arXiv subjects

Sanjay Kumar

Publications and source records attributed to Sanjay Kumar.

120 records · Page 7Linked to original sources

arXiv2007

Statistical mechanics of coil-hairpin transition in a single stranded DNA oligomer

A model of self-avoiding walk with suitable constraints on self-attraction is developed to describe the conformational behavior of a short RNA or a single stranded DNA molecule that forms hairpin structure and calculate the properties associated with coil-hairpin transition by enumerating all possible conformations of a chain of N monomers in two and three dimensions. The first and last five monomers of the chain have been allowed to pair and form the stem of the hairpin structure while the remaining monomers can form a loop. The coil-hairpin transition is found to be first order with large entropy change. While the rate of unzipping of the hairpin stem is found to be independent of the length of the loop and the dimensionality of the space, the rate of closing varies greatly with loop length and dimensionality of the space.

cond-mat.stat-mech↗

arXiv2004

Force induced triple point for interacting polymers

We show the existence of a force induced triple point in an interacting polymer problem that allows two zero-force thermal phase transitions. The phase diagrams for three different models of mutually attracting but self avoiding polymers are presented. One of these models has an intermediate phase and it shows a triple point but not the others. A general phase diagram with multicritical points in an extended parameter space is also discussed.

cond-mat.stat-mech↗

arXiv2004

Force-induced desorption of a linear polymer chain adsorbed on an attractive surface

We consider a model of self-avoiding walk on a lattice with on-site repulsion and an attraction for every vertex of the walk visited on the surface to study force-induced desorption of a linear polymer chain adsorbed on an attractive surface and use the exact enumeration technique for analyzing how the critical force for desorption $f_c(T)$ depends on the temperature. The curve $f_c(T)$ gives the boundary separating the adsorbed phase from the desorbed phase. Our results show that in two dimensions where surface is a line the force $f_c(T)$ increases monotonically as temperature is lowered and becomes almost constant at very low temperatures. In case of three-dimensions we, however, find re-entrance, i. e. $f_c(T)$ goes through a maximum as temperature is lowered. The behaviour of the polymer chain at different values of temperature and force is examined by calculating the probability distribution of the height from the surface of the vertex at which external force is applied.

cond-mat.stat-mech↗

arXiv2004

Effect of confinement on coil-globule transition

The equilibrium thermodynamic properties of a linear polymer chain confined to a space between two impenetrable walls (lines) at a distance $D$ under various solvent conditions have been studied using series analysis and exact enumeration technique. We have calculated the end to end distance of polymer chain, which shows a non-monotonic behaviour with inter wall ^M separation $D$. The density distribution profile shows a maxima at a particular value of ${(D=)}D^*$. Around this $D^*$, our results show that the collapse^M transition occurs at higher temperature as compared to its bulk value of 2d and 3d. The variation of $θ-$ temperature with $D$ shows a re-entrance behaviour. We also calculate the force of compression exerted by the walls (lines) on the polymer.

cond-mat.stat-mech↗

arXiv2002

A simple model for the relative stabilities of DNA hairpin structure

A model of self-avoiding walk with suitable constraints on self-attraction is developed to describe the conformational behaviour of a single stranded short DNA molecule that form hairpin structure. Using exact enumeration method we calculate the properties associated with coil-hairpin transition. Our results are in qualitative agreement with the experiment.

cond-mat.soft↗

arXiv2001

Collective Dynamics of One-Dimensional Charge Density Waves

The effect of disorder on the static and dynamic behaviour of one-dimensional charge density waves at low temperatures is studied by analytical and numerical approaches. In the low temperature region the spatial behaviour of the phase-phase correlation function is dominated by disorder but the roughness exponent remains the same as in the pure case. Contrary to high dimensional systems the dependence of the creep velocity on the electric field is described by an analytic function.

cond-mat.stat-mech↗

arXiv2001

The Adsorption and Collapse Transitions in a Linear Polymer Chain near an Attractive Wall

We deduce the qualitative phase diagram of a long flexible neutral polymer chain immersed in a poor solvent near an attracting surface using phenomenological arguments. The actual positions of the phase boundaries are estimated numerically from series expansion up to 19 sites of a self-attracting self avoiding walk in three dimensions. In two dimensions, we calculate analytically phase boundaries in some cases for a partially directed model. Both the numerical as well as analytical results corroborate the proposed qualitative phase diagram.

cond-mat.stat-mech↗

arXiv2000

Interpenetration of two chains different in sizes: Some Exact Results

A model of two interacting polymer chains has been proposed to study the effect of penetration of one chain in to the other. We show that small chain penetrates more in comparison to the long chain. We also find a condition in which both chains cannot grow on their own (or polymerize) but can grow (polymerize) in zipped form.

cond-mat.stat-mech↗

arXiv2000

Adsorption and collapse transitions of a linear polymer chain interacting with a surface adsorbed polymer chain

We study the problem of adsorption and collapse transition of a linear polymer chain situated in a fractal container represented by a 4-simplex lattice and interacting with a surface adsorbed linear polymer chain. The adsorbed chain monomers act as pinning sites for the polymer chain. This problem has been solved exactly using real space renormalization group transformation. The resulting phase diagram and critical exponents are given.

cond-mat.stat-mech↗

arXiv2000

Adsorbed states of a long - flexible polymer chain

A phase diagram for a surface-interacting long flexible polymer chain in a two-dimensional poor solvent where the possibility of collapse exists is determined using exact enumeration method. A model of a self-attracting self avoiding walk (SASAW) on a square lattice was considered and up to 28 steps in series were evaluated. A new adsorbed state having the conformation of a surface attached globule is found. Four phases (i) desorbed expanded, (ii) desorbed collapsed, (iii) adsorbed expanded and (iv) surface attached globule are found to meet at a point on the adsorption line.

cond-mat.stat-mech↗

arXiv2000

Multifractal Behaviour of n-Simplex Lattice

We study the asymptotic behaviour of resistance scaling and fluctuation of resistance that give rise to flicker noise in an {\em n}-simplex lattice. We propose a simple method to calculate the resistance scaling and give a closed-form formula to calculate the exponent, $β_L$, associated with resistance scaling, for any n. Using current cumulant method we calculate the exact noise exponent for n-simplex lattices.

cond-mat.stat-mech↗

arXiv1998

Electron attachment to valence-excited CO

The possibility of electron attachment to the valence $^{3}Π$ state of CO is examined using an {\it ab initio} bound-state multireference configuration interaction approach. The resulting resonance has $^{4}Σ^{-}$ symmetry; the higher vibrational levels of this resonance state coincide with, or are nearly coincident with, levels of the parent $a^{3}Π$ state. Collisional relaxation to the lowest vibrational levels in hot plasma situations might yield the possibility of a long-lived CO$^-$ state.

physics.atom-ph↗