Savitribai Phule Pune University | All Models Are False, Some Are Useful |
Name | : Dr. Anjali Kshirsagar |
Designation | : Professor in Physics, |
: Director, Interdisciplinary School of Scientific Computing | |
: Director, Centre for Modeling And Simulation | |
: University Of Pune, Pune 411007. | |
Office | : Departement of Physics, |
: University of Pune. | |
: Pune 411007, India. | |
: Tel. (91)-(020)-25691684 | |
: E-Mail : anjali@physics.unipune.ac.in | |
Residence | : 1, Khagol, 38/1 Panchavati, |
:Pashan Road, N.C.L. P.O., | |
:Pune 411008, India. | |
:Tel. (91)-(020)-15882217. | |
Date Of Birth | :November 14, 1957. |
Degree | College/University | Year Of Passing | %age of Marks | Class | Remarsk |
H.S.C. | M.P.Secondary Board Of Education, Bhopal | 1973 | 76% | First | National Merit Scholar |
B.Sc. | Devi Ahilya University,Indore | 1976 | 80% | First | Gold Medalist |
M.Sc. | University of Pune | 1978 | 62.5% | First | |
B.Sc.App.(Comp.Sc.) | University of Pune | 1986 | 'O' Grade | ||
Ph.D. | University of Pune | 1985 | Title : Band Structure Calculation of Electron momentum distribution in Pd and PdH. |
(a) Metal-hydrogen systems have received considerable attention as possible storage elements of hydrogen. Transition metals are particularly important due to their intricate band structure and Fermi surface properties. I, therefore, studied the changes in the electronic structure and electron momentum distribution (EMD) of host metal upon introduction of hydrogen. The first ab initio calculation for EMD with two atoms per unit cell using Augmented Plane Wave (APW) method was reported in my doctoral work. Compton profiles (CPs) and angular correlation of positron annihilation radiation (ACPAR) were studied by the same method to bring out the effect of positron. A proper comparison of theory with experiment is made through the Fourier transforms of EMDs, the B(r-) functions. This technique separates out the role of core and valence electrons and highlights the effect of the presence of positron.
The CP in theoretical treatment is computed within the so- called "impulse approximation" (IA) which assumes that the electron is knocked off by the photon instantaneously and goes into a continuum. But in actual practice, the electron sees different atomic potentials when it recedes away from the atom. We are trying to extract the correction term called the "Compton defect" using DFT within LDA for atoms and solids.
Interest in CP and related studies has been revived due to availability of better experimental resolutions for measuring the electron momentum distributions and increased computational power in recent years. We have employed Full potential LAPW method to calculate the Compton profiles and positron angular correlation curves in Li and Be. Our results agree with the available experimental results after applying the Lam-Platzman correction. We have also included the generalized gradient approximation (GGA) in the exchange correlation potential to improve on the theoretical results. Magnetic Compton profiles of Ni have been studied and the inclusion of GGA is found to have significant effect on the spin momentum density. Recently, Compton scattering experiments have been carried out at low-temperatures and with low incident energies for Li. We have calculated temperaturedependent CP and have included the corrections in going beyond the impulse approximation for Li. A comparison with experiments brings out the importance of such studies. We are in the process of extending these studies for transition metals where the electron-electron correlations play an important role.
We have extended the state-of-the-art band structure code to calculate the CP and the code is also rewritten in Fortran 90 ready to be included in the WIEN2K code available commercially or otherwise to the academic community.
(b) Effect of different exchange-correlation potentials on momentum space properties of atoms was systematically studied using density functional theory (DFT) within local density approximation (LDA). A part of this work was carried out at the Queen's University, Kingston, Canada, during my graduate study.
(c) Two component DFT was used to calculate the positron-ion and positron-atom binding energies with the inclusion of electron- positron correlation effects. Self-consistency was achieved by varying both the electron and positron densities. Results were obtained for first row ions and closed-shell atoms.
This work has been extended to calculate the density matrices in position and momentum spaces to gain more insight about the electron distribution. An exact Schr"odinger-like differential equation has been derived for the positron density during the bound state.
(d) Fabrication of nanostructures and study of their electronic properties is promising as it provides opportunities for practical exploitation of the quantum size effect. I worked in collaboration with the experimental group of the Department, to study the size and shape dependence of the electronic structure of II-VI semiconductor quantum dots. Medium sized clusters have been found to possess the same lattice symmetry as the bulk. However, very small clusters are found to be amorphous. Particle in a box or rigid sphere approach also qualitatively describe the correct behavior but actual shape of the dot plays an important role for such sizes. Pseudopotential method was used to predict the blue shift in the absorption spectra and also to successfully predict the sizes of the experimental QDs from their absorption spectrum. These were later compared with TEM results.
We have also used tight-binding linear muffin- tin orbital (TB-LMTO) method to study the electronic structure of small sized dots.
Currently we have been performing density functional based calculation using ab initio pseudopotentials (PAW/ USPP) to simulate the lowest energy structures of II-VI semiconductor clusters and noble metal clusters. These materials display unusual structural and electronic properties for lower dimensions. In semiconductor clusters surface reconstruction, formation of cleavage planes and doping of transition metal impurity atoms are other interesting properties. The HOMO-LUMO gap for these clusters can be tuned by changing the shape, size and by doping impurities. These also constitute important materials from device application point of view viz. LEDs, sensors etc.
We have used the VASP code, based on use of pseudopotentials and plane wave basis set, to calculate the ground state geometries and structural and electronic properties of pure and doped small clusters of II-VI semiconductor materials. The results have been useful to understand the changes in the electronic properties due to size quantization, doping and increase in the surface-to-volume ratio.
Small clusters of Zn_{n}S_{n} and CdnTen have almost identical structures with planar structures for (1 <= n <= 5) global minimum. Chalcogenide atoms prefer to stay on the outer side due to repulsion between the lone pair electrons on them. Binding energy increases with size of the cluster. Nature of bonding is analyzed by total charge density, molecular orbital charge density and electron localization function (ELF) plots.
The initial geometries of non-stoichiometric clusters were considered as fragments of the bulk with Td symmetry. It was observed that upon relaxation, the symmetry changes for the cation-rich clusters whereas the anionrich clusters retain their symmetry. It may be mentioned that the chalcogenide p lone pair repulsion drives these atoms to the surface and renders stability to the clusters. The cation-rich clusters develop a HOMO-LUMO gap due to relaxation whereas there is no considerable change in the HOMOLUMO gap of the anion-rich clusters. Thus, the symmetry of a cluster seems to be an important factor in determining the HOMO-LUMO gap. To render the surface of a quantum dot inert, passivation is essential. We have passivated the non-stoichiometric clusters using fictitious ”hydrogen”-like pseudoatoms. It was observed that passivation removed the states in the HOMO- LUMO gap region resulting in widening of the gap. The symmetry of the clusters, however, remains unchanged upon passivation. HOMO-LUMO gap opens up due to quenching of mid gap states. Passivation provides stability to the clusters as is evident from the binding energy values. Unpassivated clusters are quasi-metallic in nature. Passivation leads to removal of this metallic nature.
We have studied the smallest cage-like structure of Cd_{9}S_{9} doped with varying number of M_{n} atoms. The smallest stable stoichiometric cage-like structure Cd_{9}S_{9} of Cd_{n}S_{n} is found to have enough space to dope atoms endohedrally. Single Mn doped cage is formed with a magnetic moment of 5 _{m}u_{B} but bi-doped endohedral cage is unstable. We substitutionally doped the cage with n Mn atoms with n = 1-9. Mn substituting S is found to be a less favorable geometry than Cd substitution. On the other hand, substitutional doping is energetically favored than endohedral doping for n = 1. The magnetic moment of the cage is 5n μ_{B} up to n = 7. For larger values of n, the magnetic interactions decrease the total magnetic moment of the cluster. Doping with other transition metal atoms also depicts similar results.
(e) Clusters are being studied for their novel electronic properties like magic numbers for alkali metal clusters. The exact ground state geometry can be calculated using Car-Parrinello’s unified molecular dynamics and density functional approach by minimizing the total energy. We tried to develop algorithms for force calculations, with linearized APW (LAPW) basis set, to be used for transition metal clusters as plane wave basis with soft pseudopotentials is also not computationally economic. However, the work could not be continued due to lack of computational facilities available at that time.
We are presently investigating small neutral and cationic gold clusters to understand their fragmentation path ways and adsorption. Minimum energy structures of neutral and cationic Au _{n} ^{(0/+)}(n = 1 - 8) are found to be two-dimensional. The lowest energy structures of neutral gold clusters are planner after H_{2} or H_{2}S adsorption but the cationic gold clusters transform into three-dimensional structures at n = 7. The adsorbed molecules get adjusted such that their centers of mass lie on the plane of the gold cluster. Hydrogen sulphide adsorbed clusters with odd number of gold atoms are more stable than the neighboring even n clusters. Charge density analysis helps to understand electron transport in clusters, this is useful in single electron devices and in systems where gold clusters are used as nanoleads. Further work on semiconductor nanowires terminated with gold nonoleads is in progress.
(f) Diluted magnetic semiconductors are extensively being studied both theoretically and experimentally for basic understanding of the origin of halfmetallicity and ferromagnetism. We have used the FP LAPW method to study the changes in the band structure and density of states on the introduction of magnetic impurities in GaN. The injected spins are found to cluster together and the favorable configuration is with the spins aligned parallel to each other. From the spin-polarized electronic structure ans spin charge densities of GaN co-doped with Mn and Cr, we find that such a material is a probable candidate for spintronics applications, due to its half metallic character and possibility of above room temperature transition temperature. The band structure is more favorable than pure transition metal doped GaN for spin injection when put in a heterostructure. An ordered ferromagnetic state is more probable in co-doped system and this fact favors ferromagnetism with higher transition temperature.
I am a Senior Group Associate of the International Centre for Theoretical Physics, Trieste, Italy. I worked as a UGC Research Awardee during 2002-05.
Presently there are five research scholars working with me for their doctoral degree. Seven students have obtained their doctoral degree under my supervision and guidance. Three students have completed their M.Phil with me.
Have a cumulative teaching experience of more than 25 years, although I am working at the Department of Physics, University of Pune since 1986. I have taught core courses like Mathematical Methods for Physics, Quantum Mechanics, Classical Mechanics, Statistical Mechanics, Atomic, Molecular and Solid State Physics and advanced courses on Condensed Matter Physics and Numerical Analysis and Methods of Computational Physics. I have also conducted laboratory courses on general physics experiments and FORTRAN programming. I was instrumental in giving access to computers to our M.Sc. students in 1986 and also set up the computer laboratory which currently hosts 25 personal computers solely used by the M.Sc. students for their programming and special courses and projects.
I have conducted tutorial sessions for M.Sc. (Physics) students when I was working as a research scholar at the Department of Physics, University of Pune. I have also taught Physics and computer related courses for B.Sc. (applied) (Comp. Sc.), M.Sc. (Comp. Sc.), M.Sc. (Electronics Sc.) and a course on Computer Awareness for post-graduate students of Biomedical sciences and Health Sciences.