• Volume 39, Issue 6

      December 1992,   pages  589-671

    • Rescaled mean spherical approximation structure factor for an aqueous suspension of polystyrene spheres

      R K Pandey D N Tripathi

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      Rescaled mean spherical approximation (RMSA) has been used to calculate the structure factor for the aqueous suspension of polystyrene macroions with the interaction potential taken according to Derjaguin and Landau (1941) and Verwey and Overbeek (1948) (DLVO) model. The effects of charge over the macroion and size on the surface potential and therefore, the structure factor have been studied. The breakdown of the DLVO potential with an excess charge over the macroion (⩾800e) has been reported. The oscillation in the first peak height of structure factor versus wave vector curve with size has been correlated with the Debye length.

    • Inelastic light scattering in strontium borate glasses in the systemxSrO.(1−x)B2O3 and (SrCl2)y.[xSrO.(1−yx)B2O3]1−y

      M H Rahman B P Dwivedi Y Kumar B N Khanna

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      Raman spectra of strontium borate binary glasses in the systemxSrO.(1-x)B2O3 forx=0.20, 0.25, 0.30, 0.35, 0.40, and 0.50 and ternary glasses in the system (SrCl2)y.[xSrO.(1−yx)B2O3]1−y fory=0.10, 0.20, 0.30 and 0.40 andx=0.20 and 0.35, are reported. Raman spectra of the glasses show experimental evidence of glass network modifying nature of SrO in borate matrix. SrO causes a change of boron atom coordination number from 3 to 4 resulting in the complex structural groupings comprising of BO4 and BO3 units. Strontium cations are not easily accommodated in the glass structure and tend to break up the network at high concentration (x) of SrO, causing non-bridging oxygens. The effect of temperature variation of binary glasses has also been studied. The introduction of SrCl2 to the binary stronium borate glass causes a large change in intensity of low frequency Raman scattering whereas there is no change in the vibrational dynamics in the high frequency region. The temperature reduced Raman spectra represents true vibrational density of states. Martin-Brenig model developed for the low frequency region has been discussed to obtain the structural correlation range in the glass.

    • Magnetic field dependence ofTc and temperature dependence ofHc2 in layered superconductors with open normal state Fermi surface

      Sudhanshu S Jha A K Rajagopal

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      A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |Hc2(T)|,T ∼ 0 andTTc(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|Hc2(0)|/[-Tδ|Hc2TT]T{s0} for the case of a FS open inkz-direction with thekz-bandwidth, 4t3, very small compared to the Fermi energy,EF, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofTc and the temperature dependence of Hc2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa2Cu3O7 used elsewhere, we find reasonable values for the upper critical fieldHc2(0), the slope (dHc2/dT)Tc0, anisotropic coherence lengths ζi(T=0),i=x, y, z, and (dTc/d|H|)|H| → 0.

    • SERS studies of 1-butylaminoanthraquinone in silver sol

      V Ramakrishnan A Narayana Perumal N Krishnamurthy V J P Srivatsavoy

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      SERS studies of 1-butylaminoanthraquinone in silver sol have been made to understand the nature of adsorption. The enhancement mechanism is explained on the basis of charge transfer process. The molecule is adsorbed through the binding sites but not flat on to the surface.

    • Temperature dependent study of the single crystal electronic spectrum of octahedral Ni2+ ion subjected to distortions in weak fields

      GVR Chandramouli P T Manoharan

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      Electronic spectra of two high spind8 nickel complexes, Ni(en)3Cl2.2H2O and Ni(OAc)2.4H2O, are reported. Polarized spectra were measured at 298 K. Temperature dependent spectra were measured using unpolarized light down to 20 K. The spectra at 20 K are well resolved revealing the features of spin-forbidden transitions and vibrational fine structure on some of the bands. Some of the spectra are deconvoluted to separate out the overlapping bands. The assignments are made usingD3 symmetry for Ni(en)3Cl2.2H2O complex whileD4 symmetry is used for Ni(OAc)2.4H2O. The energies of the spectroscopic states are calculated and compared with the observed transition energies. The distortion parameters are determined.

    • An apparatus for the measurement of thermal expansion of solids at low temperatures

      Om Prakash Ashok Rao P N Dheer

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      A dilatometer, using the three terminal capacitance technique, suitable for measurement of linear thermal expansion of solids in the temperature range 1.3–300 K is described. The dialtometer is designed such that the mounting system for the specimen does not undergo any significant changes in dimensions when the specimen is heated. The apparatus, therefore, yields in principle absolute values of α, the coefficient of linear thermal expansion. The performance of the apparatus has been checked by measurements on copper in the temperature range of 77–300 K. Some preliminary results on the behaviour of α for Y1Ba2Cu3O6.9 compound in the vicinity of superconducting transition temperature,Tc are also described. The system can detect relative changes in length Δl/l0 of about 10−8. Attempts are being made to improve the sensitivity.

    • A triple axes multiple target holder assembly

      L C Tribedi S D Narvekar R G Pillay P N Tandon

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      A triple axes multiple target holder assembly has been designed and fabricated to accommodate 27 targets to be used inside a high vacuum scattering chamber. This target holder assembly has been used satisfactorily in ion solid collision experiments using heavy ion beams from an accelerator.

    • A technique for frequency stabilization of an internal mirror He-Ne laser

      H S Dahiya V T Chitnis Kowsalya V D Dandawate

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      A technique for frequency stabilization of an internal mirror He-Ne laser using phase sensitive detection without cavity length modulation is suggested. The orthogonally plane polarized modes are separated and then convered into two photoelectric signals using two photodetectors. The photoelectric signals are switched alternately so as to generate a square wave, whose amplitude is proportional to the intensity difference between the two orthogonal polarizations. A lock-in amplifier is used to detect this square wave, with the switching frequency as reference. The phase detected signal is used for thermal stabilization of the laser. The frequency stability of 5 × 10−9 was obtained with an integration time of 1 s.

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