• Volume 31, Issue 6

      December 1988,   pages  453-533

    • On the measurement of electron tunnelling time

      D K Roy Amitabh Ghosh

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      A suggestion to experimentally measure the electron tunnelling time by observing the tunnelling current cut-off as a function of the magnetic field intensity in semiconductor pn tunnel junctions, when they are placed in a crossed electric and magnetic field configuration, has been made in this paper. A simple and a rigorous quantum mechanical analysis to justify the above proposition have been presented. An order of agreement between the tunnelling time values derived from the published experimental data and our theoretical prediction has been noticed.

    • Renormalization of a gauge theory in a nonlinear gauge

      Satish D Joglekar

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      We discuss renormalization of an O(3) gauge model with the gauge fixing term given by ℒg.f.=-1/ζ|(∂μ-igA3μ)W|2-(1/2α)(∂A3)2. We utilize earlier results on the general theory of renormalization of gauge theories in quadratic gauges to prove multiplicative renormalizability of the theory together with a subtractive renormalization of gauge fixing and ghost terms. We show that this model has a double BRS invariance and that it is preserved under renormalization.

    • Magnetic symmetry and dyons

      J M S Rana O P S Negi B S Rajput

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      A self-consistent theory of dyons in Abelian and non-Abelian limits has been formulated in terms of an extra magnetic symmetry and topological magnetic charge. It has been shown that the restricted gauge potential describes the fields of dyons in terms of two regular (time-like) potentials only when recourse is made to the duality of topological (magnetic) and isocolour (electric) charges. Choosing a suitable Lagrangian density for the system of dyons in non-Abelian gauge theory, the field equations, energy-momentum tensor, Hamiltonian and momentum densities have also been derived and the conservation of the four-linear momentum and the total angular momentum has been demonstrated.

    • Masses in structure functions

      D K Choudhury A K Misra

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      We derive an approximate formula for the quark mass in ξ-formalism. Using the QCD-based form of structure function, we analyse the SLAC-MIT data to estimate the experimentally allowed ranges of quark mass. The possible variation of our results with QCD cut-off parameter, gluon distribution, hard intrinsic charm component of the proton wavefunction and the higher twist effects are also discussed.

    • Rotational analysis ofA1Π-X1Σ+ bands of As14N and As15N: perturbation studies in theA1Π state

      P Saraswathy G Krishnamurty

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      The ultraviolet bands ofA1Π-X1Σ+ system of As14N and its isotopic species As15N were excited in a quartz tube containing specpure nitrogen and traces of AsCl3 vapour, using a microwave discharge (2450 MHz). Bands of As15N were obtained using15N2 enriched to 95.5%. Rotational structure of several bands of As14N and As15N photographed under high dispersion (0.14 Å/mm) was reanalysed. Detailed studies of the observed perturbations in the levels ofν′ = 0 to 4 led to the identification of two3Σ and aC1Σ states. Molecular parametersTe, ωe, ωexe, Beandαewere determined for all the electronic states studied. Vibrational assignments of the perturbing levels were made using the isotope shift studies.

    • Low field anomaly in magnetization curves

      G Ravi Kumar P Chaddah

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      We present a calculation of the isothermal magnetization hysteresis curves appropriate to highTcsuperconductors. We discuss the nature of the low field anomaly as one goes from this strong pinning case to the weak pinning case. We show that the shape of the equilibrium (thermodynamic) magnetization curve is recovered in the limit ofJcapproaching zero.

    • Effect of pressure on the Fermi surface of potassium

      M G Ramchandani

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      The complete electronic energy band structure of bcc potassium metal has been calculated using the non-relativistic augmented plane wave method. The density of states has been calculated and results compared with available thermal, optical and X-ray data. The lattice constant has been varied to get the band structures for pressures varying from 1 to 5 kb. The Fermi surface has been studied with varying lattice constants and the results discussed in the light of results of pressure-induced changes of de-Hass van-Alphen frequency obtained by Altounian and Datars.

    • On the nature of the dhcp to fcc transition under pressure in Pr and Pr-Th alloys

      V Vijayakumar B K Godwal S K Sikka R Chidambaram

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      The results of electrical resistance (R), thermoelectric power (TEP) and X-ray diffraction measurements on praseodymium (Pr) and its alloys with thorium under pressure are reported. The maximum inR vsP curve exhibited by Pr persists only in the dhcp phase of PrTh alloy. X-ray measurements confirmed that in the alloys also the maximum inR vsP curve is due to the dhcp → fcc transition. Thus the behaviour of Pr and Pr-Th alloys is different from that of La and its alloys with Ce and Th where the maximum in theR vsP curve is electronic in origin and is exhibited by the dhcp, fcc and dist fcc phases.

    • Heat conduction through moist soils at different temperatures

      Ashok K Singh Ramvir Singh D R Chaudhary

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      A numerical model has been described to estimate the effective thermal conductivity of moist soils, considering the effective continuous medium approximation and taking all possible interactions. Numerical solutions of the exact formulations are presented. Experimental measurements have been carried out for the thermal conductivity of dune sand at different moisture contents employing the method of unsteady state line source. The predicted and measured values show reasonable agreement.

    • Warping of the bulk dispersion of InSb

      I S Nachev V Tz Nikolochovski L S Nachev

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      We calculate the warping of the bulk dispersion of InSb in thek-space using different models for the bulk band structure near the Γ point. It is shown that the dispersion of the conduction band Γ6 is well described by the simplified six-band model, while the fourteen-band model is more accurate for the valence bands.

    • Saturable absorbers for low power optical phase conjugation

      K P B Moosad V P N Nampoori

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