• S K Roy

      Articles written in Pramana – Journal of Physics

    • Frequency moments and energy shifts for dilute Mössbauer impurities in metallic solids

      S K Roy

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      Theoretical evaluations of frequency moments and second order Doppler shifts have been made for dilute57Fe impurities in the high temperature limit for a harmonic solid. The resonant energy shifts have been calculated from the Green’s function of the impure crystal containing both mass disordering and force constant change terms in the crystal Hamiltonian. High temperature frequency moments for the impurity in different metallic solids are obtained from Mc-Millan ratios using standard Mössbauerf-values at room temperature.

      The effect of mass disordering predominates over the force constant change term in the evaluations of second order Doppler shifts and hence the frequency moments for dilute57Fe impurities. The variation of frequency moments for the impurity with mass modified Debye-temperature of the hosts is shown for a number of metallic solids.

    • A theoretical study of stress-induced effects on Mössbauer impurities forming dislocations in some b.c.c. and f.c.c. metals

      S K Roy N Kundu

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      The external stress values required to move the dislocations in phase with the Mössbauer impurities, forming edge dislocations in different crystals are evaluated. The vibrating string model of dislocations is considered to calculate the average kinetic energy of dislocation. The kinetic energy of dislocation is found to alter causing an expected energy shift in the γ-ray photon emitted by the atom moving with the dislocation.

      The critical stress values calculated at low temperature are found to change with mass and Debye temperatures of the resonating atoms. The ratio of impurity site moments are also evaluated at low temperatures for a few f.c.c. and b.c.c. metals from Mössbauer data. The results are discussed in the light of bond softening or hardening due to the application of variable stresses in the glide plane. The variation of mean-square displacement, mean-square velocity and impurity moment ratios with stress values are shown for a number of atoms.

    • Anharmonic effects on lattice resonant modes of impure metallic crystals

      S K Roy N Kundu

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      The effect of lattice anharmonicity on the resonant modes of dilute impurities for Au-Cu, Au-Ag and W-Cr metallic systems are studied from the poles of the double time temperature dependent impure Green’s function of these crystals. The third order force constants used in the present work are derived assuming the systems to obey the Lennard-Jones potential. The inclusion of lattice anharmonicity has been found to increase the resonant frequency which depends upon the mass defect, force constant change parameter and the impurity concentration terms. Some interesting features about the phase shift and the change in width of the vibrational spectrum are reported at room temperature for the isotopic defects and the defects interacting with host atoms. The results are found to be in qualitative agreement with the experiments on the systems considered.

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