Theoretical science Unit

Banglore, India

Theoretical science Unit

Banglore, India

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Pati S.K.,Theoretical science Unit
Physical Chemistry Chemical Physics | Year: 2016

We present a theoretical investigation of small aggregates of quadrupolar (A-π-D-π-A or D-π-A-π-D) charge-transfer dyes, with attention focused on the role of intermolecular interactions in determining their optical properties. We tackle the theoretical issue by adopting essential-state models (ESMs), which describe an isolated molecule in terms of a minimal number of electronic states, corresponding to the resonance structures. ESMs quite naturally describe intermolecular interactions relaxing the dipolar approximation and accounting for molecular polarizabilities. The approach is applied to curcuminoid and squaraine dyes, two families of chromophores with weak and strong quadrupolar character, respectively. The method is validated against experiment and for curcuminoids also against time-dependent density functional theory. ESMs rationalize the strong ultra-excitonic effects recurrently observed in the experimental optical spectra of aggregates of highly polarizable quadrupolar dyes, offering a valuable tool to exploit the supramolecular design of material properties. © 2016 the Owner Societies.


Kumar P.,Indian Institute of Technology Patna | Yamijala S.S.R.K.C.,Chemistry and Physics of Materials Unit | Pati S.K.,Theoretical science Unit
Journal of Physical Chemistry C | Year: 2016

Optical unzipping of carbon nanotubes (CNTs) in liquid media is one of the most awaited technologies as it promises instant material transformation from CNTs to graphene nanoribbons (GNRs) and also an easy transfer of GNRs to arbitrary substrates. In the present article, we report the laser-induced optical unzipping of CNTs, dispersed in dimethylformamide (DMF) solvent. In a nutshell, laser unzipping of CNTs dispersed in liquid solvent is a photophysicochemical process where molecular interactions between CNTs and solvent are tuned by the laser irradiation and results in the formation of GNRs in a scalable manner. The proposed mechanism includes the creation of defects together with vacancies upon laser irradiation, followed by their migration toward the energetically favorable axis of the CNT - the longitudinal direction - finally leading to the unzipping/fragmentation of the nanotube. Distinct laser thresholds have been observed for each of the three events, namely, (a) the formation of the first defect, (b) vacancy migration along the longitudinal direction, and (c) fragmentation of CNTs into graphene nanosheets. Our experimental findings of the unzipping process have further been supported by the density functional theory (DFT) and density functional tight binding (DFTB) calculations performed on both single-walled and multiwalled CNTs. © 2016 American Chemical Society.


Sen S.,Chemistry and Physics of Materials Unit | Moreno J.,Louisiana State University | Jarrell M.,Louisiana State University | Vidhyadhiraja N.S.,Louisiana State University | Vidhyadhiraja N.S.,Theoretical science Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We investigate the effect of disorder on the dynamical spectrum of layered f-electron systems. With random dilution of f sites in a single Kondo insulating layer, we explore the range and extent to which Kondo hole incoherence can penetrate into adjacent layers. We consider three cases of neighboring layers: band insulator, Kondo insulator, and simple metal. The disorder-induced spectral weight transfer, used here for quantification of the proximity effect, decays algebraically with distance from the boundary layer. Further, we show that the spectral weight transfer is highly dependent on the frequency range considered as well as the presence of interactions in the clean adjacent layers. The changes in the low-frequency spectrum are very similar when the adjacent layers are either metallic or Kondo insulating, and hence are independent of interactions. In stark contrast, a distinct picture emerges for the spectral weight transfers across large energy scales. The spectral weight transfer over all energy scales is much higher when the adjacent layers are noninteracting as compared to when they are strongly interacting Kondo insulators. Thus, over all scales, interactions screen the disorder effects significantly. We discuss the possibility of a crossover from non-Fermi-liquid to Fermi-liquid behavior upon increasing the ratio of clean to disordered layers in particle-hole asymmetric systems. © 2015 American Physical Society.


Maiti M.,Theoretical science Unit | Lakshminarayanan A.,Theoretical science Unit | Lakshminarayanan A.,India Institute of Technology | Lakshminarayanan A.,eBay | And 2 more authors.
European Physical Journal E | Year: 2013

The implementation of a method for the exact evaluation of the volume and surface area of cavities and free volumes in polydisperse sphere packings is described. The generalization of an algorithm for Voronoi tessellation by Tanemura et al. is presented, employing the radical plane construction, as a part of the method. We employ this method to calculate the equation of state for monodisperse and polydisperse hard-sphere fluids, crystals, and for the metastable amorphous branch up to random close packing or jamming densities. We compute the distribution of free volumes, and compare with previous results employing a heuristic definition of free volume. We show the efficacy of the method for analyzing protein structure, by computing various quantities such as the distribution of sizes of buried cavities and pockets, the scaling of solvent accessible area to the corresponding occupied volume, the composition of residues lining cavities, etc. © EDP Sciences / Società Italiana di Fisica / Springer-Verlag 2013.


Ghosh D.,Chemistry and Physics of Materials Unit | Parida P.,University of Regensburg | Pati S.K.,Theoretical science Unit
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

Line defects in two-dimensional (2D) materials greatly modulate various properties of their pristine form. Using ab initio molecular dynamics (AIMD) simulations, we investigate the structural reconstructions of different kinds of grain boundaries in the silicene sheets. It is evident that depending upon the presence of silicon adatoms and edge shape of grain boundaries (i.e., armchair or zigzag), stable extended line defects (ELDs) can be introduced in a controlled way. Further studies show the stability of these line-defects in silicene, grown on Ag(111) surface at room-temperature. Importantly, unlike most of the 2D sheet materials such as graphene and hexagonal boron nitride, 5-5-8 line defects modify the nonmagnetic semimetallic pristine silicene sheet to spin-polarized metal. As ferromagnetically ordered magnetic moments remain strongly localized at the line defect, a one-dimensional spin channel gets created in silicene. Interestingly, these spin channels are quite stable because, unlike the edge of nanoribbons, structural reconstruction or contamination cannot destroy the ordering of magnetic moments here. Zigzag silicene nanoribbons with a 5-5-8 line defect also exhibit various interesting electronic and magnetic properties depending upon their width as well as the nature of the magnetic coupling between edge and defect spin states. Upon incorporation of other ELDs, such as 4-4-4 and 4-8 defects, 2D sheets and nanoribbons of silicene show a nonmagnetic metallic or semiconducting ground state. Highlighting the controlled formation of ELDs and consequent emergence of technologically important properties in silicene, we propose new routes to realize silicene-based nanoelectronic and spintronic devices. © 2015 American Physical Society.


Parida P.,Theoretical science Unit | Pati S.K.,Theoretical science Unit | Pati S.K.,Jawaharlal Nehru Centre for Advanced Scientific Research | Painelli A.,University of Parma
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

A current constrained approach is proposed to calculate negative differential conductance in molecular nanojunctions. A four-site junction is considered where a steady-state current is forced by inserting only the two central sites within the circuit. The two lateral sites (representing, e.g., dangling molecular groups) do not actively participate in transport but exchange electrons with the two main sites. These auxiliary sites allow for a variable number of electrons within the junction, while as required by the current constrained approach, the total number of electrons in the system is kept constant. We discuss the conditions for negative differential conductance in terms of cooperativity, variability of the number of electrons in the junction, and electron correlations. © 2011 American Physical Society.


Sissa C.,University of Parma | Manna A.K.,Theoretical science Unit | Terenziani F.,University of Parma | Painelli A.,University of Parma | Pati S.K.,Theoretical science Unit
Physical Chemistry Chemical Physics | Year: 2011

Resonance Energy Transfer (RET) is investigated in pairs of charge-transfer (CT) chromophores. CT chromophores are an interesting class of π conjugated chromophores decorated with one or more electron-donor and acceptor groups in polar (D-π-A), quadrupolar (D-π-A-π-D or A-π-D-π-A) or octupolar (D(-π-A)3 or A(-π-D)3) structures. Essential-state models accurately describe low-energy linear and nonlinear spectra of CT-chromophores and proved very useful to describe spectroscopic effects of electrostatic interchromophore interactions in multichromophoric assemblies. Here we apply the same approach to describe RET between CT-chromophores. The results are quantitatively validated by an extensive comparison with time-dependent density functional theory (TDDFT) calculations, confirming that essential-state models offer a simple and reliable approach for the calculation of electrostatic interchromophore interactions. This is an important result since it sets the basis for more refined treatments of RET: essential-state models are in fact easily extended to account for molecular vibrations in truly non-adiabatic approaches and to account for inhomogeneous broadening effects due to polar solvation. Optically forbidden (dark) states of quadrupolar and octupolar chromophores offer an interesting opportunity to verify the reliability of the dipolar approximation. In striking contrast with the dipolar approximation that strictly forbids RET towards or from dark states, our results demonstrate that dark states can take an active role in RET with interaction energies that, depending on the relative orientation of the chromophores, can be even larger than those relevant to allowed states. Essential-state models, whose predictions are quantitatively confirmed by TDDFT results, allow us to relate RET interaction energies towards allowed and dark states to the supramolecular symmetry of the RET-pair, offering reliable design strategies to optimize RET-interactions. © the Owner Societies 2011.


Sissa C.,University of Parma | Terenziani F.,University of Parma | Painelli A.,University of Parma | Manna A.K.,Theoretical science Unit | Pati S.K.,Theoretical science Unit
Chemical Physics | Year: 2012

Resonance energy transfer (RET) is investigated in a pair of polar charge-transfer (CT) chromophores, adopting essential-state models and time dependent density functional theory (TDDFT) calculations. Essential-state models describe in an efficient way linear and nonlinear optical properties of CT dyes, and prove very useful to rationalize the effects of electrostatic interchromophoric interactions on optical properties of multichromophoric systems. In this paper we adopt the same strategy developed for multichromophoric systems to investigate interchromophoric interactions responsible for RET. In the late forties, Th. Förster proposed a powerful method, based on the dipolar approximation, that directly relates the rate of the RET process to experimental accessible quantities. Here we discuss the applicability of the dipolar approximation for RET between CT dyes. The results obtained with essential-state models are confirmed by TDDFT calculations. Copyright © 2012 Published by Elsevier B.V. All rights reserved.


Maiti M.,Theoretical science Unit
The European physical journal. E, Soft matter | Year: 2013

The implementation of a method for the exact evaluation of the volume and surface area of cavities and free volumes in polydisperse sphere packings is described. The generalization of an algorithm for Voronoi tessellation by Tanemura et al. is presented, employing the radical plane construction, as a part of the method. We employ this method to calculate the equation of state for monodisperse and polydisperse hard-sphere fluids, crystals, and for the metastable amorphous branch up to random close packing or jamming densities. We compute the distribution of free volumes, and compare with previous results employing a heuristic definition of free volume. We show the efficacy of the method for analyzing protein structure, by computing various quantities such as the distribution of sizes of buried cavities and pockets, the scaling of solvent accessible area to the corresponding occupied volume, the composition of residues lining cavities, etc.


Chakraborty S.,Theoretical science Unit | Das S.K.,Theoretical science Unit
European Physical Journal B | Year: 2015

We study the dynamics of ordering in ferromagnets via Monte Carlo simulations of theIsing model, employing the Glauber spin-flip mechanism, in space dimensionsd = 2 and3, on square and simplecubic lattices. Results for the persistence probability and the domain growth arediscussed for quenches to various temperatures (Tf) below the criticalone (Tc), from differentinitial temperatures Ti ≥Tc. In long timelimit, for Ti>Tc,the persistence probability exhibits power-law decay with exponents θ ≃ 0.22 and≃ 0.18 in d = 2 and 3, respectively. For finite Ti, the early timebehavior is a different power-law whose life-time diverges and exponent decreases asTi →Tc. The two steps areconnected via power-law as a function of domain length and the crossover to the secondstep occurs when this characteristic length exceeds the equilibrium correlation length atT =Ti. Ti =Tc is expected toprovide a new universality class for which we obtain θ ≡θc ≃ 0.035 ind = 2 and≃0.105 in d = 3. The time dependenceof the average domain size ℓ, however, is observed to be rather insensitive tothe choice of Ti. © 2015, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

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