Abhijit Mookerjee

Structure, bonding, and magnetism of cobalt clusters from first-principles calculations

S. Datta, M. Kabir, S. Ganguly, B. Sanyal, T. Saha-Dasgupta, and A. Mookerjee Department of Material Sciences, S.N. Bose National Centre for Basic Sciences, JD Block, Sector-III, Salt Lake City, Kolkata 700 098, India Theoretical Magnetism Group, Department of Physics, Uppsala University, Box 530, SE-75121 Uppsala, Sweden Unit for Nanoscience and Technology, S.N. Bose National Centre for Basic Sciences, JD Block, Sector-III, Salt Lake City, Kolkata 700 098, India (Dated: February 1, 2008)

Structure, electronic properties, and magnetic transition in manganese clusters

We systematically investigate the structural, electronic, and magnetic properties of Mnn clusters n =2–2 0 within the ab initio pseudopotential plane wave method using generalized gradient approximation for the exchange-correlation energy. A new kind of icosahedral structural growth has been predicted in the intermediate size range. Calculated magnetic moments show an excellent agreement with the Stern-Gerlach experiment. A transition from ferromagnetic to ferrimagnetic Mn-Mn coupling takes place at n = 5 and the ferrimagnetic states continue to be the ground states for the entire size range. Possible presence of multiple isomers in the experimental beam has been argued. No signature of nonmetal to metal transition is observed in this size range and the coordination dependence of d-electron localization is discussed.

Averaged density of states in disordered systems

A continued fraction method is developed for calculating the averaged density of states in disordered systems. The Anderson model is treated specifically. No appeal is made to periodicity or Blochs Theorem which is its consequence. The density of states correct up to moments of order 20 is calculated using the IBM 370 computer.

Magnetism in small bimetallic Mn-Co clusters

Effects of alloying on the electronic and magnetic properties of MnxCoy (x+y=n=2-5, x=0-n) and Mn2Co11 nanoalloy clusters are investigated using the density-functional theory. Unlike the bulk alloy ...

A first-principles thermodynamic approach to ordering in Ni-Mo alloys

There is a competition between several face centered cubic (FCC)-based ordered inter-metallic phases in Ni-Mo alloys containing 8-33 at% Mo. The transformation behavior of these alloys in terms of ordering instabilities has been studied. First-principles tight-binding-linear muffin-tin orbital (TB-LMTO) method coupled with augmented space recursion (ASR) in conjunction with orbital peeling (OP) technique has been employed to extract the concentration dependent effective pair interactions. Further, the mean-field statistical mechanics based static concentration wave (SCW) model has been used to determine the free energies of these ordered phases as functions of temperature, composition and order parameter. This ASR-OP-SCW approach, applied to Ni-Mo alloy system, gives the correct ground state stability sequence as observed experimentally. Furthermore, it has been shown that such an approach can be used to study the complex transformation behavior involving several competing superstructures as well as competing first order and second order ordering processes.

Structural and optical properties of paraelectric SrTiO3

The electronic energy band structure, site- and angular-momentum-decomposed densities of states (DOS) and charge-density contours of perovskite SrTiO3 in the paraelectric cubic phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows a direct band gap of ~1.4 eV at the gamma point in the Brillouin zone. The total DOS is compared with experimental x-ray photoemission spectra. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Sr and TiO3 is mainly ionic and that the TiO3 entities bond covalently. Using the projected DOS and band structure we have analysed the interband contribution to the optical properties of SrTiO3 . The real and imaginary parts of the dielectric function and hence the optical constants (such as the reflectivity, refractive index, extinction coefficient and absorption coefficient) and the electron energy-loss spectrum are calculated. The calculated spectra are compared with the experimental results for SrTiO3 in the cubic phase and are found to be in good agreement with the experimental results in low-energy regions. The role of band-structure calculation as regards the optical properties of SrTiO3 is discussed.

Electronic structure of alloys, surfaces and clusters

Locality in Electronic Structure- An Introduction to the Recursion Method. Introduction to the Linear Band Structure Methods. Introduction to Augmented Space Methods. Electronic Structure of Disordered Alloys Using the KKR Method. Phase Stability of Binary Intermetallics: A First Principles Approach. Application of the Augmented Space Recursion to the Study of Electronic Structure and Phase Stability of Disordered Alloys. Estimation of Parameters of Many-body Model Hamiltonians from Ab Initio Calculations. Equilibrium and Non-equilibrium Statistical Mechanics of Alloys in a Face Centered Cubic Lattice. Growth, Electronic and Magnetic Structure of Rough Surfaces. Ab Initio Orbital Free Molecular Dynamics: Technique and Applications. Ab Initio Study of Transition Metal Clusters.

Emergence of noncollinear magnetic ordering in small magnetic clusters: Mn n and As@Mn n

Using first-principles density functional calculations, we have studied the magnetic ordering in pure Mn n (n=2-10,13,15,19) and As@Mn n (n=1-10) clusters. Although, for both pure and doped manganese clusters, there exists many collinear and noncollinear isomers close in energy, the smaller clusters with n≤5 have collinear magnetic ground states and the emergence of noncollinear ground states is seen for n≥6 clusters. Due to strong p-d hybridization in As@Mn n clusters, the binding energy is substantially enhanced and the magnetic moment is reduced compared to the corresponding pure Mn n clusters.

A real-space study of random extended defects in solids : Application to disordered Stone–Wales defects in graphene

Abstract We propose here a first-principles, parameter free, real space method for the study of disordered extended defects in solids. We shall illustrate the power of the technique with an application to graphene sheets with randomly placed Stone–Wales defects and shall examine the signature of such random defects on the density of states as a function of their concentration. The technique is general enough to be applied to a whole class of systems with lattice translational symmetry broken not only locally but by extended defects and defect clusters. The real space approach will allow us to distinguish signatures of specific defects and defect clusters.

Observation of large change of 7Be decay rate in Au and Al2O3 and its implications

Abstract We find from our measurements that the decay rate of 7 Be implanted in Au is lower than that implanted in Al 2 O 3 by a relatively large amount (0.72±0.07)%. This result and others have been analyzed quantitatively using the linear muffin-tin orbital (LMTO) method and Hartrees calculations. Our results are important for accurately extracting the nuclear matrix element of the astrophysically significant 7 Be + e − → 7 Li + ν reaction.