arXiv subjects

Moumita Maiti

Publications and source records attributed to Moumita Maiti.

18 recordsLinked to original sources

arXiv2019

Energy landscape description of the clustering transition for active soft spheres

For a system consisting of active soft spheres in three dimensions, we study the transition from a fluid where overlaps between particles can only occur for a short time after a collision to a state where clusters of overlapping particles persist for a long time. In order to determine the properties of the transition, we explore the energy landscape of the system in a similar way as it is done for the determination of the athermal or thermal jamming transition. Note that for zero temperature the competition of particles that attach to existing clusters and particles that detach due to thermal effects does not arise. Therefore, here we do not study such a competition because we consider systems at small or zero energy. Instead, we explore at which packing fractions and what activities cluster formation can occur at all. In case of an athermal system the transition between systems where no clusters develop at all and systems where stable clusters are found is a first order transition for packing fractions below 0.55 while the transition is continuous in case of larger packing fractions. In case of thermal systems the transition is continuous everywhere. While our approach does not deal with the real dynamics of the system, it reveals the nature of the clustering transition and it enables a deeper insight in the consequences of thermal fluctuations and the relation of the clustering transition to jamming. Though Brownian timescales diverge in athermal systems, the activity that we consider can be compared to the active velocity in other simulations if the later is measured in units of the particle size divided by an elastic time scale.

cond-mat.soft↗

arXiv2019

The thermal jamming transition of soft harmonic disks in two dimensions

By exploring the properties of the energy landscape of a bidisperse system of soft harmonic disks in two dimensions we determine the thermal jamming transition. To be specific, we study whether the ground state of the system where the particle do not overlap can be reached within a reasonable time. Starting with random initial configurations, the energy landscape is probed by energy minimization steps as in case of athermal jamming and in addition steps where an energy barrier can be crossed with a small but non-zero probability. For random initial conditions we find that as a function of packing fraction the thermal jamming transition, i.e. the transition from a state where all overlaps can be removed to an effectively non-ergodic state where one cannot get rid of the overlaps, occurs at a packing fraction of $ϕ_G=0.74$, which is smaller than the transition packing fraction of athermal jamming at $ϕ_J=0.842$. Furthermore, we show that the thermal jamming transition is in the universality class of directed percolation and therefore is fundamentally different from the athermal jamming transition.

cond-mat.soft↗

arXiv2019

Temperature dependence of the transition packing fraction of thermal jamming in a harmonic soft sphere system

The glassy dynamics of soft harmonic spheres is often mapped onto the dynamics of hard spheres by considering an effective diameter for the soft particles and therefore an effective packing fraction. While in this approach the thermal fluctuations within valleys of the energy landscape are covered, the crossing of energy barriers from one valley into another usually is neglected. Here we argue - motivated by studies of the glass transition based on explorations of the energy landscape - that the crossing of energy barriers can be attributed by an effective decrease of the glass transition packing fraction with increasing temperature T according to T^{0.2}. Furthermore, we reanalyzing data of soft sphere simulations. Since fitting scaling laws to simulation data always allows for some arbitrariness, we cannot prove based on the simulation data that our idea of a shift of the glass transition packing fraction due to barrier crossings is the only possible way to explain the discrepancies that have been observed previously. However, we show that a possible explanation of the simulation data is given by our approach to characterize the dynamics of soft spheres by both, the previously-considered temperature-dependent effective packing fraction due to the increase of the mean overlap between neighboring particles with stronger thermal fluctuations and the newly introduced increase of the glass transition packing with an increasing probability of barrier crossings.

cond-mat.soft↗

arXiv2018

Ergodicity breaking transition in a glassy soft sphere system at small but non-zero temperatures

While the glass transition at non-zero temperature seems to be hard to access for experimental, theoretical, or simulation studies, jamming at zero temperature has been explored in great detail. It is a widely discussed question whether this athermal jamming transition is related to the glass transition. Motivated by the exploration of the energy landscape that has been successfully used to describe athermal jamming, we introduce a new method to determine whether the configuration space of a soft sphere system can be explored within a reasonable timescale or not, i.e., whether the system is ergodic or effectively non-ergodic. While in case of athermal jamming for a given random starting configuration only the local energy minimum is determined, we allow the thermally excited crossing of energy barriers. Interestingly, we observe that a transition exists where the system becomes effectively non-ergodic if the density is increased. In the limit of small but non-zero temperatures the density where the ergodicity breaking transition occurs approaches a value that is independent of temperature and below the transition density of athermal jamming. This confirms recent computer simulation studies where athermal jamming occurs deep inside the glass phase. In addition, with our method we determined the critical behavior of the ergodicity breaking transition and show that it is in the universality class of directed percolation. Therefore, our approach not only makes the transition from an ergodic to an effectively non-ergodic systems easily accessible and helps to reveal its universality class but also shows that it is fundamentally different from athermal jamming.

cond-mat.soft↗

arXiv2017

Incomplete fusion analysis of $^{7}$Li-induced reaction on $^{93}$Nb within 3-6.5 MeV/nucleon

It is understood from the recent experimental studies that prompt/resonant breakup, and transfer followed by breakup in the weakly bound $^{6,7}$Li-induced reactions play a significant role in the complete-incomplete fusion (CF-ICF), suppression/enhancement in the fusion cross section around the Coulomb barrier. Investigation of ICF over CF was done by measuring cross sections of the populated residues in the $^{7}$Li-induced reaction on $^{\text{nat}}$Nb target within 3--6.5 MeV/nucleon energy region. Measured excitation functions of different channels were compared with different equilibrium (EQ) and preequilibrium (PEQ) models. The enhancement in cross sections in the proton ($\sim$ 20--30 MeV) and $α$-emitting channels, which may be ascribed to ICF, was observed in the measured energy range when compared to the Hauser-Feshbach and exciton model calculations using EMPIRE, which satisfactorily reproduces the neutron channels, compared to the Weisskopf-Ewing model and Hybrid Monte Carlo calculations. The increment of the incomplete fusion fraction was observed with rising projectile energy.

nucl-ex↗

arXiv2017

$^{\text{7}}$Li-induced reaction on $^{\text{nat}}$Mo: A study of complete versus incomplete fusion

Several investigations on the complete-incomplete fusion (CF-ICF) dynamics of $α$-cluster well-bound nuclei has been contemplated above the Coulomb barrier ($\sim$ 4-7 MeV/nucleon) in recent years. It is therefore expected to observe significant ICF over CF in the reactions induced by a weakly bound $α$-cluster nucleus at slightly above the barrier. In order to investigate CF-ICF in the loosely bound system, $^{\text{7}}$Li beam was bombarded on the $^{\text{nat}}$Mo foils, separated by the aluminium (Al) catcher foils alternatively, within $\sim$ 3-6.5 MeV/nucleon. Evaporation residues produced in each foil were identified by the $γ$-ray spectrometry. Measured cross section data of the residues were compared with the theoretical model calculations based on the equilibrium (EQ) and preequilibrium (PEQ) reaction mechanisms. The experimental cross section of $^{\text{101m,100,99m,97}}$Rh, $^{\text{97,95}}$Ru, $^{\text{99m,96,95,94,93m+g}}$Tc and $^{\text{93m}}$Mo residues measured at various projectile energies were satisfactorily reproduced by the simplified coupled channel approach in comparison to single barrier penetration model calculation. Significant cross section enhancement in the $α$-emitting channels was observed compared to EQ and PEQ model calculations throughout observed energy region. The ICF process over CF was analyzed by comparing with EMPIRE. The increment of the incomplete fusion fraction was observed with increasing projectile energies. Theoretical model calculations reveal that compound reaction mechanism is the major contributor to the production of residues. Theoretical evaluations substantiate the contribution of ICF over the CF in $α$-emitting channels.

nucl-ex↗

arXiv2017

Measurement of cross section of the residues from the $^{11}$B-induced reaction on $^{89}$Y and $^{93}$Nb: Production of $^{97}$Ru and $^{101m}$Rh

The heavy-ion induced reactions on intermediate mass targets are complex in nature, even at the low energies. To understand those nuclear reaction phenomena in detail, more experimental studies are required in a wide range of energy. Heavy-ion reactions were investigated by measuring production cross sections of the residues produced in the $^{11}$B-induced reactions on $^{89}$Y and $^{93}$Nb at low energies, near and above the barrier, and to check the effectiveness of the different nuclear models to explain them. Further, aim is also to optimize the production parameters of neutron deficient medically relevant $^{97}$Ru and $^{101m}$Rh radioisotopes produced in those reactions, respectively. The $^{11}$B-beam was allowed to impinge on $^{89}$Y and $^{93}$Nb foils supported by an aluminum catcher foil, arranged in a stack, in 27.5-58.7 and 30.6-62.3 MeV energy range, respectively. The $γ$-spectrometry was carried out after EOB to measure the activity of the radionuclides produced in each foil and cross sections were calculated. Measured cross-sectional data were analyzed in terms of compound and precompound model calculations. The measured cross sections of the residues showed good agreement with the model calculations based on the Hauser-Feshbach formulation and exciton model. A substantial preequilibrium contribution was noticed in the 3$n$ reaction channel in both the reactions. Theoretical estimations confirmed that major production yields are mostly contributed by the compound reaction process. Preequilibrium emissions contributed at the high energy tail of the 3$n$ channel for both the reactions. Moreover, an indirect signature of a direct reaction influence was also observed in the $^{90m}$Y production.

nucl-ex↗

arXiv2017

Experimental probe on the production of $^{97}$Ru from $^{7}$Li+$^{93}$Nb reaction: A study on the precompound emissions

Background: Interaction of weakly bound heavy-ions with an intermediate or heavy target is not yet understood completely due to the scarcity of experimental data. In order to develop a clear understanding of breakup fusion or preequilibrium emission even at low energy range, 3--10 MeV/nucleon, more experimental investigations are necessary. Purpose: Study of reaction mechanisms involved in the weakly bound heavy-ion induced reaction, $^7$Li + $^{93}$Nb, at low energies by measuring the production cross sections of the residual radionuclides. Method: Natural niobium ($^{93}$Nb) foil, backed by aluminium (Al) catcher, arranged in a stack was bombarded by $^{7}$Li ions of 20-45 MeV energy. Activity of the residues produced in each $^{93}$Nb target was measured by off-line $γ$-ray spectrometry after the end of bombardment (EOB) and cross sections were calculated. Experimental cross sections were compared with those computed using compound and precompound models. Results: In general, measured excitation functions of all residues produced in $^{7}$Li + $^{93}$Nb reaction showed good agreement with the model calculations based on Hauser-Feshbach formalism and exciton model for compound and precompound processes, respectively. Significant preequilibrium emission of neutrons was observed at the relatively high energy tail of the excitation function of $^{97}$Ru. Conclusions: Preequilibrium process played an important role for the enhancement of cross-section in $xn$ reaction channel over the compound reaction mechanism at higher energies for $^{7}$Li + $^{93}$Nb reaction. Additionally, indirect evidence of incomplete or breakup fusion was also perceived.

nucl-ex↗

arXiv2017

Cage properties and its implication to the existence of glass transition in hard sphere systems

In deep supercooled liquids, particles get trapped in transient cages made up of neighbouring particles. Here we define a cage from a geometrical quantity, free volume, such that the free volume of a particle is the cage volume. First we show that the relationship between the average cage volume and the structural relaxation time questions the existence of glass transition in hard sphere systems. Our observation suggests that the cage volume is zero at the transition. Further we show that cage rearrangements are strongly coupled to the single particle squared displacements. Additionally a cage can rearrange by losing its neighbours with almost no change in particle displacements. The picture presented here also supports the complex scenarios of relaxation, dynamic heterogeneity and cooperative rearrangement.

cond-mat.soft↗

arXiv2016

Friction-induced Shear thickening: a microscopic perspective

We develop a microscopic picture of shear thickening in dense suspensions which emphasizes the role of frictional forces, coupling rotational and translational degrees of freedom. Simulations with contact forces and viscous drag only, reveal pronounced shear thickening with a simultaneous increase in contact number and energy dissipation by frictional forces. At high densities, when the translational motion is severely constrained, we observe liquid-like gear-states with pronounced relative rotations of the particles coexisting with solid-like regions which rotate as a whole. The latter are stabilised by frustrated loops which become more numerous and persistent with increasing pressure, giving rise to an increasing lengthscale of this mosaique-like structure and a corresponding increase in viscosity.

cond-mat.soft↗

arXiv2015

Free volume under shear

Using an athermal quasistatic simulation protocol, we study the distribution of free volumes in sheared hard-particle packings close to, but below, the random-close packing threshold. We show that under shear, and independent of volume fraction, the free volumes develop features similar to close-packed systems -- particles self-organize in a manner as to mimick the isotropically jammed state. We compare athermally sheared packings with thermalized packings and show that thermalization leads to an erasure of these structural features. The temporal evolution, in particular the opening-up and the closing of free-volume patches is associated with the single-particle dynamics, showing a crossover from ballistic to diffusive behavior.

cond-mat.soft↗

arXiv2014

Free volume distribution of nearly jammed hard sphere packings

We calculate the free volume distributions of nearly jammed packings of monodisperse and bidisperse hard sphere configurations. These distributions differ qualitatively from those of the fluid, displaying a power law tail at large free volumes, which constitutes a distinct signature of nearly jammed configurations, persisting for moderate degrees of decompression. We reproduce and explain the observed distribution by considering the pair correlation function within the first coordination shell for jammed hard sphere configurations. We analyze features of the equation of state near jamming, and discuss the significance of observed asphericities of the free volumes to the equation of state.

cond-mat.soft↗

arXiv2014

Rheology near jamming - the influence of lubrication forces

We study, by computer simulations, the role of different dissipation forces on the rheological properties of highly-dense particle-laden flows. In particular, we are interested in the close-packing limit (jamming) and the question if "universal" observables can be identified that do not depend on the details of the dissipation model. To this end, we define a simplified lubrication force and systematically vary the range $h_c$ of this interaction. For fixed $h_c$ a cross-over is seen from a Newtonian flow regime at small strain rates to inertia-dominated flow at larger strain rates. The same cross-over is observed as a function of the lubrication range $h_c$. At the same time, but only at high densities close to jamming, particle velocity as well as local density distributions are unaffected by changes in the lubrication range -- they are candidates for "universal" behavior. At densities away from jamming, this universality is lost: short-range lubrication forces lead to pronounced particle clustering, while longer-ranged lubrication does not. These findings highlight the importance of "geometric" packing constraints for particle motion -- independent of the specific dissipation model. With the free volume vanishing at random-close packing, particle motion is more and more constrained by the ever smaller amount of free space. On the other side, macroscopic rheological observables, as well as higher-order correlation functions retain the variability of the underlying dissipation model.

cond-mat.soft↗

arXiv2011

New measurement of cross section of evaporation residues from $^{\textrm{nat}}$Pr+$^{12}$C reaction: A comparative study on the production of $^{149}$Tb

Production cross sections of evaporation residues, $^{149}$Tb, $^{150}$Tb, $^{151}$Tb and $^{149}$Gd, have been measured using the stacked foil technique followed by off-line $γ$-spectrometry in $^{12}$C induced reactions on naturally abundant mononuclidic praseodymium target in the 44-79 MeV incident energy range. Measured data have been interpreted comparing with previous measurements and theoretical prediction of nuclear reaction model code \textsc{PACE4}. About 5% and 14% of the theoretical cross sections have been measured for $^{149}$Tb and $^{150}$Tb, respectively. The new cross sections of $^{149}$Tb complement those measured earlier by $α$-spectrometry. Cross sections of $^{151}$Tb are comparable to the theory. Cumulative cross section of $^{149}$Gd sheds light on the nuclear reaction mechanism. In addition, a discussion has been made to show the feasibility of producing $^{149}$Tb in $p$- and $α$-induced reactions on gadolinium isotopes.

nucl-ex↗

arXiv2011

Production cross section of At radionuclides from $^{7}$Li+$^{\textrm{nat}}$Pb and $^{9}$Be+$^{\textrm{nat}}$Tl reactions

Earlier we reported theoretical studies on the probable production of astatine radionuclides from $^{6,7}$Li and $^{9}$Be-induced reactions on natural lead and thalliun targets, respectively. For the first time, in this report, production of astatine radionuclides has been investigated experimentally with two heavy ion induced reactions: $^{9}$Be+$^{\textrm{nat}}$Tl and $^{7}$Li+$^{\textrm{nat}}$Pb. Formation cross sections of the evaporation residues, $^{207,208,209,210}$At, produced in (HI, xn) channel, have been measured by the stacked-foil technique followed by the off-line $γ$-spectrometry at the low incident energies ($<$50 MeV). Measured excitation functions have been explained in terms of compound nuclear reaction mechanism using Weisskopf-Ewing and Hauser-Feshbach model. Absolute cross section values are lower than the respective theoretical predictions.

nucl-ex↗

arXiv2009

Competition between folding and aggregation in a model for protein solutions

We study the thermodynamic and kinetic consequences of the competition between single-protein folding and protein-protein aggregation using a phenomenological model, in which the proteins can be in the unfolded (U), misfolded (M) or folded (F) states. The phase diagram shows the coexistence between a phase with aggregates of misfolded proteins and a phase of isolated proteins (U or F) in solution. The spinodal at low protein concentrations shows non-monotonic behavior with temperature, with implications for the stability of solutions of folded proteins at low temperatures. We follow the dynamics upon "quenching" from the U-phase (cooling) or the F-phase (heating) to the metastable or unstable part of the phase diagram that results in aggregation. We describe how interesting consequences to the distribution of aggregate size, and growth kinetics arise from the competition between folding and aggregation.

cond-mat.soft↗

arXiv2009

Production of $^{93,94,95,96}$Tc through $^{7}$Li+$^{nat}$Zr and $^{9}$Be+$^{nat}$Y reactions: Measurement of excitation functions

For the first time two separate production routes of Tc radionuclides have been studied bombarding $^{7}$Li on $^{nat}$Zr and $^{9}$Be on $^{89}$Y. Excitation functions of the evaporation residues produced in those reactions have been measured using stacked-foil technique followed by the $γ$-spectrometric studies in the energy range 37-45 MeV and 30-48 MeV respectively. Measured excitation functions have been compared with those calculated using the nuclear reaction model codes PACE-II and ALICE91. Experimental results show good agreement with the theoretical predictions. Compound nuclear reaction is the key mechanism in producing evaporation residues.

nucl-ex↗

arXiv2008

An attempt to explore the production routes of Astatine radionuclides: Theoretical approach

In order to fulfil the recent thrust of Astatine radionuclides in the field of nuclear medicine various production routes have been explored in the present work. The possible production routes of $^{209-211}$At comprise both light and heavy ion induced reactions at the bombarding energy range starting from threshold to maximum 100 MeV energy. For this purpose, we have used the nuclear reaction model codes TALYS, ALICE91 and PACE-II. Excitation functions of those radionuclides, produced through various production routes, have been calculated using nuclear reaction model codes and compared with the available measured data. Contribution of various reaction mechanisms, like, direct, preequilibrium and equilibrium reactions, to the total reaction cross section has been studied using the codes. Result shows that equilibrium reaction mechanism dominates in all cases over other reaction mechanisms.

nucl-th↗