arXiv subjects

Benaoumeur Bakhti

Publications and source records attributed to Benaoumeur Bakhti.

12 recordsLinked to original sources

arXiv2026

An exact density-functional closure for the two-dimensional discrete wormlike chain

We formulate an exact density-functional description of the angularly discretized two-dimensional (2D) discrete wormlike chain (DWLC) under tension. The central result is a bond-local closure connecting the nearest-neighbor pair distribution $C_{i,i+1}^{rs}$ to the single-site angular densities $ρ_i^r$: $C_{i,i+1}^{rs}C_{i,i+1}^{00}/(C_{i,i+1}^{r0}C_{i,i+1}^{0s})=e^{\barβ\,rs}$, with the bond coupling $\barβ=βκ\varepsilon^2/a$. This relation follows directly from the connected Boltzmann weight of the quadratic bending interaction and permits exact integration of the entropy functional. Variational minimization then yields coupled self-consistent equations for the one- and two-site angular distributions, which we solve by fixed-point iteration. We establish the equivalence of this density-functional formulation to the exact transfer-matrix solution: the two approaches reproduce the angular marginals and force-extension curves to machine precision. The formulation also recovers the continuum 2D wormlike-chain behavior, including the rigid-rod and random-coil limits of the mean-square end-to-end distance. Using the segment length and persistence length taken directly from Mazur's short-DNA molecular-dynamics study, without additional fitting, the predicted bend-angle statistics agree with the simulation data within the estimated uncertainty. The closure structure extends naturally to nonharmonic local bending interactions and can also be interpreted inversely, allowing measured nearest-neighbor angular correlations to constrain effective coarse-grained bending potentials. These properties establish a direct connection between conformational statistics and local interactions and provide a density-level framework for treating interacting semiflexible-polymer systems.

cond-mat.soft↗

arXiv2026

Saturation Equations of State in Critical Gravitational Collapse: The Primordial Black Hole Threshold

The threshold and scaling laws of gravitational critical collapse depend sensitively on the matter equation of state. We investigate how these quantities are modified by a generic feature of dense matter that is absent from the radiation fluid commonly assumed in primordial black hole (PBH) studies: pressure stiffening as a maximum density is approached. As an analytically tractable proxy, we adopt the closed-form equation of state of a single-occupancy lattice gas, $p=-T\ln(1-ρ)$, which exhibits a density-dependent sound speed and a saturation density. Using general-relativistic simulations of spherically symmetric collapse, we show that this nonlinear pressure feedback increases the PBH formation threshold by $0.50\pm0.02\%$ relative to the radiation equation of state within the causal regime of the model. At the same time, the critical mass-scaling exponent remains $γ=0.357\pm0.001$, consistent with the radiation-fluid value to within our numerical precision. This agreement reflects the fact that the lattice equation of state approaches the radiation fluid at low density and remains only a mild perturbation over the near-critical regime, rather than indicating a universal critical exponent. Our results provide a proof of principle that saturation-induced stiffening can stabilize gravitational collapse and shift the PBH threshold, while introducing a linear-response framework for assessing the impact of more realistic equations of state on primordial black hole formation.

gr-qc↗

arXiv2022

Thermodynamics of interacting hard rods on a lattice

We present an exact derivation of the isobaric partition function of lattice hard rods with arbitrary nearest neighbor interactions. Free energy and all thermodynamics functions are derived accordingly and they written in a form that is a suitable for numerical implementation. As an application, we have considered lattice rods with pure hard core interactions, rods with long range gravitational attraction and finally a charged hard rods with charged boundaries (Bose gas), a model that is relevant for studying several phenomena such as charge regulation, ionic liquids near charged interfaces, and an array of charged smectic layers or lipid multilayers. In all cases, thermodynamic analysis have been done numerically using the Broyden algorithm.

cond-mat.stat-mech↗

arXiv2021

D-dimensional self-gravitating lattice gas in general relativity

Using a lattice equation of state combined with the D-dimensional Tolman-Oppenheimer-Volkoff equation and the Friedmann equations, we investigate the possibility of the formation of compact objects as well as the time evolution of the scale factor and the density profile of a self-gravitating material cluster. The numerical results show that in a $2+1$ dimensional spacetime, the mass is independent of the central pressure. Hence, the formation of only compact objects with a finite constant mass similar to the white dwarf is possible. However, in a $3+1$ dimensional spacetime, self-gravity leads to the formation of compact objects with a large gap of mass and the corresponding phase diagram has the same structure as the one for Neutron Star. The results also show that beyond certain critical central pressure, the star is unstable against gravitational collapse, and it may end in a black hole. Analysis of spacetimes of higher dimensions shows that gravity has the stronger effect in $3+1$ dimensions. Numerical solutions of the Friedmann equations show that the effect of the curvature of spacetime increases with increasing temperature, but decreases with increasing dimensionality beyond $D=3$.

gr-qc↗

arXiv2021

Interacting hard-sphere fluids in an external field

We present a new method for studying equilibrium properties of interacting fluids in an arbitrary external field. The fluid is composed of monodisperse spherical particles with hard-core repulsion and additional interactions of arbitrary shape and limited range. Our method of analysis is exact in one dimension and provides demonstrably good approximations in higher dimensions. It can cope with homogeneous and heterogeneous environments. We derive an equation for the pair distribution function. The solution, to be evaluated numerically, in general, or analytically for special cases, enters expressions for the entropy and free energy functionals. For some one-dimensional systems, our approach yields analytic solutions, reproducing available exact results from different approaches.

cond-mat.soft↗

arXiv2021

Fonctions spéciales et polynômes orthogonaux: cours et exercices corrigés

This report (written in French) is devoted to studying special functions the most used in physics. Special functions are a very broad branch of mathematics, theoretical physics, and mathematical physics. They appeared in the nineteenth century as solutions of equations in mathematical physics, particularly partial differential equations of order two and four. Their knowledge is essential for the proper handling and understanding of current problems in physics. They are also related to the art of scientific computing in physics and mathematics. Special functions are included in many computer algebra software such as Matlab, Mathematica, and Maple, and students are strongly encouraged to take part in this development which has become indispensable for the treatment of almost all current problems in physics. The manuscript contains six chapters: gamma and beta functions, Bessel functions, Fresnel error, and integral function, exponential integral, sine integral, cosine integral, and logarithm integral, orthogonal polynomials, and finally hypergeometric functions.

math.HO↗

arXiv2018

Density profiles of a self-gravitating lattice gas in one, two, and three dimensions

We consider a lattice gas in spaces of dimensionality $\mathcal{D}=1,2,3$. The particles are subject to a hardcore exclusion interaction and an attractive pair interaction that satisfies Gauss' law as do Newtonian gravity in $\mathcal{D}=3$, a logarithmic potential in $\mathcal{D}=2$, and a distance-independent force in $\mathcal{D}=1$. Under mild additional assumptions regarding symmetry and fluctuations we investigate equilibrium states of self-gravitating material clusters, in particular radial density profiles for closed and open systems. We present exact analytic results in several instances and high-precision numerical data in others. The density profile of a cluster with finite mass is found to exhibit exponential decay in $\mathcal{D}=1$ and power-law decay in $\mathcal{D}=2$ with temperature-dependent exponents in both cases. In $\mathcal{D}=2$ the gas evaporates in a continuous transition at a nonzero critical temperature. We describe clusters of infinite mass in $\mathcal{D}=3$ with a density profile consisting of three layers (core, shell, halo) and an algebraic large-distance asymptotic decay. In $\mathcal{D}=3$ a cluster of finite mass can be stabilized at $T>0$ via confinement to a sphere of finite radius. In some parameter regime, the gas thus enclosed undergoes a discontinuous transition between distinct density profiles. For the free energy needed to identify the equilibrium state we introduce a construction of gravitational self-energy that works in all $\mathcal{D}$ for the lattice gas. The decay rate of the density profile of an open cluster is shown to transform via a stretched exponential for $1<\mathcal{D}<2$ whereas it crosses over from one power-law at intermediate distances to a different power-law at larger distances for $2<\mathcal{D}<3$.

cond-mat.stat-mech↗

arXiv2017

Interacting fluids in an arbitrary external field

We present new method for studying the equilibrium properties of interacting fluids in an arbitrary external filed. The method is valid in any dimension and it yields an exact results in one dimension. Using this approach, we derive a recurrence relation for the pair distribution function of a three dimensional in-homogeneous fluids, constitute of spherical molecules with arbitrary nearest neighbour interaction that extends to two molecules diameter. By integrating this recurrence relation, we get an explicit expressions for the entropy and free energy functionals as a functionals of the density and the pair distribution function. We show that for one dimensional systems, our results coincide exactly with previously derived one using a completely different approach.

cond-mat.soft↗

arXiv2015

Monodisperse hard rods in external potentials

We consider linear arrays of cells of volume $V_\mathrm{c}$ populated by monodisperse rods of size $σV_\mathrm{c}$, $σ=1,2,\ldots$, subject to hardcore exclusion interaction. Each rod experiences a position-dependent external potential. In one application we also examine effects of contact forces between rods. We employ two distinct methods of exact analysis with complementary strengths and different limits of spatial resolution to calculate profiles of pressure and density on mesoscopic and microscopic length scales at thermal equilibrium. One method uses density functionals and the other statistically interacting vacancy particles. The applications worked out include gravity, power-law traps, and hard walls. We identify oscillations in the profiles on a microscopic length scale and show how they are systematically averaged out on a well-defined mesoscopic length scale to establish full consistency between the two approaches. The continuum limit, realized as $V_\mathrm{c}\to0$, $σ\to\infty$ at nonzero and finite $σV_\mathrm{c}$, connects our highest-resolution results with known exact results for monodisperse rods in a continuum. We also compare the pressure profiles obtained from density functionals with the average microscopic pressure profiles derived from the pair distribution function.

cond-mat.stat-mech↗

arXiv2014

Statistically interacting vacancy particles

The equilibrium statistical mechanics of one-dimensional lattice gases with interactions of arbitrary range and shape between first-neighbor atoms is solved exactly on the basis of statistically interacting vacancy particles. Two sets of vacancy particles are considered. In one set all vacancies are of one-cell size. In the other set the sizes of vacancy particles match the separation between atoms. Explicit expressions are obtained for the Gibbs free energy and the distribution of spaces between atoms at thermal equilibrium. Applications to various types of interaction potentials are discussed, including long-range potentials that give rise to phase transitions. Extensions to hard rod systems are straightforward and are shown to agree with existing results for lattice models and their continuum limits.

cond-mat.stat-mech↗

arXiv2013

Interacting hard rods on a lattice: Distribution of microstates and density functionals

We derive exact density functionals for systems of hard rods with first-neighbor interactions of arbitrary shape but limited range on a one-dimensional lattice. The size of all rods is the same integer unit of the lattice constant. The derivation, constructed from conditional probabilities in a Markov chain approach, yields the exact joint probability distribution for the positions of the rods as a functional of their density profile. For contact interaction ("sticky core model") between rods we give a lattice fundamental measure form of the density functional and present explicit results for contact correlators, entropy, free energy, and chemical potential. Our treatment includes inhomogeneous couplings and external potentials.

cond-mat.stat-mech↗

arXiv2012

Exact density functional for hard rod mixtures derived from Markov chain approach

Using a Markov chain approach we rederive the exact density functional for hard rod mixtures on a one-dimensional lattice, which forms the basis of the lattice fundamental measure theory. The transition probability in the Markov chain depends on a set of occupation numbers, which reflects the property of a zero-dimensional cavity to hold at most one particle. For given mean occupation numbers (density profile), an exact expression for the equilibrium distribution of microstates is obtained, that means an expression for the unique external potential that generates the density profile in equilibrium. By considering the rod ends to fall onto lattice sites, the mixture is always additive.

cond-mat.stat-mech↗