• S K Ghatak

      Articles written in Pramana – Journal of Physics

    • Specific heat at low temperature due to negative U centres in disordered solids

      S K Ghatak

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      The negativeU-Anderson model is considered and energy spectrum is obtained using the Gorkov’s decoupling scheme for one-electron Green’s function. The correlation of localized electron pair (bipolaron) is explicitly taken into account in this scheme. The electronic specific heat of disordered solids with negativeU-centres and having a distribution of negativeU is then calculated. At low temperature the specific heat shows linear temperature dependence, and this linearT-term is a combined effect of distributedU and of the existence of localized electron pairs.

    • Magnetization and susceptibility of disordered alloy with competitive interactions

      S K Ghatak

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      A model calculation of magnetization and susceptibility of disordered alloy (ApB1 −p) where bothA andB represent the magnetic atoms is presented. It is based on the cluster-variational method where interactions within the clusters of all possible configurations are treated exactly and the rest of the interaction is replaced by an effective variational field. The frustration effect is introduced taking the exchange interactionsJAB orJBB or both to be antiferromagnetic whereas the exchangeJAA is ferromagnetic. The results are qualitatively similar to the observed behaviour of moment and susceptibility in some metallic glasses. The critical concentration for ferromagnetic state is determined in the presence of competitive interactions.

    • Temperature dependence of electrical conductivity and thermoelectric power of Bi-Sb tapes prepared by liquid quenching

      T K Dey S K Ghatak

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      Electrical conductivity and thermoelectric power of liquid-quenched Bi-Sb tapes with Sb concentration in the range 8 to 18 at.% are reported between 77K and room temperature. Analysis of the data shows that the total electrical conductivity of these tapes is determined by a temperature-independent component due to band conduction, and a strongly temperature-dependent part due to carrier transport across or through the defect states. These defect states which appear to originate from structural imperfections and disorders due to rapid cooling, are formed close to the mobility edge within a small energy range.

    • Magnetic-field-induced valence transition in rare-earth systems

      G Gangadhar Reddy A Ramakanth S K Ghatak

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      The magnetic-field-induced valence transition in rare-earth systems has been investigated using the periodic Anderson model supplemented by the Falicov-Kimball term. This model has been solved by first decoupling the Falicov-Kimball term as proposed by Khomskii and Koharjan and then taking the limit of infinite intra-site Coulomb repulsion. The valence transition both in the absence and in the presence of magnetic field as a function of temperature is studied. It has been found that the system makes transition from non-magnetic to magnetic state when the magnetic field increases beyond a critical value Hc. The phase boundary defined in terms of reduced field Hc(T)/Hc(0) and reduced temperature T/Tv (Tv being the valence transition temperature in the absence of field) is almost independent of the position of the localized level. The results are in qualitative agreement with experimental observations in Yb- and Eu-compounds.

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