• Volume 21, Issue 3

      September 1983,   pages  159-231

    • Mechanoluminescence of coloured KCl crystals

      M Elyas S K Shukla B P Chandra

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      The gamma-irradiated KCl crystals exhibit mechanoluminescence during elastic, plastic and fracture deformation. The mechanoluminiscence (ml) intensity varies linearly with the number of newly-created dislocations and decreases with successive application and release of uniaxial pressure. The totalml intensity increases with applied pressure as well as with the temperature of the crystals. On the basis of the movement of the dislocations, the pressure and temperature dependence ofml is discussed.

    • A mean-field, effective medium theory of random binary alloys III. The ising model with competing interactions

      A Mookerjee S B Roy

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      The Ising model with competing interactions is studied in a mean field effective medium approach. The phase diagram of such model alloys is studied. We conclude that for all ratios of the competing interaction moments, a spin glass phase always exists at low temperatures for certain concentration regimes.

    • 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.

    • First passage time and escape time distributions for continuous time random walks

      V Balakrishnan M Khantha

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      We consider an arbitrary continuous time random walk (ctrw)via unbiased nearest-neighbour jumps on a linear lattice. Solutions are presented for the distributions of the first passage time and the time of escape from a bounded region. A simple relation between the conditional probability function and the first passage time distribution is analysed. So is the structure of the relation between the characteristic functions of the first passage time and escape time distributions. The mean first passage time is shown to diverge for all (unbiased)ctrw’s. The divergence of the mean escape time is related to that of the mean time between jumps. A class ofctrw’s displaying a self-similar clustering behaviour in time is considered. The exponent characterising the divergence of the mean escape time is shown to be (1−H), whereH(0<H<1) is the fractal dimensionality of thectrw.

    • CP invariance: A point of view

      Gyan Mohan

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      That the longlived componentL ofK0 has bothcp=+1 andcp=−1 modes of decay is often cited as evidence of violation ofcp invariance. The careful ones find the compelling evidence to be the non-dilution of the regeneration interference pattern when the incidentK0 beam is mixed even substantially with$$\bar K_0 $$. However the two phenomena comprehensively imply thatL has acp=+1 componentL+ and acp=−1 componentL and that the longlived component of bothK0 and$$\bar K_0 $$ are one and the sameL. This does not demand abandoningcp invariance. It does imply that$$\bar K_0 $$ is not thecp conjugate ofK0.

    • Quantum chromodynamics predictions in renormalization scheme invariant perturbation theory

      A Dhar V Gupta

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      It has recently been shown that any physical quantity ℛ, in perturbation theory, can be obtained as a function of only the renormalization scheme (rs) invariants,ρ0,ρ1,ρ2, … Physical predictions, to any given order, are renormalization scheme independent in this approach. Quantum chromodynamics (qcd) predictions to second order, within thisrs-invariant perturbation theory, are given here for several processes. These lead to some novel relations between experimentally measurable quantities, which do not involve the unknownqcd scale parameter Λ. They can therefore be directly confronted with experiments and this has been done wherever possible. It is suggested that these relations can be used to probe the neglected higher order corrections.

    • He II spectra of La, Ce and Yb: Novel features in the valence band region

      D D Sarma

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      He II photoelectron spectra of La, Ce and Yb show features which cannot be explained in terms of single electron excitations. It is proposed that these are due to formation of electron-hole paris.

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