Detailed Biographical Data
1. Name: Tara Prasad Das
2. Citizenship: U. S. A.
Bachelor of Science (Honors), Patna University, India, 1949
Master of Science, Calcutta University, India, 1951
Doctor of Philosophy Saha Institute of Nuclear Physics, Calcutta University, India, 1955
[Title of Ph.D. Thesis: "Principles and Theory of Nuclear Magnetic Resonance Experiment"]
4. Professional Positions: Prior to my permanent academic positions in Universities in the USA, I held post-doctoral research associate positions in the USA for one year in Cornell University, Department of Chemistry (1955-1956) and one year in University of California, Berkeley, Department of Physics (1956-1957), the academic position of Reader (Associate Professor) in Nuclear Physics at Saha Institute of Nuclear Physics, Calcutta, India (1957-1958) , Research Assistant Professor at Dept. of Physics, University of Illinois, Urbana, Illinois, USA (1958-1959), Research Associate Professor at Dept. of Chemistry, Columbia University, New York City, USA, (1959-1960) and Senior Research Officer at Bhabha Atomic Research Center, Bombay, India (1960-1961).
My past and present permanent (tenured) academic positions are as follows:
Associate Professor, Department of Physics, University of California, Riverside, California, USA 1961-`65
Professor, Department of Physics, University of California, Riverside, California, USA, 1965-`69
Professor, Department of Physics, University of Utah, Salt Lake City, Utah, USA, 1969-`71
Professor, Department of Physics, State University of New York at Albany, Albany, New York, USA, 1971 to present
5. Visiting Positions: I have held visiting positions as Visiting Professor or Research Scientist either during sabbatical leave periods or in the Summer breaks between Spring and Fall semesters, at a number of Institutions. These include: (a) Université Paris Sud-Orsay, France, Spring 1966, (b) Atomic Energy Research Establishment, Harwell, England, Summer 1966, (c) Institut Laue-Langevin, Grenoble, France, Summer 1977, (d) Technische Hochschule Darmstadt, Germany, Fall 1977 – Winter 1978, (e) University of Florida, Gainesville, Florida, Spring Quarter 1978, (f) Eidgenössische Technische Hochschule Zürich, Switzerland, Summer 1978, (g) Universität Mainz, Germany, Summer 1982, (h) University of Århus, Institute of Physics, Denmark, Summer 1983, (i) University of Tokyo, Faculty of Science, Meson Science Laboratory, Japan, December 1987 – January 1988, February 1990, Summer 1992, (j) Central University of Hyderabad, School of Physics, Hyderabad, India, May – June 1995 (k) Universität Zürich, Physik Department, Switzerland, July – August 1995, (l) Institute of Physical and Chemical Research (RIKEN), Japan (Eminent Scientist of RIKEN) 1996, 1997, and (m) Monbusho Senior Visiting Scientist, Institute of Materials Structure Science, Meson Science Laboratory, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan, September 1999 – January 2000, (n) Visiting Professorship in the Department of Physics, University of Central Florida, Orlando, from 1999 to the present.
6. Honors and Awards: (a) US Senior Scientist Award from Alexander von Humboldt Foundation, Germany, (Two Awards spent in two visits in Germany, at Technische Hochschule Darmstadt, 1977-78 and Universität Mainz, 1982), (b) Yamada Science Foundation Award, Japan (spent at Meson Science Laboratory, Faculty of Science, University of Tokyo, (December 1987-January 1988 and February, 1990, Summer 1992)), (c) Presidential Excellence in Research Award from State University of New York at Albany, USA (1984), (d) Jawaharlal Nehru Visiting Professorship Chair at School of Physics, Central University of Hyderabad, Hyderabad, India, Summer (1995) (e) Eminent Scientist Award [Institute of Physical and Chemical Research (RIKEN), Japan] (1996 - 1997), (f) Monbusho (Ministry of Science, Education, Culture and Sports, Japan) Visiting Foreign Scientist, Institute of Materials Structure Science, Meson Science Laboratory, High Energy Accelerator Research Organization (KEK), Japan (1999 September – 2000 January)
7. Membership in Professional Societies:
Fellow, American Physical Society
Member, New York Academy of Sciences
Member, Biophysical Society
Member, National Society of Atomic and Molecular Physics, India
Member, Orissa Science Academy, India
8. Research Interests: My research interests have been, and continue to be, focused on the first-principle understanding of electronic structures of atomic, molecular, solid-state and biological systems including the aim to explain their spectroscopic (optical, ultraviolet and infrared), magnetic and hyperfine properties. My research group, especially the Ph.D. students, and post-doctoral physicists, are trained to interact closely with experimentalists, using their theoretical predictions of properties of systems to compare with experimental results and in partnership with experimental groups try to attain a proper understanding of the mechanisms and factors that contribute to a variety of properties including magnetic and hyperfine properties, the latter being obtained by experimentalists by a variety of experimental techniques, ranging from laser techniques for atomic properties to various magnetic resonance techniques, including nuclear magnetic and quadrupole resonance and electron paramagnetic resonance, and associated refinements in these, including a variety of double resonance techniques for solid state (including systems of interest in material) and biological systems.
We also make use of results from radiative techniques like Mössbauer spectroscopy, perturbed angular correlation spectroscopy involving both g-g and b-g correlation and especially muon spin rotation (mSR). In addition, for our interests on surface properties of condensed matter systems we obtain from our electronic structure investigations the geometries of both reconstructed and unreconstructed surfaces with and without adsorbed atoms which can be tested with current methods involving Scanning Tunneling Microscopy and related techniques as well as electron diffraction and X-ray scattered wave techniques.
This emphasis on close interaction with experimental groups to use results from theory and experiment concurrently to maximize information about the electronic structures and related properties of a variety of systems has led to my worldwide collaborations (a number of them currently continuing) over the past three decades with a number of experimental and theory groups, including the Institute of Physics, Bucharest, Romania; Institut für Physikalische Chemie, Technische Hochschule Darmstadt, Darmstadt, Germany; Instituut voor Kern En Strahlingsfysika, Katholieke Universiteit Leuven, Leuven, Belgium; Instituut voor Technische Naturkunde, Technische Hogeschool, Delft, Netherlands; Institut für Physik, Johannes Gutenberg Universität Mainz, Mainz, Germany; Institut für Physik, Universität Konstanz, Konstanz, Germany; RISO National Laboratory, Roskilde University, Roskilde, Denmark; Physik Department, Technische Universität München, Garching, Germany; Physikalisches Institut der Universität Erlangen-Nürnberg, Erlangen, Germany; Fysiska Institutionen, Materialfysik, Uppsala Universitet, Uppsala, Sweden; Fakultät für Physik und Astronomie, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany; Fachbereich Physik der Philipps-Universität Marburg, Marburg, Germany; Department of Physics, Yeung Nam University, Taegu, South Korea; Indian Institute of Technology Madras, Chennai, Tamil Nadu, India; Institute of Physics, Bhubaneswar, India; School of Physics, University of Hyderabad, Hyderabad, India; Department of Physics, University of Durban-Westville, Durban, South Africa; Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan; Muon Science Laboratory RIKEN (and RIKEN-Rutherford Appleton Laboratory, Chilton, United Kingdom), Wako-Shi, Saitama, Japan; Institute of Materials Structure Science, Meson Science Laboratory, High Energy Accelerator Research Laboratory (KEK), Tsukuba, Ibaraki, Japan; Department of Physics, University of Central Florida, Orlando, Florida; Central Departments of Physics and Computer Science, Tribhuvan University, Kirtipur, Kathmandu, Nepal; Institut für Physik, Technische Universität Braunschweig, Braunschweig, Germany.
I shall list my research interests according to the materials involved rather than the experimental techniques used to obtain the data that we interpret, because often we use data in the same systems from more than one experimental technique for comparison with our theoretical results. Our journal articles in the fields of itemized research interests that are described below have been presented in the same order in the itemized list that is presented later in this link.
I. Atomic Physics
We have investigated spectroscopic and hyperfine properties using first-principle methods for electronic structures incorporating many-body and relativistic effects by relativistic and non-relativistic many-body perturbation theoretic procedures, which my students and I have helped develop and applied over the past thirty-five years.
II. Molecular and Cluster Physics
Our emphasis has been on studying hyperfine fields and electric field-gradients in small molecules including many-body effects, both because of the intrinsic interest in the nuclear magnetic and quadrupolar hyperfine properties in these systems and also to test the methods used before applying them to large systems such as biological systems like hemoglobin, cytochrome c, DNA, and chlorophyll and solid state systems where we utilize sizable clusters to simulate the entire solid. In addition, we have also studied the structures and properties of actual clusters, the emphasis being on fifth group atomic clusters by themselves and also linked to other atoms.
III. Electronic Structure and Hyperfine and Magnetic Properties of Molecular Solids
In the field of molecular solids, our interest has been primarily to study how well molecular properties can be explained by an isolated molecule, and for properties which are by their nature dependent on intermolecular interactions, how well can such interactions explain the experimentally observed values of these properties. In particular we have been interested in finding out if one can understand the relative importance of different intermolecular effects on the binding energies of molecules in molecular solids with their surroundings as well as in trying to explain the experimentally observed binding energies. Also of interest to us are the properties of impurity atoms and molecules in molecular solids.
The systems we have studied so far are first the solid halogens in some detail, both earlier by approximate methods and more recently by first-principle Hartree-Fock cluster procedure including many-body effects. We have also studied solid fourth group tetrafluorides, and hydrogen fluoride molecule as an impurity system in fluoromethanes. The properties of interest in these systems have been the nuclear quadrupole interactions, the excited nuclear state with spin 5/2 being used for 19F nucleus which has a finite nuclear quadrupole moment. The other system we have studied is the C60 fullerene system with muonium or hydrogen atom attached to a carbon atom, the properties of interest being the locations of the muonium (hydrogen) and the muon hyperfine interactions, to compare with the results of experiment which have been obtained by muon spin rotation and electron spin resonance techniques.
IV. Biologically Important Molecules
Our major efforts so far have been in the heme-based compounds such as deoxyhemoglobin, oxyhemoglobin and other hemoglobin derivatives as well as Cytochrome c and in the molecules in the bacterial and green plant Reaction Centers such as the donor molecule bacteriochlorophyll and the primary acceptor Ubiquinone in bacterial photosynthesis. For most of the last thirty-five years we have used the semi-empirical Extended Hückel procedure, but over the past ten years we have been using first-principle Hartree-Fock procedures for these large molecular systems. The Cytochrome c system is of particular interest for our theoretical investigations in view of the recent work in the Muon Science Laboratory at RIKEN and Meson Science Laboratory at the Institute of Materials Structure Science in KEK in Japan on the study of the electron transport in this compound using muon spin rotation procedures on the trapped muonium and muon systems. This procedure shows great promise for becoming an important microscopic technique for study of electron transfer in biological systems, and is also being currently utilized for similar study in DNA systems. We are also engaged in the present time in theoretical investigations in collaboration with the experimentalist group involved.
V. Physiologically and Energetically Important Molecules
We have mainly worked so far in the study of the origin of the nuclear quadrupole interactions (NQI) in these types of compounds, specifically cocaine free base, cocaine hydrochloride and heroin in the drug family and RDX and b - HMX in the area of explosive systems. Our future interests in these systems lie in: (a) understanding the temperature dependencies of the NQI which will be important in their detection during transport between places with different temperatures, (b) the influence of attached molecules or groups on the NQI frequencies, and (c) use of the calculated electron distributions in drug molecules to understand from a chemical physics point of view their physiological effects on the body.
VI. Theory of Proton Relaxivities in the Presence of Contrast Agents Used for Enhancement Effects in Magnetic Resonance Imaging
Our work has involved understanding the configurations of water molecules around paramagnetic ions like Mn2+, Mn3+, Cu2+, and Gd3+ and associated electron distributions and magnetic hyperfine fields at the protons of the solvated water molecules. A knowledge of these factors is important for understanding the enhanced proton relaxivities due to these ions when used as contrast agents in Magnetic Resonance Imaging (MRI). Our future interests include: (a) using molecular dynamics methods to obtain the correlation times that occur in proton relaxation in the presence of these ions as contrast agents and (b) the influence of organic molecules attached to the paramagnetic ions to minimize their toxic effects.
VII. Metallic Systems and Hydrogen and Muon in Metals and Alloys
Our major effort in this area in the seventies and eighties and earlier involved the use of first-principle or near first-principle band structure methods to study Knight-Shifts and relaxation times in non-magnetic metals, origin of ferromagnetism and understanding of magnetic moment distributions and hyperfine fields in ferromagnetic metals. In the latter case, the electronic and magnetic distributions in the pure metals were used to draw conclusions regarding hyperfine fields at muon and proton sites in ferromagnetic metals. Our current interests include the understanding of the locations and electron distributions associated with hydrogens, protons, muonium and muons in metallic and alloy systems using first-principle Hartree-Fock Cluster procedures to simulate the solid state systems containing these atomic-like and ionic systems as impurities. The aim is to understand the solubility and magnetic resonance properties of hydrogen and proton systems in various alloy systems that have been studied and use of muon and muonium as surrogates for proton and hydrogen to obtain information about dilute impurity systems. Additionally, we are interested in the understanding of the trapping of muons at vacancy sites in metals which is important for production of slow muon beams by techniques being developed by mSR groups for use in the future for surface studies.
VIII. Semiconductors-Properties of Impurity Atoms including Muonium and Muon inside Semiconductors
We have investigated in the past the electronic structures and magnetic hyperfine properties of a number of impurity atoms in semiconductors, especially silicon and germanium, by appropriate semiempirical procedures and the nuclear quadrupole interactions of fluorine atoms with its nucleus in an excited state, 19F*, in diamond, silicon and germanium by the Hartree-Fock Cluster procedure. The aim has been to find the locations of fluorine and test the calculated associated electron distributions by their ability to provide favorable quantitative comparison with experimentally measured nuclear quadrupole interaction parameters. We have also studied locations and magnetic hyperfine properties of normal and anomalous muonium in silicon and germanium.
Currently, we are carrying out investigations on:
(a) 69Ga and 14N nuclear quadrupole interactions to compare with experimental data from recent nuclear quadrupole interaction measurements and obtain a test of the calculated electron distributions in the GaN system using Hartree-Fock cluster procedure. In this system and other systems intermediate between ionic crystals and semiconductors, we would also like to check the relative accuracies of electronic properties obtained by the characteristic procedures that we have been using for ionic crystals and semiconductors. We are also studying muonium centers in this system to compare with results associated with hyperfine and optical properties.
(b) Similar investigations are being carried out in ZnO, which is also intermediate between ionic crystals and semiconductors. We have studied earlier the 67Zn nuclear quadrupole interaction properties in this system by the cluster procedure appropriate for ionic crystal systems and intend to test the semiconductor cluster procedure in this system and other zinc chalcogenides. Investigations on muonium centers are also in progress to compare with experimental results.
IX. Semiconductor Surfaces - Study of Electronic Structures and Magnetic Properties and Nuclear Hyperfine Interactions Associated with Adsorbed Atoms including Muon and Muonium incorporating Reconstruction Effects.
Our earlier work had been mainly concerned with locations, spectroscopic properties and nuclear quadrupole interactions associated with adsorbed atoms at surfaces, using essentially unreconstructed surfaces. Our predicted nuclear quadrupole interactions have stimulated experimental studies of these interactions by the Perturbed Angular Correlation (PAC) Techniques. While these experimental studies have shown reasonable overall agreement with our predictions, they have led to quantitative differences with the predicted nuclear quadrupole coupling constants from our calculations and shown both existence of asymmetry parameters (h) at sites which had zero values of h in the unreconstructed surfaces because of expected three-fold symmetry for (111) surfaces, as well as two sets of nuclear quadrupole interaction parameters whereas our earlier investigations had predicted one. It has been suggested that all these features could be the result of reconstruction effects of the surface, not included in our earlier investigations. Indeed we have recently demonstrated (Publication #9 in Section IX in list of publications) that these differences from our results for the unreconstructed (111) surface for adsorbed Indium atoms (where the electron capture of the 111In nucleus leads to 111Cd nucleus whose PAC signal is experimentally studied) can be explained by including reconstruction effects. Our current efforts are directed at further studies on the hyperfine properties for adsorbed atoms at reconstructed surfaces for a number of systems, including: (a) understanding of PAC data providing information on bromine atoms, containing the 77Br (77Se*) nucleus, on <111> and <100> surfaces of Silicon, (b) understanding of similar PAC data on nuclear quadrupole interaction for 111In (111Cd*) on Ga As surfaces and (c) study of magnetic hyperfine interactions associated with muonium on semiconductor surfaces, such measurements being expected to become possible soon by muon spin rotation (mSR) research groups using slow muon (and muonium) beams. A major interest in such studies is to see whether muonium, which resembles a dilute hydrogen impurity, is located at open sites on the surface, where adsorbed alkali atoms are believed to be located, or at an atop site where adsorbed halogen atoms are located, and as it appears from the available literature that hydrogen atoms are located, when there is significant coverage on silicon surfaces, or if there are some other possible sites related to the normal and anomalous muonium sites in bulk semiconductors. It will also be interesting to see if theoretical investigations can explain the expected values of muonium hyperfine constant(s) at semiconductor surfaces from slow muon mSR measurements. These investigations are expected to provide valuable insights into the natures of bonding of hydrogen (muonium) with surface atoms both in the dilute case as well as that for significant coverage.
X. Electronic Structures, and Magnetic and Hyperfine Properties of Ionic Crystals - Perfect and Imperfect
Our interests in trying to understand the electronic structures of ionic crystals, with emphasis on magnetic and hyperfine properties, have evolved over the past three and a half decades, utilizing the rapid growth and development in computing facilities (both hardware and techniques of software) during the same period. Thus, during the sixties and seventies, my students and I mainly made use of semi-empirical Born-Mayer techniques including Madelung and polarization effects to study nuclear quadrupole and magnetic hyperfine interactions in perfect and imperfect ionic crystals, the latter including solid solutions and color center systems, both electron (F-center) and hole (VK-center) centers. For the study of nuclear quadrupole interaction effects, we developed methods for accurate calculation of Sternheimer antishielding factors allowing for inclusion of self-consistency effects, to obtain these factors appropriate for electric field-gradients obtained from point charge and point dipole models. For magnetic shielding (chemical shift) effects in nuclear magnetic resonance, transferred hyperfine fields in magnetic systems and for hyperfine effects in color centers, we incorporated contributions from electron distributions in these systems by either explicit evaluation of overlap effects between ions using appropriate electronic wave-functions for the ions or electronic wave-functions, for the appropriate free molecules for VK-centers, which were molecular in nature. This was about the best possible approximation to first-principle techniques available at the time and my students and I made use of the most sophisticated procedures that were practicable at the time.
Since the 1980’s we have been using essentially complete first-principle procedures based on the Hartree-Fock Cluster Method utilizing clusters of ions of appropriate size, chosen from physical and practicability considerations, with the influence of the ions in the rest of the lattice included through their coulomb potentials experienced by the electrons in the clusters using point charges for the ions, and in addition where necessary, point dipole approximations for the external lattice ions. Among the perfect systems without impurities and defects we have studied by this procedure are Fe2O3, the superionic conductor Li3N, the chalcogenides ZnO, ZnS, ZnSe and ZnTe and ZnF2 and spinel compounds, the properties studied being magnetic hyperfine and nuclear quadrupole interactions, isomer shifts in Mössbauer spectroscopy, and influences of lattice dynamics and phase transitions on these properties. In all cases, comparison has been made with experimental results with usually good agreement, and in those cases where there are differences from experiment, these differences are used to estimate the importance of effects that have not been considered, because of the complexity in including them by first principle procedures, such as for instance many-body effects. One of the important features of our investigations in the field of nuclear quadrupole interactions is that there is no longer any need to use Sternheimer antishielding factors, because the Sternheimer effect is included directly in the cluster calculations through allowing the core electrons of the ions in question, like for instance, Fe+2, Fe+3 and Zn+2, to take part in molecular orbital formation.
We have made a start on the investigation of the nature of nuclear quadrupole interactions in 67Zn in nano-structured ZnO together with experimental collaborators, studying the Mössbauer spectra in this system and attempting a simple theoretical interpretation of the asymmetry parameter in the nuclear quadrupole interaction by preliminary exploration of the possibility of slippage of the Zn2+ ion from its axially symmetric position in the bulk crystal. A more complete theoretical interpretation is planned in our future investigations described later in this subsection.
In the field of imperfect lattice systems, we have studied:
(a) the general problem of the two-photon luminescence process in phosphors and demonstrated by arguments involving many-body effects, that such a process cannot be explained purely by a one-electron theory and that two-electron many-body effects are very important, (b) the interpretation of muon spin rotation data obtained by the mSR groups at RIKEN and KEK (High Energy Accelerator Research Laboratory at Tsukuba, Japan) in NiO and MnO, which show single mSR frequencies and hence single hyperfine fields at the muon site, in contrast to CoO and CuO where multiple hyperfine fields are observed with complex temperature dependencies of the mSR frequencies recently found for the latter. Our first-principle investigations of the spin distributions in NiO and MnO containing muon and the dynamics involving muons in the systems lead to good quantitative agreement with experiment and demonstrate that a thorough treatment of dynamic effects for the muon is necessary to explain the single hyperfine fields at the muon in these two compounds.
Our plans for the future involve more emphasis on work on imperfections in ionic crystals and nanocrystals, although we will continue efforts on improving the accuracy of theoretical investigations on perfect crystals. One direction for our efforts for the latter will include the incorporation of many-body effects, which has to be limited because of the sizes of the clusters and particularly the number of electrons involved. Another direction will be the study of larger clusters, including using pseudopotentials for all of the ions in the cluster except the central one for which hyperfine properties, both magnetic and nuclear quadrupole, are being investigated. An additional possible area would involve the use of three regions in the crystal, with the inner region involving the main cluster treated in an all-electron manner, the next region using only valence electrons treated by the pseudopotential procedure and the outermost region involving use of point charges and point dipoles producing coulomb potentials in the cluster regions. Particular emphasis for these sophistications will be made on systems involving 57mFe nuclei, where there seems to be good agreement between Mössbauer experimental data on isomer shifts and magnetic hyperfine fields but weaker agreement for nuclear quadrupole interactions.
In the field of imperfect solid state systems, we shall continue work on positive muons in transition metal oxides. In the case of MnO and NiO, our investigations will focus on the relaxation times and Knight Shifts in the paramagnetic state. Experimental data obtained in the late seventies by the mSR group at KEK and RIKEN on MnO suggests that for the spin-lattice relaxation times T1 in the paramagnetic state, the influence of the fast diffusion of the muon (which we have found from our recent first-principle investigations on the antiferromagnetic state to be very influential for explaining the observed single mSR frequency) can be rather important especially in understanding the temperature dependence of T1. It will be helpful using the results of our investigation of the dynamics of the muon in antiferromagnetic NiO and MnO to examine quantitatively how such effects can influence the spin-lattice relaxation T1 in muon spin rotation in the paramagnetic state as well as the Knight-shift. For CoO and CuO, work is planned along the lines of our recent investigations on antiferromagnetic NiO and MnO to understand the multiple values of the observed mSR frequencies, in contrast to the situation in the latter two systems, with special emphasis on the roles of distortions in the antiferromagnetic state and lattice distortions due to the presence of the muon. Another related area of our investigation on imperfect ionic crystals will be concerned with the systems involving negative muon trapped in both diamagnetic systems like alkaline earth oxides and transition metal oxides (paramagnetic state). It will be interesting to examine if as we have found from our recent work on LaCuO systems discussed in the next section, the localized paramagnetic susceptibility due to the Auger hole associated with the trapping of m¯ is the main effective contributor to the Knight shift and how the results of theory compare with available experimental data. One other area of imperfect systems we shall be working on is that of the spinel systems, where we have been successful in explaining the observed 67Zn quadrupole interactions from Mössbauer data in perfect well-annealed ZnFe2O4. In the quenched system however the quadrupole interaction is larger by about 25%, even though neutron diffraction data show only a small change in oxygen position. It will be interesting to examine if the small change in oxygen position is sufficient to explain the nuclear quadrupole interaction data or other sources need to be explored.
In the area of nanostructured materials, we will work on a number of areas. One of these will be the explanation of the observed asymmetry parameter in the nuclear quadrupole interaction for 67Zn in ZnO Mössbauer data, attempting to see from first principles Hartree-Fock Cluster investigations, if there are significant off-axial displacement of Zn or O ions from the axially symmetric locations in the bulk crystal and if these can explain the observed asymmetry parameter. A related area in nanostructured materials that we plan to study involves the systems with transition metal and rare-earth ions in nanostructured ionic crystals where there appears to be a substantial increase in oscillator strengths involved in luminescence as compared to the corresponding bulk materials. It will be interesting to study if there are significant departures from symmetry in the positions of the cations or anions in the nano-structured materials to explain the increases in oscillator strength. We are also interested in understanding nuclear quadrupole interaction data in the spinel system ZnFe2O4 where, as mentioned in the preceding paragraph, the strength of the nuclear quadrupole interaction is substantially larger than in the bulk well-annealed material and neutron diffraction data show small but significant change in oxygen position as compared to both the latter material as well as the quenched system. It will be interesting to understand the origin of the observed change in oxygen ion position in the nanostructured material and if it is sufficient to explain the observed quadrupole interaction. Since the field of nonstructured materials is becoming an increasingly important one from both basic and applied science point of views, first-principle investigations of the types planned are expected to enhance our insights into the natures of these systems and also provide a good training ground for young researchers in a field of great contemporary interest.
XI. Electron Distribution and Associated Magnetic and Hyperfine Properties in High-Tc Superconducting Systems
Our overall aim in the field of high-Tc systems has been to try to obtain a thorough understanding of the electron distributions in these systems through first-principles Hartree-Fock Cluster investigations. As in the other classes of systems we have studied, we have tried to test the accuracies of our calculated electron distributions by examining their ability to explain magnetic and hyperfine properties (both magnetic and nuclear quadrupole interaction types) associated with nuclear sites in the perfect materials obtained by magnetic and nuclear quadrupole resonance techniques. We have also studied hyperfine properties associated with interstitial sites obtainable experimentally from Muon Spin Rotation (mSR) technique using the positive muon, and again at the nuclear sites but in the presence of an Auger electron hole, through the negative muon mSR technique. The basic aim of understanding the nature of the electron distributions is for the utilization of this information for a quantitative testing of various theories proposed for the origin of the high-Tc superconducting transition.
Our earlier work on high-Tc systems has focussed on the nuclear quadrupole interactions for the 63, 65Cu, 139La, 135Ba and 17O nuclei in La2CuO4, YBa2Cu3O6 and YBa2Cu3O7 systems and magnetic hyperfine interactions in the former two systems in the antiferromagnetic state, on positive muon in La2CuO4 based systems in the antiferromagnetic state and on m- in the paramagnetic state of La2CuO4.
Our investigations on the electronic structures of the LaCuO and YBaCuO systems have provided electron distributions which have been found to lead to overall satisfactory agreement between theory and experiment for hyperfine properties, although there are some quantitative differences, especially for the 63,65Cu and 17O nuclear quadrupole interactions. The investigations on positive muon in La2CuO4 have shown that the most likely site for positive muon-trapping is about 1A away from the apical oxygen at a specific orientation of the O-m line. The hyperfine field at the positive muon is in order of magnitude agreement with experiment but is found to be twice as large as the experimental value from mSR data. For the negative muon, trapping is found inside the apical oxygen close to the nucleus, which is also shown to have a hole in the p-valence shell that appears to provide the entire localized spin susceptibility needed for the understanding of the observed stronger mSR Knight Shift data and their directional dependence with respect to the applied field. The much weaker observed Knight shift data is shown to be associated with the planar oxygen. Theory however predicts magnitude-wise stronger Knight shifts than experiment.
A part of our future efforts will be concentrated on improving the quantitative agreement between the results of first-principle investigations of the class of properties we have worked on so far and experimental results, and also extending our investigations to other high-Tc Copper Oxide superconductors. Our major efforts will be directed towards theoretical investigations related to recent experimental developments in high-Tc systems with the aim to see if we can explain these additional results quantitatively and also understand their bearing on the mechanisms for origin of the superconductivity in Copper Oxide systems. Thus, in the first area of our future efforts, we shall be working on the improvement in the agreement of nuclear quadrupole interactions for 63Cu and 17O between theory and experiment, as well as the magnetic hyperfine field at the positive muon site in La2CuO4 and resolving the quantitative difference for the Knight-shift for m- in La2CuO4 between theory and experiment. Possible effects to be studied are incorporation of many-body effects, use of larger clusters and in the case of m+ and m- in La2CuO4, the incorporation of more extensive lattice distortion effects than has been tried so far and other possible positions for m+ trapping, as well as the influence of dynamic effects for m+ in La2CuO4, because of our experience about the importance of such effects with NiO and MnO.
We shall also be studying other systems like the double-chain YBa2Cu4O8 system focussing on the 63Cu nuclear quadrupole interactions and positive muon in YBa2Cu3O7 and Tl Ca Copper oxide systems where mSR data are available and there are interesting differences between the hyperfine fields at the muon sites in the two systems although the local copper-oxygen geometries are very similar. We shall also concentrate on the Knight-shift and relaxation time data for 63Cu and 17O and their temperature dependencies from nuclear magnetic resonance experiments using our calculated electron distributions, since the interpretations of these data are of great interest, providing information on the relative importance of contributions from Korringa mechanisms and antiferromagnetic fluctuations and also for obtaining information on the validity of theories for origin of superconductivity.
In the second area, our major efforts are currently focussed on the understanding of the co-existence of magnetism and superconductivity for certain concentrations of Sr in La2CuO4, the so called 1/8 effect, and the related area of charge and spin stripe structures in these systems as well as the related La2NiO4 systems.
In La2-xSrxCuO4, one finds from neutron diffraction in the neighborhood of x=0.125, a magnetically ordered stripe-like structure composed of Cu3+ and Cu2+ in the charge-stripes and Cu2+ in spin stripes, the Cu3+ being non-magnetic. In this magnetic state in the presence of Sr, there is a single mSR frequency observed, almost equal to that in the low Sr concentration system, with magnitude about 1 per cent lower than in the latter case. We are presently studying through energy optimization, the possible trapping sites for the m+ in the presence of stripes. The aim is to see if there is only a single m+ trapping site or if there are multiple sites. In the former case, we would like to understand why the observed frequency is nearly the same as in the low Sr concentration systems and possibly explain the observed small difference in frequencies in the two cases. In case multiple sites are predicted by theory, it will be interesting to find out which particular site explains the observed mSR frequency and equally important, why no additional frequencies are observed. We also want to try to predict the expected temperature dependence of the mSR frequency and breadth of the corresponding line shape in the frequency domain. Following the analysis of the features of the mSR signal in the stripe structured systems, we shall also make theoretical predictions regarding nuclear hyperfine properties to compare with experimental observations from nuclear resonance measurements.
We plan also to study by similar methods as for the cuprate systems, the hyperfine properties associated with m+ trapped in the related (but non-superconducting) Strontium doped Lanthanum Nickelate systems. Of particular interest are features like the much larger observed mSR frequency than in the cuprate system and the two frequencies in the stripe-structured nickelate system. As far as the former feature is concerned, it would be interesting to understand whether the higher observed mSR frequency in the nickelate system can be explained by the larger magnetic moment of Ni2+ alone or other factors such as differences in trapping positions between the Nickelate and Cuprate systems, differing covalency effects leading to different hyperfine contributions, or the influence of the magnetic Ni3+ ions (in contrast to the non-magnetic Cu3+ ions) play important roles. It will also be interesting to see if these effects, as well as the existence of more than one trapping site in the nickelate systems, can explain the two observed mSR frequencies.
XII. Organic Ferromagnets
We are currently working on a class of systems involving ferromagnetism at very low temperatures, less than 1oK, which do not involve any transition metal ions, the ferromagnetism being expected to arise from exchange interactions between free radical molecules arranged in two-dimensional layer-like structures, belonging to a class of compounds called TEMPO-derivatives, involving organic ligands attached to the TEMPO-group with a chair-like structure including a NO group, the latter carrying, almost entirely, the single unpaired electron-spin in the system. We have carried out first-principle Hartree-Fock investigations on the electronic structure in a number of systems in this class of molecular solids and obtained the associated spin distributions in the molecules in the absence of any attached muon or muonium. By studying the electronic structures in the central molecule where a muon or muonium is attached, we have determined the possible trapping sites for the muon or muonium. Then, carrying out a comparison between the hyperfine fields at the muon in the trapped muon or muonium sites in the central molecule using the spin distributions in the central molecule and the other molecules which do not have muon or muonium, and the experimental muon spin rotation frequency, we have identified two possible sites, one for a muonium and the other for a muon, which give good agreement with experiment and provide the easy axis of the magnetic moments in the ferromagnetic state. This easy axis has been verified to be correct in one of this class of solids (4-(p-Chlorobenzylidene amino)-TEMPO) from dipole-dipole interactions between the spin distributions on the molecules in the molecular solid in the ferromagnetic state. These agreements in the easy axis direction by the use of two different techniques and between the mSR frequencies obtained by experiment and theory act as a good check on the accuracy of the calculated electronic structure and on the dipole-dipole interaction mechanism as the determining factor for the magnetic anisotropy that lead to the origin of the direction of the easy axis for the magnetic moment in the ferromagnetic state.
We are now investigating if the dipole-dipole interaction is significant in importance in determining the Curie temperature or whether the Heisenberg interactions mechanism is mainly responsible. The reason for examining the dipole-dipole interaction as a possible mechanism is the very small Curie temperature of about 0.4 K. We are also testing the procedures we have used for the 4-(p-Chlorobenzylidene amino)-TEMPO for the system NPNN, the very first organic ferromagnet that was discovered. This system has had two mSR frequencies observed experimentally and we are applying the same electronic structure and other methods that we have used for the TEMPO-system to assign to the trapped muon and muonium sites the two observed hyperfine fields at the muon. We also intend to study the origin of the Curie temperature for the same reasons as for the TEMPO system. It is expected that this comprehensive theoretical study of two different organic ferromagnet systems without any transition metal ions will give us valuable general insights into the origins of the properties of organic ferromagnets.
Subsequent to the completion of this work we shall be working on another class of ferromagnets involving high-spin clusters of paramagnetic ions separated from each other through organic molecular systems. These systems have recently been studied experimentally by mSR technique, and have technological importance as controlled ferromagnetic and antiferromagnetic materials that can provide large magnetic moments in the ordered magnetic state.
The muon catalyzed fusion (mCF) technique is becoming a viable technique for generation of energy. With the mCF technique, instead of having the deuteron and triton collide at high temperatures as in hot fusion, they are brought close to each other by bonding through a negative muon m-, forming a H2+ like molecule about 200 times smaller than the conventional one bonded through an electron. After the fusion event occurs, the m- moves away, attaching itself to another (DT) pair and causing a chain-reaction type fusion events until it either decays in its natural decay process to an electron with a life-time of 2 microseconds or is absorbed possibly by a nucleus like a 3He generated from tritium by b-decay. The most promising environment for a long sustained chain reaction is in the solid D-T mixture but there seems to also be increased accumulation of 3He in the solid as compared to the liquid or gaseous environment.
My research group, interacting closely with the experimental mCF group at RIKEN in Japan (and RIKEN-Rutherford Appleton Laboratory in the United Kingdom), has been working on the trapping of 3He related systems in solid hydrogen with the aim to understand the trapping process and the attachment of m- to 3He related traps.
We have nearly completed a comprehensive analysis of 3He+ ion-trapping in solid hydrogen and have found a number of features, including some observed experimentally, that are associated with the trapping process. Thus, we have found that the trapping strengths at tetrahedral and octahedral interstitial sites are nearly equal, that the trapping of 3He+, and consequently the accumulation of 3He+ in mCF trapping measurements, should be much greater in the solid than in the liquid, that at larger concentrations of trapped 3He+ there is a tendency towards bubble formation and hence a reduction in the strength of m- capture by the accumulated He and that vibrational effects on He trapping are small so that one expects no difference in trapping strengths for 4He and 3He.
Our current and future efforts on He trapping in solid hydrogen will be directed at the following areas, namely, trapping of (3He-H)+, likely to arise from b-decay of a 3H bound to a proton or deuteron., the formation of neutral 3He from charge exchange between 3He+ or (3He-H)+ and solid hydrogen, the capture process for m- by He+ and (He-H)+ ions and neutral He0, the influence of the solid hydrogen environment on the Ka/Kb intensity ratio in X-ray emission from m-mesic Helium atom and study of the strength of fusion for 3H and another 3H or 2H through H2+ molecule formation through binding by m- in solid hydrogen. This program is proving to be valuable for both the technical aspects and results of He-trapping in solid hydrogen as well as for the basic aspects of 3He systems impurities in solid hydrogen. Additionally, the understanding of the trapping process between accumulated 3He+ or 3He0 and the m- or any entity involving m- with 2H or 3H and 3He should be helpful in the understanding of the other m--He interactions involving the 4He produced in the fusion reaction and m- which also has important inhibition effect on the chain reaction associated with mCF. Current experimental mCF measurements indicate that there seems to be some release of m- by the m--4He process the ratio of which could be different between the gaseous and solid hydrogen phases. The theoretical understanding of both the trapping and release process of m- by m--4He could enable experimental augmentation of the m--4He unit formation allowing the mCF process in the solid to be stronger and therefore more cost-effective for energy production.
XIV.Study of Structures of Silicon – Rare Earth and Si-Ge – Rare Earth Systems and Photoluminescence and Other Properties
Systems involving Rare Earth impurities in Silicon and Silicon-Germanium systems, especially Erbium impurities, have become rather important because of their importance in applications for optoelectronic integrated circuits on silicon chips and optocommunications. From earlier experimental and theoretical investigations, the broad features of the mechanisms for the 1.54 mm photoluminescence (PL) signal in Er-Si and Er-Si/Ge systems with frequency only slightly dependent on the environment, and the factors that could influence the photoluminescence efficiency, are emerging. But there remain a number of factors to be understood definitively, among them: (a) the possible locations of Er in the presence and absence of co-dopants of the second period, (b) the natures of the wave-functions of the impurity levels associated with the presence of the Er ions in the Si or SiGe host, the multiplet 4f-levels for the Er3+ ion and the co-dopant environment of the Er3+ ion, (c) the mechanisms of the excitation process from the semiconductor valence band to the impurity band, and the energy transfer from the impurity level to excited multiplet levels of the Er3+ ion, (d) the influence of the co-dopant ions in enhancing the 1.54 mm photoluminescence signals and improving its temperature quenching, (e) the differences between the photoluminescence signals for the Er3+ impurities in Si-Ge alloy systems and Si system including the more complex photoluminescence signal and very different temperature quenchings. My students and I are now involved in a comprehensive research program to contribute to a thorough understanding of all these features of the Er-Si and Er-SiGe systems, the primary aim being of course the understanding of the factors contributing to the strength of the PL and to enhance it in order to make its technical applications feasible and efficient.
Our investigations currently are concerned with the determination of the sites for Er in Silicon and Silicon/Germanium alloy. We are studying six possible sites, namely tetrahedral interstitial, hexagonal interstitial, substitutional, intrabond region, hexagonal interstitial sites with a neighboring vacancy or interstitial to compare with available information from earlier channeling studies and additional channeling studies in progress by a collaborating experimental group in our department. Following this we shall study the geometry in the presence of co-dopant atoms, carbon, nitrogen, oxygen and fluorine, including the positions of the Er and the associated positions of the co-dopant atoms and their numbers to compare with available information from EXAFS, PL and electron spin resonance studies and channeling studies in progress in our department. The wave-functions from our investigations will be analyzed carefully to draw conclusions about the mechanisms and their strengths for electronic transitions from valence band to the Er-related donor type levels, the energy transfer from the latter to excited 4f configurations of Er, and the transitions from the latter to the ground state of Er. We shall also be testing the wave-functions for our calculated ground state configuration for Er in Si and SiGe by using them to study electron spin resonance (EPR) properties like g-tensors, Er nuclear hyperfine interactions and transferred hyperfine interactions for adjacent host and co-dopant nuclei and also properties like the isomer shift, magnetic hyperfine and nuclear quadrupole interactions for 166Er to be obtained from Mössbauer investigations to be carried out by an experimentalist group at University of Central Florida who will be interacting with us.
Future investigations are planned for the locations of Er and co-dopant ions in Si and SiGe hosts and PL properties, as well as EPR and Mössbauer (166Er) properties in strained systems involving hydrostatic and uniaxial pressures. We will also be studying similar properties as we are studying in Si and SiGe systems in the Si and SiGe Multiple Quantum well systems with Er, involving layers of Si, SiGe with and without Er impurities and light emitting diode properties of these systems.
XV. Studies of Electronic Structures and Associated Properties of Nano-Structured Materials We have worked before, and are currently working on, a number of areas of nanostructured materials science. Thus, as pointed out in Section III, we have worked in the past in the attachment of muonium (hydrogen) to the C60 fullerene system and the associated hyperfine field at the muon. We plan to continue this work in other fullerene systems with lower symmetry where similar data on attached muonium are now available and also the hyperfine properties of 13C nuclei. We also plan to investigate corresponding properties of carbon nanotubes and other atoms implanted in this system of substantial current interest. At the present time we are studying the nanostructured ionic system involving Zinc Oxide in trying to understand the evidence from 67Zn nuclear quadrupole interaction from Mössbauer measurement that there is loss of the axial symmetry at the Zinc site in bulk systems. In the near future we plan to work on the locations of Erbium in multiquantum well systems involving arrays of layers of Si-Er and Si layers and similar systems with Si replaced by SiGe and also on the electronic structures of these systems to understand the features of the photoluminescence in these systems, especially the strong enhancements compared to corresponding bulk systems.
XVI.Electronic Structures and Associated Properties of Elementary Chalcogens and Glass Transitions in Chalcogen Systems
In the past, we have studied the electronic structures of Chalcogen semimetals, both crystalline and amorphous, by semi-empirical self-consistent charge extended Hückel (SCCEH) procedure, as well as orthogonalized plane-wave band structure procedure (crystalline chain systems). Using the band-structure procedure we have interpreted both Fermi surface and optical properties as well as nuclear quadrupole interactions and isomer shift in tellurium studied by Mössbauer measurements for 125Te nucleus. We have used the semi-empirical SCCEH procedure for chain systems in crystalline materials as well as amorphous materials and ring systems in amorphous materials to make semiquantitative comparison with experimental Mössbauer data. We are presently using first-principle Hartree-Fock cluster procedure to obtain the electronic structures to compare theoretical predictions with 125Te Mössbauer data in chains and rings of both tellurium and selenium and also 77Se nuclear quadrupole interaction data from Perturbed Angular Correlation measurements in the same systems with the Se and Te atoms being either in their own chains and rings or as alloys in chains and rings of the other chalcogen.
We have also started investigations on glass formation in pure chalcogen systems Se, Te and alloys like SexTe1-x and the influence of impurity atoms on the glass transition temperature. There are experimental results available on the influence of antimony and lead impurities on Se-Te alloy system from Differential Scanning Calorimetry (DSC) measurements. These results show very different behaviors with impurity concentration. There are theories suggested for the dependence of glass transition temperature which depend on the relative strengths of bonds between the host atoms themselves and between host atoms and impurity atoms. We shall be testing these theories quantitatively by our first-principles Hartree-Fock cluster calculations.
These investigations will involve a knowledge of the locations of the impurity atoms and the electronic wave-functions associated with the impurity-host systems. The accuracy of the impurity locations and the associated electronic wave-functions obtained by our investigations will be tested by their ability to explain quantitatively the results of Mössbauer spectroscopy measurements being carried out by our experimental collaborators. This test will of course act as a check on the accuracy of our results and conclusions regarding the mechanisms and factors influencing glass transitions in chalcogen glasses. In addition to antimony and lead impurities we shall also study other impurities like tin which is currently under experimental study by Differential Scanning Spectroscopy.
9. Past and Present Ph.D. Students and Post-Doctoral Research Associates, Research Disciplines and Professional Positions:
I have had forty-nine (49) students complete their theses for their Ph.D. degrees under my guidance and currently have four (4) more working with me for their Ph.D. degrees.
Of the forty-nine past students who have received their Ph.D. degrees, twenty-eight (28) have worked in the field of condensed matter physics and materials science, twelve (12) have worked in atomic physics, two (2) in molecular physics and seven (7) in biophysics. Of the four present students, two are working on projects in both solid state and materials science and biophysics, one in only solid state physics and materials science and another in only biophysics.
As regards the activities of my past students,
twenty-six (26) went into faculty positions at universities, with six of them in universities abroad, one each in Australia, Malaysia and Greece and three in India,
five (5) joined national laboratories, with three in the US and two abroad, one of them reaching the position of President of the Government Advanced Energy Research Institute in South Korea and another the Director-General of Science and Technology Research Program of the Government of Nigeria,
thirteen (13) went into research positions in Industry in the USA and abroad, one becoming the Vice-President in Indian Metals and Ferro-Alloys Corporation in Thiruvelli in Orissa, India and another is the CEO of Software Corporation of America in the Washington D.C. area, and
five (5) have gone into Medical Physics positions in prominent Hospitals in the USA.
As regards Post-Doctoral Research Associates, I have had eighteen (18) scientists work with me in this capacity, five (5) in Atomic Physics and twelve (12) in Solid State Physics and Materials Science and one in Biophysics. One of the Research Associates in Solid State Physics and Materials Science also did Post-doctoral work in Biophysics.
Of the eighteen Post-Doctoral Physicists,
seven (7) hold University faculty positions both in the US and abroad, in the latter case, one in India and two in China,
three (3) in National Laboratories, all abroad, in France, India and Nigeria, the one in India has held the position of Director General of Council of Scientific and Industrial Research,
two (2) in Medical Physicist positions in the US, and
six (6) in Research Positions in Industry in the US and abroad, in the latter case, one in India.
10. Past and Current International Collaborations:
My research group has been carrying on international collaborations with groups in other countries over the past twenty-seven years, primarily with experimental research groups in a number of areas of condensed matter and materials science and atomic physics (and over the past four years in Biophysics), and also with some theoretical research groups in the same areas. In the beginning, in the mid nineteen seventies and early eighties, the collaborating groups were comprised of the solid state theory and experimental groups in Institut für Physikalische Chemie in Technische Hochschule Darmstadt, primarily in the theory of electronic structures and magnetic and hyperfine properties of metallic systems as well as some ionic crystal systems with the experimental group working on nuclear magnetic and quadrupole resonance measurements on similar systems as well as molecular solids, an experimental atomic Physics Group in the Universität Mainz in Germany, with whom we have interacted on the interpretation of their experimental data on hyperfine properties of heavy atomic systems using relativistic many-body perturbation theory, a theoretical group in the Technische Universiteit Delft in Netherlands with whom we have interacted very closely in developing the relativistic many-body perturbation theory of atomic systems with emphasis on hyperfine properties, and three research groups in Institute of Physics in Bucharest, Romania, with two theory groups, one involved in magnetic and hyperfine properties of metallic systems and another in magnetic and hyperfine properties of impurity centers in semiconductors and an experimental group involved in optical properties of ionic crystal systems.
Subsequently, in addition to continuing our collaboration with the Technical University of Delft, we started interacting with other research groups in Germany, Belgium, Denmark, South Africa, India, South Korea and Japan. In Germany, at the Technische Universität München we have collaborated extensively with an experimental group on interpretation of nuclear hyperfine interactions and Mössbauer isomer shift data in ionic crystals and subsequently on the interpretation of hyperfine interaction results in nano-structured ionic crystals, at Heidelberg and Marburg Universities, we have interacted with an experimental group on hyperfine properties of liquid metals and intercalated systems in graphite, in University of Erlangen-Nürnberg with an experimental group on nuclear quadrupole interactions of excited 19F* nuclei in condensed matter systems and at University of Konstanz on the interpretation of nuclear quadrupole interaction data on radioactive atoms implanted on semiconductor surfaces obtained by the experimental group there. In Belgium, we have interacted with an experimental group at Katholieke Universiteit in Leuven on hyperfine interactions of impurities in semiconductors studied by Mössbauer measurements. In Denmark, we have interacted with an experimental group in University of Roskilde on hyperfine interactions in biological systems using data obtained by them by Perturbed Angular Correlation technique and with an experimental group in University of Århus on nuclear quadrupole interactions involving excited 19F* nuclei in condensed matter systems. In India we have been in interaction since 1995 with a number of Universities, namely an experimental group at Indian Institute of Technology Madras in Chennai on magnetic properties and hydrogen storage in Laves Phase compounds and alloys, another experimental group at the Institute of Physics in Bhubaneswar on electronic structures of adsorbed atoms at semiconductor surfaces and associated nuclear quadrupole interactions and other spectroscopic properties and with University of Hyderabad with both an experimental group on the interpretation of nuclear magnetic resonance data in liquid crystal systems and with a theory group on the pion channeling process in semiconductors. In South Africa, we have been interacting with an experimental group on 19F* nuclear quadrupole interactions in semiconductor systems. In South Korea, we have been interacting with a theory group at Yeungnam University in Taegu, on hyperfine properties of sixth-group semi-conductor systems and their alloys and the electronic structures associated with adsorbed atoms at III-V semiconductor surfaces and associated nuclear quadrupole interactions. In Japan, with the Muon Science Laboratory at the Institute of Physical and Chemical Research (RIKEN) at Wako-shi, Saitama Ken and the Meson Science Laboratory at Institute of Material Structure Science, High Energy Accelerator Research Organization (KEK) at Tsukuba, Ibaraki Ken, (previously including the same group at the Faculty of Science, University of Tokyo) we have had since 1987, and are currently continuing, extensive collaboration on a variety of problems on interactions of muons with solid state materials. Among the problems we have worked on are the interaction of positive and negative muons with high-temperature superconducting materials, attempting to study the location (or locations) of the muons and interpret interesting data obtained by the KEK-MSL and RIKEN Muon Science groups. Similar work with emphasis on dynamics of the muon in addition to its trapping sites and associated electron distributions is being carried out on transition metal oxides, again to interpret hyperfine data from mSR experiments. Related investigations are in progress for the interaction of muons with spin gap systems including ionic potassium copper halide systems, the experimental work on these systems having been carried out by the group in RIKEN. Investigations are also planned for the location of muon and associated hyperfine properties involving organic compounds containing transition metal atoms which undergo Haldane magnetic phase transitions, on muonium and muon in organic ferromagnets without transition metal atoms and on muonium in organic polymer systems. Our work on the latter system involves theoretical efforts under way on the electronic structure of the heme unit and its immediate environment and on muonium trapping at the amino acid group sites in the protein chain in Cytochrome c and DNA systems to understand quantitatively the recent results on electron transport in these systems, resulting from the break up of the trapped muonium, that have been obtained by the RIKEN and KEK-MSL groups through mSR studies. Lastly, we are also working on the trapping of helium entities in solid hydrogen to understand the depletion of negative muons in solid tritium and deuterium-tritium mixtures associated with muon catalyzed fusion (mCF) experiments being carried out by the group at the RIKEN Muon Science Laboratory. This, as explained earlier in subsection XIII of Section 8 on Research Interests, is part of a research program connected with energy production from fusion of tritium atoms among themselves, and of deuterium and tritium atoms, which are bound at relatively short distances by the negative muon. Efforts are also under progress for the understanding in solid hydrogen of the sticking of m- to the 4He atom produced as a results of the fusion process, especially the regeneration effect for m- which appears to be very important for the mCF technique to the economic efficiency of the mCF process as an energy source.
We have started a recent collaboration since Sept. 2001 between my research group in SUNY Albany jointly with an experimental group in University of Central Florida, Orlando and the faculties of Central Physics and Computer Science Departments in the leading University of Nepal, Tribhuvan University (TU), Kirtipur Campus, Kathmandu, interested in Solid State Physics and Materials Science and Environmental Science. This Research Program is jointly sponsored by the US National Science Foundation and the University Grants Commission of Nepal. There are three areas of research that we are involved in. The first is in the fields of Optoelectronics and Photonics involving rare-earth impurities, specifically Erbium in Silicon where our interests and planned efforts are described in subsection XIV of Section 8 dealing with Research Interests. The aim is to understand from a first-principle electronic structure point of view using the Hartree-Fock Cluster procedure the photoluminescence associated with the 1.54 mm line from the transition between two multiplet 4f levels of Er and how to enhance its intensity. The second area is the study of influence of impurity atoms on the temperature of glass transition in chalcogen glasses involving Se-Te alloy, described in detail in Section XVI. The aim here is to understand quantitatively the nature of the changes in glass transition temperatures as functions of the concentration of impurities from a first-principle electronic structure point of view. Some of the experimental results involved have been obtained by experimentalists at the Central Physics Dept. of TU. The third project we are working on involves the ultraviolet absorption by Ozone in the Kathmandu Valley adjacent to the Himalaya mountains. The aim is to understand the results of the measurements being carried out by an experimental group at TU on the ultraviolet absorption as a function of frequency in the wave-length regions (l < 303 nm) and (303 nm < l < 317 nm), which are respectively parts of the Hartley and Huggins bands. The theoretical work is being carried out using the Hartree-Fock-Roothaan procedure (augmented by inclusion of many-body effects), and the accuracy of the electronic wave-functions for the ground state are being checked by comparing the binding energy, geometry and related properties with available experimental data and results of earlier investigations.
One of the aims of the collaborative effort with Nepal is of course to enhance our understanding of some contemporary research areas in solid state physics and materials science and environmental science. The other aim is to help the TU group gain expertise in computational methods for electronic structure investigations in materials science and molecular physics to interpret the results of their own experimental studies and other available experimental results in the literature. For our group it allows expansion in the research areas we are interested in at SUNY Albany and the opportunity to interact with a number of highly motivated and experienced researchers in fields of mutual interest. This collaborative effort also allows us to interact jointly with the TU group and the University of Central Florida (UCF) on the interpretation of hyperfine interaction data in the optoelectronic, photonic and chalcogen glass systems being obtained by the UCF and TU groups.
Lastly, our current investigations on m+ hyperfine interactions in stripe-structured Lanthanum Cuprate and Nickelate systems is being carried out in collaboration with scientists at Technische Universität Braunschweig, Germany who are carrying out mSR measurements in these systems.
In addition to the benefits to scientific understanding of systems important in materials research and energy production, the collaborative programs with active international groups have proven very beneficial to graduate students in my research group and to their counterparts in the international groups in terms of the completeness and breadth of their research training, by providing the opportunity for mutual interaction with research groups with different backgrounds and training. In this respect, eight of my atomic theory students have received their Ph. D. degrees working on joint research projects with the atomic theory group at Technical University of Delft in Netherlands and two others from collaborative projects with the experimental atomic hyperfine interaction group at Universität Mainz in Germany. Two Ph. D. students from our group have received their Ph. D. degrees working on joint projects with the experimental Mössbauer group at Technische Universität München in Germany. One other Ph. D. student has received his Ph. D. degree from work on projects of joint interest to the electronic structure of metals theory group at Technische Hochschule Darmstadt in Germany and two others have received their Ph. D. thesis degrees on properties of adsorbed atoms on reconstructed semiconductor surfaces in interaction with experimental surface studies groups at Institute of Physics, Bhubaneswar, India and University of Konstanz in Germany. With the research groups at RIKEN, Muon Science Laboratory at Wako-shi and the Meson Science Laboratory at Institute of Materials Structure Science at KEK in Tsukuba-Shi in Japan, six of my students have already received their Ph. D. degrees working on joint projects on Muon (both positive and negative) interactions with high-Tc systems and alkali halides and on Muon interactions in ionic transition metal compounds. Currently one of my Ph.D. students is working on magnetic properties of organic ferromagnets including muon and muonium interactions with these systems. This work is being carried out in collaboration with the experimental groups at RIKEN and KEK and also with a past Ph.D. student of mine who is currently in a faculty position at Tohoku University in Sendai, Japan. One other Ph.D. student is carrying out extensive collaboration on both muon and muonium interactions with heme systems as well as DNA both with the RIKEN and KEK-MSL groups as well as a group at Yamanashi University in Japan. This student is also working on helium trapping in solid hydrogen in the project described earlier (in subsection XIII of Section 8 on Research Interests), important for muon catalyzed fusion studies.
Abstracts Presented at National and International Conferences: A list of Abstracts of papers that my students, collaborators and I have presented at National and International Meetings since 2001 can be found under this link. The Conferences listed are typical of Conferences where we presented papers since the mid 1960`s. In addition to these Conferences we have also presented papers at some other Conferences on topics involving research interests of myself and my group.
Publications (Books, Reviews and Journal Articles): I have authored three books (two jointly with one co-author in each case and one by myself), sixteen review articles (either by myself or with others as co-authors) and 396 research articles in leading refereed journals in Physics, Chemistry, Biophysics and Biochemistry, of which 380 are already published or in press and 16 others submitted for publication, involving all areas of my interests in atomic physics, condensed matter physics and material science and biophysics. Please follow this link for a list of the books I have authored co-authored and recent reviews and papers (and a few earlier ones) in my various areas of interest by my research group, collaborators at other centers and myself.
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