• Volume 11, Issue 2

      August 1978,   pages  119-232

    • Propagation of longitudinal ultrasonic waves in RbH2PO4 near its ferroelectric phase transition temperature

      G P Singh B K Basu

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      Ferroelectric phase transition in RbH2PO4 has been investigated using propagation of longitudinal acoustic waves along the polar axis near the transition temperature. The velocity of this mode is continuous across the transition temperature. Velocity data in the ferroelectric phase are analyzed in terms of coupled soft modeacoustic mode model of Pytte to obtain the temperature dependence of the soft mode frequency. The attenuation data in the ferroelectric phase show power law dependence. It follows scaling behaviour of the type predicted by Kawasaki from the mode-mode coupling theory and the dynamical scaling.

    • Surface plasmon dispersion relation for spherical metal particles

      Satish B Ogale V N Bhoraskar P V Panat

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      Using the hydrodynamical model, we have obtained the surface plasmon dispersion relation for spherical metallic particles in the following two cases: (1) a sharp surface cut off in electron density and (2) a diffused electron density at the surface. The diffused density is modelled with a step function. The diffuse nature of the electron density at surface of the metal particle is necessary to understand the experimental result for particles with small radii. Shift in the absorption frequency is estimated and found to be small.

    • Electrical conduction and size effect in bismuth films

      A Rahman P C Mahanta

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      The size effect on electrical conduction is observed in bismuth films in the region of film thickness above 2000 Å. Applying Fuchs-Sondheimer theory and assuming specularity parameterp = 0·8, the value of bulk resistivity and mean free path are obtained as 6·692 × 104 ohm-cm and 6000 Å respectively.

    • Microhardness studies in alkali halide mixed crystals

      U V Subba Rao V Hari Babu

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      Microhardness measurements done in KCl, KBr and in different compositions of KCl-KBr mixed crystals show that it varies nonlinearly with composition. In order to investigate the nature of defects, several techniques such as etching, ionic conductivity and dielectric loss have been employed which showed that the mixed cristals of KCl-KBr are more defective, containing a high concentration of dislocations, low-angle grain boundaries and vacancies as compared to the end products KCl and KBr. These imperfections appear to be responsible for the nonlinear variation of microhardness in mixed crystals. The microhardness studies also revealed that the difference in size of the ions constituting the mixed system are responsible for the internal strains which in turn give rise to imperfections affecting the microhardness of mixed crystals.

    • A polarised neutron study of magnetic moment density in Cu2MnAl

      V C Rakhecha R Chakravarthy N S Satya Murthy

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      A polarised neutron study of the ferromagnetic Heusler alloy Cu2Mn0.863Al1.057 has been made. It has been concluded that the magnetic moment density is primarily situated on the Mn ions. On assigning the Mn-moment value, the observed magnetic form factor is found to be in good agreement with the Mn2+ free ion form factor calculated by Watson and Freeman. A slight asphericity has been observed in the moment density. It is estimated that there are about 3% excess 3d-electrons in the Eg states compared to spherical distribution. There is evidence of a very small positive polarisation of the Cu atoms. No appreciable conduction electron polarisation is found.

    • Spontaneous creation of massive spin half particles by a rotating block hole

      B R Iyer Arvind Kumar

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      The techniques of second quantization in Kerr metric for the scalar and neutrino (massless) fields are extended to the massive spin half case. The normal modes of Dirac field in Kerr metric are obtained in Chandrasekhar’s representation and the field is quantized as usual by imposing equal-time anti-commutation relations. The vacuum expectation value of energy-momentum tensor is evaluated asymptotically, leading to the result that a Kerr black hole spontaneously creates, in addition to scalar and neutrino quanta, massive Dirac particles in the classical superradiant modes.

    • Dielectric investigation of relaxation in some heterocyclic compounds

      N K Mehrotra S B L Misra S K S Somevanshi

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      The dielectric relaxation mechanism in five heterocyclic compounds in dilute solutions using microwave technique is reported. Measurements have been made at different temperatures in order to calculate dipole moments and free energies of activation for the process of dipole orientation and viscous flow. It has been observed that the relaxation time is very closely related with the molecular parameters, such as size, shape and nature of the solute molecule.

    • A link between the second-order Jahn-Teller effect and the highest occupied molecular orbital postulate for molecular shapes

      Anjuli S Bamzai B M Deb

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      For a closed-shell molecule, a connection is drawn between two recent models for molecular shapes, namely, those based on the second-order Jahn-Teller (SOJT) effect and the highest occupied molecular orbital (HOMO) postulate respectively. Two necessary and sufficient conditions are derived within the molecular orbital framework for the approximation inherent in the SOJT model to be valid. One of these conditions is akin to the HOMO postulate.

    • A bag model study ofD mesons

      K Babu Joseph M Sabir M N Sreedharan Nair

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      An investigation of the newly discovered charmed mesonsD 0 andD +, particularly their non-leptonic decay modes, is carried out in the framework of the MIT bag model. The amplitude for a number of two-body final state decays are explicitly evaluated and compared with other available estimates.

    • Shape of spectrum with ensemble of 2-body random hamiltonians

      V Potbhare

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      Monte-Carlo calculations ind-s shell space have been done using two-body random interactions, to obtain ensemble-averagem-particle scalar moments up to fourth order. A shift of the spectrum shape from semi-circle to Gaussian with respect to the increase in number of particles can be clearly seen in terms of the ensemble-averaged fourth moment.

    • Eigenvalue density for ensemble of 2-body random hamiltonians with non-zero mean for matrix elements

      V Potbhare V K B Kota

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      We obtain an expression for the ensemble-averaged moments inm-particle space of 2-body random Hamiltonians with same non-zero mean for the matrix elements, in the limit ofN → ∞,m ≫ 2. The eigenvalue density function can then be immediately obtained in terms of the eigenvalue density (a Gaussian whenm ≫ 2) for zero mean ensembles. The results of Monte-Carlo calculations for iso-scalar rotationally invariant 2-body ensembles have also been given.

    • A mean-field, effective medium theory of random magnetic alloys I: The random ising model

      Abhijit Mookerjee

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      Starting from a physical RKKY interaction model for magnetic alloys and a valid probabilistic description of a random substitutional alloy, the possible phases have been studied in a mean field, effective medium theory. Most of the detailed objections raised against former treatments of these systems have been removed.

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