arXiv subjects

S. Bhattacharya

Publications and source records attributed to S. Bhattacharya.

At least 163 records · Page 9Linked to original sources

arXiv2011

Systematics of Anti magnetic rotation in even-even Cd isotopes

The lifetimes for the high spin levels of the yrast band of $^{110}$Cd has been measured. The estimated B(E2) values decrease with increase in angular momentum. This is the characteristic of Anti magnetic rotation as reported in $^{106,108}$Cd. However, alignment behavior of $^{110}$Cd is completely different from its even-even neighbors. A model based on classical particle plus rotor has been used to explore the underlying systematics and develop a self consistent picture for the observed behavior of these isotopes.

nucl-ex↗

arXiv2011

Strain induced band gap deformation of H/F passivated graphene and h-BN sheet

Strain induced band gap deformations of hydrogenated/fluorinated graphene and hexagonal BN sheet have been investigated using first principles density functional calculations. Within harmonic approximation, the deformation is found to be higher for hydrogenated systems than for the fluorinated systems. Interestingly, our calculated band gap deformation for hydrogenated/fluorinated graphene and BN sheets are positive, while those for pristine graphene and BN sheet are found to be negative. This is due to the strong overlap between nearest neighbor π orbitals in the pristine sheets, that is absent in the passivated systems. We also estimate the intrinsic strength of these materials under harmonic uniaxial strain, and find that the in-plane stiffness of fluorinated and hydrogenated graphene are close, but larger in magnitude as compared to those of fluorinated and hydrogenated BN sheet.

cond-mat.mtrl-sci↗

arXiv2010

Monte-Carlo phase diagram of a Hubbard-Peierls model in the search for spin crossover transition in $π$-conjugated polymers

We present a Monte Carlo study of the finite temperature properties of an extended Hubbard-Peierls model describing one dimensional $π$-conjugated polymers. The model incorporates electron-phonon and hyperfine interaction and it is solved at the mean field level for half filling. In particular we explore the model as a function of the strength of electron-electron and electron-phonon interactions. At low temperature the system presents a diamagnetic to antiferromagnetic transition as the electron-electron interaction strength increases. At the same time by increasing the electron-phonon coupling there is a transition from a homogeneous to a Peierls dimerized geometry. As expected such a Peierls dimerized phase disappears at finite temperature as a result of thermal vibrations. More intriguing is the interplay between the electron-phonon and the electron-electron interactions at finite temperature. In particular we demonstrate that for a certain region of the parameter space there is a spin-crossover, where the system transits from a low-spin to a high-spin state as the temperature increases. In close analogy to standard spin-crossover in divalent magnetic molecules such a transition is entropy driven. Finally we discuss the role played by the hyperfine interaction over the phase diagram.

cond-mat.mes-hall↗

arXiv2010

The third conformer of graphane: A first principles DFT based study

We propose, on the basis of our first principles density functional based calculations, a new isomer of graphane, in which the C-H bonds of a hexagon alternate in 3-up, 3-down fashion on either side of the sheet. This 2D puckered structure called 'stirrup' has got a comparable stability with the previously discovered chair and boat conformers of graphane. The physico-chemical properties of this third conformer are found to be similar to the other two conformers of graphane with an insulating direct band gap of 3.1 eV at the Γ point. Any other alternative hydrogenation of the graphene sheet disrupts its symmetric puckered geometry and turns out to be energetically less favorable.

cond-mat.mtrl-sci↗

arXiv2010

Electronic and Structural Analysis of a Stable Hydrogenated BN sheet (BHNH): A First Principles Based Approach

From first-principles density functional calculations, we study the structural and electronic properties of a stable hydrogenated BN sheet, having formula unit BHNH. In the optimized BHNH structure, the H atoms stabilize on the B and N sites, alternating themselves on both sides of the BN-plane in specific periodic manner, giving rise to different BHNH conformers, viz. chair, boat and stirrup. The chair and boat conformers resemble in structure to those of graphane (CH). We propose a new conformer, called 'stirrup' conformer, that turns out to be the most stable, albeit marginally with respect to the boat conformer. All these BHNH conformers are insulator, with band gap varying between ~ 3.0 eV to 4.5 eV.

cond-mat.mtrl-sci↗

arXiv2010

Extreme nuclear shapes examined via Giant Dipole Resonance lineshapes in hot light mass system

The influence of alpha clustering on nuclear reaction dynamics is investigated using the giant dipole resonance (GDR) lineshape studies in the reactions 20Ne (Elab=145,160 MeV) + 12C and 20Ne (Elab=160 MeV) + 27Al, populating 32S and 47V, respectively. The GDR lineshapes from the two systems are remarkably different from each other. Whereas, the non alpha-like 47V undergoes Jacobi shape transition and matches exceptionally well with the theoretical GDR lineshape estimated under the framework rotating liquid drop model (RLDM) and thermal shape fluctuation model (TSFM) signifying shape equilibration, for alpha cluster 32S an extended prolate kind of shape is observed. This unusual deformation, seen directly via -decay for the first time, is predicted to be due to the formation of orbiting di-nuclear configuration or molecular structure of 16O+ 16O in 32S superdeformed band.

nucl-ex↗

arXiv2010

Band crossing in Shears band of $^{108}$Cd

The level lifetimes have been measured for a Shears band of $^{108}$Cd which exhibits bandcrossing. The observed level energies and B(M1) rates have been successfully described by a semi-classical geometric model based on shear mechanism. In this geometric model, the bandcrossing in Shears band has been described as the reopening of the angle between the blades of a shear.

nucl-ex↗

arXiv2010

Optical Redshift and Richness Estimates for Galaxy Clusters Selected with the Sunyaev-Zel'dovich Effect from 2008 South Pole Telescope Observations

We present redshifts and optical richness properties of 21 galaxy clusters uniformly selected by their Sunyaev-Zel'dovich signature. These clusters, plus an additional, unconfirmed candidate, were detected in a 178 square-degree area surveyed by the South Pole Telescope in 2008. Using griz imaging from the Blanco Cosmology Survey and from pointed Magellan telescope observations, as well as spectroscopy using Magellan facilities, we confirm the existence of clustered red-sequence galaxies, report red-sequence photometric redshifts, present spectroscopic redshifts for a subsample, and derive R_200 radii and M_200 masses from optical richness. The clusters span redshifts from 0.15 to greater than 1, with a median redshift of 0.74; three clusters are estimated to be at z > 1. Redshifts inferred from mean red-sequence colors exhibit 2% RMS scatter in sigma_z/(1+z) with respect to the spectroscopic subsample for z < 1. We show that M_200 cluster masses derived from optical richness correlate with masses derived from South Pole Telescope data and agree with previously derived scaling relations to within the uncertainties. Optical and infrared imaging is an efficient means of cluster identification and redshift estimation in large Sunyaev-Zel'dovich surveys, and exploiting the same data for richness measurements, as we have done, will be useful for constraining cluster masses and radii for large samples in cosmological analysis.

astro-ph.CO↗

arXiv2009

Imaging ferroelectric hysteresis and domain wall pinning

Simultaneous imaging of the piezoresponse phase, amplitude and bare surface topography of displacive ferroelectric thin films by scanning probe microscopy directly shows the nature of domain wall pinning and its relation to morphological disorder. Strong and stable pinning of walls occurs at grain boundaries while weak and unstable pinning occurs within the grains. The results show that polarization reversal consists of both nucleation of domains and spreading of domain walls, the latter being the dominant process in multi-grain regions. A characteristic single-grain switching in faceted grains is observed as the motion of a single domain wall parallel to one of the facets.

cond-mat.mes-hall↗

arXiv2009

Observation of isomeric decays and the high spin states in doubly-odd 208Fr

Neutron deficient isotopes of Francium (Z=87, N=121-123) as excited nuclei were produced in the fusion-evaporation reaction: 197Au(16O,xn)[213-x]Fr at 100 MeV. The gamma-rays from the residues were observed through the high sensitivity Germanium Clover detector array INGA. The decay of the high spin states and the isomeric states of the doubly-odd 208Fr nuclei, identified from the known sequence of ground state transitions, were observed. The half lives of the 194(2) keV isomeric transition, known from earlier observations, was measured to be 233(18) ns. A second isomeric transition at 383(2) keV and half life of 33(7) ns was also found. The measured half lives were compared with the corresponding single particle estimates, based on a the level scheme obtained from the experiment.

nucl-ex↗

arXiv2009

Polymer desorption under pulling: first order phase transition without phase coexistence

We show that when a self-avoiding polymer chain is pulled off a sticky surface by force applied to the end segment, it undergoes a first-order thermodynamic phase transition albeit without phase coexistence. This unusual feature is demonstrated analytically by means of a Grand Canonical Ensemble (GCE) description of adsorbed macromolecules as well as by Monte Carlo simulations of an off-lattice bead-spring model of a polymer chain. Theoretical treatment and computer experiment can be carried out both in the constant-force f statistical ensemble and in the constant-height h ensemble. We find that the force-assisted desorption undergoes a first-order dichotomic phase transition whereby phase coexistence between adsorbed and desorbed states does not exist. In the f-ensemble the order parameter (the fraction of chain contacts with the surface) is characterized by huge fluctuations when the pulling force attains a critical value f_D. In the h-ensemble, in contrast, fluctuations are always finite at the critical height h_D. The derived analytical expressions for the probability distributions of the basic structural units of an adsorbed polymer, such as loops, trains and tails, in terms of the adhesive potential and f or h, provide a full description of the polymer structure and behavior upon force-assisted detachment. In addition, one finds that the hitherto controversial value of the universal critical adsorption exponent ϕdepends essentially on the extent of interaction between the loops adsorbed chain so that ϕmay vary within the limits 0.39 < ϕ< 0.59.

cond-mat.soft↗

arXiv2009

Dielectric Anomalies in a New Manganocuprate, Gd3Ba2Mn2Cu2O12

Dielectric response has been studied for a new manganocuprate, Gd3Ba2Mn2Cu2O12 (Gd3222) as a function of temperature (100 - 300 K) and frequency (75 kHz to 1 MHz). The dielectric constant (e) exhibits a two step increase (two peaks) in e(T) with increasing temperature from 100 to 300 K. The first peak is seen around 150 K and the second one around 210 K (both for 75 kHz). Increasing frequency shifts both the peaks to higher temperature side. The behavior of dielectric constant (e) and dielectric loss (tan_delta) matches with glassy behavior observed in many dipolar molecules.

cond-mat.mtrl-sci↗

arXiv2009

Polymer Chain Adsorption on a Solid Surface: Scaling Arguments and Computer Simulations

We examine the phase transition of polymer adsorption as well as the underlying kinetics of polymer binding from dilute solutions on a structureless solid surface. The emphasis is put on the properties of regular multiblock copolymers, characterized by block size M and total length N as well as on random copolymers with quenched composition p of sticky and neutral segments. The macromolecules are modeled as coarse-grained bead-spring chains subject to a short-ranged surface adhesive potential. Phase diagrams, showing the variation of the critical threshold for single chain adsorption in terms of M and p are derived from scaling considerations in agreement with results from computer experiment. Using both scaling analysis and numerical data from solving a system of coupled Master equations, we demonstrate that the phase behavior at criticality, and the adsorption kinetics may be adequately predicted and understood, in agreement with the results of extensive Monte Carlo simulations. Derived analytic expressions for the mean fraction of adsorbed segments as well as for Probability Distribution Functions of the various structural building blocks (i.e., trains, loops, tails) at time t during the chain attachment process are in good agreement with our numeric experiments and provide insight into the mechanism of polymer adsorption.

cond-mat.soft↗

arXiv2009

Sticking dynamics of deformable colloids

The sticking of a soft polystyrene colloidal particle to a planar glass plate was studied by a microrheological technique using an optical tweezer to trap the particle and a piezoelectric-stage to position the plate and to sinusoidally drive it in-planeThe sticking process was well-described by a crossover of the mechanical coupling between the plate and the particle from being dominantly viscous to becoming dominantly elastic. By varying three parameters in the experiment, namely, the strength of the tweezer potential, the strength of the particle-plate interaction and the strength of the oscillatory drive, three different regimes of dynamics - stuck, non-stuck and ageing were observed, corresponding to the coupling that is elastic, viscous and viscoelastic, respectively. The observations of [1] can be illustrated better by a parametric Nyquist plot of the real and imaginary parts of the effective rigidity modulus of the coupling medium. Upon contact between the particle and the plate, these parameters change abruptly from their bulk values, regardless of whether the particle is stuck or non-stuck in the terminal steady state. In the ageing regime, where sticking evolves gradually in time, the system shows a parametric plot significantly different from what would be expected from a single-relaxation process. A tendency of clustering of the data in the Nyquist plot provides an interesting contrast between truly many-body systems and one with a few degrees of freedom.

cond-mat.soft↗

arXiv2009

Pulling an adsorbed polymer chain off a solid surface

The thermally assisted detachment of a self-avoiding polymer chain from an adhesive surface by an external force applied to one of the chain ends is investigated. We perform our study in the "fixed height" statistical ensemble where one measures the fluctuating force, exerted by the chain on the last monomer when a chain end is kept fixed at height $h$ over the solid plane at different adsorption strength $ε$. The phase diagram in the $h - ε$ plane is calculated both analytically and by Monte Carlo simulations. We demonstrate that in the vicinity of the polymer desorption transition a number of properties like fluctuations and probability distribution of various quantities behave differently, if $h$ rather than $f$ is used as an independent control parameter.

cond-mat.soft↗

arXiv2009

Performances of linseed oil-free bakelite RPC prototypes with cosmic ray muons

A comparative study has been performed on Resistive Plate Chambers (RPC) made of two different grades of bakelite paper laminates, produced and commercially available in India. The chambers, operated in the streamer mode using argon, tetrafluroethane and isobutane in 34:59:7 mixing ratio, are tested for the efficiency and the stability with cosmic rays. A particular grade of bakelite (P-120, NEMA LI-1989 Grade XXX), used for high voltage insulation in humid conditions, was found to give satisfactory performance with stable efficiency of > 96% continuously for more than 130 days. A thin coating of silicone fluid on the inner surfaces of the bakelite RPC is found to be necessary for operation of the detector.

physics.ins-det↗

arXiv2009

Development of linseed oil-free bakelite resistive plate chambers

In this paper we would like to present a few characteristics of the Resistive Plate Chambers (RPC) made of a particular grade of bakelite paper laminates (P-120, NEMA LI-1989 Grade XXX), produced and commercially available in India. This particular grade is used for high voltage insulation in humid conditions. The chambers are tested with cosmic rays in the streamer mode using argon, tetrafluroethane and isobutane in 34:59:7 mixing ratio. In the first set of detectors made with such grade, a thin coating of silicone fluid on the inner surfaces of the bakelite was found to be necessary for operation of the detector. Those silicone coated RPCs were found to give satisfactory performance with stable efficiency of >90% continuously for a long period as reported earlier. Results of the crosstalk measurement of these silicone coated RPC will be presented in this paper. Very recently RPCs made with the same grade of bakelite but having better surface finish, are found to give equivalent performance even without any coating inside. Preliminary results of this type of RPCs are also being presented.

physics.ins-det↗

arXiv2009

Study of timing properties of single gap high-resistive bakelite RPC

The time resolution for several single gap (2 mm) prototype Resistive Plate Chambers (RPC) made of high resistive (bulk resistivity ~ 10^10 - 10^12 ohm cm), 2 mm thick matt finished bakelite paper laminates with silicone coating on the inner surfaces, has been measured. The time resolution for all the modules has been found to be ~ 2 ns at the plateau region.

physics.ins-det↗