Groupe de Physique Statistique

Equipe 106, Institut Jean Lamour

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Statistical Physics and Low Dimensional Systems 2011

Atelier des groupes Physique Statistique et Surfaces et Spectroscopies de l'institut Jean Lamour

mercredi 18 mai 2011 - vendredi 20 mai 2011

Programme de l'atelier

mercredi 18 mai 2011 Conférences plénières 14:30 - Mark Goerbig, Graphene: a two-dimensional electron system of Dirac fermions

Physique StatistiqueSystèmes de basse dimension
mercredi 18 mai 2011
15:30 - Coffee Break
16:00 - Lorenz Bartosch
16:45 - Gwendal Fève
19:00 - Party
jeudi 19 mai 2011
09:30 - Maxime Clusel
10:15 - Coffee Break
10:45 - Elisabeth Agoritsas
11:30 - Peter Holdsworth
12:30 - Lunch break
14:00 - Guillaume Roux
14:45 - Emmanuel Fort
15:30 - Coffee Break
16:00 - Peter Barmettler
vendredi 20 mai 2011
09:30 - Christian von Ferber
10:15 - Coffee Break
10:45 - Martin Michael Müller
11:30 - Jean-Charles Walter
12:30 - Lunch break
mercredi 18 mai 2011
16:00 - Coffee Break
16:30 - Antonio Tejeda
17:00 - Jean-Yves Veuillen
18:00 - Daniel Malterre
19:00 - Party
jeudi 19 mai 2011
09:00 - Andres Santander
09:30 - Mattia Mulazzi
10:00 - Valerio Olevano
10:30 - Coffee Break
11:00 - Christophe Brun
11:30 - Cédric Tournier-Colletta
12:30 - Lunch break
14:00 - Jérôme Faure
14:30 - Azzedine Bendounan
15:00 - Frederick Schiller
15:30 - Coffee Break
16:30 - Stéphane Pons
17:00 - Miguel Gonzalez-Barrio
vendredi 20 mai 2011
09:30 - Oliver Grönig
10:00 - Guillaume Schull
10:30 - Coffee Break
11:00 - Bruno Grandidier
11:30 - Laurent Limot
12:30 - Lunch break

Orateurs

Physique StatistiqueSystèmes de basse dimension
Elisabeth Agoritsas (Genève)
Temperature-induced crossovers in the static roughness of a one-dimensional interface
Interfaces are ubiquitous in nature, and display a large variety of characteristic lengthscales and different specific microphysics, ranging from domain walls in ferromagnets or ferroelectric materials, to the spread of imbibing coffee on a napkin. However, the generic framework of the "disordered elastic systems" allows to tackle theoretically the static and dynamic properties of such interfaces, by retaining essentially two physical ingredients in competition: the elasticity of the interface and the disorder of its underlying medium, blurred by thermal fluctuations at finite temperature. We actually explored the consequences of a finite correlation length of disorder, or alternatively a finite interface width (as it is always the case in experimental systems) on the geometrical fluctuations of a static 1D interface, by computing analytically its roughness using a replica approach and a Gaussian variational method (with full replica-symmetry breaking), and then probing its different scaling regimes depending on the lenghtscale of observation. The specific case of a 1D interface is in particular of special interest both for the experimental domain walls in thin ferromagnetic films, and for the theoretical study of 1+1 directed polymers in random media, which is just one of the many statistical physics models on which the 1D interface problem can be mapped.
Peter Barmettler (University of Geneva)
Local dissipation in quantum gases
We study the complex dynamics of a one-dimensional Bose gas subjected to a dissipative local defect which induces one-body atom losses. In experiments these atom losses occur for example when a focused electron or light beam or a single trapped ion is brought into contact with a quantum gas. We discuss how within such setups one can measure or manipulate densities locally and specify the excitations that are induced by the defect. In certain situations the localized dissipation can be used to generate entanglement in a controlled way.
Lorenz Bartosch (University of Frankfurt)
Scaling theory of the Mott transition and breakdown of the Grüneisen scaling near a finite-temperature critical end point
We discuss a scaling theory of the lattice response in the vicinity of a finite-temperature critical end point. The thermal expansivity is shown to be more singular than the specific heat such that the Grüneisen ratio diverges as the critical point is approached, except for its immediate vicinity. More generally, we express the thermal expansivity in terms of a scaling function which we explicitly evaluate for the two-dimensional Ising universality class. Recent thermal expansivity measurements on the layered organic conductor kappa-(BEDT-TTF)_2 X close to the Mott transition are well described by our theory.
Maxime Clusel (Montpellier)
A simple granocentric model for packing of polydisperse spheres
Gwendal Fève (ENS)
Current correlations of an on-demand single electron emitter
Ballistic electronic transport along the Quantum Hall edge states of two-dimensional electron gases presents strong analogies with the propagation of photons which have been illustrated, for example, by the realization of Mach-Zehnder interferometers [1]. Provided that one can produce on-demand single electronic states, these analogies can be pushed to quantum optics, based on the manipulation of single particles. Some celebrated quantum optics experiment, such as the single particle Hanbury-Brown and Twiss or the two particles Hong-Ou Mandel [2], could then be implemented in solid state with electrons. The achievement of these experiments relies both on the ability to produce these single electron states and on the ability to measure the output correlations between single electron beams. We present the use of short time current-current correlations to characterize a recently realized electron source that periodically emits a single electron propagating along a chiral edge channel [3]. When exactly a single charge is emitted at each cycle of the source, the current noise reduces to a fundamental limit given by the fluctuations in the emission time of a single charge [4]. This jittering , caused by the random delay between the trigger and the tunnel emission of a single particle [4,5], was already observed in the context of single photon emitters but is observed here for the first time in the context of electronics. It demonstrates the perfect emission of a single particle at each cycle of the source. In the opposite limit where the probability of charge emission per cycle is low, usual shot noise is recovered. [1] Y. Ji et al., Nature 422, 415 (2003). [2] S. Ol'khovskaya et al., Phys. Rev. Lett. 101, 166802 (2008). [3] G. Fève et al., Science 316, 1169 (2007). [4] A. Mahé et al. Phys. Rev. B 82, 201309 (2010) [5] M. Albert et al., Phys. Rev. B 82, 041407 (2010).
Emmanuel Fort (ESPCI, Paris)
Path-memory dynamics: emergence of quantum behaviours at the macroscopic scale
We have shown recently that a droplet bouncing on a vertically vibrated liquid interface can become dynamically coupled to the surface waves it excites. It thus becomes a self-propelled "walker", a symbiotic object formed by the droplet and its associated wave. Through several experiments we will address one central question. How can a continuous and spatially extended wave have a common dynamics with a localized and discrete droplet? We will show that in all cases (diffraction, interference, tunnelling, etc?) where the wave is split, a single droplet has an apparently random response but that a deterministic behaviour is statistically recovered when the experiment is repeated. The truncation of the wave is thus shown to generate an uncertainty in the drop?s motion. Finally, in another set of experiments analogous to Landau experiments, we demonstrate that when the walker has an orbiting motion, the possible radii of the orbit are discrete. We will show how these properties result from what we call the walker's "path-memory". The limits in which these results can be compared to those at quantum scale will be discussed.
Peter Holdsworth (ENS Lyon)
Magnetic Monopoles in Spin Ice
Model spin ice shows the remarkable property of fractionalization of magnetic moments into effective magnetic monopoles. The development of experimental signatures of the presence of monopoles in spin ice materials, Holmium and Dysprosium Titanate is hence one of the most engaging and exciting challenges of condensed matter physics. In this talk I will review recent work presenting such signatures, through diffuse neutron scattering and mu-SR experiments, using the Wein effect. I will also present a theory of diffusive monopole dynamics, motivated by magnetic relaxation measurements on both materials. I will show that the relaxation rate of the underlying string network, analagous to Dirac strings, is in quantitative agreement with magnetic relaxation in Dy2Ti2O7, providing an explanation for the non-standard nature of spin freezing in spin ice compounds and giving strong evidence for the presence of monopoles.
Martin Michael Müller (Metz)
Membrane-Mediated Interactions -- Entropic Forces on Curved Membranes
Particles embedded in a fluctuating interface experience forces and torques mediated by the deformations and by the thermal fluctuations of the medium. In this talk I will focus on the toy problem of two cylindrical particles adhering to a lipid membrane. We will see that the entropic contribution strongly enhances the curvature-mediated repelling force between the two cylinders. This is contrary to the usual attractive Casimir force in the absence of curvature-mediated interactions. For large separation between the cylinders, the renormalization of the surface tension of a flat membrane due to thermal fluctuations is retrieved.
Guillaume Roux (LPTMS Paris Sud)
From slow to sudden quenches in the Bose-Hubbard model
We review recent results on the physics of quenches in quantum many-body systems throughout the example of the one-dimensional Bose-Hubbard model. The finite-rate quench is first discussed with the focus on the scaling of the heat put in the system with the ramp time and transport phenomena in the case of inhomogeneous systems encountered in trapped cold-atoms setups. The nature of the energy distribution in the sudden quench limit is eventually addressed, showing the role of integrability and finite-size effects in the observed non-thermalisation.
Christian von Ferber (Coventry University)
Effects of excluded volume and correlated disorder on the shapes of self-avoiding walks
We apply the field theoretical renormalization group to analyze universal shape properties of long polymer chains modelled as self-avoiding walks in a correlated environment. Many analytical calculations focus on the scaling exponents that govern conformational properties of polymer macromolecules. Here, we consider observables that are related to universal ratios. These are universal in the sense that given a polymer in a good solvent, they are independent of the chemical structure of the macromolecules and the details of the solvent. In particular we focus on ratios characterising the deviation from the spherical shape as well as the relation between the end-end distance and the gyration radius of the polymer. These questions have a long history going back to Kuhn's work on random walks 1934 with the aim to understand the viscosity of polymer solutions. Here, we address the question of the influence of excluded volume and correlated disorder on the shapes acquired by the long flexible macromolecules. This question may be of relevance for the understanding of the behavior of macromolecules in colloidal solutions, near microporous membranes, or even in a biological environment. To this end, we consider a model of polymers in D dimensions in an environment with structural obstacles, characterized by a pair correlation function h(r), that decays with distance r according to a power law: h(r) ~ 1/r^a . We apply the field-theoretical renormalization group approach and expand in both (4-D) and (4-a) to estimate ratios that characterise the end-end distance, the gyration radius and rotationally invariant measures of the deviation from the spherical shape. [V Blavatska, C von Ferber, and Yu. Holovatch; Effects of disorder on the shapes of macromolecules, Condensed Matter Physics (2011)]
Jean-Charles Walter (KU Leuven)
Non-equilibrium effects in DNA microarrays: a multiplatform study
Microarrays are devices which consist of tens of thousands single stranded DNA sequences tethered on the surface. They are an efficient tool allowing (among other applications) the analysis of gene expression experiments on a large scale. However, the knowledge of hybridization in microarrays is still not well understood. A better understanding of the physical process involved in such devices would allow to use them (e.g. for gene expression or detection of mutations) into more precise tools based on thermodynamics rather than statistical methods. In the past, the Langmuir model, a two-state model, has been used to describe the kinetics of the hybridization process (i.e. binding between a single strand in solution with a strand fixed on the microarray). However, recently it has been shown that under certain circumstances the equilibration time exceeds the actual experimental time resulting in a deviation from the Langmuir isotherm. The latter has been observed by Hooyberghs et al (PRE, 2010) in Agilent microarrays who present an extension of the two-state Langmuir model to a three-state kinetic model. This model predicts namely an effective temperature greater than the experimental one in an intermediate regime. In order to check the robustness of this model with respect to the microarray platform, we analyze data obtained with Agilent microarrays, Affymetrix microarrays and Codelink activated slides. It confirms that data can be described by the three-state model involving an effective temperature.
Azzedine Bendounan (Synchrotron SOLEIL)
Time/Angle resolved photoemission and NEXAFS experiments at TEMPO beamline of Synchrotron SOLEIL
TEMPO beam-line at Synchrotron SOLEIL provides soft-X-ray light with variable polarization and tunable photon energy in the range from 40 eV to 1500 eV. Well-equipped with surface science techniques, the experimental end-station is dedicated to study dynamic processes of electronic and magnetic properties of materials. The specific characteristics of the installation related to the time dependent experiments are the basis of the planned pump/probe experiments combining laser and synchrotron radiation pulses. Also, high quality NEXAFS measurements together with angle resolved photoemission experiments in variable temperature and pressure environments are routinely given at TEMPO beam-line. Here, I will give a description of the beam-line installation and present recent results of the group members.
Christophe Brun (INSP Paris)
Decay mechanisms of quantum-well states and Superconductivity in ultrathin Pb islands grown on Si(111): a scanning tunneling spectroscopy study
A clear picture of the decay mechanisms of low-energy electronic excitations has been obtained in several types of bulk metals as well as at various metal surfaces [1]. However, due to various technical limitations, few studies have reported so far detailed lifetime investigations of metallic quantum well states [2]. Only one scanning tunneling spectroscopy (STS) study reported a quantitative linewidth analysis of a QWS metal system, Yb(111)/W(110) [3], but the results were subsequently questioned by a combined time-resolved two-photon photoemission and DFT study on bulk Yb [4]. Using low temperature STS in ultrahigh vacuum (UHV) environment, we studied the linewidth of unoccupied quantum well states (QWS) in Pb nanocrystals of thicknesses between 7 and 22 monolayers, deposited on Si(111) and grown in UHV on two different Pb/Si interfaces. The Pb/Si(111) system is very suited to study thickness dependent properties because the thickness of the Pb film or islands can be controlled at the atomic monolayer, due to very prononced quantum size effects occuring in this system [5]. A quantitative analysis allowed us to determine the various decay contributions of the QWS excitations in terms of electron-electron, electron-phonon and interface-defects scattering. These quantities are in good agreement with the corresponding ab initio calculated quantities [6]. In addition, the superconducting energy gap was measured as a function of island thickness between 5ML and 60 ML. It is found to decrease with decreasing island thickness, from about the bulk value for 60 ML to about 60% of the bulk value for 5ML islands. The observed reduction with decreasing island thickness was rationalized employing DFT calculations for free-standing Pb films [7] and is in good agreement with ex-situ magnetic susceptibility measurements on Ge-capped Pb ultrathin filmsgrown on Si(111) [8]. References [1] E.V. Chulkov et al. Chem. Rev. 106, 4160 (2006) [2] J.J. Paggel, T. Miller, and T.-C. Chiang, Science 283, 1709 (1999) [3] D. Wegner, A. Bauer, and G. Kaindl, Phys. Rev. Lett. 94, 126804 (2005) [4] V. P. Zhukov et al. Phys. Rev. B 76, 193107 (2007) [5] Z. Zhang et al. Phys. Rev. Lett. 80, 5381 (1998) [6] I.-P. Hong et al. Phys. Rev. B 80, 081409 (R) (2009) [7] C. Brun et al. Phys. Rev. Lett. 102, 207002 (2009) [8] M. M. Ozer et al. Nature Physics 2, 173 (2006)
Jérôme Faure (Ecole Polytechnique)
FEMTO-ARPES: Time-resolved photo-emission on bismuth
Mark Goerbig (LPS Orsay)
Graphene: a two-dimensional electron system of Dirac fermions
Graphene is not only a fascinating material because it is the first truly two-dimensional crystal, but also in view of its amazing electronic properties. Indeed, low-energy electrons in graphene behave in a manner that is better known in high-energy rather than in condensed matter physics, because they are described by a Dirac equation for massless ultra-relativistic fermions, though with the Fermi velocity playing the role of the speed of light. Here, we review some of the physical consequences of the relativistic nature of graphene electrons, namely those related to their chiral properties in a magnetic field. These chiral properties, such as the absence of backscattering for a smooth potential, yield a very particular particle-hole excitation spectrum with novel collective excitations. The particle-hole excitation spectrum shows striking differences when compared with that of conventional two-dimensional electron systems, as a consequence of the ultra-relativistic character of graphene electrons.
Symmetry decoupled surface states in Au(100)
The Shockley surface state, common among the (001)-terminated surfaces as Cu, Ag and Au, is very sensitive to surface modifications, such as gas adsorption or surface reconstruction, due to its localization at the surface. The Au(100) reconstruction can be described as a floating, corrugated hexagonal layer on top of the bulk-terminated, square-symmetry substrate, similar to other 5d metal reconstructed surfaces, such as Ir(100) and Pt(100). The reconstruction superperiodicity, as determined by STM and LEED, is (5x26). The substrate Shockley surface state survives the reconstruction and becomes an interface surface state. Fermi surface and E vs. k photoemission mesaurements reveal that the 3D bulk sp bands and the 2D interface state replicate with the superperiodicity of the reconstruction due to Umklapp processes. Moreover, we observe the signature of a quasi 1D electronic state, confined within the reconstruction fringes, that also replicates with the superperiodicity of the reconstruction.
Bruno Grandidier (IEMN - Lille)
Coulomb energy determination of a single Si dangling bond
Adding or removing electrons from materials costs energy. This supply of energy increases as the size of the materials shrinks and has been beautifully demonstrated when current is flowing through artificial atoms, such as small metal particles and semiconductor quantum dots [ , , ]. Indeed, in this confined structures, the tunneling conductance displays resonances, that arise entirely or partly from Coulomb repulsion between the confined electrons. A Coulomb blockade mechanism also occurs when electrons are transferred through the bound states of a single atom [ ], but coupling a single atom to electrical leads through tunnel junctions and switching its charge state is still a difficult task [ ]. In addition, depending on the dielectric environment and the chemical bonding of the atom, changing its electron population may lead to significant relaxation effects. Such effects modify the intra-atomic Coulomb repulsion energy, also called the Hubbard U splitting, and a precise knowledge of the effective U* energy on an atomic scale basis is still missing. Point defects in semiconductor materials may consist of a single atom, that exhibits energy levels in the band gap region of the materials. Due to their localized character, these deep levels can trap up to two electrons on the same state. Therefore, they form an important system to study U*, because a change of the trapped electron population may significantly affect the nature and degree of electrical conductivity in electronic devices. Here, with scanning tunneling microscopy (STM), we investigate U* for a protypical point defect, a single Si dangling bond (DB) at the surface of a B-doped Si(111)-?3x?3 R30° surface. The cancellation of the energy dependence of the probability transmission allows to characterize the transition between the single particle electronic spectrum and the shell-filling spectrum, that is obtained when the inelastic current through the dangling bond ground state approaches saturation. From the different charge states of the Si dangling bond, the effective correlation of a single Si dangling bond is measured.
Oliver Grönig (EMPA Thun- Swiss)
Surfaces with modulated surface potential for templated adsorption and self-assembly
abstract coming soon
Laurent Limot (Strasbourg)
Inviting Shockley surface states into molecular electronics
The performance of modern electronics has increased steadily thanks to the ongoing miniaturization of its components. For more than a decade now, hybrid metal-molecule devices have been under strong scrutiny as they represent a possible new route towards an ultimate miniaturization of electronic elements. Research in this field, also known as molecular electronics, has been driven by an assortment of promising technological advancements, both fundamental and applied in nature. In this talk we will in particular focus on the interplay of molecules with a twodimensional electron gas common to many noble metals, namely the Shockley surface states. Ever since the mapping of their standing wave pattern on the close-packed (111) surfaces by means of scanning tunneling microscopy and spectroscopy (STM and STS), these states turned out to be an ideal playground for a variety of STM experiments. Here we will use STM and STS to demonstrate two things: 1) The localization of Shockley surface states by molecules with a metallic center; 2) The feasibility of using Shockley surface states and a C60 molecule to engineer a negative differential resistance (NDR) at the molecular level.
Daniel Malterre (IJL - Nancy)
Symmetry breaking and gap opening in two-dimensional hexagonal lattices
The inhibition in wave propagation at band gap energies plays a central role in many areas of technology such as electronics (electron gaps), nanophotonics (light gaps) and phononics (acoustic gaps) among others. Here we demonstrate that metal surfaces featuring free-electron-like bands may become semiconducting by periodic nanostructuration. We combine scanning tunneling spectroscopy and angle-resolved photoemisssion to accurately determine the energy-dependent local density of states and band structure of the Ag/Cu(111) noble metal interface patterned with an array of triangular dislocations, demonstrating the existence of a 25 meV band gap that extends over the entire surface Brillouin zone. We prove that this gap is a general consequence of symmetry reduction in close-packed metallic overlayers, in particular we show that the gap opening is due to the symmetry lowering of the wave vector group at the $K$ point from $C_{3v}$ to $C_{3}$.
Mattia Mulazzi (Wuerzburg)
Strong correlations in f-electron systems: from the bulk to surface science
This contribution is devoted to the presentation of some recent results in the field of f-electron systems. While 50 years have passed since the explanation of the Kondo effect, the physics lanthanoids is far from being dead. On the contrary, new surprising effects are reported and old problems remain unsolved, like for instance, the hidden order phase in URu2Si2. The interest in f-electron systems lies in the fact that many phenomena co-exist and become important or negligible as functions of one or more order parameters, which can be the external pressure or the magnetic field or the temperature. While the physics of these compounds is very interesting, the application of spectroscopic techniques may yield doubtful results since the surface sensitivity of the latter. For this reason we applied a new approach to the field, i.e. the growth of artificial surface alloys that retain the structure of the bulk compounds, while featuring other advantages. First, advanced surface science methodologies allow both the creation and the characterisation of reproducible samples (not granted for cleaved samples); second, the reduced thickness of the alloy, ranging up to a few tens of nanometers, may reduce the dimensionality of the electronic structure and thus allow easier theoretical approaches. We will present our photoemission results for the CePt5 surface alloy grown on Pt(111) showing that the high- and low-energy degrees of freedom give different information about the band structure and the many-particle phenomena determining its low-temperature behaviour. Similarly, recent high-resolution photoemission data on URu2Si2 show a progress in the understanding of the hidden order phase by showing new electron bands not previously reported. The confirmation of the experimental results from the theoretical calculations opens the way to the study of strong correlations and interactions between ordered magnetic impurities in non-magnetic hosts by surface science methods and to the creation of artificial compounds at surfaces featuring unusual properties.
Valerio Olevano (Institut Neel - Grenoble)
Electronic structure by ab initio many-body GW: spectroscopy and quantum transport from 0-temperature to non-equilibrium Green's function theory
Ground-state equilibrium properties of materials, such as the atomic structure, the total energy and the electronic density, have been extensively investigated from ab initio by successful density-functional theory (DFT). On the other hand DFT cannot describe excited-state properties, such as the excitations sampled in ARPES spectroscopy or the conductance measured out-of-equilibrium in quantum transport experiments. One needs to go beyond. A possibility is represented by the many-body Hedin's GW approximation [1], both in the framework of 0-temperature and also in non-equilibrium Green's function (NEGF) theory [2]. We will present ab initio GW calculations of the electronic structure, the momentum distribution and the conductance. The systems will range from 3D standard Fermi liquid metals (Na) and more exotic Mott or excitonic insulators (VO2, TiSe2), to low dimensional systems, such as 2D graphene, 1D nanowires and 0D organic molecules. Our results will be compared to ARPES, IXSS, and quantum transport experiments. [1] L. Hedin, Phys. Rev. 139, A796 (1965). [2] L. P. Kadanoff and G. Baym, Quantum Statistical Mechanics.
Stéphane Pons (EPFL - Lausanne)
Unconventional Rashba spin-orbit interactions at surfaces
abstract comming soon
Andres Santander (Paris Sud)
A metallic two-dimensional electron gas with universal subbands at the surface of insulating SrTiO3
Similar to silicon that is the basis of conventional electronics, strontium titanate (SrTiO3) is the bedrock of the emerging field of oxide electronics. SrTiO3 is the preferred template to create exotic two-dimensional (2D) phases of electron matter at oxide interfaces, exhibiting metal-insulator transitions, superconductivity, or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs) remains elusive, although its determination is crucial to understand their remarkable properties. In this talk, we present our angle-resolved photoemission spectroscopy (ARPES) results showing that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3, independent of bulk carrier densities over more than seven decades, including the undoped insulating material [A. F. Santander-Syro et al., Nature 469, pp 189-193, 2011]. Our data unveil a remarkable electronic structure consisting on multiple subbands of heavy and light electrons. The analysis of the data shows that this 2DEG is confined within a region of ~5 unit cells with a sheet carrier density of ~0.33 electrons per a^2 (a is the cubic lattice parameter). The similarity of this 2DEG with those reported in SrTiO3-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices, and a novel route to generate 2DEGs at surfaces of functional oxides.
Frederick Schiller (San Sebastian)
Surface state engineering by surface alloying
Surface states of noble metals are model systems of free electron scattering in periodic structures. At the pure (111) surfaces the characteristic surface state Fermi wavelength kF is much smaller than the Brillouin zone (BZ) boundaries. Therefore the influence of the 2-5 Å periodicity of the surface network is not noted. Superstructures derived from reconstructions, steps, or decorations may change this situation. Here I will show that the surface state kF dimensions of dislocations networks, like the ones found in the monolayer thick Ag and Au films on Cu(111) and Ni(111), respectively, are similar to the superstructure induced smaller BZ. Such a situation results in gap openings at the BZ boundaries and can even influence the lattice constant of the superstructures. For 1ML Ag/Cu(111) the usual ring-like Fermi surface of the pure (111) noble metals transforms into small hole pockets around the K points. One can further modify this electronic structure by additional alloying with a third material, e.g., a small amount of Au shifts the surface band structure and the band gap below the Fermi energy creating electron pockets around the M points.
Guillaume Schull (IPCMS - Strasbourg)
Atomic-scale engineering of molecular contacts
abstract coming soon
Antonio Tejeda (IJL - Nancy - Synchrotron SOLEIL)
First observation of nearly ideal dispersion on graphene onto C-face SiC
First observation of nearly ideal dispersion on graphene onto C-face SiC Graphene was ?rst isolated by chemical exfoliation in 1961 [1] and later shown to grow epitaxially on SiC in 1975 [2] and 1998 [3]. However, the interest on its electronic properties began on 2004 [4] due to its potential for carbon electronics. Graphene grown on the (000-1) (C-face) of SiC exhibits all the transport properties of an isolated graphene sheet [5], so it should exhibit the ideal linear dispersion that has been ellusive to observe in other graphene preparations. This system presents several graphene sheets rotated by different angles, giving rise to a non Bernal stacking. Such rotations break the stacking symmetry of graphite and lead to each single sheet behaving like an isolated graphene plane. We have shown that all the graphene rotated sheets leave the linear dispersion unaffected [6,7]. The system exhibits the most ideal linear dispersion observed up to date and accomplishes the requirements for carbon electronics development: compatibility with mass production and standard lithographic techniques, scalability and a nearly ideal dispersion. [1] H.P. Von Boehm et al., Z. Naturforschg. b 17, 150 (1962). [2] A.J. Van Bommel et al., Surf. Sci. 48, 463 (1975). [3] I. Forbeaux et al., Phys. Rev. B 58, 16396 (1998). [4] C. Berger et al. 2004 J. Phys. Chem. B, 108, 19912 (2004). K. Novoselov et al., Science 306, 666 (2004). [5] C. Berger et al., Science 312, 1191 (2006). [6] M. Sprinkle et al., Phys. Rev. Lett. 103, 226803 (2009). [7] M. Sprinkle et al., J. Phys. D: Appl. Phys., 43, 374006 (2010).
Cédric Tournier-Colletta (IJL - Nancy)
Lifetime of electronic quasiparticles confined in silver nanopyramids
Within the framework of quantum many-body theory, an extra electron (or hole) added to a crystal is dressed and forms a quasiparticle. A characteristic feature of quasiparticles is their lifetime ?/? which is due to the scattering of the bare electron by the other electrons, phonons, impurities and defects. According to Fermi liquid theory (FLT), the contribution of e-e interaction to the spectral width ? must vanish as the energy of the quasiparticle goes to EF. Experimentally, by mean of scanning tunneling spectroscopy (STS), one can measure the energy dependence of the spectral width by confining the excitations in nanometric resonators [1]. Nevertheless, the non-ideal reflection at the island edges induces an extra contribution to the spectral width, which generally dominates all other contributions [2][3]. Here we present a low-temperature (5K) STS study on electronic quasiparticles confined on peculiar quantum resonators, namely silver hexagonal nanopyramids. By considering resonators of suitable lateral size (L~20-30 nm) and analysing many quantum well states of different symmetry, spectral widths are shown to exhibit a distinct minimum in the vicinity of EF, as expected in the FLT [4]. By basing on many-body lifetime GW calculations [5][6], we access ultimately the lossy boundary contribution and demonstrate that electronic confinement is more efficient in nanopyramids compared to simple adatom islands [7][8].
Jean-Yves Veuillen (Grenoble - Insitut Neel)
Reconstruction dependent graphene substrate interaction investigated by STM and ab-inito calculations.
The interaction of a graphene layer with the environment may alter the electronic properties of the material. Specifically, for supported layers the coupling with the substrate is actually an important issue. We address this question in the case of graphene monolayers grown by preferential sublimation of SiC substrates. The hexagonal SiC wafers have two different faces, called C and Si faces, which exhibit quite different behaviour. It is well established that on the Si face a strong interaction between the first graphitic layer and the SiC surface takes place [1,2], leading to a strong modification of the ? bands and to the disappearance of the Dirac cones [3]. On the C face the graphene/substrate coupling was found to be much weaker [3,4], and to depend on the SiC surface reconstruction [4]. We shall present STM and STS data, complemented by ab-initio calculations, which indicate that the interaction is small (resp. moderate) for the (3x3) (resp. (2x2)) reconstructed surface [5,6]. We shall also discuss the role of interface defects in the doping of the surface graphene layer [7]. [1] Kim S et al., Phys. Rev. Lett. 100, 176802 (2008) [2] Varchon F et al., Phys. Rev. B 77, 235412 (2008) [3] Emtsev K Vet al., Phys. Rev. B 77, 155303 (2008) [4] Hiebel Fet al., Phys. Rev. B 78, 153412 (2008) [5] Magaud L et al., Phys. Rev. B 79, 161405(R) (2009) [6] Hiebel F et al., Phys. Rev. B 80, 235429 (2009) [7] F. Hiebel et al., Phys. Rev. B 83, 075438 (2011)

Communication par poster

Olivier Collet (Ecole des Ponts ParisTech)
How does water fold proteins so fast.

Comité d'organisation

 Bertrand Berche Christophe Chatelain Olivier Collet Yannick Fagot-Révurat Jean-Yves Fortin Malte Henkel Dragi Karevski Bertrand Kierren Daniel Malterre Luc Moreau Loic Turban Tomasz Wydro