• Volume 39, Issue 4

October 1992,   pages  1-404

• Effect of meson exchange current and three-body force on the charge form factor of3He

We report a calculation of meson exchange current contribution to the charge form factor of3He by the hyper spherical harmonic expansion method with the inclusion of two-pion exchange three nucleon force. Results indicate that the charge form factor is changed appreciably in the right direction at high momentum transfer, due to the inclusion of the meson exchange current.

• Measurement of diffusion coefficients of thallium ion in H2O and D2O systems at different concentrations

Sliding cell method, developed in our laboratory, has been used to measure the inter diffusion coefficient of thallium ion in thallous sulphate solution over a wide concentration range using both water and heavy water as solvent at 35°C. The results have been analysed from the point of view of both ion-ion and ion-solvent interactions. The comparison of the diffusivities of the same ion in D2O and H2O electrolyte solutions at the same temperature indicate that the addition of salt affects the two solvents differently.

• Melting temperature variation with concentration of alkali halides in mixed crystal systems

Diffusion-vibration theory of melting (Sharmaet al 1991) has been extended to study the variation in the melting temperature of mixed ionic crystals with concentration. The melting temperature varies non-linearly with concentration in the KClxBr1−x, RbClxBr1−x, KxRb1−xBr and NaClxBr1−x mixed alkali halides and shows a sharp increase in melting temperature for values ofx&gt;0.5 which is in good agreement with the experimental values. This behaviour has been explained on the basis of present propounded theory.

• The effect of low intra-sublattice repulsion on phase diagram of YBa2Cu3O6+x: A Monte Carlo simulation study

We report the results of Monte Carlo simulation of the phase diagram and oxygen ordering in YBa2Cu3O6+x for low intra-sublattice repulsion. At low temperatures, apart from tetragonal (T), orthorhombic (OI) and ‘double cell’ ortho II phases, there is evidence for two additional orthorhombic phases labelled here asOI andOIII. At high temperatures, there was no evidence for the decomposition of theOI phase into theT andOI phases. We find qualitative agreement with experimental observations and cluster-variation method results.

• Covalent binding parameters and the ground state wavefunctions of vanadyl ion complexes

The Steven’s method of molecular orbitals for octahedral complexes containing transition metal ions has been used for estimating the binding parameters and interpreting theg factors of VO2+ ion in single crystals. The expressions forg factors have been given in terms ofK andK taking into account the tetragonal crystalline field and covalent binding effects. Computations show thatK should be less than 0.066 in order to fit the experimentalg values. The ground state wave function (GSWF) of VO2+ ion doped in different single crystals has been estimated using crystal field theory. The GSWF is found to be indxy state with slight admixture of the excited states$$d_{x^2 - y^2 }$$,dxz anddyz. The hyperfine interaction parameterP and Fermi contact termX have also been estimated.

• Non-linear conduction in a quasi one-dimensional conductor at low temperatures

The current-voltage characteristics of a quasi one-dimensional organic system having asymmetric donor molecule like Qn-(TCNQ)2 pellet with stoichiometry 1:2 grown from acetonitrile as a solvent have been studied atT=69 K. The characteristic curves show a pronounced deviation from ohmicity beyond a certain value of current. For higher values of currents a negative differential resistance region is observed.

• Ground-state recovery of coumarin dyes by pump-probe technique with picosecond pulses

Rotation-free transmission measurements governed by ground-state recovery of coumarin 102, coumarin 138 and coumarin 339 dyes in ethanol solutions were performed for the first time by pump-probe technique using 35 ps pulses of Nd:YAG laser at 355 nm. Recovery of absorption is characterized by simple exponential behaviour with relaxation times of 4.0, 3.7 and 3.0 ns respectively for C 102, C 138 and C 339. These values are comparable to the lifetimes of the first excited singlet states.

• Two photon optogalvanic spectrum of theD-X system of HgBr

Two photon laser optogalvanic spectrum of theD-X system of HgBr molecule has been recorded using Nd-YAG laser pumped dye laser for the first time. A large number of new bands involving higher vibrational levels have been observed. Isotopic heads due to the two bromine isotopes have been observed and used in fixing the vibrational assignment. Vibrational constants for theX andD states of the two isotopic molecules have been determined.

• Measurement of photoabsorption and fluorescence cross-sections for CS2 at 188.2–213 and 287.5–339.5 nm

The photoabsorption and fluorescence cross-sections for carbon disulphide have been measured in the 188.2–213 and 287.5–339.5 nm spectral regions using an argon mini-arc light source. The absorption cross-sections have been measured with an accuracy of ±4.2% whereas the most probable error estimated in the case of fluorescence cross-sections is ±5.1%. The fluorescence quantum yields for CS2 have also been obtained in the two spectral regions.

• Performance of 2-hole YBCO rf SQUID in flux-locked-loop mode at 77 K

A 2-hole rf SQUID has been fabricated using naturally present grain boundary Josephson weaklinks in a microbridge of bulk Y-Ba-Cu-O superconductor. Periodic oscillations in voltage-flux characteristics have been observed up to 80.5 K. The spectral density of flux noise of the SQUID is 8 × 10−4 Φ0/√Hz at 100 Hz and 77 K for open-loop mode. The SQUID has been successfully operated in flux-locked-loop mode at 77 K, demonstrating the feasiblity of the device for practical applications. In the flux-locked-loop mode the stability as well as flux noise of the SQUID has been found to improve considerably as compared to that in the open-loop mode. Various SQUID parameters have also been estimated and reported here.

• An automated thermal relaxation calorimeter for operation at low temperature (0.5 K&lt;T&lt;10 K)

We describe an automated calorimeter for measurement of specific heat in the temperature range 10 K&gt;T&gt;0.5 K. It uses sample of moderate size (100–1000 mg), has a moderate precision and accuracy (2%–5%), is easy to operate and the measurements can be done quickly with He4 economy. The accuracy of this calorimeter was checked by measurement of specific heat of copper and that of aluminium near its superconducting transition temperature.

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