• D Sen

      Articles written in Pramana – Journal of Physics

    • Iodine stabilised He-Ne Laser

      V D Dandwate P N Puntambekar D Sen

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      A single mode He-Ne laser operating at 6328 Å is used with an iodine cell in the cavity to detect the absorption components of iodine falling within the gain curve of the laser line. Experimental details are given for locking the frequency of the laser line with one of the hyperfine components of the iodine absorption line, using a servo-control system. The system uses the technique of detecting the first and third harmonics of the modulation frequency.

    • Pseudopotential theory of effective interaction in simple metal

      A Sarkar D Sen S Sengupta

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      The second order pseudopotential theory suggests the possibility of a break-up of the total energy of simple metals into a purely volume-dependent part and an effective central pairwise interaction between ions. In the present paper finite contributions for these two parts of the energy have been extracted in a form convenient for calculation. Using the local Heine-Abarenkov model potential, a reliable effective ion-ion interaction is generated and the volume-dependent energy is calculated for Al. The relative contributions of the effective interaction and the volume-dependent energy term to various metallic properties are also calculated. The importance of volume dependence on the effective interaction is also discussed.

      The interactomic force constants upto eighth neighbour are derived from the effective interaction and it is found that the force constants beyond the third neighbour are negligibly small. This result is also confirmed by the calculation of dispersion curves with force constants obtained from the effective interaction upto the third neighbour which is found to reproduce the results of the full pseudopotential calculations. The force constants obtained are also used to study some finite temperature properties of Al in the quasi-harmonic approximation and the limitations of the theory are pointed out.

    • A simple approach to evaluation of lattice sums

      Subinay Dasgupta D Sen S Sengupta

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      It is shown that starting from a Fourier transform relation one can derive, in a surprisingly simple manner, all the well-known results of lattice summation, that have been obtained so far by a complicated use of the Ewald theta transformation. We show that the Ewald transformation follows directly from the Fourier transform relation.

    • Unified study of lead: Energy-dependent pseudopotential calculation

      A Sarkar D Sen S Sengupta

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      A Unified study of lattice-mechanical properties of lead using energy-dependent pseudopotential is carried out. Energy dependence in pseudopotential is considered through the effective mass approximation; the pseudopotential model chosen is the local Heine-Abarenkov model potential. Properties studied include cohesive energy, equilibrium lattice parameter, second-order elastic constants, pressure derivative of second-order elastic constants, equation of state (atT=0 K), phonon-dispersion and effective two-body interaction. The results show fairly good agreement with experiment especially with a modified Heine-Abarenkov potential.

    • Classical limit of scattering in quantum mechanics—A general approach

      D Sen A N Basu S Sengupta

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      The classical and quantum physics seem to divide nature into two domains macroscopic and microscopic. It is also certain that they accurately predict experimental results in their respective regions. However, the reduction theory, namely, the general derivation of classical results from the quantum mechanics is still a far cry. The outcome of some recent investigations suggests that there possibly does not exist any universal method for obtaining classical results from quantum mechanics. In the present work we intend to investigate the problem phenomenonwise and address specifically the phenomenon of scattering. We suggest a general approach to obtain the classical limit formula from the phase shiftδl, in the limiting value of a suitable parameter on whichδl depends. The classical result has been derived for three different potential fields in which the phase shifts are exactly known. Unlike the current wisdom that the classical limit can be reached only in the high energy regime it is found that the classical limit parameter in addition to other factors depends on the details of the potential fields. In the last section we have discussed the implications of the results obtained.

    • Electromagnetism according to geometric algebra: An appropriate and comprehensive formulation

      D SEN

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      While the Maxwell’s equations describe the electric and magnetic fields developed by electrically charged particles and/or currents, the Lorentz force law completes the picture of classical electromagnetic theory by defining the force acting on a localised charge (or charge distribution) moving in the field. However, the standard formulation using vector algebra suffers from several inadequacies and unwarranted features. Clifford’s geometric algebra or more specifically space–time algebra, i.e geometric algebra in 4D Minkowski space–time, provides anelegant, compactified and comprehensive description by removing the discrepancies of the earlier formulation. It provides an invariant description, in the appropriate space–time setting, in terms of the combined electromagnetic field without reference to any inertial system. Moreover, using elementary geometric calculus, it facilitates direct analytical introduction of the putative concept of magnetic monopole and renders the equations for both the constituent fields, symmetric and inhomogeneous. In terms of the single space–time force equation, space–time algebra also encapsulates both the Lorentz force equation and the electromagnetic power equation.

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