arXiv subjects

Mesfin Asfaw

Publications and source records attributed to Mesfin Asfaw.

14 recordsLinked to original sources

arXiv2014

Thermally activated barrier crossing rate for a coupled system moving in a ratchet potential

We explore the dependence of the thermally activated barrier crossing rate on various model parameters for a dimer that undergoes a Brownian motion on a piecewise linear bistable potential employing the method of adiabatic elimination of fast variable. By introducing a different model system and approaches than the previous works \cite{c4,c5}, not only we recapture the previous results but we further show that systematic elimination of the fast changing variable leads to an effective Kramers type potential. It is shown that for rigid dimer, the escape rate $R$ monotonously decreases with $k$. On the other hand, in the presence of time varying force, the signal to noise ratio (SNR) attains a pronounced peak at particular barrier height $U_{0}$.

cond-mat.stat-mech↗

arXiv2010

Noise-created bistability and stochastic resonance of impurities diffusing in a semiconductor layer

We investigate the dynamics of impurities walking along a semiconductor layer assisted by thermal noise of strength $D$ and external harmonic potential $V(x)$. Applying a nonhomogeneous hot temperature in the vicinity of the potential minimum may modify the external potential into a bistable effective potential. We propose the ways of mobilizing and eradicating the unwanted impurities along the semiconductor layer. Furthermore, the thermally activated rate of hopping for the impurities as a function of the model parameters is studied in high barrier limit. Via two state approximation, we also study the stochastic resonance (SR) of the impurities dynamics where the same noise source that induces the dynamics also induces the transition from mono-stable to bistable state which leads to SR in the presence of time varying field.

cond-mat.mes-hall↗

arXiv2010

Thermally activated barrier crossing and stochastic resonance of a flexible polymer chain in a piecewise linear bistable potential

We study the stochastic resonance (SR) of a flexible polymer chain crossing over a piecewise linear bistable potential. The dependence of signal to noise ratio $SNR$ on noise intensity $D$, coupling constant $k$ and polymer length $N$ is studied via two state approximation. We find that the response of signal to the background noise strength is significant at optimum values of $D_{opt}$, $k_{opt}$ and $N_{opt}$ which suggests novel means of manipulating proteins or vesicles. Furthermore, the thermally activated barrier crossing rate $r_{k}$ for the flexible polymer chain is studied. We find that the crossing rate $r_{k}$ exhibits an optimal value at an optimal coupling constant $k_{opt}$; $k_{opt}$ decreases with $N$. As the chain length $N$ increases, the escape rate for the center of mass $r_{k}$ monotonously decreases. On the other hand, the crossing rate for the portion of polymer segment $r_s$ increases and saturates to a constant rate as $N$ steps up.

cond-mat.stat-mech↗

arXiv2010

Lateral phase separation of confined membranes

We consider membranes interacting via short, intermediate and long stickers. The effects of the intermediate stickers on the lateral phase separation of the membranes are studied via mean-field approximation. The critical potential depth of the stickers increases in the presence of the intermediate sticker. The lateral phase separation of the membrane thus suppressed by the intermediate stickers. Considering membranes interacting with short and long stickers, the effect of confinement on the phase behavior of the membranes is also investigated analytically.

cond-mat.soft↗

arXiv2009

Stochastic Resonance of a Flexible Chain Crossing over a Barrier

We study the stochastic resonance (SR) of a flexible polymer surmounting a bistable-potential barrier. Due to the flexibility that can enhance crossing rate and change chain conformations at the barrier, the SR behaviors manifest many features of an entropic SR of a new kind, such as the power amplification peaks at optimal chain lengths and elastic constants as well as the optimal noise strengths. The pronounced peaks that emerge depending on the chain lengths and conformation states suggest novel means of manipulating biopolymers, such as efficient separation methods, within undulating channels.

cond-mat.soft↗

arXiv2009

Adhesion of membranes via receptor-ligand complexes: Domain formation, binding cooperativity, and active processes

Cell membranes interact via anchored receptor and ligand molecules. Central questions on cell adhesion concern the binding affinity of these membrane-anchored molecules, the mechanisms leading to the receptor-ligand domains observed during adhesion, and the role of cytoskeletal and other active processes. In this review, these questions are addressed from a theoretical perspective. We focus on models in which the membranes are described as elastic sheets, and the receptors and ligands as anchored molecules. In these models, the thermal membrane roughness on the nanometer scale leads to a cooperative binding of anchored receptor and ligand molecules, since the receptor-ligand binding smoothens out the membranes and facilitates the formation of additional bonds. Patterns of receptor domains observed in Monte Carlo simulations point towards a joint role of spontaneous and active processes in cell adhesion. The interactions mediated by the receptors and ligand molecules can be characterized by effective membrane adhesion potentials that depend on the concentrations and binding energies of the molecules.

cond-mat.soft↗

arXiv2008

Adhesion-induced lateral phase separation of multi-component membranes: the effect of repellers and confinement

We present a theoretical study for adhesion-induced lateral phase separation for a membrane with short stickers, long stickers and repellers confined between two hard walls. The effects of confinement and repellers on lateral phase separation are investigated. We find that the critical potential depth of the stickers for lateral phase separation increases as the distance between the hard walls decreases. This suggests confinement-induced or force-induced mixing of stickers. We also find that stiff repellers tend to enhance, while soft repellers tend to suppress adhesion-induced lateral phase separation.

cond-mat.soft↗

arXiv2008

Competing jump cycles for vacancy diffusion in binary alloys

The mean-first-passage-times (MFPTs) for a vacancy that diffuses (via one- and six-jump cycles) in a two dimensional ordered binary alloy are evaluated using the properties of random walks on networks. We investigate the effect of temperature and relative barrier height on the ratio between the MFPTs of the two cycles. We find that the six-jump cycle takes shorter time than the one-jump cycle for the range of parameters considered.

cond-mat.mtrl-sci↗

arXiv2008

Unbinding transitions of multicomponent membranes and strings

We present a theory of unbinding transitions for membranes that interact via short and long receptor/ligand bonds. The detail of unbinding behavior of the membranes is governed by the binding energies and concentrations of receptors and ligands. We investigate the unbinding behavior of these membranes with Monte Carlo simulations and via a comparison with strings. We derive the scaling laws for strings analytically. The exact analytic results provide scaling estimate for membranes in the vicinity of the critical point.

cond-mat.soft↗

arXiv2008

Modeling an efficient Brownian heat engine

We discuss the effect of subdividing the ratchet potential on the performance of a tiny Brownian heat engine that is modeled as a Brownian particle hopping in a viscous medium in a sawtooth potential (with or without load) assisted by alternately placed hot and cold heat baths along its path. We show that the velocity, the efficiency and the coefficient of performance of the refrigerator maximize when the sawtooth potential is subdivided into series of smaller connected barrier series. When the engine operates quasistatically, we analytically show that the efficiency of the engine can not approach the Carnot efficiency and, the coefficient of performance of the refrigerator is always less than the Carnot refrigerator due to the irreversible heat flow via the kinetic energy.

cond-mat.stat-mech↗

arXiv2006

Membrane adhesion via competing receptor/ligand bonds

The adhesion of biological membranes is controlled by various types of receptor and ligand molecules. In this letter, we present a statistical-mechanical model for membranes that interact via receptor/ligand bonds of two different lengths. We show that the equilibrium phase behavior of the membranes is governed by an effective double-well potential. The depths of the two potential wells depend on the concentrations and binding energies of the receptors and ligands. The membranes are unbound for small, and bound for larger potential depths. In the bound state, the length mismatch of the receptor/ligand bonds can lead to lateral phase separation. We derive explicit scaling laws for the critical points of unbinding and phase separation, and determine the prefactors by comparison with Monte Carlo results.

cond-mat.soft↗

arXiv2006

Exploring the operation of a tiny heat engine

We model a tiny heat engine as a Brownian particle that moves in a viscous medium in a sawtooth potential (with or without load) assisted by $\it {alternately}$ placed hot and cold heat baths along its path. We find closed form expression for the steady state current as a function of the model parameters. This enables us to deal with the energetics of the model and evaluate either its efficiency or its coefficient of performance depending upon whether the model functions either as a heat engine or as a refrigerator, respectively. We also study the way current changes with changes in parameters of interest. When we plot the phase diagrams showing the way the model operates, we not only find regions where it functions as a heat engine and as a refrigerator but we also identify a region where the model functions as neither of the two.

cond-mat.stat-mech↗

arXiv2005

Energetics of a simple microscopic heat engine

We model a microscopic heat engine as a particle hopping on a one-dimensional lattice in a periodic sawtooth potential, with or without load, assisted by the thermal kicks it gets from alternately placed hot and cold thermal baths. We find analytic expressions for current and rate of heat flow when the engine operates at steady state. Three regions are identified where the model acts either as a heat engine or as a refrigerator or as neither of the two. At quasistatic limit both efficiency of the engine and coefficient of performance of the refrigerator go to that for Carnot engine and Carnot refrigerator, respectively. We investigate efficiency of the engine at two operating conditions (at maximum power and at optimum value with respect to energy and time) and compare them with those of the endoreversible and Carnot engines.

cond-mat.stat-mech↗

arXiv2002

An adjustable Brownian heat engine

A microscopic heat engine is modeled as a Brownian particle in a sawtooh potential (with load) moving through a highly viscous medium driven by the thermal kick it gets from alternately placed hot and cold heat reservoirs. We found closed form expression for the current as a function of the parameters characterizing the model. Depending on the values these model parameters take, the engine is also found to function as a refrigerator. Expressions for the efficiency as well as for the refrigerator performance are also reported. Study of how these quantities depend on the model parameters enabled us in identifying the points in the parameter space where the engine performs either with maximum power or with optimized efficiency. The corresponding efficiencies of the engine are then compared with those of the endoreversible and Carnot engines.

cond-mat.stat-mech↗