• SANKAR P SANYAL

Articles written in Pramana – Journal of Physics

• Nonlinear lattice dynamical theory for valence transition in rare earth compounds

A nonlinear lattice dynamical theory is proposed to explain the mode softening in intermediate valence compound Sm0.75 Y0.25 S. In this theory we consider the breathing motion of the electron shells of Sm-ion to be nonlinear, and depend on temperature, resulting in strong electron-ion coupling. We calculate the salient features of the nonlinear breathing potential, which resembles theφ4 potential for second order ferro-electric transitions. We also calculate the temperature dependence of LO(L) frequency for this compound, showing that at transition temperature (about 200 K) this mode freezes.

• Anharmonic oxygen vibration and mode softening in Tl2Ba2Ca2Cu3O10 superconductor

The softening of the IR active 300 cm−1 phonon mode in Tl-2223 superconductor, aroundTc has been explained by considering a sixth order polarization potential at the off-center oxygen ion site in the Tl-O plane, and by using a nonlinear lattice dynamical theory. The present theory explains, more or less satisfactorily, the unusual temperature dependence of oxygen ion vibration and frequency shifts at higher temperature. The existence of strong nonlinear electron-phonon interaction atTc has been emphasized.

• Effect of strain on vibrational modes in strained layer superlattices

We have investigated the lattice vibrational properties of a two-component strained layer semiconductor superlattice (GaAs)n1 (GaSb)n2 using a one-dimensional linear chain model and transfer matrix method [1]. Effect of strain, arising due to the lattice mismatch (∼ 7%) has been considered explicitly in the equation of motion. We show for the first time that the optical vibrational frequency increases (in the case of (GaAs)4 (GaSb)n) or decreases (in the case of (GaAs)n(GaSb)4) with increase of layer thicknessn, in either type of superlattices. Raman scattering measurements on some other similar systems support our findings.

• Lattice vibrational properties of uranium pnictides

Lattice vibrational properties of uranium pnictides have been studied using breathing shell model (BSM) which includes breathing motion of electrons of the U-atoms due tof−d hybridization. The phonon dispersion curves of U-pnicitides calculated from the present model agree reasonably well with the measured data. A comparison has been made between BSM and our results reported earlier obtained from three-body force rigid ion model to reveal the importance of the short-range electron-phonon interactions in these compounds. We also report, for the first time, the two phonon density of states and specific heat for these compounds.

• Electronic structure and high pressure phase transition in LaSb and CeSb

The electronic structure and high pressure structural phase transition in lanthanum and cerium antimonides have been investigated using the tight binding LMTO method. Calculation of the total energy reveals that the simple tetragonal structure is stable at high pressure for both the compounds. In LaSb, the calculated values of the equilibrium cell volume and the cell volume at which phase transition occurs agree with the experimental results. However, in CeSb, the agreement is not so good. We have also predicted the most favouredc/a value in the simple tetragonal phase for these compounds. Further, we present the calculated results on the electronic structure of these systems at the equilibrium as well the reduced cell volumes.

• Investigation of phonon anomalies in intermediate valence compounds SmS and Sm0·65Y0·25S

Phonon anomalies in two intermediate valence compounds (IVC), SmS and Sm0·75Y0·25S have been investigated using breathing shell model (BSM). The BSM includes breathing motion of electron shells of the rare earth atom due tofd hybridization. The phonon dispersion curves of IVC, calculated from the present model, agree well with the measured data. One-phonon density of states calculated from the present model compares well with the Raman spectra.

• Phonons and periodons in IV–VI semiconductor superlattices

We have calculated the phonon and periodon dispersion relations in IV–VI semi-conducting bulk PbTe and SnTe and their superlattice structure. The model used here is a one-dimensional lattice which includes harmonic interactions up to second neighbours as well as on-site nonlinear electron-ion interactions at the anion site. We calculate the phonon and periodon dispersion relations in bulk and PbTe-SnTe superlattice for the transverse optic and acoustic modes using the transfer matrix method. Our analysis has predicted correct nature of the folding of acoustic and confinement of optical phonons at various frequency intervals corresponding to pass and stop bands of the superlattices.

• Phonon properties of rare earth ytterbium pnictides

We report for the first time the complete phonon dispersion curves for the ytterbium pnictide compounds (YbN, YbP and YbAs) using a breathing shell model to establish their predominant ionic nature. The calculated results also show that this group of rare earth compounds does not show any elastic and phonon anomalies which are the characteristic features of other rare earth compounds. We emphasize the need for further Raman and neutron scattering measurements.

• Phonon dispersion in quasiperiodic semiconductor superlattices

The phonon spectra of unstrained and strained quasiperiodic semiconductor superlattices (QSSL) have been calculated using one-dimensional linear chain model. We consider two types of quasiperiodic systems, namely cantor triadic bar (CTB) and Fibonacci sequences (FS), constituting of AlAs, GaAs and GaSb of which the latter two have a lattice mismatch of about 7%. The calculations have been made using transfer matrix method and also with and without the inclusion of strain. We present the results on phonon spectra of two component CTB and two as well as three component FS semiconductor superlattices (SSL), thickness and order dependence on LO mode of GaAs, effect of strain on LO frequency of GaAs. The calculated results show that the strain generated due to lattice mismatch reduces significantly the magnitudes of the confined optical phonon frequency of GaAs.

• Nonlinear dynamics of a two-dimensional lattice

The dynamics of the nonlinear excitations in a two-dimensional (2D) φ4-diatomic lattice, with nonlinear on-site electron-phonon coupling at the polarizable ion site has been presented, without considering the self consistent phonon approximation. One of the major results obtained from our calculations is in the understanding of continuous structural phase transition, where we have obtained the minimum in soft mode frequency at a soft mode temperatureTs (&gt;Tc), not at critical temperatureTc. This occurs due to the anisotropy of such 2D systems.

• Nonlinear travelling waves in φ6 polarizable model

We present a complete theoretical analysis of the periodic and non-periodic travelling waves in a diatomic chain model, in the continuum limit by incorporating nonlinear sixth order polarization potential (φ6) at the anion site. We have formulated a nonlinear lattice dynamical theory in which various energy curves are obtained for different types and magnitudes of the core-shell force constants. For periodic solutions, we have obtained two types of commensurate wave amplitudes which propagate in the opposite direction with respect to each other. For nonperiodic solutions, we have obtained various travelling excitations such as kink, antikink, excitons etc. for different values of the mass ratio and velocity parameter. The dipole moment per unit charge for SrTiO3 has been calculated and it is found that the nonlinear excitations in this model carry large amount of energy as compared to those obtained from harmonic and anharmonic optical phonons in the φ4-polarizable model.

• Phonon dispersion in aluminium arsenide and antimonide

The phonon dispersion curves for aluminium arsenide and antimonide have been investigated by using a deformation bond approximation model. The results obtained from this model are compared with the experimental values wherever it is available. Since there is no complete experimental phonon dispersion curves for AlAs, we could not compare our calculated results, but the results of AlSb have been compared with the inelastic neutron scattering measurements at 15 K. However, we compare the phonon frequencies of AlAs and AlSb at critical points of the Brillouin zone obtained by our calculations and Raman spectroscopy measurements. This model predicts the phonon modes satisfactorily in all the symmetry directions of the Brillouin zone (BZ). The spectrum has similar features as observed in other III–V compound semiconductors.

• Phonon and thermal properties of achiral single wall carbon nanotubes

A detailed theoretical study of the phonon and thermal properties of achiral single wall carbon nanotubes has been carried out using force constant model considering up to third nearest-neighbor interactions. We have calculated the phonon dispersions, density of states, radial breathing modes (RBM) and the specific heats for various zigzag and armchair nanotubes, with radii ranging from 2.8 Å to 11.0 Å. A comparative study of phonon spectrum with measured Raman data reveals that the number of Raman active modes for a tube does not depend on the number of atoms present in the unit cell but on its chirality. Calculated phonon modes at the zone center more or less accurately predicted the Raman active modes. The radial breathing mode is of particular interest as for a specific radius of a nanotube it is found to be independent of its chirality. We have also calculated the variation of RBM and G-band modes for tubes of different radii. RBM shows an inverse dependence on the radius of the tube. Finally, the values of specific heat are calculated for various nanotubes at room temperature and it was found that the specific heat shows an exponential dependence on the diameter of the tube.

• An investigation on the stability of the structural and electronic properties of $ErX_{3} (X = Ga, In\,and\,Sn)$ intermetallic compounds

First-principle computations on structural and electronic properties of cubic rare-earth $\rm{ErX_{3} (X = Ga, In\,and\,Sn)}$ intermetallic compounds have been accomplished using the full-potential linearised augmented plane wave (FP-LAPW) method within the framework of density functional theory (DFT). For the exchange correlation, we used local spin density approximation (LSDA) plus Hubbard parameter $U (LSDA+U)$ approach because of the strong on-site Coulomb repulsion between the localised $\rm{RE}-4 f$ states. Calculated ground-state properties such as lattice constant ($a_{0}$) and other parameters with exchange correlation functional are found compatible with the experimental results. The electronic properties have been determined in terms of band structures, total and partial density of states (DOSs) and Fermi surfaces, which demonstrate the metallic behaviour of all the compounds. Also, the effect of Hubbard potential on this is discussed in detail. The bonding descriptions of these compounds have also been evaluated from charge density difference plots, which display the presence of metallic and mixed covalent–ionic bonding. The determined magnetic moments explain the ferromagnetic behaviour of these compounds.

• Pramana – Journal of Physics

Volume 94, 2020
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Posted on July 25, 2019