arXiv subjects

A. Benyoussef

Publications and source records attributed to A. Benyoussef.

At least 19 recordsLinked to original sources

arXiv2024

Insights into the magnetocaloric effect of Gadolinium: A DFT Exploration of Structural, Electronic, and Magnetic Features in Bulk and Film configurations

Gadolinium stand as the favored choice among magnetic refrigerant materials for numerous active magnetic regenerator (AMR) prototypes due to its remarkable ability to exhibit a substantial change in magnetic entropy. This unique characteristic arises from its status as one of the elemental ferromagnets with a high Curie temperature, closely aligning with room temperature conditions, and undergoing a second-order magnetic phase transition. In this comprehensive study, we employ density functional theory (DFT) calculations to explore the structural, electronic, and magnetic properties of both Gadolinium bulk and film configurations. Our primary objective is to gain a deeper understanding of the intricate physics underlying the intriguing magnetocaloric features observed in Gadolinium. This investigation provides valuable insights into the potential applications and the broader implications of Gadolinium in the realm of magnetic refrigeration technology.

cond-mat.mtrl-sci↗

arXiv2021

Electronic properties of the Rutile-type dioxide SnO2 material doped by sulfur element: DFT study

In this work, we study the effect of doping the Rutile-type dioxide SnO2 material by the non-metal Sulfur (S) atoms on the electronic properties. In fact, we have used the ab-initio method applied on the basis of the Density Functional Theory (DFT) using the Quantum Espresso code. Through the density of states and the band structure calculations for different concentrations have been deduced. When doping the SnO2 material with 6% of Sulfur (S), we found a perfect symmetry between up and down spin states in the total DOS confirming the non-magnetic behavior of this material doped SnO2 with 6% of Sulfur. It is also worth to note that the SnO2 material doped with 6% of Sulfur, exhibits a semiconductor of the P-type. Moreover the band gap decreases when increasing the concentration of doping the SnO2 by Sulfur. Our results are in good agreement with the existing literature both experimental and theoretical.

cond-mat.mtrl-sci↗

arXiv2021

DFT and Monte Carlo simulations of the equiatomic quaternary Heusler Alloy CoFeCrP

In this work, we study the equiatomic quaternary Heusler Alloy CoFeCrP using two methods: DFT and Monte Carlo simulations. The DFT method allowed us to illustrate the structural, electronic and magnetic properties of this alloy. The ground state phase diagrams have been presented to show the stable configurations in different physical parameter planes. On the other hand, the Monte Carlo simulations, performed under the Metropolis algorithm, permitted to deduce the critical the behavior of the equiatomic quaternary Heusler alloy CoFeCrP. The structural properties results show that the phase of type I, of this alloy is the most stable configuration. In addition, the band structures, and density of states calculations results show that this compound exhibits a half-metallic character with a 100 % of spin polarization (SP) at the Fermi-level. The total magnetic moment of the Heusler compound is found to be 4.00 mu_B. Moreover, it is found that the Slater-Pauling is well described for this alloy. Our results show that this material is a potential candidate for the spintronic applications. This is due to its half-metallicity, its high spin moments, its complete SP polarization and its high Curie temperature.

cond-mat.mtrl-sci↗

arXiv2020

Engineering the magnetic and magnetocaloric properties of PrVO3 epitaxial oxide thin films by strain effects

Combining multiple degrees of freedom in strongly-correlated materials such as transition-metal oxides would lead to fascinating magnetic and magnetocaloric features. Herein, the strain effects are used to markedly tailor the magnetic and magnetocaloric properties of PrVO3 thin films. The selection of appropriate thickness and substrate enables us to dramatically decrease the coercive magnetic field from 2.4 T previously observed in sintered PVO3 bulk to 0.05 T for compressive thin films making from the PrVO3 compound a nearly soft magnet. This is associated with a marked enhancement of the magnetic moment and the magnetocaloric effect that reach unusual maximum values of roughly 4.86 uB and 56.8 J/kg K in the magnetic field change of 6 T applied in the sample plane at the cryogenic temperature range (3 K), respectively. This work strongly suggests that taking advantage of different degrees of freedom and the exploitation of multiple instabilities in a nanoscale regime is a promising strategy for unveiling unexpected phases accompanied by a large magnetocaloric effect in oxides.

cond-mat.mtrl-sci↗

arXiv2020

From Mott Insulators to Quantum Metals

High critical temperature cuprate superconducting materials are composed of copper oxide layers and interlayer charge reservoirs. When not doped, these cuprates are antiferromagnetic insulators. We propose to design new materials by combining alternating layers of parents of hole-doped and electron-doped of these cuprates and modifications thereof. Our goal is to find undoped cuprates that can be either an antiferromagnetic insulator or a quantum metal. The term quantum metal means a metal characterized by long range antiferromagnetic order or only strong antiferromagnetic correlations, i.e., it is thus a stable ground state against any other perturbations. The new metallic states sought here could be precursors to new superconducting states in the absence or presence of doping. Using the density functional theory, we report on two compounds {La}{Pr}CuO4 and {La}{V}CuO4 that illustrate the different physics described above. The curly brackets mean that the preparation of these compounds shall be done by depositing a layer containing Pr, then one CuO2 layer, then finally the La layer in {La}{Pr}CuO4 for example. The configuration formed by the positions of the charge reservoir atoms with respect to the CuO2 layer is an important factor in the new procedure we propose here. This paper reports on the X-ray diffraction, electronic, optical, and magnetic properties of these hypothetical materials. We found that {La}{Pr}CuO4 is a Mott insulator, but {La}{V}CuO4 is an undoped correlated quantum metal with long-range order. Our calculations were performed using the linearized plane wave method (FP-LAPW) implemented within the Wien2k software.

cond-mat.str-el↗

arXiv2020

Effect of Orientation of Cation CH3NH3PbI3 on Ambipolarity, Open-Circuit Voltage, and Excitons Lifetime

The effect of the orientation of the cation MA (i.e., CH3NH3) on the physical properties of MAPbI3 are investigated using the density functional theory. We report that the Fermi energy level exhibits a large variation with MA orientation, and it is this Fermi level variation that makes this material unique and so different from the non-organic perovskites. The Fermi level variation with orientation of MA is proposed to be responsible for the experimentally observed intrinsic open-circuit voltage V_OC, ambipolarity, and long lifetime of the excitons. Based on our results, ferroelectric domains in the low-temperature orthorhombic phase or clusters of MA molecules not rotating in unison (as a consequence of e.g. impurities and/or defects) in the higher-temperature tetragonal and cubic phases are proposed to be responsible for ambipolarity. This is because any given two adjacent domains or clusters with different MA s average orientations may have very different Fermi levels, causing them to behave as an effective diode. Also, the measured significant lifetime of excitons in MAPbI3 finds a natural explanation using this effective diode. Thus, our work provides a unified manner for explaining at least these three important properties in terms of the rotation-induced Fermi level variations. We used the density functional theory to get our results.

cond-mat.mtrl-sci↗

arXiv2019

Chemical synthesis and magnetic properties of monodisperse cobalt ferrite nanoparticles

In this work, a successful synthesis of magnetic cobalt ferrite (CoFe2O4) nanoparticles is presented. The synthesized CoFe2O4 nanoparticles have a spherical shape and highly monodisperse in the selected solvent. The effect of different reaction conditions such as temperature, reaction time and varying capping agents on the phase and morphology is studied. Scanning transmission electron microscopy showed that the size of these nanoparticles can be controlled by varying reaction conditions. Both X-ray diffraction and energy dispersive X-ray spectroscopy corroborate the formation of CoFe2O4 spinel structure with cubic symmetry. Due to optimized reaction parameters, each nanoparticle was shown to be a single magnetic domain with diameter ranges from 6 to 16 nm. Finally, the magnetic investigations showed that the obtained nanoparticles are superparamagnetic with a small coercivity value of about 315 Oe and a saturation magnetization of 58 emu/g at room temperature. These results make the cobalt ferrite nanoparticles promising for advanced magnetic nanodevices and biomagnetic applications.

cond-mat.mtrl-sci↗

arXiv2018

Bethe lattice approach study of the mixed spin-$\frac{1}{2}$ and spin-$\frac{7}{2}$ Ising model in a longitudinal magnetic field

The magnetic properties of the mixed spin-$\frac{1}{2}$ and spin-$\frac{7}{2}$ Ising model with a crystal-field in a longitudinal magnetic field are investigated on the Bethe lattice using exact recursion relations. The ground-state phase diagram is constructed. The temperature-dependent one is displayed in the case of uniform crystal-field on the $(k_{\text{B}}T/|J|, D/|J|)$ plane in the absence of the external constraint for lattice coordination numbers $z = 3, 4, 6$. The order parameters and corresponding response functions as well as the internal energy are calculated and examined in detail in order to feature the real nature of phase boundaries and corresponding temperatures. The thermal variations of the average magnetization are classified according to the Néel nomenclature.

cond-mat.stat-mech↗

arXiv2017

Ground state study of a double core-shell dendrimer nanostructure

Based on a dendrimer graph theory, we investigate the ground state phase diagrams of a nanostructure formed by three spins: $σ=1/2$, S=1 and q=3/2. This nanostructure is built on a dendrimer with triangular core geometry. More precisely, we evaluate the effect of different physical parameters including the exchange interactions, the external magnetic and the crystal fields. By varying these parameters, we analytically elaborate the ground state phase diagrams. Concretely, we found that the existence of the crystal field increases the number of stable phases. The total number of possible phases is 24=2x3x4, which are: ($\pm$1/2,$\pm$1,$\pm$3/2), ($\pm$1/2,0,$\pm$3/2), ($\pm$1/2,$\pm$1,$\pm$1/2) and ($\pm$1/2,0,$\pm$1/2). All these phases are found to be stable, expect two special phases: (+1/2,0,-3/2) and (-1/2,0,+3/2), which are masked by the other phases in all the ground state phase diagrams.

cond-mat.stat-mech↗

arXiv2014

The plasma protein fibrinogen stabilizes clusters of red blood cells in microcapillary flows

The supply of oxygen and nutrients and the disposal of metabolic waste in the organs depend strongly on how blood, especially red blood cells, flow through the microvascular network. Macromolecular plasma proteins such as fibrinogen cause red blood cells to form large aggregates, called rouleaux, which are usually assumed to be disaggregated in the circulation due to the shear forces present in bulk flow. This leads to the assumption that rouleaux formation is only relevant in the venule network and in arterioles at low shear rates or stasis. Thanks to an excellent agreement between combined experimental and numerical approaches, we show that despite the large shear rates present in microcapillaries, the presence of either fibrinogen or the synthetic polymer dextran leads to an enhanced formation of robust clusters of red blood cells, even at haematocrits as low as 1%. Robust aggregates are shown to exist in microcapillaries even for fibrinogen concentrations within the healthy physiological range. These persistent aggregates should strongly affect cell distribution and blood perfusion in the microvasculature, with putative implications for blood disorders even within apparently asymptomatic subjects.

physics.bio-ph↗

arXiv2011

Monte Carlo simulation of magnetic phase transitions in Mn doped ZnO

The magnetic properties of Mn-doped ZnO semi-conductor have been investigated using the Monte Carlo method within the Ising model. The temperature dependences of the spontaneous magnetization, specific heat and magnetic susceptibility have been constructed for different concentrations of magnetic dopant Mn and different carrier concentrations. The exact values of Mn concentration and carrier concentration at which high temperature transition occurs are determined. An alternative for the explanation of some controversies concerning the existence and the nature of magnetism in Mn diluted in ZnO systems is given. Other features are also studied.

cond-mat.mtrl-sci↗

arXiv2008

Study of the mixed Ising spins (1/2,3/2) in a random crystal field

We study the magnetic properties of a mixed Ising ferrimagnetic system, in which the two interacting sublattices have spins $σ$, $(\pm 1/2)$ and spins $S$, $(\pm 3/2,\pm 1/2)$ in the presence of a random crystal field, with the mean field approach. The obtained results show the existence of some interesting phenomena, such as the appearance of a new ferrimagnetic phase namely the partly ferrimagnetic phase $(m_σ=\frac{-1}{2},m_S=+1)$ and consequently the existence of three topologically different types of phase diagrams. The effect of increasing the exchange interaction parameter $J$, at very low temperature is investigated. The transitions shown in these phase diagrams are in good agreement with those obtained in the ground state case.

cond-mat.stat-mech↗

arXiv2005

Interface disorder and layer transitions in Ising thin films

The disorder and layer transitions in the interface between an Ising spin-1/2 film denoted $(n)$, and an Ising spin-1 film denoted $(m)$, are studied using Monte Carlo simulations. The effects of both an external magnetic field, acting only on the spin-1/2 film, and a crystal magnetic field acting only on the spin-1 film, are studied for a fixed temperature and selected values of the coupling constant $J_p$ between the two films. It is found that for large values of the constant $J_p$, the layers of the film $(n)$, as well as those of the film $(m)$, undergo a first order layering transition. On the other hand, the only disordered layer of the film $(n)$ is that one belonging to the interface films $(n)/(m)$, for any values of the crystal field $Δ$. We show the existence of a critical value of the crystal field $Δ_c$, above which this particular layer of the film $(n)$ is disordered. We found that $Δ_c$ depends on the values of the constant coupling $(J_p)$ between the two films.

cond-mat.stat-mech↗

arXiv2005

Phase diagrams in the Blume-Emery-Griffiths Ising films

We study the spin-1 Ising model with bilinear and biquadratic exchange interactions and single-ion crystal field. In addition to the four usual phases: disordered DIS, ferromagnetic FER, antiquadrupolar AQU and ferrimagnetic FRI, we found three new phases in the case of a thin magnetic film, namely: the sublattice A non magnetic phase NMA, the sublattice B non magnetic phase NMB and the global non magnetic phase NMG. These phases are studied, for each layer of the film, either in the temperature-crystal field plane $(T,Δ)$ or in the biquadratic coupling-crystal field plane $(d,Δ)$, for diferent film thicknesses. On the other hand, the thermal behaviour of the layer quadrupolar moments $q_{A}, q_{B}$ and layer magnetisations $m_{A}, m_{B}$ are investigated for negative values of the biquadratic coupling and crystal field. It is also found that for fixed values of the biquadratic coupling, the temperature and the crystal field, each layer of the film can belong to a differnt phase. To illustrate this situation, an example is given for $d=-3.0$, $Δ=-3.0$ and $T=1.3$.

cond-mat.stat-mech↗

arXiv2005

Crossover component in non critical dissipative sandpile models

The effect of bulk dissipation on non critical sandpile models is studied using both multifractal and finite size scaling analyses. We show numerically that the local limited (LL) model exhibits a crossover from multifractal to self-similar behavior as the control parameters $h_{ext}$ and $ε$ turn towards their critical values, i.e. $h_{ext} \to 0^+ $ and $ε\to ε_c$. The critical exponents are not universal and exhibit a continuous variation with $ε$. On the other hand, the finite size effects for the local unlimited (LU), non local limited (NLL), and non local unlimited (NLU) models are well described by the multifractal analysis for all values of dissipation rate $ε$. The space-time avalanche structure is studied in order to give a deeper understanding of the finite size effects and the origin of the crossover behavior. This result is confirmed by the calculation of the susceptibility.

cond-mat.stat-mech↗

arXiv2005

Effect of the inhomogeneity of substrate on layering transitions

We study the surface substrate inhomogeneity effect on the wetting of a spin-1/2 Ising ferromagnetic film in an external magnetic field $H$, using Monte Carlo simulations. It is found the inhomogeneity leads to the formation of islands of positive spins in each layer. However, depending on the values of $H$, for a fixed surface magnetic field $H_s$, each layer exhibits three different phases; totally wet, nonwet and partially wet (PW). In the latter case, we show the existence of three distinct configurations namely: A configuration (PWTD) in which the layer is partially wet with a total disconnection between islands; (PWPD) a configuration in which the layer is partially wet with a partial disconnection between islands; (PWTC) a configuration in which the layer is partially wet and the islands are totally connected. Furthermore, we show that an increase of $H_s$ values breaks the bonds connecting some islands of the phase (PWTC) which leads to an expansion of the (PWPD) region. On the other hand, the frequency distribution and the mean size of island are investigated in the (PWTD) region for fixed values of temperature $T$, $H_s$ and $H$. Moreover, we show the existence of 3D-islands resulting from the formation of islands in each layer.

cond-mat.mtrl-sci↗

arXiv2005

The Effect Of Delay Times On The Optimal Velocity Traffic Flow Behavior

We have numerically investigated the effect of the delay times $τ_f$ and $τ_s$ of a mixture of fast and slow vehicles on the fundamental diagram of the optimal velocity model. The optimal velocity function of the fast cars depends not only on the headway of each car but also on the headway of the immediately preceding one. It is found that the small delay times have almost no effects, while, for sufficiently large delay time $τ_s$ the current profile displays qualitatively five different forms depending on $τ_f$, $τ_s$ and the fractions $d_f$ and $d_s$ of the fast and slow cars respectively. The velocity (current) exhibits first order transitions at low and/or high densities, from freely moving phase to the congested state, and from congested state to the jamming one respectively accompanied by the existence of a local minimal current. Furthermore, there exist a critical value of $τ_f$ above which the metastability and hysteresis appear. The spatial-temporal traffic patterns present more complex structure

cond-mat.other↗

arXiv2005

Surface Effects On Wetting and Layering Transitions

Wetting phenomena plays an interesting role in the technological development of materials. Recently, much attention has been directed to the study of magnetic solid films. To understand, theoretically, the effect of surface on wetting and layering transitions in these systems, we will give in this chapter a presentation of several relevant recent theoretical works realized on wetting and layering transitions in which we explain how the nature of surface can affect the behavior of wetting and layering transitions. Using different spin systems models and different numerical and approximate methods such as mean field and effective field theories, real space renormalization group technique, transfer matrix method and Monte-Carlo simulations, it is found that the wetting and layering transitions depend on the nature of the surface magnetic field, the surface coupling strength, the surface crystal field, the geometry of surface, the in-homogeneity of substrate and the quantum fluctuations at the surface�

cond-mat.mtrl-sci↗