Research Summary

Experimental and theoretical determinations of the material response at high pressure and temperature are critical for understanding thermal, mechanical, chemical, and electronic properties of solids. Quite often, material models describe some properties very well, while others are at best qualitatively understood. In this light, my research interests during my graduation days have been centered on studying the local dynamics of solids that occur due mainly to thermal agitations at the microscopic level. In the process, I got myself involved in the following activities that have potential applications in the field of crystal dynamics. These are respectively:

lattice dynamics modeling
molecular dynamics simulations
inelastic neutron scattering experiments
First principles electronic structure study

Extensive utilization of these mechanisms has been made to study the atomistic dynamics in two solid systems alongwith the theoretical investigation of interlayer exchange coupling in one multilayer system of current interest viz.

various crystalline polymorphs of ZnCl₂
fluoroscheelites LiYF₄ and LiYbF₄
Fe/Nb multilayers

While the inelastic neutron scattering study in one of the polymorphs (viz. α-ZnCl₂) has helped convincingly in validating the realistic lattice dynamical model that was due, molecular dynamics study on one of the important laser host materials (viz. LiYF₄), has been able to determine the hitherto unknown structure of the second high-pressure phase in it. It may be noted that high-pressure thermodynamic properties such as the equation of state (EOS), and phase diagram of solids establish fundamental relationships between thermodynamic variables (e.g. pressure, density, temperature etc.) and provide closure of the hydrodynamic equations, which explicitly ensure conservation of mass, energy and momentum. Materials models are hence used often to determine the dynamic mechanical response especially in regimes of high pressures and energies.

On another count, the works during my graduation study seem to have served satisfyingly well. It is known that during lattice vibrations, the electronic charge cloud associated with the ions undergoes deformation. However, the physics of this deformation still continues to receive attention in exploring significant crystal properties. Hence, developing a realistic model, with the aid of as few variable parameters as possible, has always been a physicist's choice of concern. To some extent, the works accomplished so far by me poise to carry this wisdom. Simple models of interatomic potentials within the framework of quasi-harmonic approximation have been formulated in deriving various equilibrium thermodynamic properties (e.g. free energy, heat capacity etc.) of some important compounds. Various experimental data have fairly supported the results out of those models. Following are the summarized version of some critical observations gathered during my research study towards the PhD degree:

The occurrence of softening in two successive modes (one acoustic and the other optic mode of E_g symmetry) in γ-ZnCl₂ along the principal axis of symmetry [001] with the Λ₅ representation in the Brillouin zone implies lattice instability towards a possible second order phase transition. Successive investigations further suggest that this transformation has the maximum probability to result in a structure other than any of the remaining crystalline phases of ZnCl₂.
A rigid ion model set up for LiYF₄ comprising as few as five variable parameters has the potential to study the thermodynamic properties in any interesting fluoroscheelite system of ABF₄ (A: metal; B: rare earth) kind.
The possible reason for the discontinuity observed in the Raman spectra of LiYF₄ is analyzed lattice dynamically as being due to a second order phase transition initiated by a dynamic instability of a transverse acoustic phonon mode at a small wave vector along the [130] direction.
When the ambient scheelite phase of LiYF₄ transforms upon compression into a new phase, the low temperature structure has slight distortions compared to the high-temperature (>300 K) one. It is found that the space group of the low-temperature energy minimized structure is P21/c, while that of the high-temperature simulated and experimentally observed one is I2/a.
A possible P-T phase diagram of LiYF₄, as constructed at the end of my PhD works, provides a qualitative picture of how its various high-pressure phases may coexist. Diffusivities and molecular reorientations are also studied in course of my research works.

My hitherto research works conform well to the fact that while the inelastic scattering of neutrons opens up the possibility of characterizing a solid by the way of phonon frequency distributions as well as phonon dispersion relations, lattice dynamics allows one to further these in the interpretation of associated important thermodynamical properties. One of the most salient features that my research study may offer lies essentially in its explicit ability to investigate (e. g. through MD simulation) whether there would be any change in the structural behaviour even at a pressure, which may be too extreme (e.g. 100 GPa) to be accessible experimentally. In addition, the carefully tailored complementary lattice dynamical calculations would help one determine the dynamically stable structure (with proper space group) of any potential high-pressure crystalline phase (as has been the present case with LiYF₄), which has a fair chance to be otherwise subverted due to poor or insufficient scattering signals, if measured through conventional instruments.

One particular area of research work that has attracted me a lot during my PhD works is to effectively carry out first principles lattice dynamics (using the Density Functional Perturbation Theory) as well as molecular dynamics simulations to calculate more accurately crystal properties associated with various dynamical phenomena especially in magnetic and superconducting nano-crystalline substances, which would certainly be of paramount interest in days to come. During the last couple of months of my post PhD assignments I have worked on magnetic multilayers am now looking forward to extending my ab initio works to quantum dots and other nano clusters. I have recently studied the mechanism of interlayer exchange coupling in Fe/Nb multilayer system through self consistent approach by means of linear-muffin-in-orbitals method in the generalized gradient approximation. It has been observed that the coupling oscillation comprises both short- and long-periods due to the Fermi surface curvatures of the intervening layer along the [0, 0, π/2a] direction. The Vernier effect observed in the higher harmonics of long periods in the Fe₃Nb₁₆ heterostructure signifies the less dominance of the Ruderman-Kittel-Kasuya-Yosida kind of mechanism in Fe/Nb system. The dependence of the interlayer coupling amplitude on Fe moments has also been studied. For small spacer thickness of Nb, the exchange coupling oscillates rapidly with Fe moments, favouring the quantum well model as the feasible mechanism behind the oscillatory behaviour in Fe/Nb multilayers. Quantum well dispersions around the Fermi level demonstrate further that the sp majority-spin bands contribute largely to the formation of quantum well states, which has been analyzed quantitatively by making use of the phase accumulation model.

As far as experiments are concerned, though I am used to working in moderate-flux steady-state nuclear reactors for neutron studies in materials of scientific/technological interest, I am in addition looking forward to carrying out a handful of experiments in coming years making use of high-intensity synchrotron X-rays and possibly of accelerator based muon sources, which had not been available so far to me as our country does not have such facilities in operation to date.

I. I. T. Kanpur | Department of Physics |