Shreemoyee Ganguly

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)

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 ...

Unusual structure and magnetism in manganese oxide nanoclusters

We report an unusual evolution of structure and magnetism in stoichiometric MnO clusters based on an extensive and unbiased search through the potential-energy surface within density functional theory. The smaller clusters, containing up to five MnO units, adopt two-dimensional structures; and regardless of the size of the cluster, magnetic coupling is found to be antiferromagnetic in contrast to previous theoretical findings. Predicted structure and magnetism are strikingly different from the magnetic core of Mn-based molecular magnets, whereas, they were previously argued to be similar. Both of these features are explained through the inherent electronic structures of the clusters.

A cartoon in one dimension of the hydrogen molecular ion

To illustrate the basic methodology involved in the quantum mechanics of molecules, a one-dimensional caricature of the hydrogen molecular ion (H+2) is presented, which is exactly solvable, in the Born–Oppenheimer approximation, in terms of elementary functions. The purpose of the exercise is to elucidate in a simple setting the standard approach to the electronic and vibrational states of molecules, a subject of interest to students of physics, chemistry and biology.

Magnetic and electronic crossovers in graphene nanoflakes

Manipulation of intrinsic magnetic and electronic structures of graphene nanoflakes is of technological importance. Here we carry out systematic study of the magnetic and electronic phases, and its manipulation in graphene nanoflakes employing first-principles calculation. We illustrate the intricate shape and size dependence on the magnetic and electronic properties, and further investigate the effects of carrier doping, which could be tuned by gate voltage. A transition from nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance, which we identify to be originated from the armchair defect at the junctions of two sublattices on the edge. Electron, or hole doping simultaneously influences the magnetic and electronic structures, and triggers phase changes. Beyond a critical doping, crossover from antiferromagnetic to ferromagnetic phase is observed for the flakes without sublattice imbalance, while suppression of magnetism, and a possible crossover from magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous to magnetic phase changes, a semiconductor to (half) metal transition is observed, upon carrier doping.

A local-density approximation for the exchange energy functional for excited states: The band-gap problem

We present excited-state density functional theory (DFT) to calculate band gap for semiconductors and insulators. For the excited-state exchange-correlation functional, we use a simple local-density approximation (LDA) like functional and it gives the result which is very close to experimental results. The linear muffin-tin potential is used to solve the self-consistent Kohn-Sham equation.

Time‐Independent Excited‐State Density Functional Theory

We review the current status of time‐independent density‐functional theory for an individual excited‐state. The theory is built upon generalized constrained‐search formulation that maps a given excited‐state density ρ(r) to the corresponding many‐body wavefunction. This process obtains the wavefunction Ψ[ρ;ρ0] as a bi‐functional of the excited‐state desnity ρ(r) and the ground‐state density ρ0(r). However, the important issues in excited‐state density‐functional theory are whether an accurate exchange‐correlation functional can be constructed to get excited‐state energy and whether a unique Kohn‐Sham system can be constructed for a given excited‐state. In this paper we show that the answer to both the questions is in the affirmative. The existence of excited‐state Kohn‐Sham system is demonstrated by constructing it for the excited‐states of some members of Boron isoelectronic series. In connection with the excited‐state functional, we present recently constructed exchnage energy fucntional for excited‐sta...

Manipulating magnetism of MnO nano-clusters by tuning the stoichiometry and charge state

We study the composition dependent evolution of geometric and magnetic structures of MnO clusters within density functional theory. A systematic and extensive search through the potential energy surface is performed to identify the correct ground state, and significant isomers. We find that the magnetic structures in these MnO clusters are complex, which has been explained using the intrinsic electronic structure of the cluster, and analyzed using model Hamiltonian with parameters obtained from maximally localized Wannier functions. The calculated vertical displacement energies of off-stoichiometric MnO clusters compare well with the recent experimental results. Interestingly, the charged state of the cluster strongly influences the geometry and the magnetic structure of the cluster, which are very different from the corresponding neutral counterpart. Further, the importance of electron correlation in describing simple Mn-dimer and MnO clusters has been discussed within Hubbard model and hybrid exchange-correlation functional.In this paper, we have studied the composition dependent evolution of geometric and magnetic structures of MnO clusters within density functional theory. The magnetic structures are determined by the competition between direct and superexchange interactions, which have been analyzed by the parameters obtained from maximally localized Wannier functions. The intrinsic electronic structures of the clusters have been thoroughly studied by looking into the hybridization (quantified using the Hybridization Index) and charge transfer scenario. Further, the importance of electron correlation in describing simple Mn-dimer and MnO clusters has been discussed within the Hubbard model and hybrid exchange-correlation functional. Our calculated vertical detachment energies of off-stoichiometric MnO clusters compare well with the recent experimental results. Interestingly, the charged state of the cluster strongly influences the geometry and the magnetic structure of the cluster, which are very different from the corresponding neutral counterpart. We have demonstrated that the exchange interaction between Mn atoms can be switched between ferromagnetic and anitiferromagnetic ones by changing the charge state and hence can be useful for spin-based information technology.

Effect of Fe doping in the structural, electronic and magnetic properties of CoCr2O4 : insights from ab initio calculations

CoCr2O4 has attracted significant attention recently due to several interesting properties such as magnetostriction, magnetoelectricity etc. More recent experiments on Fe substituted CoCr2O4 observ ...

A study of magnetism in disordered Pt–Mn, Pd–Mn and Ni–Mn alloys: an augmented space recursion approach

In this paper we shall study three binary alloy systems, one constituent of which is Mn. The other constituents are chosen from a particular column of the periodic table: Ni(3d), Pt (4d) and Pd (5d). As we go down the column, the d-bands become wider, discouraging spin-polarization. In a disordered alloy, the situation becomes more complicated, as the exchange interaction between two atoms is environment dependent. We shall compare and contrast their magnetic behaviour using robust electronic structure techniques. In all three alloy systems conjectures are made to explain experimental data. In this paper we shall examine whether there is any basis to these conjectures.