Dr. Kumar - RESEARCH

I am working in the field of heterogeneous/homogeneous catalysis focusing on chemical reactions catalyzed by solid state catalysts/catalysts in liquid or gaseous forms.

I worked on defects and their role in electronic transport in 2D materials. I studied how thermodynamic defect transition levels can be engineered using strains and similar mechanisms. I also explored nanoribbons, nanowires and defects in them for exploring their usefulness in electrical applications.

Previously I worked on thermoelectric response of 2D materials using Boltzmann transport equation for electrons and phonons. I have excellent knowledge of the nuts and bolts of the implementations of these equations.

I have developed programs for calculating the phonons and their interactions using model calculation and first principles method.

PhD. Thesis: I have studied the Lattice dynamics of complex inorganic oxides using DFT (Density Functional Theory). Both ways i.e. finite difference technique and density functional perturbation theory have been used. Structural, elastic, bonding and dielectric properties in oxides have also being investigated using first principles approach. During my PhD work, I also did experimental work involving sample preparation of oxides in their bulk form, XRD, Raman and IR spectroscopy, and More.

Code development:

- Developed "esta" software package for analyzing, pre-processing, and post-processing data from different software packages such as vasp, quantum-espresso, gaussian, GRRM, xTB, and so on. It can automatically generate input files for vasp, quantum-esspresso, and other codes as well can do some model calculations. It is written in python3, Fotran90, and some part in C. In future, it will be able to perform calculations for predicting electronic and transport properties.

abilities of the code:
- automatic input generations for different electronic structure softwares
- output analysis of different file formats such xml, yaml, json, and so on,
- vibrational and thermodynamical analysis of atomic and molecular systems
- zone center phonon calculations based on gradient input (to be expanded to whole zone)
- model calculations of lattice thermal conductivity (first principles calculation is experimental)
- tight binding calculation of bulk materials
- point group detection (space group detection and other stuff carried out using spglib)
- lattice transformation analysis
- machine learning prediction of *important* physical property

more to be added to esta package!!!

EStA software package

Complex oxides:

The interplay between competing energy scales can results in interaction between charge, spin and orbital degrees of freedom. Oxygen octahedrda, if there in the oxides, is generally tilted or show buckling nature, which results AFD structural disordering.

This disordering can couple to other modes, like vibrational modes at low energy scale or other modes, resultingin structural or electronic phase transition. Structural phase transition from one phase to another phase, say from cubic to tetragonal, can be easily predicted using experiments/DFT, in which there is splitting of vibrational modes into multiple modes in low symmetry structure. Whereas, electronic phase transition can be detected by doing conductivity measurements, or other experimental techniques along with DFT analysis of the electronic states at atomic level in the bulk, or at interface etc. depending upon the nature of the material and its phase.

Here I quote few lines from a Review on thin films of perovskite-type complex oxides by Hanns-Ulrich Habermeier, Materialstoday, 10 (2007): Among the solid inorganic materials, oxides display perhaps the most diverse range of properties. The Nature of cation-oxygen bonding determines their electronic properties..... This bonding results in an interplay between localized and itinerant characters for the electrons, which is superimposed with strong electron-electron correlations and lattice effects. Additionally, the interaction between charge, orbital, spin, and lattice degrees of freedom can cause drastic changes in the electronic states upon subtle extrinsic perturbations.

Why Density Functional theory?

Density functional theory is being used widely to investigate the ground state properties of atoms, molecules, and the condensed matter systems. It relies on the ground state charge density as a basic variable rather than the wave function. In principle DFT is an exact theory. However we have to resort to approximations to exchange-correlation energy functional which is currently the bottleneck to all first principles calculations.There are number of approximation to the exchange-correlation term like LDA (local density approximation), GGA (generalized gradient approximation): PBE, PBErev, PBE0, PBEsol etc and so on.

BEWARE: Use of a particular functional to system under study is VERY-VERY crucial to have a sensible and reliable prediction of properties.

For More Information: follow these links

http://www.vega.org.uk/video/programme/23

http://web.physics.ucsb.edu/~kohn/

http://en.wikipedia.org/wiki/Density_functional_theory