# Groupe de Physique Statistique

## Equipe 106, Institut Jean Lamour

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### Statistical Physics and Low Dimensional Systems 2009

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

## mercredi 13 mai 2009 - vendredi 15 mai 2009

#### Programme de l'atelier

mercredi 13 mai 2009 Conférences plénières 14:30 - Wolf Dieter Schneider, Spectroscopic manifestation of low dimensional physics: a local view 15:30 - Silke Biermann, Electronic structure of correlated materials: a dynamical mean field perspective 16:00 - Coffee Break 16:40 - Ernesto Medina, Chiral Electron Transport: Scattering Through Helical Potentials 17:20 - Christian von Ferber, Shapes of embedded networks

Physique StatistiqueSystèmes de basse dimension
jeudi 14 mai 2009
09:00 - Bernard Derrida
09:40 - Rudolph Hilfer
10:20 - Coffee Break
10:50 - Bertrand Delamotte
11:30 - Ralph Kenna
12:30 - Lunch break
14:30 - Dragi Karevski
15:30 - Poster Session
16:30 - Coffee Break
17:00 - Vivien Lecomte
17:40 - Alexander Lopez
vendredi 15 mai 2009
09:00 - Jean-Yves Fortin
09:40 - Arnaud Ralko
10:20 - Coffee Break
10:50 - Sudhir Jain
11:30 - Wolfhard Janke
12:30 - Lunch break
14:30 - Ferenc Iglói
15:10 - Cécile Monthus
mercredi 13 mai 2009
18:00 - Poster Session
jeudi 14 mai 2009
09:00 - Dimitri Roditchev
09:30 - Oliver Groening
10:00 - Stephano Rusponi
10:30 - Coffee Break
11:00 - Jorge Lobo Checa
11:30 - Bertrand Kierren
12:30 - Lunch break
14:30 - Evgeny Chulkov
15:00 - Coffee Break
15:30 - Friedrich T Reinert
16:00 - Stephane Pons
16:30 - Claude Monney
17:00 - Poster Session
vendredi 15 mai 2009
09:00 - Enrique Ortega
09:30 - Andres Santander
10:00 - Antonio Tejeda
10:30 - Coffee Break
11:00 - A.K. Shulka
11:30 - Jose Avila

#### Orateurs

Physique StatistiqueSystèmes de basse dimension
Bertrand Delamotte (Paris 6)
Non Perturbative Renormalization Group and the Kardar-Parisi-Zhang equation
The Non Perturbative Renormalization Group (NPRG) is by now a well established and widly tested method thanks to which several results unreachable by perturbative means have been obtained for systems either at or out of thermal equilibrium. We review the method and show how NPRG allows us to obtain the full phase diagram of the Kardar-Parisi-Zhang equation in all dimensions including the fixed point describing the strong coupling (rough phase) with exponents in reasonable agreement with numerical ones.
Bernard Derrida (ENS Paris)
Current fluctuations in non equilibrium systems
This talk will present a series of recent results on the fluctuations of the current in non-equilibrium diffusive systems. When the system is maintained in a non-equilibrium steady state by contact with two reservoirs at unequal densities, the fluctuations of the current have in general a non-Gaussian distribution which can be computed exactly for diffusive systems [1,2,3]. For systems at equilibrium on a ring geometry, the cumulants of these fluctuations take a universal scaling form which can be understood by several theoretical approaches such as the Bethe ansatz or fluctuating hydrodynamics [4]. Some recent results concerning non steady state initial conditions will also be discussed [5]. [1] T. Bodineau, B. Derrida, Phys. Rev. Lett. 92, 180601 (2004) Current fluctuations in non-equilibrium diffusive systems: an additivity principle [2] T. Bodineau, B. Derrida, Phys. Rev. E 72, 066110 (2005) Distribution of current in non-equilibrium diffusive systems and phase transitions [3] T. Bodineau, B. Derrida, C. R. Physique 8, 540-555 (2007) Cumulants and large deviations of the current through non-equilibrium steady states [4] C. Appert, B. Derrida, V. Lecomte, F. Van Wijland, Phys. Rev. E 78, 021122 (2008) Universal cumulants of the current in diffusive systems on a ring [5] B. Derrida, A. Gerschenfeld, Current Fluctuations of the One Dimensional Symmetric Simple Exclusion Process with Step Initial Condition cond-mat/0902.2364
Jean-Yves Fortin (Nancy Université)
Examples of a Grassmann technique applied to classical spin systems in 2D
In this talk, we will present an analytical method using Grassmann techniques to study the fermionic theory underlying general spin-S Ising models in two dimensions. The simplest case S=1/2 was understood a long time ago and is known to be equivalent to a massive free fermion theory possessing a second-order transition when the mass vanishes. Here we try to extend the method for general spin-S with the additional presence of a splitting field in the Hamiltonian. One of our motivation comes from the fact that there is little knowledge about the fermionic theory of such models and the relation to the free fermion theory of the S=1/2 2D Ising model. In particular we can show that the Blume-Capel model can be exactly mapped onto an interacting fermion model, with a bare mass depending on the value of the splitting field. The location of the points that make this bare mass vanish is very close to the numerical results found by Monte-Carlo method in the region of the second-order transition. The extension to spin-S models and a method to find the bare mass in general is presented, with an accuracy of less than 1% for the second-order transition lines in these models. The particular values of the transition points at zero splitting field are then compared to high and low-temperature expansions.
Rudolph Hilfer (Stuttgart)
Excess Wings in Glass Formers
Ferenc Iglói (KFKI, Budapest)
Nonequilibrium dynamics of fully frustrated Ising models at T=0
We consider two fully frustrated Ising models: the antiferromagnetic triangular model in a field of strength, $h=H T k_B$, as well as the Villain model on the square lattice. After a quench from a disordered initial state to $T=0$ we study the nonequilibrium dynamics of both models by Monte Carlo simulations. In a finite system of linear size, $L$, we define and measure sample dependent relaxation time, $t_r$, which is the number of Monte Carlo steps until the energy is relaxed to the ground-state value. The distribution of $t_r$, in particular its mean value, $\langle t_r(L) \rangle$, is shown to obey the scaling relation, $\langle t_r(L) \rangle \sim L^2 \ln(L/L_0)$, for both models. Scaling of the autocorrelation function of the antiferromagnetic triangular model is shown to involve logarithmic corrections, both at $H=0$ and at the field-induced Kosterlitz-Thouless transition, however the autocorrelation exponent is found to be $H$ dependent.
Sudhir Jain (Aston University)
Persistence in the Zero-Temperature Dynamics of the Q-State Potts Model
To follow
Wolfhard Janke (ITP Leipzig)
Fractals meet Fractals: Self-Avoiding Random Walks on Percolation Clusters
We consider self-avoiding walks (SAWs) on the backbone of percolation clusters in space dimensions $d=2, 3, 4$. Applying numerical simulations based on the PERM chain-growth algorithm, we estimate the critical SAW exponents and show that the whole multifractal spectrum of singularities emerges in exploring the peculiarities of the model. We obtain estimates for the set of critical exponents, that govern scaling laws of higher moments of the distribution of percolation cluster sites visited by SAWs, in a good correspondence with an appropriately summed field-theoretical $\varepsilon=6-d$-expansion. V. Blavatska and W. Janke, {\em Scaling Behavior of Self-Avoiding Walks on Percolation Clusters\}, Europhys. Lett. {\bf 82}, 66006 (2008); {\em Multifractality of Self-Avoiding Walks on Percolation Clusters\/}, Phys. Rev. Lett. {\bf 101}, 125701 (2008); {\em Walking on Fractals: Diffusion and Self-Avoiding Walks on Percolation Clusters\/}, J. Phys. {\bf A42}, 015001 (2009).
Dragi Karevski (Nancy Université)
Qantum Non-Equilibrium Steady States Induced by Repeated Interactions
We study the steady state properties of a finite XY chain coupled at its boundaries to quantum reservoirs. The reservoirs are made of free spins that interact one after the other with the boundary sites of the chain. We show that the dynamical equations governing the time evolution of the two-point correlation matrix are of Lindblad type in the continuous interaction limit. Under XY coupling with the reservoir spins, the steady state correlations are calculated exactly. It is shown that the relevant physical quantities characterizing completely the steady state are the magnetization profile and the associated current. Except at the boundary sites, the magnetization is equal to the average of the reservoirs magnetizations. The steady state current, proportional to the difference in the reservoirs magnetizations, shows a non-monotonous behavior with respect to the system-reservoir coupling strength, with an optimal current state for a finite value of the coupling. Finally, the steady state density matrix is shown to be of generalized Gibbs form. At small current linear irreversible thermodynamics predictions are recovered.
Ralph Kenna (Coventry University)
The Site-Diluted Ising Model in Two and Four Dimensions
The Ising model with uncorrelated, quenched random-site or random-bond disorder has been controversial in both two and four dimensions. In these dimensions, the leading exponent $\alpha$, which characterizes the specific-heat critical behaviour, vanishes and no Harris prediction for the consequences of quenched disorder can be made. In the two-dimensional case, the controversy is between the strong universality hypothesis which maintains that the leading critical exponents remain the same as in the pure case and the weak universality hypothesis, which favours dilution-dependent leading critical exponents. In the four-dimensional case unusual corrections to scaling characterize the model, and the precise nature of these corrections has been debated. Here both versions of the model are subject to finite-size scaling analyses, paying special attention to the implications for multiplicative logarithmic corrections. The analysis is fully supportive of the scaling relations for logarithmic corrections and of the strong scaling hypothesis in the 2D case. Progress is also made in determining the correct 4D scenario.
Vivien Lecomte (Université de Genève)
Depinning transition for domain walls with an internal degree of freedom
We examine the dynamics of a domain wall subject to a pinning potential, in situations where the position of the wall is coupled to an internal degree of freedom (e.g. a spin phase, in magnetic domain walls). We investigate the corresponding depinning transition, which displays several novel features when compared to standard cases. At zero temperature, there exists a bistable regime for low forces, with a logarithmic behavior close to the transition. For weak pinning, there occurs a succession of bistable transitions corresponding to different modes of the phase evolution, separated by topological transitions. At finite temperature, using techniques from stochastic dynamical systems, we show that the force-velocity characteristics is non-monotonous, as an effect of the zero-temperature topological transitions.
Alexander Lopez (IVIC, Caracas)
Mach Zehnder spin interferometer for Rashba and Dresselhaus media: Exact solutions for perfect spin filtering
We address spin filtering through quantum spin interference in a medium that has both Rashba and Dresselhaus spin-orbit couplings. We propose an experimentally feasible electronic Mach Zehnder Interferometer and solve for the perfect spin filtering conditions. We find two broad solutions, one where filtering is achieved in the original incoming basis that is purely a non-Abelian solution, and the other where one needs a tilted axis to observe the polarized output spinor. Both solutions apply for arbitrary incoming polarization, and are only limited by the randomness of the incoming spinor state.
Ernesto Medina (IVIC, Caracas)
Chiral Electron Transport: Scattering Through Helical Potentials
We present a model for the transmission of spin-polarized electrons through oriented chiral molecules, where the chiral structure is represented by a helix. The scattering potential contains a confining term and a spin-orbit contribution that is responsible for the spin-dependent scattering of the electrons by the molecular target. The differential scattering cross-section is calculated for right- and left-handed helices and for arbitrary electron spin polarizations. We apply our model to explain chiral effects in the intensity of photo-emitted polarized electrons transmitted through thin organic layers. These are spin-active molecular interfaces that exhibit electron dichroism and a number of remarkable magnetic properties. In our model, differences in intensity are generated by the preferential transmission of electron beams whose polarization is oriented in the same direction as the sense of advance of the helix.
Cécile Monthus (CEA Saclay)
Dynamics of random systems : strong disorder renormalization approach
To understand the non-equilibrium dynamics of random systems, we will describe a strong disorder renormalization procedure in configuration space, that we define for any master equation with transitions rates $W ({\cal C} \to {\cal C}')$ between configurations. The idea is to eliminate iteratively the configuration with the highest exit rate $W_{out} ({\cal C})= \sum_{{\cal C}'} W ({\cal C} \to {\cal C}')$ to obtain renormalized transition rates between the remaining configurations. The multiplicative structure of the new generated transition rates suggests that, for a very broad class of disordered systems, the distribution of renormalized exit barriers defined as $B_{out} ({\cal C}) \equiv - \ln W_{out}({\cal C})$ will become broader and broader upon iteration, so that the strong disorder renormalization procedure should become asymptotically exact at large time scales. As an example of application, we have checked numerically this scenario for the non-equilibrium dynamics of a directed polymer in a two dimensional random medium.
Arnaud Ralko (Grenoble)
Emergence of Generic Mixed Phases in Rokshar-Kivelson Models
The phase diagram of Rokhsar-Kivelson models, which are used in fields such as superconductivity, frustrated magnetism, cold bosons, and the physics of Josephson junction arrays, is revisited. From an extended height effective theory, we show that one of two simple generic phase diagrams contains a mixed phase that interpolates continuously between columnar and plaquette states. Applied to the square lattice frustrated Heisenberg antiferromagnet, we have extended the Rokhsar-Kivelson (RK) loop-expansion to derive a generalized Quantum Dimer Model containing only connected terms up to arbitrary order. Our results suggest that the Heisenberg model is a physical realization of such a mixed phase, in the parameter region of maximum frustration.
Christian von Ferber (Coventry)
Shapes of embedded networks
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Jose Avila (Soleil)
Electronic structure of advanced materials using spacially resolvec photoemission
Microscopy of photoemission SPEM (Scanning PhotoEmission Microscope) is a powerful characterization technique of heterogeneous systems using the photoelectric effect, i.e. the generation of electrons by a system subjected to an electromagnetic radiation. Contrary to the PEEM microscopy, where the image is obtained by collecting the photoelectrons through an electronic column of optics, the imagery here is generated by a simple nanometer sweeping of the samples and focusing the incident light. ANTARES is one of the first beamlines in Europe that is attempting to extend such demanding technique to the low energy domain, 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 core level of light elements of advanced materials with a nanometer spatial resolution. The possibility of achieving synchrotron light beams from a few tens of nm to some µm, makes possible the visualization of density of states, chemical heterogeneities likewise magnetic order distributions getting the cartography of the samples on the basis of contrast produced by the photoemission response. This setup is being outfitted with several absorption detectors in order to complementarily obtain, in situ, the imagery based on the absorption contract. The SPEM microscope at ANTARES beamline will operate in two alternative modes, the imagery and the spectroscopy. In the first mode, the material will be monitored using a synchronized sweeping of the sample with regard to the focused beam, by simultaneously detecting photoelectrons with a given kinetic energy. The second mode allows the determination of the photoemission and absorption spectra at a selected nanometer portion of the sample. I will give several examples of Scanning X-Ray microscopic studies of a range of heterogeneous systems with applications mainly in corrosion, heterogeneous chemical and structural aggregates and massive systems as well as the growth of metals and characterization of catalysts. Finally, an overview of the utilization limits of the x-ray microscopes will be detailed together with the future developments based on the last generation of detectors and new focalization optics.
Silke Biermann (Ecole Polytechnique - Palaiseau)
Electronic structure of correlated materials: a dynamical mean field perspective
During the last years new methods for the description of the electronic structure of correlated materials have been developed. The combination of dynamical mean field techniques with density functional theory ("LDA+DMFT") allows for the calculation of electronic properties of materials from first principles, taking into account the effect of strong Coulomb interactions. After a brief review of the theoretical aspects, we will describe some recent advances on transition metal oxides and f-electron materials.
Evgeny Chulkov (San Sebastian)
DECAY OF EXCITED ELECTRONS IN SURFACE STRUCTURES
Interaction between lattice and electron subsystems as well as interaction within each of these subsystems is crucial to understand mechanism of single-particle excitation dynamics, i.e. lifetime of excitations. The lifetime sets the duration of excitation and in combination with the velocity determines the mean free path, a measure of influence of the excitation. Interest to the study of excited particles dynamics is motivated by an important role that excited electrons and holes play in many processes, e.g. in energy and charge transfer in bulk materials, at surfaces, across interfaces, and at nanosystems. In this presentation I discuss recent theoretical results on the decay of excited electrons and holes at clean metal surfaces, at single adatoms, at islands as well as at overlayers and free standing thin films. Different decay mechanisms and different kinds of interactions - elastic and inelastic electron- electron (e-e) interaction as well as electron-phonon (e-ph) interaction - are analysed. E-ph decay channel is shown to be important for all systems considered. In the e-e decay channel the electron (hole) decay can be realized via creation of electron-hole pairs or plasmon excitation. Dimensionality effects in the lifetime of electrons and holes on metal surfaces and the role of screening and intra- (inter-) band transitions are also discussed.
Oliver Groening (Thun - Switzerland)
Electron Scattering in Carbon Nanotubes
Abstract_Groening
Bertrand Kierren (Nancy Université)
Electron confinement in nano resonators: life time investigation and coupling effects.
We will focus on the Shockley state confinement in quantum resonators. In the first part i will show that using original design, it is possible to built nano resonators with high reflection coefficient, making possible life time analysis. I will show that a Fermi liquid behaviour can be observed and electron/hole and electron/phonon contributions will be discussed. In te second part of the talk, i will present recent results of STS obtained in chlorine based quantum corrals. The study from single isolated corral to chain of multiple corrals will be presented. The possible coupling between quantum wells will be discussed.
Jorge Lobo Checa (Centre d'Investigaciò en Nanociè)
Surface state confinement imposed by a supramolecular porous network: Band formation from coupled quantum dots
The properties of crystalline solids can to a large extent be derived from the scale and dimensionality of periodic arrays of coupled quantum systems like atoms and molecules. Periodic quantum confinement in two dimensions has been elusive on surfaces mainly because of the challenge to produce regular nanopatterned structures capable of trapping electronic states. The practical implementation of this type of confinement will be shown by demonstrating that the two dimensional free electron gas of the Cu(111) surface state can be trapped within the pores of an organic nanoporous network. The confinement units can be regarded as a regular array of quantum dots. Moreover, a shallow dispersive electronic band structure is formed, which is indicative of electronic coupling between neighbouring pore states.
Konrad Matho (Institut Néel - Grenoble)
On kinks and waterfalls in ARPES
Angle resolved photoemission spectroscopy (ARPES) allows to reveal a regime change in the momentum dependence of low energy fermionic excitations, as the distance from the Fermi surface increases. From gentle dispersion crossovers in most systems, more accentuated "kinks" or even "waterfalls", i.e.: discontinuities as function of k - k_F, have been observed. They are attributed to the presence of strong correlations among the fermions or strong electron-lattice coupling. I shall present the simplest generic model for a complex selfenergy function that allows to classify such phenomena and analyse the ARPES data quantitatively. The main driving parameter is the ratio of coherent spectral weight over total low energy weight, including the incoherent part. Further, I shall discuss the influence of asymmetry between particle- and hole-like excitations and, time permitting, the evolution of "kinks" as function of temperature.
Claude Monney (Fribourg)
Temperature dependence of the exciton condensate phase in 1T-TiSe2
At the temperature of 200K, the quasi-two-dimensional system 1T-TiSe2 undergoes a phase transition towards a charge density wave (CDW) phase, which is accompanied by a periodic lattice distortion (PLD) [1]. We propose the exciton condensate phase model as the purely electronic origin of the CDW phase of 1T-TiSe2 [2,3]. Measured angle-resolved photoemission spectroscopy data are compared with photoemission intensity maps generated by the spectral function within this model [4,5]. We also discuss features of the model which allow extracting from the experiment the temperature dependence of the corresponding order parameter. Finally, we show recent results on the coupling of the exciton condensate to the lattice, which demonstrate that the presence of condensed excitons are capable of producing the observed PLD. References: [1] F. J. Di Salvo et al.., Phys. Rev. B, 14 (4321), 1976 [2] D. Jérôme et al., Phys. Rev., 158 (462), 1967 [3] L.V. Keldysh et al., Sov. Phys. Solid State, 6 (2219), 1965 [4] H. Cercellier et al., Phys. Rev. Lett., 99 (146403), 2007 [5] C. Monney et al., Phys. Rev. B, 79 (045116), 2009
Enrique Ortega (San Sebastian)
Investigating Shockley surface states in curved noble metal surfaces: interplay between structure and electronic states
Vicinal surfaces with periodic arrays of steps and terraces are among the simplest lateral nanostructures. They are useful as growth templates for self-assembled arrays of nanostructures, but also attractive playgrounds for testing the fundamental properties of electrons at one- or two-dimensional (1D or 2D) superlattices. The most studied case is that of vicinal noble metal surfaces, where repulsive scattering gives rise to a rich surface electronic structure and phenomena that depend on the terrace size (lattice constant) d. On the one hand, the Shockley-like surface state that characterizes these surfaces is observed to evolve from a quasi-1D state to a 2D band as a function of d. On the other hand, surface states are characterized by relatively large (15-40AA) Fermi wavelengths in noble metal surfaces, and hence may exhibit 1D Fermi surface nesting with reciprocal lattice vectors pi/d. In the latter case, the question arises whether a charge density wave (CDW)-like, structural/electronic interplay occurs, i.e., whether such Fermi surface nesting and subsequent Fermi gap opening triggers structural instabilities in the step superlattice. We use curved crystals to investigate the evolution of surface states as a function of d, namely the miscut angle (alpha) in a single sample. In particular we utilize Au and Cu curved surfaces that cover a alpha=+-15° range of miscuts around the [111] direction (alpha=0°). The local structure is determined by Scanning Tunneling Microscopy (STM), whereas surface bands are mapped locally for each miscut angle by Angle Resolved Photoemission (ARPES) by scanning a 100 mum synchrotron light spot.
Stephane Pons (EPFL - Lausanne)
Titel Pons
Abstract Pons
Friedrich T Reinert (Wuerzburg)
Modifing the Rashba Splitting of Metallic Surface States
This talk will summarize recent experiments by photoemission spectroscopy (ARUPS) on metallic surfaces, that reveal a considerable Rashba splitting as a consequence of the changed local symmetry at the surface. The splitting is due to the spin-orbit coupling of the contributing electronic states and depends strongly on the atomic composition of the surface, as e.g. in a surface alloy. It shall be discussed, how these states depend on surface modifications as induced by electron doping, local geometry, or by the coverage with a dielectric overlayer.
Dimitri Roditchev (INSP - CNRS, Université Paris 6)
Ultimate Vortex Confinement
Authors: Tristan Cren, Denis Fokin, François Debontridder, Vincent Dubost and Dimitri Roditchev We report a detailed Scanning Tunneling Spectroscopy (STS) study of a superconductor in a strong vortex confinement regime. This is achieved in a thin nano-island of Pb having a size D about three times the coherence length, and a thickness h such that h<< D <
Stephano Rusponi (Lausanne)
XAS study of individual magnetic atoms on insulating substrates
Andres Santander (ESPCI - Paris)
Angle resolved electronic structure of electron-doped SrTiO3: new hints for old puzzles
SrTiO3 is considered the cornerstone of oxide electronics. It is often used as a substrate for thin-film growth of other transition-metal oxides, and is the preferred template to create novel 2D phases of electron matter at oxide interfaces. SrTiO3 is also a model system for strongly-correlated electron physics: upon electron-doping the Ti 3d band, it goes from a band insulator to a correlated metal, and turns into a Mott-Hubbard insulator when all the Sr is substituted by La. However, despite the technological and fundamental importance of this material, key questions as the precise experimental determination of its momentum-resolved band structure and Fermi surface remain unsolved. Indeed, the electronic structure near the Fermi level (EF) of doped SrTiO3 is not that of a doped band insulator: besides the appearance of coherent states at EF, a broad peak of incoherent states centred at about 1.3 eV forms in its band-gap. These in-gap states, obviously not explained by a rigid shift of the bands with increasing doping, are observed already at dopings as low as x ~ 0.05 (about 1 electron every 20 unit cells), where electron correlations are expected to be small. Thus, they cannot correspond either to precursors of the “lower Hubbard band” of the d1 insulator. The nature of these in-gap states has also been a long-standing puzzle in the physics of SrTiO3. Using angle-resolved photoemission spectroscopy (ARPES) we have studied in detail the electronic structure of SrTiO3 doped with oxygen vacancies. We determined that the Fermi surface, centred at Gamma, is composed of a central spherical-like sheet surrounded by cigar-like sheets. We observed the degeneracy lift in the conduction band induced by the cubic-to-tetragonal transition occurring at 110°K. Furthermore, we observed that the in-gap states do not disperse, indicating that they originate from localized states, rather than from electron-correlations.
Wolf Dieter Schneider (Lausanne)
Spectroscopic manifestation of low dimensional physics: a local view
The interest in nanostructured materials, consisting of building blocks of a small number of atoms or molecules, arises from their promising new optic, catalytic, magnetic and electronic poperties, which are fundamentally different from their macroscopic bulk counterparts: small is different. Here we present selected examples from our laboratory, which elucidate local aspects of physics in low dimensions investigated by photoemission (PES) and by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS): Kondo effect of supported magnetic adatoms [1-3], electronic properties (electron confinement, lifetimes of surface states and of quantum well states, supraconductivity) of ultrathin supported metal islands [4-8], self-assembly, melting and electronic structure of two-dimensional adatom superlattices stabilized by long-range electronic interactions [9-12], two-dimensional supramolecular self-assembly, chirality, and electronic structure of small organic molecules [13-17], inelastic tunneling processes leading to vibrational excitations of supported molecules[18] and to local fluorescence and phosphorescence [19-21] enabling chemical recognition at the molecular scale. [1] F. Patthey. B. Delley, W. D. Schneider and Y. Baer, Phys. Rev. Lett. 55, 1518 (1985). [2] J. Li, W.-D. Schneider, R. Berndt, B. Delley, Phys. Rev. Lett. 80, 2893 (1998). [3] M. Ternes, A. J. Heinrich, and W.-D. Schneider, J. Phys.: Cond. Matter 21, 053001 (2009). [4] F. Patthey and W.-D. Schneider, Phys. Rev. B 50, 17560 (1994). [5] J. Li, W.-D. Schneider, R. Berndt, S. Crampin, Phys. Rev. Lett. 80, 3332 (1998) [6] J. Li, W.-D. Schneider, R. Berndt, O. R. Bryant, and S. Crampin, Phys. Rev. Lett. 81, 4464 (1998). [7] C. Brun, I.-P. Hong, F. Patthey, I. Yu. Sklyadneva, R. Heid, P. M. Echenique, K. P. Bohnen, E. V. Chulkov, and W.-D. Schneider, Phys. Rev. Lett. (2009), in press. [8] I.-P. Hong, C. Brun, F. Patthey, and W.-D. Schneider, (2009), submitted. [9] F. Silly, M. Pivetta, M. Ternes, F. Patthey, J. P. Pelz, and W.-D. Schneider, Phys. Rev. Lett. 92, 016101 (2004). [10] M. Ternes, C. Weber, M. Pivetta, F. Patthey, J. P. Pelz, T. Giamarchi, F. Mila, and W.-D. Schneider, Phys. Rev. Lett. 93, 146805 (2004). [11] F. Silly , M. Pivetta, M. Ternes, F. Patthey, J. P. Pelz, and W.-D. Schneider, New J. Phys. 6, 16 (2004). [12] N. N. Negulyaev, V. S. Stepanyuk, L. Niebergall, P. Bruno, M. Pivetta, M. Ternes, F. Patthey, and W.-D. Schneider, (2009), submitted. [13] M. Böhringer, K. Morgenstern, W.-D. Schneider, R. Berndt, F. Mauri, A. De Vita, R. Car, Phys. Rev. Lett. 83, 324 (1999). [14] M.-C. Blüm, E. Cavar, M. Pivetta, F. Patthey, W.-D. Schneider, Angew. Chem. Int. Ed. 44, 5334 (2005). [15] M.-C. Blüm, M. Pivetta, F. Patthey, and W.-D. Schneider, Phys. Rev. B 73, 195409 (2006). [16] M. Pivetta, M.-C. Blüm, F. Patthey, and W.-D. Schneider, Angew. Chem. Int. Ed. 47, 1076 (2008). [17] M. Pivetta, M.-C. Blüm, F. Patthey, and W.-D. Schneider, J. Chem. Phys. B 113, 4578 (2009). [18] M. Pivetta, M. Ternes, F. Patthey, and W.-D. Schneider, Phys. Rev. Lett. 99, 126104 (2007). [19] R. Berndt, R. Gaisch, J.K. Gimzewski, B. Reihl, R.R. Schlittler, W.-D. Schneider,M. Tschudy, Science 262, 1425 (1993). [20] E. Cavar, M.-C. Blüm, M. Pivetta, F. Patthey, M. Chergui, and W.-D. Schneider, Phys. Rev. Lett. 95, 196102 (2005). [21] F. Rossel, M. Pivetta, F. Patthey, and W.-D. Schneider, Optics Express 17, 2714 (2009).
A.K. Shulka (Institut Jean Lamour - Nancy)
Quantum size effects in the thin metal films grown on quasicrystalline surfaces
We have studied the growth and electronic structure of Ag films on 5-fold surface of icosahedral (i)-Al-Cu-Fe quasicrystal using scanning tunneling microscopy (STM), ultra-violet photoemission spectroscopy (UPS) and low energy electron diffraction (LEED). Magic height islands of Ag are observed by STM and these are related to the quantum size effects (QSE). LEED pattern shows that Ag grows in 5-fold twinned nanocrystallites with Ag(111) structure for > 7 monolayers and Ag(111) structure is well supported by the UPS spectra of bulk-like thick Ag films. Observations of Ag sp quantum well states (QWS) in the UPS spectra of Ag/i-Al-Cu-Fe confirmed the QSE. Mismatch in the symmetry of electronic states has been attributed to the origin of the QWS.
Antonio Tejeda (Nancy Université / synchrotron S)
Tailoring electronic and magnetic properties in structured Au surfaces
Tailoring electronic and magnetic properties at the nanoscale presents both technological and fundamental interest. Magnetic interactions among nanostructures and confinement conditions for electronic states can be studied in surface systems. Ideal playgrounds to study these aspects are vicinal and faceted surfaces of noble metal surfaces. In these systems, it is possible to induce magnetic stripes and high density magnetic islands lattices by self-organisation. We will show how the lattices influence the nearly-free Shockley state at the (111) face of noble metals via the modifications of the surface potential. The superposition of the potential of steps and that of magnetic islands leads to a two dimensional lattice of interacting quantum boxes for the surface state.

#### Communication par poster

Mathias Aust (Leipzig)
Free Energy Inherent Structures in Spin Glass Models
One important feature of the glass phase of spin glasses is its rugged energy landscape. While at zero temperature the (possibly degenerate) ground state dominates, local energy minima with higher minimum energy and the entropy of the corresponding valleys become important at higher temperatures. Especially the configurational entropy (the complexity) of the spin glass at different temperatures is of interest. These quantities can be obtained from Monte Carlo simulations using the free energy inherent structure (FEIS) approach introduced in Coluzzi et al., A new method to compute the configurational entropy in glassy systems, Eur. Phys. J. B 32, 495 (2003). This method is applied to the Sherrington-Kirkpatrick (SK) model and the Edwards-Anderson Ising (EAI) model, both with a bimodal distribution for the couplings. The application of the multicanonical algorithm allows to simulate bigger system sizes for the SK model than before, while the EAI model has not been studied by this method before.
Hendrik Bentmann (Wuerzburg)
Rashba spin-orbit coupling in surface alloys: Bi/Cu(111) vs. Bi/Ag(111)
We investigate the Rashba spin-orbit splitting of the surface states on the surface alloys Bi/Cu(111) and Bi/Ag(111) by angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT). The splitting on Cu is found to be significantly smaller than on Ag. Our calculations show that this is due to different relaxations of the Bi atom. We also study the effect of Na adsorption on Bi/Cu(111). It turns out to be a simple method to tune the surface electronic structure. We discuss implications for future investigations and applications.
Silke Biermann (Laboratoire Ecole Polytechnique)
Generalized Hedin's equations and $\sigma$ G $\sigma$ W approximation for quantum many-body systems with spin-dependent interactions
Hedin's equations for the electron self-energy and the vertex of a quantum many-particle system have originally been derived for systems with Coulomb interaction. Here, we derive a generalized set of Hedin's equations for quantum many-body systems containing spin-dependent interactions, e.g. spin-orbit and spin-spin interactions. The corresponding spin-dependent GW approximation is constructed. This work should open the way to describing the interplay of correlations and spin-dependent interactions, in systems such as quantum dots or wires, as well as in interface and surface problems. [1] F. Aryasetiawan, S. Biermann, Phys. Rev. Lett. 100 (2008) 116402. [2] F. Aryasetiawan, S. Biermann, J. Phys.: Condens. Matter 21 (2009) 064232.
Nelson Bolívar (UCV Caracas)
Non-Abelian formulation for Rashba and Dresselhaus coupling and the gauge invariance of the spin currents
A gauge theory interpretation for the Rashba and Dressehaus Hamiltonians is briefly discussed. Special attention is draw to the fact that Pauli Hamiltonian and the non Abelian gauge formulation of Rashba and Dresselhaus spin orbit interactions cannot be written correctly in a general gauge invariant form: a gauge invariant Hamiltonian does not allow a locally gauge invariant spin current or spin polarization. We argue that the correct formulation demands a gauge symmetry breaking term that allows for locally gauge invariant matter spin current.
Hervé Cercellier (Grenoble)
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Mario Collura (Institut Jean Lamour)
Gradient critical phenomena in the Ising quantum chain: surface behaviour
We consider the behaviour of a critical system in the presence of an arbitrary power-law variation of the couplings. We develop a scaling theory for some bulk or surface operator profile induced by the gradient perturbation which involves a characteristic lenght given by the width of the surface region. The scaling predictions are tested using the Ising quantum chain in a power-law varying transverse field which corresponds to the extreme anisotropic limit of the classical two-dimensional Ising model. The surface magnetization can be evaluated for arbitrary values of the couplings and we study the finite-size behaviour for different limiting cases. In the linearly varying field the quantum Hamiltonian can be diagonalized exactly in the scaling limit where the eigenvalue problem is the same as for the quantum harmonic oscillator.
Xavier Durang (Institut Jean Lamour)
Ageing in bosonic contact and pair-contact processes with L\'evy flights
Ageing in systems without detailed balance is studied in bosonic contact and pair-contact process with L\'evy diffusion. In the ageing regime, the dynamical scaling of the two-time correlation function and two-time response function is found and analyzed. Exact results for exponents and scaling functions are derived. The behaviour of the fluctuation-dissipation ratio is analyzed and a crossover time from relaxation regime to ageing regime is defined.
Helene Grandclaude (CEA Saclay)
Finite-size effects in the degree statistics of scale-free networks
In this poster we present a study of the degree statistics of scale-free growing networks. The main emphasis is on the cutoff effects which take place whenever the system is finite. We focus on a one-parameter generalization of the famous Barabàsi-Albert model, which exhibits a continuously varying growth exponent. We consider the evolution of the degree statistics for two different initial conditions, corresponding to a tree and a rooted tree. We determine among other quantities the exact finite-size scaling functions for both initial states.
Taras Holovatch (Universite Henri Poincare Nancy-)
Network harness: bundles of routes in public transport networks
Public transport routes sharing the same grid of streets and tracks are often found to proceed in parallel along shorter or longer sequences of stations. Similar phenomena are observed in other networks built with space consuming links such as cables, pipes, neurons, etc. In the case of PTNs this behavior may be easily worked out on the basis of sequences of stations serviced by each route. To quantify this behavior we use the recently introduced notion of network harness [1,2]. It is described by the harness distribution P(r; s): the number of sequences of s consecutive stations that are serviced by r parallel routes. For certain PTNs that we have analysed we observe that the harness distribution may be described by power laws. These power laws observed indicate a certain level of organization and planning which may be driven by the need to minimize the costs of infrastructure and secondly by the fact that points of interest tend to be clustered in certain locations of a city. This effect may be seen as a result of the strong interdependence of the evolutions of both the city and its PTN. To further investigate the significance of the empirical results we have studied one- and two-dimensional models of randomly placed routes modeled by different types of walks. While in one dimension an analytic treatment was successful, the two dimensional case was studied by extensive simulations showing that the empirical results for real PTNs deviate significantly from those expected for randomly placed routes. [1] C. von Ferber, Yu. Holovatch, V. Palchykov. Condens. Matter Phys. Vol.8, No. 1(41), 225 (2005) [2] C. von Ferber, T. Holovatch, Yu. Holovatch, and V. Palchykov. Physica A 380, 585 (2007); Eur. Phys. J. B 68 2, 261 (2009)
Monika Möddel (Leipzig)
Conformational Transitions of Polymer Adsorption Transitions at Attractive Substrates
Conformational phases of a semiﬂexible off-lattice homopolymer model near an attractive substrate are investigated by means of multicanonical computer simulations. In our polymer-substrate model, nonbonded pairs of monomers as well as monomers and the substrate interact via attractive van der Waals forces. We analyze thermal ﬂuctuations of energetic and structural quantities and adequate docking parameters as a function of the temperature. Introducing a solvent parameter that is related to the strength of the surface attraction, we construct and discuss the solubility-temperature phase diagram. Apart from the main phases of adsorbed and desorbed conformations, several other phase transitions such as the freezing transition between energy-dominated crystalline low-temperature structures and globular entropy-dominated structures are identiﬁed.
Jean-Charles Walter (Nancy université)
Aging in the fully-frustrated XY model
We present results on the aging of the fully frustrated XY model by means of Monte Carlo simulations. The ground state has a continuous degeneracy like the XY model associated to the rotation of all the spins and a discrete symmetry linked to the so-called chirality, a variable associated to the circulation of the spins around a plaquette. The two transitions occur at different temperatures. It is established that the spins perform an XY-like transition. However, the transition of the chirality was in debate in the literature. A recent article brings convincing arguments in favour of the Ising universality class. First the system is the ground state and heated to the low temperature phase. We measure the autocorrelation which displays aging. A fit of this function give an estimation of the exponent eta associated to the spin transition which is a function of the temperature in the low-temperature phase, like the XY model. Our results are in good agreement with the theoretical prediction. Then we have performed a quench of the system from infinite temperature to the critical temperature of the chirality. Surprisingly, the universal values are different from Ising. With the same protocol, we have also calculated universal values associated to the spins.
Johannes Ziroff (Wuerzburg)
Organic molecules on noble metal(111) surfaces probed by surface-state ARUPspectroscopy
The modification of the noble metal (111) surface states by an adsorbed monolayer of large pi-conjugated molecules (PTCDA, NTCDA, CuPc) was investigated by high-resolution angle-resolved photoelectron spectroscopy. We determined binding energy, band mass, and Rashba splitting and discuss the results in the context of rare-gas adsorption on noble metals. This comparison allows the determination of the bonding strength of the adsorbates.We will also present a superstructure model for the NTCDA/Au(111) system, deduced from low energy electron diffraction images in combination with band-backfolding of the Tamm and Shockley states. The coverage dependent evolution of the surface states was also investigated for the three molecules, giving evidence for a dilute-phase growth mode of the CuPc molecules.

#### Comité d'organisation

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