• Volume 35, Issue 4

October 1990,   pages  1-403

• Observation of rf SQUID characteristics in YBCO bulk at 77 K

RF SQUID behaviour due to grain boundary weak links in a bulk YBCO is observed at 77 K using modified commercial rf electronics. Porous samples with lowIc are found to show this characteristic whereas dense samples with higherIc do not show SQUID behaviour.V-B modulation characteristic is found to be better when the rf pumping frequency is kept slightly higher than the resonance frequency of the tank circuit. Designing of coil for tank circuit with appropriateQ has been found to be very crucial for seeing the SQUID behaviour. Estimation of parameters such as coupling constant, mutual inductance, inductance and radius of the SQUID loop, have been made and their significance is discussed. Flux noise spectrum of the bulk rf SQUID in flux locked mode is also reported.

• Fluctuating hydrodynamics of a fluid with internal rotation

The fluctuating hydrodynamics theory of a fluid possessing internal rotation is set up following the Landau-Lifshitz approach.

• Scaling of the shear viscosity of the system nitrobenzene —n-heptane in the critical region

The temperature dependence of shear viscosity of the system nitrobenzene-n-heptane has been studied near the critical concentration. The critical exponent of the shear viscosity Φ was calculated from the empirical formula$$\eta = \eta _{id} \varepsilon _{sp} ^{ - \Phi }$$ and compared with the theoretical and experimental results obtained for other critical systems. The shear viscosity satisfies scaling law relations similar to those previously established for equilibrium properties.

• Microwave losses in ABO3 type perovskites

Using Green’s function method, microwave lossess were theoretically calculated for BaTiO3, SrTiO3 and KTaO3 ferroelectric perovskites. In microwave range, an increase in frequency is followed by an increase in the dielectric loss. In the paraelectric phase, the dielectric loss decreases with increasing temperature showing the Curie-Weiss behaviour of the tangent loss.

• Effect of Sb substitution on the superconductivity of Bi2(Ca,Sr)n+1CunO4n+2 (n=1, 2 and 3) systems

Effect of incorporation of Sb in place of Bi in the bismuth cuprate superconductors has been examined. The nominal compositions studied are MCa1·5Sr1·5Cu2O8+δ and MCa2Sr2Cu3O10+δ, whereM=Bi2−xSbx or Bi1·5−xSbxPb0·5. Different preparative routes such as the ceramic method, the matrix route as well as the melt route were employed to prepare the materials. No indication of either Sb entering the lattice or enhancement ofTc is noted from resistivity, magnetic susceptibility and microwave absorption measurements.

• Unusually large shift in low-temperature susceptibility peak of Fe2.8Ti0.2O4 with frequency

The temperature dependence of magnetic susceptibility,x, of titanomagnetites (Fe3−xTixO4, 0⩽x⩽0.2) at different frequencies in the range of 13 Hz to 117 kHz is reported. Whereas the position of thex-peak of Fe3O4 at 120 K shows no significant shift over this frequency range, that of Fe2.8Ti0.2O4 shifts from 70 K at 13 Hz to 115 K at 117 kHz. This seems to be the first observation of such large shift in the position of ax-peak observed at low-temperatures is attributed to the occurrence of an isotropic point (temperature at which first order anisotropy constantK1=0) associated with change in crystal structure. In such a case a shift inx-peak with frequency is not expected normally. Thus, the observed large shift in the position ofx-peak with frequency is an unusual phenomenon and some of its repercussions are discussed.

• Donor electron in a quantum well under the influence of an electric field

The ionization energies and the polarizabilities of a donor in an isolated well of a quasi two dimensional (Q2D) GaAs/Ga1−xAlxAs heterostructure have been obtained for different well widths including electron-lattice coupling. A wave function that properly reduces to the hydrogenic function in the limiting case has been used. For fields of the order of 105 V/m, the ionization energies decrease slightly with electric fields for all well widths (10 nm to 50 nm) studied. Also for a given electric field, as the well width increases, the ionization energy decreases. For fields of the order of 107 V/m and for smaller well widths (&lt;10 nm), the ionization energy generally increases with electric field. The results also show that for electric fields of this order, no donor bound state associated with the lowest subband is possible for well widths greater than 20 nm. The polarizabilities estimated using the expression for the dipole operator show that as the well width increases, the polarizability values also increase and do not show any abnormal behaviour.

• Interaction of dimethyl sulphoxide with chloranil by ESR and optical methods

The molecular charge-transfer absorption bands have been observed in solution of dimethyl sulphoxide (DMSO) and tetrachloro-p-benzoquinone (chloranil) in chloroform and cyclohexanone. The stability constant, the thermodynamic parameter of the complex and the extinction co-efficient of CT bands at 350 nm in chloroform and 360 nm in cylohexanone have been estimated on the assumption of the 1:1 complex formation. Detection of ESR signal of the solution of chloranil in pure DMSO (g ≈ 2) indicates the formation of chloranil radical anion, the precursor being the charge-transfer complex, which dissociates partly in high dielectric constant DMSO.

• Spectroscopically determined electronegativity values for heavy elements

A new scale of electronegativity based on X-ray spectroscopic data was earlier presented by Mande and others for the elements3Li to54Xe. The present paper reports an extension of this scale carried out for most of the heavier elements lying between55Cs and92U. The present approach is based on the physical interpretation of electronegativity of an atom as the attractive electrostatic force it exerts at a distance equal to its covalent radius. To estimate this force, experimental spectroscopic data have been employed. The merits of such an approach have been pointed out recently by Mullay and may be of interest in the context of the intimate relation found between electronegativity and superconducting transition temperature.

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