied. Dielectric constant increases with increase in the applied electric field and decreases with increasing temperature in PbTiO₃ and KNbO₃ crystals. Anharmonicity is necessary in these crystals to observe these effects. <br/> <br/>Page(s): 47-51 Tue, 29 Dec 2009 22:58:59 GMT