Groupe de Physique Statistique

Equipe 106, Institut Jean Lamour

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Grp Travail
Theses, Postes

Statistical Physics and Low Dimensional Systems 2008

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

mercredi 21 mai 2008 - vendredi 23 mai 2008

Programme de l'atelier

Conférences plénières
mercredi 21 mai 2008
12:00 - Barbecue - Opening
14:30 - Jose Texeira, Hydrogen bonds in water and consequences for its unusual behaviour
15:15 - Clement Sire, The out of equilibrium physics of poker tournaments
15:45 - Fernando Flores, Electronic transport in carbon nanotubes: diffusive and localized regimes
16:15 - Coffee Break
16:45 - Fabien Alet, Statistical physics of dimers
17:15 - Gerard Le Caer, Local fluctuations of eigenvalues of random matrices from the beta-Hermite ensemble
17:45 - Jose Ortega, The In/Si(111)-4x1 to 8x2 reversible phase transition

Physique StatistiqueSystèmes de basse dimension
jeudi 22 mai 2008
08:30 - Welcome Coffee
09:00 - Jean-Christian Angles d'Auriac
09:20 - Wolfhard Janke
09:40 - Ralph Kenna
10:00 - Sandro Wenzel
10:20 - Coffee Break
11:00 - Frédéric van Wijland
11:30 - David Dean
12:00 - Ludovic Berthier
12:30 - Lunch break
14:00 - Paolo de los Rios
14:30 - Dominik Horinek
15:00 - Mounir Tarek
15:30 - Christian von Ferber
16:00 - Coffee Break
16:40 - Kay Wiese
17:10 - Haye Hinrichsen
17:40 - Laurent Joly
vendredi 23 mai 2008
08:30 - Welcome Coffee
09:00 - Ferenc Iglói
09:20 - Sreedhar Dutta
09:40 - Viktor Eisler
10:00 - Rosemary Harris
10:20 - Coffee Break
11:00 - Jean-Marc Luck
11:30 - Pasquale Calabrese
12:00 - Frederic Mila
12:30 - Lunch break
14:00 - Marco Picco
14:30 - Alberto Rosso
14:50 - Sakuntala Chatterjee
jeudi 22 mai 2008
08:30 - Welcome Coffee
09:00 - Patrick Lefevre
09:30 - Etienne Janod
10:00 - Claude Monney
10:30 - Coffee Break
11:00 - Frederic Cherioux
11:30 - Laurent Simon
12:00 - Boris Andrushechkin
12:30 - Lunch break
14:30 - Maria-Carmen Asensio
15:00 - Enrique Garcia Michel
15:30 - Ad Ettema
16:00 - Coffee Break
16:30 - Christophe Brun
17:00 - Dimitri Roditchev
17:30 - Gautier Mahieu
18:00 - Muriel Sicot
18:30 - Poster Session
vendredi 23 mai 2008
08:30 - Welcome Coffee
09:00 - Safia Ouazi
09:30 - Jean-Yves Veuillen
10:00 - Pascal Turban
10:30 - Coffee Break
11:00 - Véronique Brouet
11:30 - Yannick Fagot-Revurat
12:00 - Antonio Tejeda
12:30 - Lunch break
14:30 - Markus Klein
15:00 - Andres Santander
15:30 - A.K. Shukla
16:00 - Stephane Pons
16:30 - Coffee Break


Physique StatistiqueSystèmes de basse dimension
Fabien Alet (Université Paul Sabatier, Toulou)
Statistical physics of dimers
I will present recent results on the statistical physics of dimers, especially in three dimensions. Already at the non-interacting level, dimers on bipartite lattices show an unexpected behavior with the apparition of dipolar correlations. I will review how this can be understood within a "fictitious electromagnetism" analogy. Turning on interactions between dimers gives rise to unconventional phase transitions and emerging symmetries, which cannot be understood within the standard Landau theory of phase transitions.
Jean-Christian Angles d'Auriac (Grenoble)
Birational mapping from the Chiral Potts model
We introduce discrete dynamical systems originating from 2-dimensional lattice statistical mechanics models. These dynamical systems are birational mappings which show a reduced degree growth. All the mappings associated to the q-state Potts model with q a prime or the square of a prime are found. For the other cases, a symbolic enumerative algorithm is used for q small. A two parameters integrable family is found.
Ludovic Berthier (Labo des Colloides, Verres et Na)
Self-diffusion and collective dynamics in glass-forming liquids
Pasquale Calabrese (Universita' di Pisa)
Dynamical correlation functions of the one-dimensional Bose gas
The momentum- and frequency-dependent correlation functions (one-body and density-density) of the one-dimensional interacting Bose gas (Lieb-Liniger) are obtained for any value of the interaction parameter. In the repulsive regime, we use a method that mixes algebraic Bethe ansatz and numerics to reconstruct the correlators to high accuracy for systems with finite but large numbers of particles. For attractive interactions, the correlations are computed analytically. Our results are discussed, with particular emphasis on their applications to quasi-one-dimensional atomic gases.
Sakuntala Chatterjee (Forschungszentrum Jülich)
Diffusion of Hydrocarbon Mixture in One Dimensional Zeolite Channel
Zeolite channels can be used as effective hydrocarbon traps. Earlier experiments (Czaplewski {\sl et al.}, 2002) show that the presence of large aromatic molecules (toluene) block the diffusion of light hydrocarbon molecules (propane) inside the narrow pore of a zeolite sample. As a result, the desorption temperature of propane is significantly higher in the binary mixture, than in the single component case. We propose a simple lattice gas model to describe the diffusion of propane and toluene in a one dimensional zeolite channel. We model the system by a two component symmetric exclusion process in an open channel and our Monte Carlo simulations show that taking into account the single file diffusion of the two species with diffusivities that show an Arrhenius dependence on temperature, one can explain the qualitative features of the temperature programmed desorption profile observed in experiments.
Paolo de los Rios (Laboratoire de Biophysique Stati)
Spectral coarse graining of networks
David Dean (Université Paul Sabatier, Toulou)
Diffusion in a non-Gaussian potential in one dimension
Sreedhar Dutta (KIAS, Seoul)
Crossovers in model-A systems due to nonequilibrium perturbations
Non-conserved N-Vector models are subjected to spacial-anisotropic nonequilibrium perturbations. It is believed that these perturbations are irrelevant in model-A systems. We show, using Renormalization group analysis, that the perturbations, though might be marginally irrelevant, can make the models cross over to dynamic Ising class.
Viktor Eisler (Freie Universität Berlin)
Entanglement in a periodic quench
I will present results on the evolution of entanglement entropy in the simple case of periodically switching the dimerization in a chain of free electrons. It is shown that the entropy of a given segment evolves in a stepwise manner accompanied by additional slow oscillations and reaches an extensive asymptotic value. The case of full dimerization is particularly instructive and relations to equilibrium lattice models will also be pointed out.
Rosemary Harris (Queen Mary, University of London)
Non-equilibrium collective behaviour of market traders
We introduce a stochastic heterogeneous interacting-agent model for the short-time non-equilibrium evolution of excess demand and price in a stylized asset market. We consider a combination of social interaction within peer groups and individually heterogeneous fundamentalist trading decisions which take into account the market price and the perceived fundamental value of the asset. The resulting excess demand is coupled to the market price. Rigorous analysis reveals that this feedback may lead to price oscillations, a single bounce, or monotonic price behaviour. The model is a rare example of an analytically tractable interacting-agent model which allows us to deduce in detail the origin of these different collective patterns. For a natural choice of initial distribution the results are independent of the graph structure that models the peer network of agents whose decisions influence each other.
Haye Hinrichsen (Universität Würzburg)
Boundary-induced nonequilibrium phase transition into absorbing states
Non-equilibrium phase transitions may be induced by the dynamics of a single site at the boundary. As an example we consider a one-dimensional model of diffusing particles, where a single site at the boundary evolves according to the dynamics of a contact process. As the rate for offspring production at this site is varied, the model exhibits a phase transition from a fluctuating active phase into an absorbing state. The talk discusses the universal properties of the transition, approximation methods and related models.
Dominik Horinek (Technische Universität München)
Forces between biomolecules and hydrophobic surfaces
Adsorption of biomolecules at interfaces is determined by an interplay of solvation forces and direct interations with with surface. There are no analytical models for the solvation forces in such complex systems, but atomistic simulations can describe all forces that determine surface adsorption. We study peptide adsorption forces and friction forces on a hydrophobic and a hydrophilic diamond surface by molecular dynamics simulations with explicit solvent. The analysis of the friction forces show that equilibrium conditions are achieved on the hydrophobic surfaces. The resulting free energies compare well with AFM experiments. The resulting free energy is decomposed into contributions of different origin. This decomposition reveals that the "hydration force" has several contributions that partly compensate. On hydrophilic surfaces, friction effects are stronger than on the hydrophobic surfaces, and the simulations cannot be done under equilibrium conditions. We present a thorough analysis of the friction effects on peptide motion on the hydrophilic and the hydrophobic surface.
Ferenc Iglói (Theoretical Solid State Physics,)
Nonequilibrium critical dynamics of the two-dimensional Ising model quenched from a
The universality class, even the order of the transition, of the two-dimensional Ising model depends on the range and the symmetry of the interactions (Onsager model, Baxter-Wu model, Turban model, etc.), but the critical temperature is generally the same due to self-duality. Here we consider a sudden change in the form of the interaction and study the nonequilibrium critical dynamical properties of the nearest-neighbor model. The relaxation of the magnetization and the decay of the autocorrelation function are found to display a power law behavior with characteristic exponents that depend on the universality class of the initial state.
Wolfhard Janke (Institut für Theoretische Physik)
Universal aspects of Ising droplets
Within the Ising lattice gas model a large-scale Monte Carlo study of equilibrium droplets is discussed and compared with analytical predictions. Emphasis is placed on universal aspects of the condensation/evaporation transition in two dimensions. To this end extensive simulations for square lattices with nearest-neighbour (NN) and next-nearest-neighbour (NNN) interactions as well as for NN triangular lattices are compared with each other and confronted with the theory, which a priori is applicable only for asymptotically large NN square lattices. By using exact values when available or carefully determined numerical estimates for a few system parameters (magnetization, susceptibility, Wulff interface free energy), we indeed find for all three systems a characteristic, universal behaviour at the condensation/evaporation transition when suitably scaled variables are employed.
Laurent Joly (Université Lyon 1)
Wetting on nanorough surfaces
I will present a free-energy approach to the dynamics of a fluid near a nano-structured surface. The model accounts both for the static phase equilibrium in the vicinity of the surface (wetting angles, Cassie-Wenzel transition) and the dynamical properties like liquid slippage at the boundary. This method bridges the gap between phenomenological phase-field approaches and more macroscopic lattice-Boltzmann models.
Ralph Kenna (Applied Mathematics Research Cen)
Scaling Analysis of the Site-Diluted Ising Model in Two Dimensions
A combination of numerical and analytical advances are applied to analyze the scaling behaviour of the site-diluted Ising model in two dimensions, paying particular attention to the multiplicative logarithmic corrections in that model. The analysis focuses primarily on the odd sector, and in particular on Lee-Yang zeros, which are determined to high accuracy. Scaling relations for logarithmic corrections are used to connect to the even sector, and a first analysis of the density of zeros yields information on the specific-heat. The analysis is fully supportive of the strong scaling hypothesis and scaling relations for logarithmic corrections are verified.
Gerard Le Caer (Université de Rennes-I)
Local fluctuations of eigenvalues of random matrices from the beta-Hermite ensemble
The local fluctuations of adequately rescaled eigenvalues of some fundamental random matrix ensembles such as the three classical Gaussian ensembles, define universality classes in the limit of large matrix size. Indeed, they depend ultimately on the matrix symmetries and are independent on the details of the probability distributions of matrix elements. These properties hold not only for much broader classes of random matrices but they do apply to many problems in various areas of physics and of mathematics (1), some of which appearing as being unrelated to random matrices. The random matrices from the beta-Hermite (beta-HE) ensemble are real symmetric and tridiagonal (2,3) where beta, which can take any positive value, is the reciprocal of the temperature in the classical 2D electrostatic interpretation of eigenvalues. The distribution of eigenvalues coincide with those of the three Gaussian ensembles for beta = 1, 2, 4 respectively. The use of the beta-HE ensemble results in an incomparable speed up and efficiency of numerical simulations of all spectral characteristics of large random matrices. The evolution with b of the distribution P(s) of the spacing ‘s’ between nearest-neighbour levels of unfolded spectra of random matrices from the beta-HE is investigated by Monte Carlo simulations. Generalised gamma (GG) distributions are shown to be excellent approximations of P(s) for any beta while being still simple (3). They account both for the level repulsion in s**b when s--> 0 and for the whole distribution shape in the range of ‘s’ which is accessible to experiment or to most numerical simulations. GG distributions are found too to be excellent approximations of the simulated distributions of the n th-order spacing (n=1,..,4) , which is the spacing between two levels separated by n consecutive levels, for any beta and for any n. The simple logarithmic behaviour of the spacing correlation functions discussed by Relano et al. (4) for the Gaussian ensembles is actually a direct and simple consequence of the Gaussian distributions of the n th-order spacings with a variance depending linearly on ln(n), even for small values of n. (1) M. L. Mehta, Random Matrices, third ed. , Academic Press, New York, 2004. (2) I. Dumitriu and A. Edelman, J. Math. Phys. 43 (2002) 5830. (3) G. Le Caër and R. Delannay, J. Phys. A: Math. Theor. 40 (2007) 1561; G. Le Caër, C. Male and R. Delannay, Physica A 383 (2007) 190; C. Male, G. Le Caër, and R. Delannay Phys. Rev. E 76 (2007) 042101; G. Le Caër, C. Male and R. Delannay, Physica A (2007) submitted; G. Le Caër, C. Male and R. Delannay, in preparation. (4) A. Relano, J. Retamosa, E. Faleiro and J.M.G. Gomez, Phys. Rev. E 72 (2005) 066219.
Jean-Marc Luck (CEA Saclay)
A growth process driven by records
Frederic Mila (Ecole Polytechnique de Lausanne)
Hidden order in spin liquids
The concept of spin liquid has been introduced to describe magnets that do not possess long-range magnetic order at zero temperature. However, the low-energy physics of these systems is often not limited to short-range magnetic correlations but controlled by some kind of exotic (non-magnetic) order. In this talk, I will describe two such examples of spin liquids with respectively quadrupolar and topological order.
Marco Picco (LPTHE, Universite Paris 6)
Universality of the strong disorder fixed points in the two-dimensional random-bond Ising model
Alberto Rosso (LPTMS, Orsay)
First passage problems for anomalous walkers
The motion of a tagged monomer in a long polymer is a remarkable example of sub-diffusion. Although the free motion of this anomalous walker can be studied analytically, its behavior inside an absorbing interval has been approached only numerically. First passage time and absorption problems are important for applications and give a better understanding of the nature of this anomalous diffusion. As a device for capturing part of these effects we replace the finite interval with an harmonic absorption. This problem can be treated analytically. The differences between this model and the diffusion fractional equations will be discussed.
Clement Sire (LPT, Université Paul Sabatier, T)
The out of equilibrium physics of poker tournaments
We present a simple model of Texas hold'em poker tournaments which retains the two main aspects of the game: a) the minimal bet grows exponentially with time; b) players have a finite probability to bet all their money. The distribution of the fortunes of players not yet eliminated is found to be independent of time during most of the tournament, and reproduces accurately data obtained from Internet tournaments and world championship events. This model also makes the connection between poker and the persistence problem widely studied in physics, as well as some recent physical models of biological evolution, and extreme value statistics.
Mounir Tarek (EDAM, UHP Nancy)
Proteins and their hydration water glassy dynamics
Hydration water is important for protein structure and stability, and influences profoundly the dynamics of the protein molecule. Sub-nanosecond dynamics of hydration water is different from that of bulk water (slowing down of translational and rotational motion). The anomalous behavior of water at the protein surface has been a subject of investigation by both inelastic neutron scattering and molecular dynamics (MD) simulations. Hydration water, as well as proteins, display dynamical characteristics that can be described in terms of alpha relaxation models similar to those respectively of glasses and dense supercooled liquid. Here, we present analyses based on MD simulations of water single particle and collective density fluctuations in protein crystals, aimed at deciphering the origin of the interplay between protein dynamics and that of their hydration water.
Jose Texeira (CEA Saclay)
Hydrogen bonds in water and consequences for its unusual behaviour
Since more than 30 years the puzzle of supercooled water remains object of debate. The more recent developments have been achieved almost exclusively by computer simulations of the molecular dynamics using different "effective" potentials. Some of them demonstrate the existence of liquid-liquid phase transitions and of a critical point under temperature-pressure conditions not accessible to real experiments. It is not obvious that such results apply to supercooled water. We use coherent quasi-elastic neutron scattering in an original way that partly discriminates the dynamics of different partial components of the scattering law. The results confirm that the dynamics of hydrogen bonds follows an Arrhenius behaviour. We argue that the dynamics of supercooled water between the homogeneous nucleation temperature (not accessible to simulations) and the glass transition is determined by the dynamics of the bonds similarly to beta relaxation in polymer melts.
Frédéric van Wijland (Laboratoire Matière et Systèmes)
Phase transitions in kinetically constrained models of glass formers
We consider kinetically constrained models of glass formers. These share many features in common with atomistic glasses. We show that their glassy behavior is the byproduct of a phase transition that occurs in the space of all temporal realizations, unlike more conventional phase transitions occurring in configuration space.
Christian von Ferber (Coventry University)
Star polymers and DNA denaturation in correlated environments
In contrast to the prediction of the naive Harris criterion for the polymer limit of the O(n) vector model weak random disorder does not change the scaling behaviour of polymers in solution. Here we show that however correlated environments may have significant and quantitatively measurable impacts on the scaling properties of polymers and branched polymer structures such as star polymers. These result in experimentally accessible effects on their osmotic behaviour in differently correlated environments. We calculate these quantitative effects by appropriately extending a field theoretic renormalization group approach for spin models in long range correlated disorder. New universal scaling exponents are found for linear and star polymers in such environments with changing their entropic behaviour as well as the effective interactions between these polymers. Surprisingly, these effects have opposite signs for linear and star polymers resulting in quantitatively measurable static segregation behaviour. Consequences for the order of the denaturation transition of double stranded DNA coils in a correlated environment are discussed within the Poland-Scheraga model.
Sandro Wenzel (Leipzig)
Evidence of Unconventional Universality Class in a Two-Dimensional Dimerized Quantum Heisenberg Model
The two-dimensional J-J' dimerized quantum Heisenberg model is studied on the square lattice by means of (stochastic series expansion) quantum Monte Carlo simulations as a function of the coupling ratio alpha=J'/J. The critical point of the order-disorder quantum phase transition in the J-J' model is determined as alpha=2.5196(2) by finite-size scaling for up to N=96x96 quantum spins. By comparing six dimerized models we show, contrary to the current believe, that the critical exponents of the J-J' model are not in agreement with the three-dimensional classical Heisenberg universality class which gives support to the notion of non-trivial critical excitations at the quantum critical point.
Kay Wiese (Laboratoire de Physique Théoriqu)
How to calculate and measure the effective action and the avalanche size distribution for disordered systems
Using methods of functional renormalization, we show how to measure the effective action or disorder correlator of elastic systems in a random environment. The disorder correlator, predicted to be cuspy due to shocks, is the central ingredient of functional renormalization, which is confirmed by numerical and experimental results. Building on these achievements, we compute the avalanche size distribution.
Boris Andrushechkin (A.M.Prokhorov General Physics In)
Local structure of Ag(100) surface in reaction with I2: combined STM and DFT study
The interest to halogen-metal interactions has been initiated in the 1970-s mainly due to importance of their technological applications in catalysis, microelectronics and photography. A specific feature of such class of reactions is formation of monolayer of chemisorbed halogen at the first stage of reaction, whereas nucleation and growth of halide occurs on the second stage. Recently, for iodine adsorption on copper surfaces, we have reported the existence of the interface layer between halide and the substrate [1]. It is of interest, that the structure of the interface corresponds to the structure of saturated iodine monolayer. [1]. Here, we present combined STM and DFT study of iodine interaction with Ag(100). This system appears to be a good model system for the study of halide growth, since the saturated iodine monolayer forms a simple c(2×2) structure and the growth mode of AgI film is close to 2D [2]. At monolayer stage of reaction, we have found the faceting of the step edges as iodine adsorbs. At coverage of 0.5 ML both STM and LEED data evidently demonstrate the formation of the commensurate c(2×2) lattice. Our DFT calculations confirm that iodine prefers to adsorb into the four-fold hollow sites. Further dosing leads to the local etching of the terraces (appearance of the dark lines parallel to atomic rows of the c(2×2) structure) and formation of 2D AgI islands near the step edges. In addition to the atomic modulation, STM images of the islands show a distinct superstructure with a unit cell (2√2×12√2)R45º. According to DFT calculations, the local minimum corresponds to a new structure obtained as a result of relaxation of two bilayers of wurtzite structure on top of the c(2×2) iodine monolayer on Ag(100). New AgI structure is rather unusual: it contains coupled planes of silver in the middle and iodine planes on the borders («sandwich»-like structure). Note also, that during relaxation of atomic layers, the c(2×2) structure does not exhibit a noticable distortion. To confirm the new AgI structure, we have simulated STM image. Simulated STM image appears to be in the excelent correspondence with the experimental one. [1] B V Andryushechkin et al 2004 Surf. Sci. 566 203; 2006 JETP Lett. 83 162. [2] U Bardi, G Rovida 1983 Surf. Sci. 128 145.
Maria-Carmen Asensio (Synchrotron SOLEIL)
ANTARES: A Soft x-Ray Scanning Photoemission Microscope beamline at SOLEIL dedicated to the study of Low Dimensional Systems
A wide range of structural and chemical imaging techniques are now available for research in Low Dimensional Systems. In particular, synchrotrons offer various spectrometers equipped with micrometric or even nanometric beam sizes. It is clear that the highly elevated flux of the third generation synchrotron sources are essentially profited in the area of the X-ray microscopy. In such context, Microscopy of photoemission SPEM (Scanning PhotoEmission Microscope) is one of these powerful microscopic techniques, based on the contrast originated by the photoelectric effect. Contrary to the PEEM microscopy, the imagery in SPEM is generated by a simple nanometric sweeping of the samples and a focusing the incident light. ANTARES is one of the first beamlines in Europe that is attempting to extend such demanding technique to the domain of low energy, where the states of the valence band can be detected with high precision as well as their dispersions using angle resolved photoemission (ARPES). The objective is to be able to determine the electronic band structure and the Fermi surface together with the chemical shift of core level of light elements with a nanometer spatial resolution. In essence, this new beamline of SOLEIL will fit the existing emptiness between the atomic information provided by the STM spectroscopy and the low-spatially resolved data supplied by traditional ARPES and NEXAFS techniques. Several examples of Scanning X-Ray microscopic studies will be described. Finally, an overview of the utilization limits of different x-ray microscopes will be detailed together with future developments based on the detectors of last generation and new focalization optics.
Véronique Brouet (Paris Sud)
ARPES studies of misfit cobaltates
In Na cobaltates, NaxCoO2, the number of electrons in triangular Co planes can be changed by varying the amount of intercalated Na atoms. The phase diagram as a function of x is very rich, including superconducting and magnetic phases, metal-insulator transitions, etc. The evolution of the electronic properties with x is counter-intuitive, especially, strong magnetic correlations seem to develop when the number of magnetic Co ions (Co4+) decreases. The role of the Na potential in this behavior is often questioned; it is believed that it could trap some electrons or holes. We compare this behavior to that of misfit cobaltates, where the same Co planes are found, but with different intercalated structures.
Christophe Brun (EPFL - Lausanne)
Thin Superconducting Lead Islands Studied by Low-Temperature Scanning Tunneling Microscopy and Spectroscopy
The physical properties of conventional bulk superconductors have been studied for many decades and are understood in great detail [1]. Nevertheless, the superconducting properties are expected to be modified as the superconductor evolves from a three-dimensional (3D) to a 2D or 1D system. In general, the superconducting transition temperature (Tc) is found to be continuously reduced as the film thickness of the 2D superconductor is decreased [2]. Transport measurements have recently established that ultrathin lead films on Si(111) substrate show a decrease of Tc from 30 ML to 15 ML from about 6.25K to 5.0K, whereas bulk Tc = 7.2K [3]. This Tc versus thickness dependancy was later on confirmed by using different growth conditions and by measuring the magnetic susceptibility of Ge capped Pb/Si(111) films [4]. Nevertheless two recent scanning tunneling spectroscopy (STS) studies on the same system did not confirm such a thickness dependent behavior of the superconducting gap for thin islands [5,6]. Motivated by these contradictory results, we studied the properties of thin superconducting lead islands on Si(111) employing low-temperature STM and STS. [1] See for example M. Tinkham, Introduction to Superconductivity, 2nd Ed., McGraw-hill, New-York, 1996 [2] D.B. Haviland, Y. Liu, and A. M. Goldman, Phys. Rev. Lett. 62, 2180 (1989) [3] Y. Guo et al., Science 306, 1915 (2004) [4] M.M. Ozer, J.R. Thompson, and H.H. Weitering, Nature Physics 2, 173 (2006) [5] D. Eom, S. Qin, M.-Y. Chou, and C. K. Shih, Phys. Rev. Lett. 96, 027005 (2006) [6] T. Nishio, M. Ono, T. Eguchi, H. Sakata, and Y. Hasegawa, Appl. Phys. Lett. 88, 113115 (2006)
Frederic Cherioux (FEMTO ST Besançon)
Stable room temperature molecular assembly on silicon surfaces
Younes Makoudi, Mohamed El Garah, Frank Palmino, Eric Duverger, and Frédéric Chérioux*

The adsorption of functional molecules on semiconductors plays a vital role for the emerging field of nanoelectronic. In this context, molecular and supramolecular ordering, which are key-steps for the development of complexes architectures, are controlled by a balance between intermolecular forces and molecule-substrate interactions. Here we propose two strategies for the adsorption of conjugated molecules on semi-conductive interfaces at room temperature:
1-The used of an passivated surface, SiB(111)- 3x3 surface, known to be less reactive than another Si reconstructions. The high resolution STM images show that two of the three thienyl moieties of 2,4,6-tri(2’-thienyl)-1,3,5-triazine are adsorbed on top of two Si atoms of the 3x3 reconstruction. [1]
2-2) Zwitterionic organic dipoles have been deposited on Si(111)-7x7 and imaged by STM. DFT calculations demonstrate that the group acts as an electrostatic shield for the protection of the -skeleton of organic molecules versus the dangling bonds of semiconductor surfaces. [2]
Ad Ettema (Specs Nanotechnology)
Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms
In january 2002 Markus Greiner, Olaf Mandel, Tilman Esslinger, Theodor W. Hansch and Immanuel Bloch published their results of a Bose Einstein condensate of ultra cold Rb atoms trapped in an optical lattice of a magneto-optical trap (Nature, Vol 415, p39 (2002)). This work shows a beautiful example of a many particle phenomenon which is analogous to the Mott-Hubbard metal-insulator transition for electrons in solids. The tuning of the potential height in the Bose gas is equivalent to the tuning of the Hubbard U in electron systems. This important result has generated much impact both in atomic physics as in solid state physics with more than 1300 citations.
Yannick Fagot-Revurat (Nancy Université)
Redefinition of the ground state of alkali/Si(111) semiconducting surfaces
The narrowing of the bands and the reduction of screening effects at surface allow to obtain 2D Mott insulators involving sp-surface dangling bonds at semi-conducting surfaces as evidenced in alkali/Si(111), SiC(0001) or more recently in Sn/Ge(111). The triangular topology characterizing the rac(3)xrac(3) surface reconstruction, common to all these materials, has been predicted to exhibit magnetic properties originating in the frustrated triangular lattice. Therefore, as experimentally observed recently in layered cobaltates, doping this 2D frustrated magnets should lead to the occurrence of a superconducting phase at low temperature. We report here on new low energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES) studies of alkali/Si(111) previously established as a strongly correlated semi-conducting surface. We unambigously demonstrate : (i) the U/W>>1 Mott criteria for the alkali-induced half-filled dangling bond surface band ; (ii) a strong Franck-Condon broadening of this lower Hubbard band evidencing the strong coupling to the lattice (iii) the occurrence of a novel surface phase transition from a metastable rac(3)xrac(3) towards a dimerized 2rac(3)xrac(3) phase below 270 K. This allows us to redefine the ground state of alkali/Si(111) interface as a bi-polaronic insulator (BPI) rather than a Mott-Hubbard one (MHI). This formation of bi-polarons is theoretically expected in case of strong on-site repulsion U and strong coupling to the lattice g with g>U in the Holstein-Hubbard model but seldom observed. Consequences on expected frustrated magnetism and the possible observation of superconductivity upon carrier doping will be discussed.
Fernando Flores (Universidad Autonoma de Madrid)
Electronic transport in carbon nanotubes: diffusive and localized regimes
A fully self-consistent analysis of Boltzmann´s equations for electrons and phonons is used to study how the resistance of single-walled carbon nanotubes evolves as a function of its length. We demonstrate that, whereas for short tubes electronic transport is dominated by scattering with hot optical phonons, acoustical phonons play the leading role when the nanotube is very long. In this limit of long tubes, we also analyze the interplay between the diffusive and the localized transport regimes when the electron mean free path and the localization length due to impurities are comparable.
Enrique Garcia Michel (Universidad Autonoma de Madrid)
Electronic and structural properties across a surface phase transition in Sn/Cu(100)
The Cu(100) surface covered with 0.5 ML Sn undergoes a temperature induced phase transition from a (root2xroot2 )R45º structure above 360 K to a (3root2xroo2)R45º phase below, i.e. the unit cell becomes three times larger with two rotational surface domains. The surface electronic structure is dominated by a Sn induced surface state, which is observed in the surface M point bulk band gap with angle resolved photoemission. The formation of the low temperature phase is related to gapping and nesting of the Fermi surface, and thus it has been interpreted as the stabilization of a surface charge density wave. The structural changes across the phase transition are analyzed using temperature dependent STM and surface x-ray diffraction (SXRD). The SXRD analysis enables a precise determination of the atomic positions in both the high temperature and low temperature phases, thanks to a new minimization procedure based in a genetic algorithm, and includes also an analysis of the critical behaviour. The nature of the phase transition and of the atomistic mechanisms behind it will be discussed on the basis of all the information collected.
Etienne Janod (Nantes)
Origin of the metal-insulator transition and of the quantum spin gap in the vanadium bronze SrV6O15
The mixed valence quasi-1D vanadium bronzes AV6O15 (A=Na, Li, Ag, Ca, Sr) show numerous unusual properties, such as a metal-insulator transition, a charge ordering, superconductivity under pressure or a quantum spin gap state. We will present results obtained from structural determination, band structure calculations, reflectivity measurements, photoemission, magnetic susceptibility and inelastic neutron scattering on SrV6O15. The problems of the driving force of the metal-insulator transition and of the origin of quantum spin gap in SrV6O15 will be discussed on the basis of these results.
Markus Klein (Würzburg)
Fermi surface topology in an ordered CePt5 surface alloy
The physical properties of Kondo systems are determined by interactions between localized f-states and conduction electrons. Of particular interest are low-dimensional systems as they can serve as elementary model systems. Due to its surface sensitivity angle-resolved photoelectron spectroscopy (ARUPS) is an excellent tool to study directly the electronic structure of a two-dimensional Kondo system. This requires highly ordered and ultra thin singlecrystalline films. We have prepared singlecrystalline CePt5 surface-alloys by in situ deposition of Ce on a Pt(111) surface. Our ARUPS results show a temperature and wave-vector dependent Kondo-resonance and other hybridization effects. I will discuss our spectroscopic results with the help of data from isostructural LaPt5 films, LDA+U and DMFT calculations.
Patrick Lefevre (Synchrotron SOLEIL)
CASSIOPEE: a synchrotron beamline for high resolution and spin resolved photoemission
SOLEIL is the new french synchrotron facility. It will provide an intense and highly brilliant photon beam from infrared to X-rays for french and foreign scientists in extremely various research fields such a s solid state physics, geology, chemistry, biology... CASSIOPEE is a beamline dedicated to photoemission on solids and surfaces in the 10-1500 eV photon energy range. It hosts a high energy resolution angular photoemission setup and a spin resolved photoemission experiment. These two experimental chambers are coupled with a preparation chamber allowing the preparation and the characterization of surfaces and thin films. In this talk, I will present the characteristics and performances of the CASSIOPEE beamline.
Gautier Mahieu (ISEN Phisics, 41 Bvd Vauban Lill)
Tunneling spectroscopy of semiconductor nanocrystals in superlattices
Colloidal semiconductor nanocrystals (NCs) are quantum-size-effect tunable and offer an abundance of available surface area for optic and electronic devices. Scanning tunnelling microscopy is an ideal tool to characterize nanoscale object such individual NCs deposited on a metallic surface. Generally the NCs have to be attached to a conducting substrate to get stable substrate/NC/tip junction, but here we will show that NCs are better stabilized in the tunnelling junction between a tip and a conductive substrate, when they self-assemble to form a monolayer of nanocrystals. By using scanning tunnelling spectroscopy at low temperatures, we will investigate the transport through individual nanocrystals in the monolayer and show that tunnelling currents with higher intensities are driven through the NCs in a monolayer with respect to the tunnelling currents used on isolated NCs. From theoretical calculations, we will discuss the origin of the linewidth for the resonances observed in the differential conductance spectra. Depending on the chemical nature of the NCs, different mechanisms, including a strong electron-phonon coupling, a large intervalley coupling or a variable degree of electronic coupling between the NCs, are found to be involved in the broad resonances with widths in the range of tens of meV.
Claude Monney (Université de Neuchatel)
BCS-like phase transition in TiSe2
Claude Monney, Hervé Cercellier, Florian Clerc, Corsin Battaglia, Eike Schwier, Clément Didiot, Michael Gunnar Garnier, Hans Beck and Philipp Aebi (Université de Neuchâtel), Luc Patthey (Swiss Light Source, PSI, Villigen) Helmut Berger, Laszlo Forro (Institut de Physique de la Matière Complexe, EPFL) 1T-TiSe2 is a transition metal dichalcogenide well known for its (2x2x2) charge density wave (CDW) phase whose origin is discussed. Recently, with help of angle-resolved photoemission spectroscopy, we gave strong support for spontaneous condensation of excitons as the driving force of this CDW phase. This was achieved by comparing our measurements with simulated intensity maps based on a BCS-like model called the excitonic insulator phase. The results are presented with a special attention to the temperature dependence of the order parameter for the excitonic insulator phase.
Jose Ortega (Universidad Autonoma de Madrid)
The In/Si(111)-4x1 to 8x2 reversible phase transition
Safia Ouazi (EPFL)
The role of interface for the magnetic properties of nanostructures
Safia Ouazi, Géraud Moulas, Stefano Rusponi and Harald Brune
Stephane Pons (EPFL and Nancy université-CNRS)
Hybridization induced spin gaps in a assymetric quantum well system
We show that a BiAg surface alloy grown on a thin film of Ag leads to the interplay between the electronic states of the surface alloy and the quantum well states developing in the Ag layer. This interaction produces a set of spin-split bands and leads also to openings of spin-dependent gaps in the band structure.
Dimitri Roditchev (Jussieu)
Metal-Insulator Transition Induced on the Nanometer Scale by Electric Pulses
The understanding of the Mott Metal-Insulator transition remains one the most puzzling questions of the Solid State Physics. While this transition under the effect of temperature or pressure was deeply studied, the appearance in a solid of a non-volatile metallic state after the application of electric pulses is much less known. Such an electric pulse induced Metal-Insulator transition was first observed at the Institut des Matériaux Jean Rouxel at Nantes (IMN) in single crystals of some transition metal chalcogenides, in which the resistance switches over many orders of magnitude. Recent STM/STS experiments provided at INSP on single crystals of GaTa4Se8 evidenced that the observed modification of the transport properties was connected to the changes in the electronic structure in the bulk material. STM data revealed the nanometer scale Electronic Phase Separation [1], with the co-existence of metallic and insulating regions. Moreover, it was showed that the material is sensitive to the local electric field created by the STM tip. The latter effect strongly complicates the study of the samples by STM/STS but, at the same time, offers the possibility of a controlled modification of the electronic structure of the matter at the nanometer scale. Owing this effect, it was possible to “write” at the surface of the material with an extremely high resolution (about 3nm) even at room temperature. The achieved writing density exceeded 1Tbit/cm2. Thus, the study of novel Mott insulators appears also promising for possible applications in high storage density RRAM devices.

1. C. Vaju et al. “Electric Pulse driven Electronic Phase Separation, Insulator - Metal Transition and Possible Superconductivity in a Mott insulator”. Advanced Materials, 2008 (to appear)
Andres Santander (Paris-Sud and ESPCI)
Some microscopic aspects of the hidden-order transition in URu2Si2 studied by ARPES
A.K. Shukla (INPL Nancy)
Electronic structure of Al-Mn alloys
The influence of sp - d hybridization on the electronic structure of different Al-Mn alloys has been studied by photoelectron spectroscopy. Experimental evidence of pseudogap in a crystalline binary Hume Rothery alloy is observed. The pseudogap varies systematically with Mn concentration. The sp - d hybridization alone, even in absence of Hume-Rothery mechanism, can produce the pseudogap. Existence of the pseudogap, suppression of the Mn 2p satellite and decrease in the Doniach - Šunjić asymmetry parameter are the consequences of the sp - d hybridization. An in situ method of preparing these alloys by annealing a Mn adlayer on Al(111) is presented.
Muriel Sicot (Eindhoven University of Technolo)
Atomic and molecular hydrogen superstructures on Co(111) nanoislands grown on Cu(111)
Surface structures and related electronic properties of flat Co nanoislands supported on Cu(111) exposed to molecular hydrogen at low temperature have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS). First, prior to hydrogen adsorption, the Co islands and the Cu substrate exhibit surface states. Moreover, the Co islands are shown to be magnetic at T=5K by spin-polarized STM. Second, the Co/Cu(111) system is exposed to a low dose of hydrogen. At adsorption temperature Tads around 10K, STM/STS analysis show that Cu is a hydrogen free surface. Contrary to Cu, STM images of the Co surface show a hydrogen adlayer forming a p(2 x 2) superstructure. The corresponding STS reveals a shift of the Co d surface state from -0.3 eV to -0.5 eV. At Tads=78K, the p(2 x 2) superstructure is again observed on Co. These results suggest that the p(2 x 2) structure is induced by dissociative chemisorption of hydrogen on cobalt. Preferential adsorption areas on a Co nanoisland were monitored by sequential STM-imaging carried out at T=78 K. The real-time STM observation enabled visualization of the adsorption process. It clearly demonstrates that hydrogen first adsorbs at the edges prior to fill progressively the inner region of the island. Third, the Co/Cu(111) system is exposed to a high dose of H2. At Tads around 10K, STS spectrum measured on both Co and Cu reveals a prominent peak of differential conductance centered around zero bias ascribed to the presence of physisorbed molecules. Our results suggest that H2 first dissociatively chemisorbs on Co forming an atomic H adlayer. Subsequently, molecules impinging on that H-terminated Co surface are physisorbed. Upon high exposures, different phases on Co are formed and a mixture of (2 x 2) and (3 x 3) phases is identified on Cu. A model describing the adsorption sites of the molecules is proposed.
Laurent Simon (Mulhouse)
Long time evolution of order on a hierarchy of lengthscales in self-organized monolayers of supramolecular polymers
I will present an experimental low temperature STM study of the self-organization process of supra-molecular polymers based on bis-urea substituted toluene molecules deposited on gold (111) surfaces. Our work clearly shows that specific interactions between molecules via hydrogen bonds which are essential for the formation of linear supra-molecular polymers in solution (3D), are not sufficient to allow for a lateral 2D organization exhibiting long range order. To obtain such ordering, comparatively weak coupling between lateral side groups (alkyl chains) becomes important. Such weak interactions have not been taken into account when designing the molecules at the chemistry level. We also show that supra-molecular polymers in 2D are able to disassemble to reform allowing the annihilation of domains boundaries and improving long range order.

F. Vonau et al, PRL 94 (2005) 066103, F. Vonau et al, PRL 99 (2007) 086103.
Antonio Tejeda (Université Henri Poincaré)
Atomic structure of Sn/Ge(111)-(3 x 3) : a combined photoemission and STM study
The interface formed by 1/3·ML of Sn on Ge(111) exhibits a (3x3) reconstruction below 200 K, which has been a subject of interest since several years. Recently, this interface has been shown to stabilize a Mott insulator phase below 30 K [1]. The structure of the (3x3) reconstruction is still controversial in spite of the many efforts devoted to understanding its properties. X-ray diffraction measurements, theoretical calculations and STM experiments support a surface unit cell with one Sn atom in a higher position than the other two (1U2D model). However, since the Sn 4d line shape could not be understood within this structure, some experiments were interpreted with a unit cell consisting of two high Sn atoms per unit cell (2U1D model) [2]. Our work consists of a combined study of STM and high resolution photoemission from the Sn 4d CL line shape and the Fermi surface [3]. The high resolution measurements of the Sn 4d CL allow us to find a new interpretation of the line shape and of the origin of the different components. Our results determine unambiguously a 1U2D structure within the initial state picture. The new CL decomposition found is also supported by STM images. Valence band photoemission and Fermi surface measurements further discard the 2U1D structure as the most stable configuration [4]. [1] R. Cortés, et al, Phys. Rev. Lett. 96 (2006) 126103. [2] T.-L. Lee, et al, Phys. Rev. Lett. 96 (2006) 46103; M. E. Dávila, et al, Phys. Rev. B 70 (2004) 241308 [3] A. Tejeda, et al. Phys. Rev. Lett. 100 (2008) 026103. [4] A. Tejeda, et al. J. Phys. Cond. Mat. 19 (2007) 355008.
Pascal Turban (Université de Rennes I)
Ballistic electron emission microscopy (BEEM) investigation of epitaxial Au/GaAs(001) Schottky contacts
Sophie Guézo, Sergio Di Matteo, Pascal Turban Ballistic electron emission microscopy (BEEM) is a unique experimental tool which allows characterization of the electronic properties of buried interfaces with nanometric lateral resolution. In this talk, we will discuss experimental BEEM results on Au/GaAs(001) Schottky contacts. In BEEM experiments, a STM tip is used to locally inject a hot electron current IT into the Au gate of the Schottky contact, with an energy defined by the applied tip bias VT. A part of this current travels ballistically across the metal layer and reaches the Au/GaAs interface. This ballistic current may directly enter into the conduction band of the semiconductor, provided the electron energy overcomes the local Schottky barrier height B. In that case, a BEEM collector current IB (typically a few percents of IT) is measured at the backside of the the GaAs substrate. BEEM spectroscopy measurements on the Au/GaAs(001) model system present typical fingerprints of hot electron injection into the Γ, X and L valleys of the GaAs conduction band. These spectroscopic signatures are classically analyzed in the framework of a simple free-electron model and are attributed to the presence of a high density of scattering centers at the Au/GaAs(001) interface, resulting in the non-conservation of the transverse wave vector at the metal/semiconductor interface [1]. We reconsider this analysis with respect to a growth study which demonstrates high-quality MBE epitaxial growth of (110)-oriented Au layer on GaAs(001). We will discuss how the original epitaxial relationship between metal and semiconductor has to be taken into account for an improved theoretical understanding of the Au/GaAs BEEM spectra based on a realistic band-structure description of the system [2]. [1] W. J. Kaiser and L. D. Bell, Phys. Rev. Lett. 61, 2368 (1988). [2] P.L. de Andres, F.J. Garcia-Vidal, K. Reuter, F. Flores, Prog. Surf. Sci. 66, 3 (2001).
Jean-Yves Veuillen (Institut Nééel - Grenoble)
Rotational disorder in few layer graphene (FLG) films on 6H-SiC(000-1).
The graphene layers grown on the polar faces of hexagonal SiC substrates have characteristics similar to ideal (free standing) systems. This has been demonstrated for epitaxial layers grown on the Si terminated face (6H-SiC(0001)) by means of surface science techniques such as ARPES or STM. Quite surprisingly, single layer graphene behaviour has also been found in FLG films grown on 6H-SiC(000-1) substrates by (magneto) transport and optical experiments (See W. de Heer et al. Solid State Comm. 143, 92 (2007) for a review). This behaviour has been ascribed to the presence of stacking faults in the films, as shown by X-ray reflectivity experiments (J. Hass et al., Phys. Rev. B 75, 214109 (2007)). Analytical (J. Lopes dos Santos et al., Phys. Rev. Lett; 99, 256802 (2007)) and ab-initio calculations -some performed in our lab- have confirmed that a twist between adjacent planes lead to an effective electronic decoupling of the layers which restores a graphene-like behaviour. We have investigated by STM the early stages of graphitisation of the C terminated face (6H-SiC(000-1)) of SiC. We observe superlattices with periods in the nanometer range on FLG, thus directly demonstrating the presence of stacking faults, and we show that they influence the apparent sublattice asymmetry in agreement with theoretical results. This rotational disorder is present in the monolayer range which suggests that the graphene-substrate interaction is quite different on the Si and C faces of the substrate.

Communication par poster

Rafik Addou (LSG2M, UMR 7584 CNRS-INPL)
Surface Structure Investigations of the Orthorhombic Al13Co4 approximant
We present a study of the (100) surface of the orthorhombic Al13Co4 approximant phase using scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and x-ray photoelectron diffraction (XPD). This complex metallic alloy is an approximant phase of the decagonal quasicrystalline phase. The LEED pattern of the (100) surface exhibits a pseudo-10-fold symmetry and lattice parameters consistent with the bulk model. Two different types of surface terminations are observed depending on preparation conditions: either a well ordered atomically flat termination corresponding to simple bulk truncation or a rough layer which partially covers the first one.
Jose Avila (Synchrotron SOLEIL)
Charge Density wave transition in quasi-2D KMo6O17 purple bronze
High resolution angle-resolved photoemission of quasi-2D KMo6O17 purple bronze has been performed in the range from room temperature to 130 K, slightly above the charge density wave (CDW) transition (Tc = 110 K), and down to 35 K (well below Tc). In this paper we report a detailed study of how electronic band structure is affected by this transition driven by the hidden nesting scenario. The expected spectroscopic fingerprints of the CDW phase transition have been found and discussed according to the hidden one dimension and the development of a quasi-commensurate CDW. The excellent agreement between theory and our experimental results makes of potassium purple bronze a reference system for studying this type of instabilities
Azzedine Bendounan (LNS-SLS, Paul Scherrer Institut)
Xe-induced hybridization in a spin-split surface band: Xe/Bi/Ag(111)
We show here ARPES and LEED investigations on the adsorption of Xe on Bi/Ag(111) surface alloy [1]. We found that the rare gas induces a long-range 9*9 reconstruction and the hybridization of spz and pxy surface bands that are non-interacting in the bare alloy. By contrast, adsorption does not increase the size of the splitting. This points to the in-plane potential gradient, relatively insensitive to the adsorbate-induced compression of the surface state wave function, as the likely origin of the giant spin-orbit splitting in the alloy. [1]: L. Moreschini et al. Phys. Rev. B 77, 115407 (2008).
Murtaza Bohra (LSG2M, UMR 7584 CNRS-INPL)
Verwey transition in Fe3O4 thin films: effect of substrate temperature
Fe3O4 with spinel structure is a promising material for spintronic devices because of its half metallic nature and high Curie temperature of 853 K. Additionally, Fe3O4 shows curious metal-insulator transition at temperature of Tv = 120 K, named as Verwey transition, where magnetite transforms from cubic to monoclinic crystal structure due to freezing of electron hopping. [1-3] Observation of the Verwey transition by means of magnetic study is an interesting way to deduce the purity of films since this transition is signature of Fe3O4. Fe3O4 thin films were deposited by PLD at various substrate temperatures (Ts) on fused quartz substrates. XRD and Micro-Raman spectroscopy studies exhibited that films deposited at Ts below 500°C are single phase Fe3O4 while those deposited at higher Ts (500-850° C) are mixture of FeO, g-Fe2O3 and Fe3O4 phases. By varying Ts from room temperature (RT) to 350-500° C the orientation of Fe3O4 films can be varied from (ll0) to (lll). The 4pMs value does not show monotonous increase with Ts but goes through a maximum value of 5000 G (80% of bulk value) for Ts of 350° C. Temperature dependence of magnetization (M-T) study shows that, as Ts increases from RT to 850° C, the position of Verwey transition temperature changes from 70 K to 120 K and then to spread over a wider temperature range. The origin of the Verwey transition and its shift will be discussed on the basis of these results.
Clement Didiot (Université de Neuchâtel)
New structural model for Si(331) - 12x1
C. Battaglia, K. Ga·l-Nagy, C. Monney, E. Schwier, C. Didiot, M.G. Garnier, P. Aebi The Si(331) - 12x1 reconstruction is known to be the only confirmed planar silicon surface with a stable reconstruction located between (111) and (110) directions. Here we report the study of the atomic structure by the combination of LEED and high resolution STM measurements and propose a new structural model for this surface reconstruction.
Vincent Fournée (CNRS-INPL)
Quantum size effect in thin films on complex aluminides
Two different phenomena uniquely combine on quasicrystalline substrates: quantum size effects (QSEs) and trapping effects. The trapping effect is a consequence of an ordered array of specific sites of the substrates which present a strong adsorption energy and act as nucleation centre during thin film growth. This is of interest for a larger research community, where significant work is being done on heteroepitaxial systems, with the idea of fabricating nanostructures on solid surfaces for technological applications. In general, exploitation of natural growth morphology is considered as an alternative to optical lithography to obtain self-organized patterns of nanosized features, such as quantum dots in semiconductor heteroepitaxy. Quantum size effects relate to the appearance of discrete electronic states arising from the confinement of the electron within the film thickness. QSE is expected to modulate greatly the electronic structure near the Fermi level with the film thickness, and consequently its physical and chemical properties. Therefore controlling the film thickness offers the possibility to engineer the electronic structure. On quasicrystalline substrate, QSE are manifest at room temperature and even above. The poster will review experimental findings on both effects, observed by STM and photoemission spectroscopy.
Wolf-Dieter Schneider (EPFL)
Electronic structure and dynamics of quantum-well states in thin films of Pb on Si(111)
Electrons confined in a thin metal film on a surface form standing-wave eigenstates in the direction perpendicular to the surface. These discrete electronic eigenstates are known as quantum well states (QWs). Their eigenenergies crucially depend on the film thickness, because the Fermi wavelength (f) is typically ~1nm. Consequently, as a function of film thickness, the electronic density of states (DOS) at the Fermi level Ef is modulated in an oscillatory manner when a QWs passes through the Fermi level. This oscillatory behavior of the electronic properties at Ef affects many of the physical and chemical properties of a thin film, such as, e.g., the local work function, the chemical reactivity, the thermodynamic stability or even the superconducting properties at low temperature. Since their discovery by electron tunneling [1] QWs have been extensively studied by various techniques. Of particular interest is the analysis of the linewidths of the QWs, because it gives access to intrinsic properties of the thin metal film in terms of the lifetime of these electronic states [2]. Here, we have chosen to investigate quantum well states in thin Pb films on a Si(111) substrate by scanning tunneling microscopy (STM and spectroscopy (STS) because of its high energy resolution at low temperature, its ability to probe states on both sides of the Fermi surface, and its high-spatial resolution. The sample was prepared by evaporating Pb from a W filament onto either (i) a clean Si(111)-7x7 substrate at room temperature (RT) [3] or onto a (ii) reconstructed or -3x 3-Pb single layer phase on Si(111) substrate at RT [4]. Substrate (ii) is obtained after evaporation of more than 1.3 ML Pb on the Si(111)-7x7 surface followed by thermal annealing to 415 oC. On these two different substrates, showing a different Pb-Si(111) interface, Pb nanocrystals were grown with various sizes (30~800 nm in diameter) and thicknesses (4~40ML). On both samples (i) and (ii) our STM results show that the nanocrystals are atomically flat on the surface and are forming wedges in agreement with the results of Altfeder et al. [3]. For both samples the buried Si(111)/Pb interface as well as a weak Pb atomic corrugation is clearly observed [5]. While the nanocrystal grown on sample (i) shows the buried Si(7x7) periodicity, the one grown on sample (ii) exhibits a hexagonal Moiré pattern. The electronic properties of these nanocrystals were characterized by STS measurements of the QWs at 50K and at 5K. Using the phase accumulation model [6], the dispersion relation E(k) along the L direction and the energy dependent phase shifts (E) were extracted from the QWs energies. The analysis of the linewidth of the QWs was performed following Paggel et al. [7]. We find that for a given QW energy, the measured linewidth increases with decreasing thickness, underlining the role of the interface scattering in this system. Our results clearly show that the interface scattering term is much smaller on sample (ii) than on sample (i), pointing to a structurally more homogeneous interface in the former case. Support by the Swiss National Science Foundation is gratefully acknowledged. References [1] R.C. Jaklevic, J. Lambe, M. Mikkor, and W.C. Vassel, Phys. Rev. Lett. 94 126804 (1971) [2] T. C. Chiang Surface Science Reports 39 181 (2000) [3] I. B. Altfeder, K. A. Matveev, and D. M. Chen Phys. Rev. Lett. 78 2815 (1997) [4] M. H. Upton, C. M. Wei, M. Y. Chou, T. Miller, and T. C. Chiang Phys. Rev. Lett. 93 026802 (2004) [5] I. B. Altfeder, V. Narayanamurti, and D. M. Chen Phys. Rev. Lett. 88 206801 (2002) [6] P.M. Echenique, and J.B. Pendry J. Phys. C : Solid State Phys. 11 33 (1978) [7] J. J. Paggel, T. Miller, and T. C. Chiang Science 283 1709 (1999)

Comité d'organisation

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

Nos partenaires

Université Henri Poincaré, Nancy Université Institut National Polytechnique de Lorraine, Nancy Université
Institut Jean Lamour Comité National de la Recherche Scientifique
Labo. de Physique des Matériaux Société Francaise de Physique
Conseil Régional de Lorraine Université France-Allemande
Grand Nancy C'Nano
Specs Nanotechnology

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