Computational condensed matter physics has become an integral part of modern day material science. The ability of computational physics to model and simulate the real life scenarios offers a great deal of advantage in understanding the underlying physics, especially in the nanotechnology area. The elegant nature of modeling and simulating the experimental scenarios is to me the most attractive part of this field. It is the ability to examine the nanoscopic structure of materials which gives me impetus to pursue the research in this exciting field.
I have been involved in atomic and electronic structure calculations, developing and employing various state-of-art packages based on the formulations such as Density Functional Theory, Hartree Fock Theory, Configuration Interaction method and Solvation techniques throughout my doctoral studies as well as in my PostDoc works at
National Institute for Nanotechnology and
University of Nebraska - Lincoln. Using these techniques I have been exploring various nanoscopic systems:
with graphene based systems as my personal love!
Below is the brief description of my work. Also have a look at my
ResearcherId page for my publications and more. A list of my collaborators is at the bottom of the page.
Magnetism in Random Alloys
The Fe_{1-x}Mn_{x}Pt system exhibits five magnetic structures, including a ferromagnetic phase, two antiferromagnetic phases, and two noncollinear ones [1]. The latter are characterized by superpositions of orthogonal magnetic amplitudes at two commensurate magnetic wave vectors. Here we study the magnetic interaction in this system and construct the magnetic phase diagram based on total energy calculations within the vector-DLM (disordered local moment) method representing partially ordered magnetic states in the coherent potential approximation (CPA). This method has been implemented within the linear muffin-tin orbital basis. The calculated total energies are fitted to polynomial functions of the relevant order parameters (magnetization amplitudes at the relevant wave vectors for the two components), which are subsequently used to evaluate the free energy in the mean-field approximation. The interaction between spatially orthogonal amplitudes is evaluated by noncollinear total energy calculations for representative alloy configurations. The phase diagram is constructed by minimizing the free energy at each concentration and temperature. The structure of the resulting phase diagram agrees with neutron diffraction measurements, demonstrating the applicability of the method to the studies of magnetic phase diagrams of intermetallic alloys with competing ferromagnetic and antiferromagnetic interactions.
[1] A. Z. Menshikov et al., J. Magn. Magn. Mater. 65, 159 (1987).
Graphene and its hydrogenation.
GraphAne - the hydrogenated graphene has been subject of interest lately. I worked on a few problems based on hydrogenation of graphene.
In the latest density functional study, we demonstrate that single-side-hydrogenated
graphene (termed as
SSHGraphene) is a semiconductor with an indirect band gap of 1.35 eV, which
is in between the gapless graph
ene and wide band-gap
graphane
and surprisingly close to silicon. We show
that its electronic
structure and lattice characteristics are substantially different from
those of graphene, graphone, or
graphane. The lattice parameter and C-C bond length are found to be lengthened by
15%
of those of graphene. Our binding-energy analysis confirms that such a
single-sided hydrogenation leads to energetically stable material,
making it a promising candidate as an organic semiconductor.
Phys. Rev. B 84, 041402(R) (2011) (Rapid Communications)
Earlier, we have probed the transformation of graphene upon hydrogenation to graphane within the framework of density functional theory. By analysing the electronic structure for 18 different hydrogen concentrations, we brought out some novel features of this transition. Our results showed that the hydrogenation favored clustered configurations leading to the formation of compact islands. The analysis of the charge densities and electron localization functions (ELF) indicated that, as hydrogen coverage increases, the semi-metal turns into a metal, (showing a delocalized charge density,) then transforms into an insulator. The metallic phase was spatially inhomogeneous in the sense it contained islands of insulating regions formed by hydrogenated carbon atoms and metallic channels formed by contiguous bare carbon atoms. It turned out that it is possible to pattern the graphene sheet to tune the electronic structure. For example, removal of hydrogen atoms along the diagonal of the unit cell, yielding an armchair pattern at the edge, gave rise to a bandgap of 1.4 eV. We also showed that a weak ferromagnetic state exists even for a large hydrogen coverage whenever there was a sublattice imbalance in the presence of an odd number of hydrogen atoms.
J. Phys.: Condens. Matter 22 465502,(2010)
This work is featured in “IOP Select November-2010”
In first problem we carried out ab initio electronic structure calculations on graphane having single
and double vacancy defects. Our
analysis of the density of states reveals that such vacancies induce the
mid-gap states and modify the band gap. The induced states are due to
the unpaired electrons on carbon atoms surrounding the vacancy.
Interestingly, the placement and the number of such states is found to
be sensitive to the distance between the vacancies. It turns out that
such vacancies also induce a local magnetic moment.
J. Phys. Chem. C, 2009, 113 (50), pp 21063–21067
Solvation - RISM
So far most of my research work has been using density functional
theory. However one problem with density functional theory is that the
number of atoms handled in the theory is seriously restricted by the
availability of the computational resources. Hence when it comes to
handling the mesoscopic systems like complex solute-solvent systems the
treatment by other methods are more useful. In my current work I
collaborate with experimental group who are interested in finding out
the optimum size of certain nanocrystals in given a solvent. We address
this issue by a method called 3D-Reference Index Site Model (RISM).
3D-RISM is based on integral equations and calculates the solvation
energies by solvent distribution. Our results are in excellent agreement
with the experiments and it provides in depth profiles of the
distribution of the molecules around the nanocrystals. I am fortunate to
be a part of the group which has done a fundamental contribution to
the RISM theory.
Amid all the application based projects my personal interest lies with
the development of new algorithms and codes. In my current tenure I
also work on the development of the RISM code. We are currently writing a
manuscript which we believe is a fascinating contribution in the RISM
technique. We demonstrated that the existing RISM methods can be made
highly efficient by intelligent treatment of correlation functions,
thereby saving substantially on computation memory and time.
Organic Solar Cells
Organic solar cells are attractive for
their cost effectiveness and portability . However their
efficiency remains less than 10% which is problematic for widespread
commercialization. A variety of experimental and theoretical works
are underway in pursuit of better materials in order to increase the
efficiency. In order to improve the efficiency it is important
to understand the underlying mechanism that governs the charge
transfer across the interface of acceptor-donor pair. In this work
we examined one such acceptor-donor pair. We studied the ground state and excited state geometry, and detailed
electronic structure of the germafluorene based polymer PGFDTBT and
C_{70} interface. The charge transfer was studied by analyzing
the charge densities of individual orbitals near the HOMO and LUMO.
Our results indicated that the charge transfer at the ground state of
Polymer:C_{70} interface was negligible, while the efficient
charge transfer from polymer to C_{70} took place upon
excitation. The transfer was enabled by a "bridging state"
which is energetically in between the "hole state" and the
"charge separated" state.
(To be appeared in ECS-Transactions)
Quantum Dots
As a part of the work I developed the programs based on Density Functional method, Hartree Fock method and Configuration Interactions method, in order to examine the effects of impurity in confined electron gas (Quantum Dots). This work was done in two parts.
In the first part, we used density functional formulations for upto 20 electron quantum dots of varying sizes with and without an impurity. We observed the emergence of localized Wigner-molecule-like behavior along with the wall-like feature in the charge density. However, unlike the parabolic quantum dots, we do not observe the analog of concentric rings. We also observed the typical broken symmetry configurations noted in earlier reports for quantum dots. The impurity induced the localized magnetic moment which, in many cases, generated the spin-polarized configurations with the antiferromagnetic coupling. An examination of the magnetic states
indicated that the presence of impurity might change the ground state of quantum dot from magnetic to nonmagnetic and vice-versa. We also observed that the localized charge at the center sharpens the walls. (
Phys. Rev. B
76,
085340
(2007))
In the second part as continuation we performed a full configuration-interaction study on a square quantum dot containing up to six electrons in the presence of an attractive impurity. The magnetic ordering in the dot was analyzed using appropriate pair-correlation functions. We found that a change in the size of the quantum dot could change the nature of the impurity from nonmagnetic to magnetic. In the low-density regime, the impurity trapped one electron and the magnetic moment on the localized peaks outside the impurity fluctuated from negative to positive going through zero as a function of number of electrons. We also observed that the impurity changes the charge densities of excited states of two-electron quantum dot significantly, which in the absence of the impurity were almost similar. Our study also showed that in the strongly correlated regime the configuration-interaction approach yields ∼20% more localization than density-functional theory. It had also been observed that only a small fraction of the total number of Slater determinants were required to produce ∼99% of the converged charge density. (
Phys. Rev. B
78,
125414
(2008))
Others
Confinement effects on gallium clusters inside carbon nanotubes.
Confinement is known to impose dramatic effects on the nanosystems. In the present work we have carried a systematic investigation of ground state geometries of gallium clusters confined inside carbon nanotubess. For small clusters the geometry is influenced by Ga-C interactions e.g. there is substantial increment in the bond lengths of dimer and trimer in sufficiently large tubes. On the other hand, for the larger clusters the intra-cluster binding is significant. In such cases the physical confinement plays a significant role in determining the geometry. The lowest energy geometry of all the clusters tend to become 1D as the diameter of the is reduced. Our calculations demonstrate that it is indeed possible to choose the radius of the tube for which the confined cluster (n > 6) can take either 1D, 2D or 3D. For the larger cluster we also find that the interaction between the cluster and the tube is rather weak and is van der Waal-like. For many clusters inside the tube of intermediate diameter there are several nearly degenerate states.
Angular momentum transport in quasi-Keplerian accretion disks.
There
have been several physically motivated, simple, treatments in popular
textbooks that derive the form of the viscous torque between neighboring
annuli in accretion disks. Some of these treatments have been shown to
be wrong, and we understood that some of the corrections proposed in the
literature are also wrong. Now we have proposed simple, physically
motivated arguments about the transport of matter and the angular
momentum in the accretion disks. (The work is published in : J. of Astrop. Astron., vol 25, pg 81).
For a complete list of publications please see the
ResearcherID page