Advanced Materials Research Unit

Salt Lake, India

Advanced Materials Research Unit

Salt Lake, India
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Alam A.,University of Illinois at Urbana - Champaign | Saha-Dasgupta T.,Advanced Materials Research Unit | Mookerjee A.,Advanced Materials Research Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Using first-principles density-functional calculations, we studied the electronic structure and phase stability of the pseudobinary alloys. The calculations have been carried out using the augmented space recursion based on the all electron, tight-binding linear muffin-tin orbital basis. This work, in particular, provides a further generalization of our earlier developed technique to the case of systems of multiple sublattices with varying degree of disorder on them. We showcase the feasibility of our formalism by applying to a face-centered tetragonal-based pseudobinary (Ni1-x Ptx) 3 Al alloy system. Based on the calculation of effective pair interactions and their lattice Fourier transform, our phase stability search yields two stable superordered structures, namely, L 10 -type Ni2 PtAl and L 10 -type NiPt2 Al of which the latter has been observed experimentally. An estimate of the minima in the effective pair potential surface V (k) predicted the order-disorder transition temperatures of the two stable structures to be ∼1027 and ∼1379K, respectively. The results are in agreement with the previous findings, proving the effectiveness of augmented space recursive technique in dealing with systems of multiple sublattices with varying degree of disorder on them. Our calculated additional physical quantities such as short-range order maps can be compared with future experimental studies. © 2010 The American Physical Society.


Rahaman M.,KTH Royal Institute of Technology | Rahaman M.,Sn Bose National Center For Basic Science | Ruban A.V.,KTH Royal Institute of Technology | Mookerjee A.,Advanced Materials Research Unit | Johansson B.,KTH Royal Institute of Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We investigate the effect of global magnetization on the effective cluster interactions and order-disorder phase transition in Fe xCo 1-x alloys. The effective cluster interactions are obtained by the screened generalized perturbation method as it is implemented in the exact muffin-tin orbitals formalism within the coherent potential approximation. The ordering transition from the high-temperature disordered body-centered cubic alloy to the ordered B2 phase is determined by Monte Carlo simulations. The calculated transition temperatures are in good agreement with the available experimental data for the effective interactions, which correspond to the experimentally observed magnetization at the order-disorder phase transition. © 2011 American Physical Society.


Alam A.,University of Illinois at Urbana - Champaign | Chouhan R.K.,Advanced Materials Research Unit | Mookerjee A.,Advanced Materials Research Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

There has been increasing evidence about the effects of short-range order (or local chemical environment effects) on the lattice dynamics of alloys, which eventually affect the vibrational entropy difference among various phases of a compound, and hence their relative stability. In this article, we present an ab initio calculation of the lattice dynamics and the vibrational entropy of disordered systems with short-range order. The features in the phonon density of states were found to change systematically with chemical short-range order in the alloy. Plausible explanations for our smaller value of vibrational entropy of mixing compared to experiment are given in some detail. A general trend of the magnitude of vibrational entropy of mixing is explained by making a connection to the phonon lifetime broadening, an intrinsic property of any multiple scattering phenomenon. We illustrate the method by applying it to a body-centered cubic Fe 1-xCr x alloy. © 2011 American Physical Society.


Sengupta A.,Heinrich Heine University Düsseldorf | Sengupta S.,Indian Association for The Cultivation of Science | Sengupta S.,Advanced Materials Research Unit | Menon G.I.,Chennai Mathematical Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We study a simple model for the depinning and driven steady-state phases of a solid tuned across a polymorphic phase transition between ground states of triangular and square symmetry. The competition between the underlying structural phase transition in the pure system and the effects of the underlying disorder, as modified by the drive, stabilizes a variety of unusual dynamical phases. These include pinned states which may have dominantly triangular or square correlations, a plastically flowing liquidlike phase, a moving phase with hexatic correlations, flowing triangular and square states and a dynamic coexistence regime characterized by the complex interconversion of locally square and triangular regions. We locate these phases in a dynamical phase diagram and study them by defining and measuring appropriate order parameters and their correlations. We demonstrate that the apparent power-law orientational correlations we obtain in our moving hexatic phase arise from circularly averaging an orientational correlation function which exhibits long-range order in the (longitudinal) drive direction and short-range order in the transverse direction. This calls previous simulation-based assignments of the driven hexatic glass into question. The intermediate coexistence regime exhibits several distinct properties, including substantial enhancement in the current noise, an unusual power-law spectrum of current fluctuations and striking metastability effects. We show that this noise arises from the fluctuations of the interface separating locally square and triangular ordered regions by demonstrating a correlation between enhanced velocity fluctuations and local coordinations intermediate between the square and triangular. We demonstrate the breakdown of effective "shaking temperature" treatments in the coexistence regime by showing that such shaking temperatures are nonmonotonic functions of the drive in this regime. Finally we discuss the relevance of these simulations to the anomalous behavior seen in the peak effect regime of vortex lines in the disordered mixed phase of type-II superconductors. We propose that this anomalous behavior is directly linked to the behavior exhibited in our simulations in the dynamical coexistence regime thus suggesting a possible solution to the problem of the origin of peak effect anomalies. © 2010 The American Physical Society.


Franzrahe K.,University of Konstanz | Nielaba P.,University of Konstanz | Sengupta S.,Indian Association for The Cultivation of Science | Sengupta S.,Advanced Materials Research Unit
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

In soft matter systems the local displacement field can be accessed directly by video microscopy enabling one to compute local strain fields and hence the elastic moduli in these systems using a coarse-graining procedure. We study this process in detail for a simple triangular, harmonic lattice in two dimensions. Coarse-graining local strains obtained from particle configurations in a Monte Carlo simulation generates nontrivial, nonlocal strain correlations (susceptibilities). These may be understood within a generalized, Landau-type elastic Hamiltonian containing up to quartic terms in strain gradients. In order to demonstrate the versatility of the analysis of these correlations and to make our calculations directly relevant for experiments on colloidal solids, we systematically study various parameters such as the choice of statistical ensemble, presence of external pressure and boundary conditions. Crucially, we show that special care needs to be taken for an accurate application of our results to actual experiments, where the analyzed area is embedded within a larger system, to which it is mechanically coupled. Apart from the smooth, affine strain fields, the coarse-graining procedure also gives rise to a noise field (χ) made up of nonaffine displacements. Several properties of χ may be rationalized for the harmonic solid using a simple "cell model" calculation. Furthermore the scaling behavior of the probability distribution of the noise field (χ) is studied. We find that for any inverse temperature β, spring constant f, density ρ and coarse-graining length Λ the probability distribution can be obtained from a master curve of the scaling variable X=χβf/ρ Λ2. © 2010 The American Physical Society.


Kabir M.,Massachusetts Institute of Technology | Mukherjee S.,Advanced Materials Research Unit | Saha-Dasgupta T.,Advanced Materials Research Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Stone-Wales (SW) transformation is a key mechanism responsible for the growth, transformation, and fusion in fullerenes, carbon nanotubes, and other carbon nanostructures. These topological defects also substantially alter the physical and chemical properties of the carbon nanostructures. However, this transformation is thermodynamically limited by very high activation energy (∼7eV in fullerenes). Using first-principles density functional calculations, we show that the substitutional boron doping substantially reduces the SW activation barrier (from ∼7 to 2.54 eV). This reduction is the largest in magnitude among all the mechanisms of barrier reduction reported to date. Analysis of bonding charge density and phonon frequencies suggests that the bond weakening at and around the active SW site in B heterofullerenes is responsible for such a reduction. Therefore, the formation of the SW defect is promoted in such heterofullerenes and is expected to affect their proposed H2 storage properties. Such substitutional doping also can modify the SW activation barrier in carbon nanotubes and graphene nanostructures and can catalyze isomerization, fusion, and nanowelding processes. © 2011 American Physical Society.


Datta S.,Advanced Materials Research Unit | Kabir M.,Massachusetts Institute of Technology | Saha-Dasgupta T.,Advanced Materials Research Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Using first-principles density-functional-theory-based calculations, we analyze the structural stability of small clusters of 3d late transition metals. We consider the relative stability of the two structures: layer-like structures with hexagonal closed packed stacking and more compact structures of icosahedral symmetry. We find that the Co clusters show an unusual stability in hexagonal symmetry compared to the small clusters of other members, which are found to stabilize in icosahedral-symmetry-based structure. Our study reveals that this is driven by the interplay between the magnetic-energy gain and the gain in covalency through the s-d hybridization effect. Although we have focused our study primarily on clusters with 19 atoms, we find this behavior to be general for clusters with between 15 and 20 atoms. © 2011 American Physical Society.


Datta S.,Advanced Materials Research Unit | Kabir M.,Massachusetts Institute of Technology | Mookerjee A.,Advanced Materials Research Unit | Saha-Dasgupta T.,Advanced Materials Research Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

With a goal to produce a giant magnetic moment in a Mn13 cluster that will be useful for practical applications, we have considered the structure and magnetic properties of a pure Mn13 cluster and substitutionally doped it with X=Ti, V, Cr, Fe, Co, Ni atom respectively to produce Mn12X clusters. We find that Ti and V substitutions in a Mn13 cluster are the most promising in terms of gaining substantial binding energy as well as achieving a higher magnetic moment through ferromagnetic alignment of the atom-centered magnetic moments. This has been demonstrated in terms of energetics and electronic properties of the clusters. For comparison, we have also studied the effect of N capping of Mn13 cluster, predicted in an earlier work, as a means of producing stable giant magnetic moments in Mn clusters up to cluster sizes of five Mn atoms. © 2011 American Physical Society.


Ganguly S.,Advanced Materials Research Unit | Kabir M.,Massachusetts Institute of Technology | Sanyal B.,Uppsala University | Mookerjee A.,Advanced Materials Research Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

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. © 2011 American Physical Society.


Das A.,se National Center For Basic Science | Chakrabarti J.,se National Center For Basic Science | Chakrabarti J.,Advanced Materials Research Unit | Ghosh M.,se National Center For Basic Science
Biophysical Journal | Year: 2013

We extract the thermodynamics of conformational changes in biomacromolecular complexes from the distributions of the dihedral angles of the macromolecules. These distributions are obtained from the equilibrium configurations generated via all-atom molecular dynamics simulations. The conformational thermodynamics data we obtained for calmodulin-peptide complexes using our methodology corroborate well with the experimentally observed conformational and binding entropies. The conformational free-energy changes and their contributions for different peptide-binding regions of calmodulin are evaluated microscopically. © 2013 by the Biophysical Society.

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