arXiv subjects

L. Simon

Publications and source records attributed to L. Simon.

10 recordsLinked to original sources

arXiv2022

Flat band and Lifschitz transition in long-range ordered supergraphene obtained by Erbium intercalation

Dispersionless energy bands are a peculiar property gathering increasing attention for the emergence of novel photonic, magnetic and electronic properties. Here we report the first observation of a graphene superstructure n-doped up to the Lifshitz transition and exhibiting a flat band, obtained by ordered Erbium intercalation between a single layer graphene and SiC(0001). STM experiments reveal large graphene areas characterized by a long-range ordered hexagonal superstructure with a lattice parameter of 1.40 nm, rotated by 19 degrees with respect to the original lattice. Angle Resolved Photoelectron Spectroscopy measurements show that this graphene structure exhibits Dirac cones with perfect linear dispersion, and a Dirac point at -1.72 eV +/- 0.02 under the Fermi level, which is one of the highest doping levels ever obtained solely by intercalation. Fermi surface measurements show that the Lifshitz transition has been reached, and that a wide flat band is generated around the M point. We propose that this modification of the band structure is the effect of an induced spin-orbit coupling. This system provides a playground to study the interaction between a novel magnetic order mediated by pi-band states, and a divergent density of states at the Fermi level.

cond-mat.mes-hall↗

arXiv2016

English-Medium Instruction Calculus: Is flipping helpful?

This paper addresses some of the experiences encountered and some of the strategies utilized while using flipped classroom pedagogy in an undergraduate Single Variable Calculus (SVC) course in a Confucian Heritage Culture (CHC) environment. Due to issues related to instructor-student communication and students' learning styles, a theoretical framework is built upon three components for learning environments: inverted Bloom's taxonomy for educational learning objectives, English-medium instruction (EMI) and integrating technology in the classroom. In this study, four different types of instruction were designed; three classes were flipped while the fourth acts as a control. Type A means entirely flipped with instructor-created videos and integrating CAS {\sl Maxima}. Type B means entirely flipped with third-party videos. Type C is similar to Type A, but instead of the entire course being flipped, a single topic is flipped. Type D uses the traditional lecture style. By considering exam scores and students' feedback, both quantitative and qualitative aspects of the flipped pedagogy were investigated. Quantitative analysis indicates a statistically significant difference in the exam scores between instruction Type A and Type C as determined by one-way ANOVA ($F(3,306) = 2.67$, $p = 0.0477$) with small practical significance of the effect size ($\eta^2 = 0.0255$). Qualitative findings suggest that although students' engagement and communication with instructors have been enhanced, some challenges related to language and culture, along with minimizing competition and learning about a new technological tool remain to persist.

math.HO↗

arXiv2016

Covalent functionalization by cycloaddition reactions of pristine, defect-free graphene

Based on a low temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. These studies showed that the cycloaddition reactions can be carried out on the basal plane of graphene, even when there are no pre-existing defects. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloaddition were possible on free standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

cond-mat.mes-hall↗

arXiv2012

High Van Hove singularity extension and Fermi velocity increase in epitaxial graphene functionalized by gold clusters intercalation

Gold intercalation between the buffer layer and a graphene monolayer of epitaxial graphene on SiC(0001) leads to the formation of quasi free standing small aggregates of clusters. Angle Resolved Photoemission Spectroscopy measurements reveal that these clusters preserve the linear dispersion of the graphene quasiparticles and surprisingly increase their Fermi velocity. They also strongly modify the band structure of graphene around the Van Hove singularities (VHs) by a strong extension without charge transfer. This result gives a new insight on the role of the intercalant in the renormalization of the bare electronic band structure of graphene usually observed in Graphite and Graphene Intercalation Compounds.

cond-mat.mtrl-sci↗

arXiv2011

Distributed Reasoning in a Peer-to-Peer Setting: Application to the Semantic Web

In a peer-to-peer inference system, each peer can reason locally but can also solicit some of its acquaintances, which are peers sharing part of its vocabulary. In this paper, we consider peer-to-peer inference systems in which the local theory of each peer is a set of propositional clauses defined upon a local vocabulary. An important characteristic of peer-to-peer inference systems is that the global theory (the union of all peer theories) is not known (as opposed to partition-based reasoning systems). The main contribution of this paper is to provide the first consequence finding algorithm in a peer-to-peer setting: DeCA. It is anytime and computes consequences gradually from the solicited peer to peers that are more and more distant. We exhibit a sufficient condition on the acquaintance graph of the peer-to-peer inference system for guaranteeing the completeness of this algorithm. Another important contribution is to apply this general distributed reasoning setting to the setting of the Semantic Web through the Somewhere semantic peer-to-peer data management system. The last contribution of this paper is to provide an experimental analysis of the scalability of the peer-to-peer infrastructure that we propose, on large networks of 1000 peers.

cs.AI↗

arXiv2011

Fourier Transform Scanning Tunneling Spectroscopy: the possibility to obtain constant energy maps and the band dispersion using a local measurement

We present here an overview of the Fourier Transform Scanning Tunneling spectroscopy technique (FT-STS). This technique allows one to probe the electronic properties of a two-dimensional system by analyzing the standing waves formed in the vicinity of defects. We review both the experimental and theoretical aspects of this approach, basing our analysis on some of our previous results, as well as on other results described in the literature. We explain how the topology of the constant energy maps can be deduced from the FT of dI/dV map images which exhibit standing waves patterns. We show that not only the position of the features observed in the FT maps, but also their shape can be explained using different theoretical models of different levels of approximation. Thus, starting with the classical and well known expression of the Lindhard susceptibility which describes the screening of electron in a free electron gas, we show that from the momentum dependence of the susceptibility we can deduce the topology of the constant energy maps in a joint density of states approximation (JDOS). We describe how some of the specific features predicted by the JDOS are (or are not) observed experimentally in the FT maps. The role of the phase factors which are neglected in the rough JDOS approximation is described using the stationary phase conditions. We present also the technique of the T-matrix approximation, which takes into account accurately these phase factors. This technique has been successfully applied to normal metals, as well as to systems with more complicated constant energy contours. We present results recently obtained on graphene systems which demonstrate the power of this technique, and the usefulness of local measurements for determining the band structure, the map of the Fermi energy and the constant-energy maps.

cond-mat.mes-hall↗

arXiv2010

Superlattice of resonators on monolayer graphene created by intercalated gold nanoclusters

Here we report on a "new" type of ordering which allows to modify the electronic structure of a graphene monolayer (ML). We have intercalated small gold clusters between the top monolayer graphene and the buffer layer of epitaxial graphene. We show that these clusters perturb the quasiparticles on the ML graphene, and act as quantum dots creating a superlattice of resonators on the graphene ML, as revealed by a strong pattern of standing waves. A detailed analysis of the standing wave patterns using Fourier Transform Scanning Tunneling Spectroscopy strongly indicates that this phenomenon can arise from a strong modification of the band structure of graphene and (or) from Charge Density Waves (CDW)where a large extension of Van Hove singularities are involved.

cond-mat.mes-hall↗

arXiv2009

Analytical study of the propagation of acoustic waves in a 1D weakly disordered lattice

This paper presents an analytical approach of the propagation of an acoustic wave through a normally distributed disordered lattice made up of Helmholtz resonators connected to a cylindrical duct. This approach allows to determine analytically the exact transmission coefficient of a weakly disordered lattice. Analytical results are compared to a well-known numerical method based on a matrix product. Furthermore, this approach gives an analytical expression of the localization length apart from the Bragg stopband which depends only on the standard deviation of the normal distribution disorder. This expression permits to study on one hand the localization length as a function of both disorder strength and frequency, and on the other hand, the propagation characteristics on the edges of two sorts of stopbands (Bragg and Helmholtz stopbands). Lastly, the value of the localization length inside the Helmholtz stopband is compared to the localization length in the Bragg stopband.

physics.class-ph↗

arXiv2008

Symmetry of standing waves generated by a point defect in epitaxial graphene

Using scanning tunneling microscopy (STM) and Fourier Transform STM (FT-STM), we have studied a point defect in an epitaxial graphene sample grown on silicon carbide substrate. This analysis allows us to extract the quasiparticle energy dispersion, and to give a first experimental proof of the validity of Fermi liquid theory in graphene for a wide range of energies from -800 meV to +800 meV. We also find evidence of a strong threefold anisotropy in the standing waves generated by the defect. We discuss possible relations between this anisotropy, the chirality of the electrons, and the asymmetry between graphene's two sublattices. All experimental measurements are compared and related to theoretical T-matrix calculations.

cond-mat.mes-hall↗

arXiv2004

GATE : a simulation toolkit for PET and SPECT

Monte Carlo simulation is an essential tool in emission tomography that can assist in the design of new medical imaging devices, the optimization of acquisition protocols, and the development or assessment of image reconstruction algorithms and correction techniques. GATE, the Geant4 Application for Tomographic Emission, encapsulates the Geant4 libraries to achieve a modular, versatile, scripted simulation toolkit adapted to the field of nuclear medicine. In particular, GATE allows the description of time-dependent phenomena such as source or detector movement, and source decay kinetics. This feature makes it possible to simulate time curves under realistic acquisition conditions and to test dynamic reconstruction algorithms. A public release of GATE licensed under the GNU Lesser General Public License can be downloaded at the address http://www-lphe.epfl.ch/GATE/.

physics.med-ph↗