Bhalchandra Pujari

The Fe_1-xMn_xPt 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).

Published in : Phys. Rev. Lett. 115, 057203 (2015)

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

Using ab-inito molecular dynamics as well as Nudge Elastic Band simulations we examine the transfer of proton across a complex Organometallic compound. The compound synthesized by the experimental group at the Department of Chemistry shows the promising conductivity as a function of humidity. With the aid of ab-initio calculations we compute the activation energy along with the electronic structure calculations. The theoretical as well as computational results are in excellent agreement with each other.
Paper in communication

Materials under high pressure and temperatures are of great interest, especially in astrophysical applications. Among terrestrial and extraterrestrial environments, it is quite common to have materials under extreme physical conditions. It is known that the matter inside the mantle of the Earth can be under more than 100 GPa pressure and is at temperatures about 900 C. Extraterrestrial environment offers even more exotic situations. For example even at the surface of the Venus, temperature and pressure can reach as high as 467 C and 9.3 MPa. It is a daunting task to know the exact nature of the surface experimentally, because, it is difficult to simulate such extreme situations in the laboratory and most of the astrophysical objects are still beyond thorough observations. We employ evolutionary algorithm coupled with state of the art density functional theory, to study the crystal structure under extreme conditions that mimics the extreme astrophysical situations, mainly Venus. The exact nature of the surface of Venus is still under intense investigation and theoretical predictions are warranted.

Lead zirconate titanate (PZT), a ceramic perovskite material is used as a component in ultra- sound transducers, ceramic capacitors, STM/AFM actuators, FRAM chips and sensors. PZT has also been used in the manufacture of ceramic resonators for reference timing in electronic circuitry. PZT is usually not used in its pure form but doped with either acceptors to create anion vacancies or with donors to create cation vacancies. Acceptor doping creates hard PZT and donor doping creates soft PZT, which are differentiated based on their piezoelectric constants. These piezoelectric constants are proportional to the polarization i.e. the electric field generated per unit of mechanical stress. Usually soft PZT has a higher piezoelectric constant but larger losses in material whereas hard PZT has lower piezoelectric constant and lower losses in material. With this view, we are interested in performing first principles density functional theory (DFT) based calculations on undoped and doped PZT materials.

So far most of my research work has been using density functional theory. However one of problems 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.

We developed a technique to decrease memory requirements when solving the integral equations of three-dimensional (3D) molecular theory of solvation, a.k.a. 3D reference interaction site model (3D-RISM), using the modified direct inversion in the iterative subspace (MDIIS) numerical method of generalized minimal residual type. The latter provides robust convergence, in particular, for charged systems and electrolyte solutions with strong associative effects for which damped iterations do not converge. The MDIIS solver (typically, with 2 × 10 iterative vectors of argument and residual for fast convergence) treats the solute excluded volume (core), while handling the solvation shells in the 3D box with two vectors coupled with MDIIS iteratively and incorporating the electrostatic asymptotics outside the box analytically. For solvated systems from small to large macromolecules and solid–liquid interfaces, this results in 6- to 16-fold memory reduction and corresponding CPU load decrease in MDIIS. We illustrated the new technique on solvated systems of chemical and biomolecular relevance with different dimensionality, both in ambient water and aqueous electrolyte solution, by solving the 3D-RISM equations with the Kovalenko–Hirata (KH) closure, and the hypernetted chain (HNC) closure where convergent. This core–shell-asymptotics technique coupling MDIIS for the excluded volume core with iteration of the solvation shells converges as efficiently as MDIIS for the whole 3D box and yields the solvation structure and thermodynamics without loss of accuracy. Although being of benefit for solutes of any size, this memory reduction becomes critical in 3D-RISM calculations for large solvated systems, such as macromolecules in solution with ions, ligands, and other cofactors.
Journal of Computational Chemistry Volume 33, Issue 17, pages 1478–1494, 30 June 2012