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Search Results: 1 - 10 of 1064 matches for " Adel Benlagra "
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Model of Quantum Criticality in He3 bilayers Adsorbed on graphite
Adel Benlagra,Catherine Pepin
Physics , 2007, DOI: 10.1103/PhysRevLett.100.176401
Abstract: Recent experiments on He3 bilayers adsorbed on Graphite have shown striking quantum critical properties at the point where the first layer localizes. We model this system with the Anderson lattice plus inter-layer Coulomb repulsion in two dimensions. Assuming that quantum critical fluctuations come from a vanishing of the effective hybridization, we can reproduce several features of the system, including the apparent occurrence of two quantum critical points (QCP), the variation of the effective mass and coherence temperature with coverage.
Doping-induced band shifts and Lifshitz transitions in heavy-fermion metals
Adel Benlagra,Matthias Vojta
Physics , 2012, DOI: 10.1103/PhysRevB.87.165143
Abstract: For some heavy-fermion compounds, it has been suggested that a Fermi-surface-changing Lifshitz transition, which can be driven, e.g., by varying an applied magnetic field, occurs inside the heavy- fermion regime. Here we discuss, based on microscopic calculations, how the location of such a transition can be influenced by carrier doping. Due to strong correlations, a heavy band does not shift rigidly with the chemical potential. Intriguingly, we find that the actual shift is determined by the interplay of heavy and additional light bands crossing the Fermi level: doped carriers tend to populate heavy and light bands equally, despite the fact that the latter contribute a small density of states of excitations only. This invalidates naive estimates of the transition shift based on the low-temperature specific heat of the heavy Fermi liquid. We discuss applications of our results.
He3 bi-layers as a simple example of de-confinement
Adel Benlagra,Catherine Pépin
Physics , 2008, DOI: 10.1103/PhysRevB.79.045112
Abstract: We consider the recent experiments on He3 bi-layers, showing evidence for a quantum critical point (QCP) at which the first layer localizes. Using the Anderson lattice in two dimensions with the addition of a small dispersion of the f-fermion, we modelize the system of adsorbed He3 layers. The first layer represents the f-fermions at the brink of localization while the second layer behaves as a free Fermi sea. We study the quantum critical regime of this system, evaluate the effective mass in the Fermi liquid phase and the coherence temperature and give a fit of the experiments and interpret its main features. Our model can serve as well as a predictive tool used for better determination of the experimental parameters.
Finite-temperature spectra and quasiparticle interference in Kondo lattices: From light electrons to coherent heavy quasiparticles
Adel Benlagra,Thomas Pruschke,Matthias Vojta
Physics , 2011, DOI: 10.1103/PhysRevB.84.195141
Abstract: Recent advances in scanning tunneling spectroscopy performed on heavy-fermion metals provide a window onto local electronic properties of composite heavy-electron quasiparticles. Here we theoretically investigate the energy and temperature evolution of single-particle spectra and their quasiparticle interference caused by point-like impurities in the framework of a periodic Anderson model. By numerically solving dynamical-mean-field-theory equations, we are able to access all temperatures and to capture the crossover from weakly interacting c and f electrons to fully coherent heavy quasiparticles. Remarkably, this crossover occurs in a dynamical fashion at an energy-dependent crossover temperature. We study in detail the associated Fermi-surface reconstruction and characterize the incoherent regime near the Kondo temperature. Finally, we link our results to current heavy-fermion experiments.
Kondo lattices with inequivalent local moments: Competitive vs. co-operative Kondo screening
Adel Benlagra,Lars Fritz,Matthias Vojta
Physics , 2011, DOI: 10.1103/PhysRevB.84.075126
Abstract: While standard heavy fermion metals feature a single spin-1/2 local moment per unit cell, more complicated systems with multiple distinct local moments have been synthesized as well, with Ce_3Pd_20(Si,Ge)_6 being one example. Here, we discuss the physics of a Kondo lattice model with two local-moment sublattices, coupled with different Kondo couplings to conduction electrons. The phase diagram will be strongly modified from that of the standard Kondo lattice if the characteristic screening temperatures of the distinct moments are well separated. Therefore, we investigate the interplay between the two Kondo effects using a local self-energy approximation via slave bosons. We find that the two Kondo effects can either compete or co-operate depending on the conduction-band filling. In the regime of competition, small differences in the two Kondo couplings can lead to huge differences in the respective Kondo scales, due to non-trivial many-body effects. We also study the low-temperature properties of the collective heavy Fermi-liquid state and propose a connection to depleted Kondo lattice systems.
Activation gap in the specific heat measurements for He$^3$ bi-layers
Adam Ran?on-Schweiger,Adel Benlagra,Catherine Pépin
Physics , 2010, DOI: 10.1103/PhysRevB.83.073102
Abstract: Recently, attention has been given to a system of He$^3$ bi-layers where a quantum criticality similar to the one in heavy fermion compounds has been observed [Science 317, 1356 92007)] In our previous analysis [Phys. Rev. B 79, 045112 (2007)], based on the Kondo breakdown scenario, we addressed successfully most of the features observed in that experiment. Here, we consider the activation energy $\Delta$ observed experimentally in the specific heat measurements at low temperatures in the heavy Fermi liquid phase. Within our previous study of this system, this is identified with the gap opening when the upper hybridized band is emptied due to a strong hybridization between the nearly localized first layer and the fluid second one. We discuss the successes and limitations of our approach. An additional prediction is proposed.
Two-channel pseudogap Kondo and Anderson models: Quantum phase transitions and non-Fermi liquids
Imke Schneider,Lars Fritz,Frithjof B. Anders,Adel Benlagra,Matthias Vojta
Physics , 2011, DOI: 10.1103/PhysRevB.84.125139
Abstract: We discuss the two-channel Kondo problem with a pseudogap density of states, $\rho(\w)\propto|\w|^r$, of the bath fermions. Combining both analytical and numerical renormalization group techniques, we characterize the impurity phases and quantum phase transitions of the relevant Kondo and Anderson models. The line of stable points, corresponding to the overscreened non-Fermi liquid behavior of the metallic $r=0$ case, is replaced by a stable particle-hole symmetric intermediate-coupling fixed point for $0\rmax$, this non-Fermi liquid phase disappears, and instead a critical fixed point with an emergent spin--channel symmetry appears, controlling the quantum phase transition between two phases with stable spin and channel moments, respectively. We propose low-energy field theories to describe the quantum phase transitions, all being formulated in fermionic variables. We employ epsilon expansion techniques to calculate critical properties near the critical dimensions $r=0$ and $r=1$, the latter being potentially relevant for two-channel Kondo impurities in neutral graphene. We find the analytical results to be in excellent agreement with those obtained from applying Wilson's numerical renormalization group technique.
Analyticity of Semigroups generated by Degenerate Mixed Differential Operators  [PDF]
Adel Saddi
Advances in Pure Mathematics (APM) , 2011, DOI: 10.4236/apm.2011.13010
Abstract: In this paper we are interested in studying the dissipativity of degenerate mixed differential operators involving an interface point. We show that, under particular interface conditions, such operators generate analytic semigroups on an appropriate Hilbert space . To illustrate the results an example is discussed.
Mild Solutions of Fractional Semilinear Integro-Differential Equations on an Unbounded Interval  [PDF]
Adel Jawahdou
Applied Mathematics (AM) , 2013, DOI: 10.4236/am.2013.47A007
Abstract: In this paper, we study the existence of mild solutions for fractional semilinear integro-differential equations in an arbitrary Banach space associated with operators generating compact semigroup on the Banach space. The arguments are based on the Schauder fixed point theorem.
A Decision Tree Classifier for Intrusion Detection Priority Tagging  [PDF]
Adel Ammar
Journal of Computer and Communications (JCC) , 2015, DOI: 10.4236/jcc.2015.34006
Abstract: Snort rule-checking is one of the most popular forms of Network Intrusion Detection Systems (NIDS). In this article, we show that Snort priorities of true positive traffic (real attacks) can be approximated in real-time, in the context of high speed networks, by a decision tree classifier, using the information of only three easily extracted features (protocol, source port, and destination port), with an accuracy of 99%. Snort issues alert priorities based on its own default set of attack classes (34 classes) that are used by the default set of rules it provides. But the decision tree model is able to predict the priorities without using this default classification. The obtained tagger can provide a useful complement to an anomaly detection intrusion detection system.
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