Groupe de Physique Statistique

Equipe 106, Institut Jean Lamour

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Grp Travail
Theses, Postes

Statistical Physics and Low Dimensional Systems 2006

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

mercredi 17 mai 2006 - vendredi 19 mai 2006

Programme de l'atelier

Conférences plénières
mercredi 17 mai 2006
14:30 - Ferenc Iglói, Strong Griffiths singularities in random systems and their relation to extreme value statistics
15:30 - Harald Brune, Magnetism of nanostructures at surfaces

Physique StatistiqueSystèmes de basse dimension
mercredi 17 mai 2006
17:00 - Pasquale Calabrese
jeudi 18 mai 2006
09:00 - Yurij Holovatch
09:30 - Christian von Ferber
10:00 - Elmar Bittner
11:00 - Daniel Cabra
11:30 - Wolfhard Janke
12:00 - Rob Hagemans
14:30 - Des Johnston
15:00 - Rosemary Harris
15:30 - Heiko Rieger
16:00 - Cécile Appert
17:00 - Michael Bachmann
17:30 - Florian Baumann
vendredi 19 mai 2006
09:00 - Tomasz Wydro
09:30 - Paolo de los Rios
10:00 - Enrico Carlon
11:00 - Ralph Kenna
11:30 - Pierre Pujol
12:00 - Victor Martín-Mayor
14:30 - Vivien Lecomte
15:00 - Andrea Gambassi
15:30 - Raoul Santachiara
16:00 - Sébastien Léonard
mercredi 17 mai 2006
17:00 - Coriolan Tiusan
17:30 - Oliver Groening
jeudi 18 mai 2006
09:00 - Véronique Brouet
09:30 - Philipp Aebi
10:00 - Hervé Guyot
11:00 - Konstantin Eltsov
11:30 - Peter Zeppenfeld
12:00 - Wolf Dieter Schneider
15:00 - Marco Grioni
15:30 - Friedrich Reinert
16:00 - Daniel Malterre
17:00 - Pascal Ruffieux
17:30 - Bertrand Kierren
vendredi 19 mai 2006
09:00 - Enrique Garcia Michel
09:30 - Guy Le Lay
10:00 - Jean-Yves Veuillen
11:00 - Antonio Tejeda
11:30 - Hervé Cercellier
12:00 - Nadine Witkowski
14:30 - Azzedine Bendounan
15:00 - Vladimir Yu. Yurov


Physique StatistiqueSystèmes de basse dimension
Cécile Appert
Systems with Markov dynamics: thermodynamic formalism and dynamic entropies
Dynamic entropies (in particular the Kolmogorov-Sinai entropy) are convenient tools to characterize the dynamic complexity of trajectories in nonequilibrium systems. They can be obtained in the frame of the so-called thermodynamic formalism which was first introduced for continuous systems and then extended to discrete time Markov processes. It turned out that the case of continuous time Markov dynamics cannot be obtained by sending the discrete time step to zero. Here we propose a formulation of the formalism that allows to apply it to continuous time Markov dynamics. We show that it can be rephrased in terms of large deviations of an observable. As an illustration of our approach, we shall apply it on various examples.
Michael Bachmann
Adsorption phenomena in hybrid organic-inorganic interfaces
The interest in understanding polymer adsorption at substrates has grown quite recently with the development of high-resolution experimental equipment allowing for studying the technologically important problem of substrate-binding specificity of synthetic peptides. In our study of simple hybrid models, we investigate how solubility of the surrounding solvent and temperature influence the substrate-binding of nongrafted polymers in a cavity with an attractive surface. Applying a suitably adapted variant of the multicanonical chain-growth algorithm for self-avoiding walks, we performed simulations of lattice polymers with up to 200 monomers and obtained the entire temperature-solubility pseudo-phase diagram of the hybrid system within a single simulation. We clearly separated expected thermodynamically stable phases dominated by the respective adsorbed and desorbed collapsed and random-coil conformations. Another central aspect of our study is the discussion of pseudo-phases that specifically depend on finite-size properties such as the precise number of monomers or, for peptides, the sequence of residues.
Florian Baumann
Ageing at surfaces: The semi-infinite spherical model
Ageing phenomena and dynamical scaling behaviour have been considered in many translationally invariant systems. An interesting question is what happens if spatial translation invariance is broken in one direction, that is if we introduce a spatial surface. Numerical investigations have been done on this question and it turned out that surface ageing exponents, surface scaling functions and a surface fluctuation-dissipation ratio can reasonably be defined in close analogy to the bulk case[1]. In this talk I wish to add some exact results to the discussion by considering the semi-infinite kinetic spherical model [2]. I do this for both Dirichlet and Neumann boundary conditions at the surface, which corresponds to the ordinary transition and special transition point (in mean-field approximation) respectively. I give the exact results for the two-time surface correlation and response functions in the dynamical scaling regime as well as the surface fluctuation-dissipation ratio. I also study the low-temperature phase of this model. The results show that for the case of Dirichlet boundary conditions the value of the non-equilibrium surface exponent $b_1$ does not vanish, in contrast to the usual bulk value of systems undergoing phase ordering. [1] M. Pleimling, Phys. Rev. B 70 104401, (2004). [2] F. Baumann and M. Pleimling, to appear in J.Phys. A: Math. Gen.
Elmar Bittner
The evaporation/condensation transition of Ising droplets
In recent analytical work, Biskup et al. studied the behaviour of $d$-dimensional finite-volume liquid-vapour systems at a fixed excess $\delta N$ of particles above the ambient gas density. By identifying a dimensionless parameter $\Delta (\delta N)$ and a universal constant $\Delta_\mathrm{c}(d)$, they showed that for $\Delta < \Delta_c$ the excess is absorbed in the background (``evaporated'' system), while for $\Delta > \Delta_c$ a droplet of the dense phase occurs (``condensed'' system). Also the fraction $\lambda_\Delta$ of excess particles forming the droplet is given explicitly. Furthermore, they argue that the same is true for solid-gas systems. By making use of the well-known equivalence of the lattice-gas picture with the spin-$1/2$ Ising model, we performed Monte Carlo simulations of the Ising model with nearest-neighbour couplings on a square lattice with periodic boundary conditions at fixed magnetisation, corresponding to a fixed particles excess. To confirm the analytical results, we measured the largest minority droplet, corresponding to the solid phase, at various system sizes ($L=40, \dots, 640$). Using analytic values for the spontaneous magnetisation $m_0$, the susceptibility $\chi$ and the Wulff interfacial free energy density $\tau_\mathrm{W}$ for the infinite system, we were able to determine $\lambda_\Delta$ numerically in very good agreement with the theoretical prediction.
Daniel Cabra
The influence of phonons on low dimensional magnetic systems
Pasquale Calabrese
Entanglement entropy and Quantum Field Theory
A systematic study of entanglement entropy in relativistic quantum field theory is discussed. For the case of a 1+1-dimensional critical system, whose continuum limit is a conformal field theory with central charge c, the result S_A\sim(c/3) log(l) is re-derived, and it is extended to many other cases: finite systems, finite temperatures, and when A consists of an arbitrary number of disjoint intervals. For such a system away from its critical point, when the correlation length \xi is large but finite, the result S_A\sim N(c/6)\log\xi is shown, where N is the number of boundary points of A. I will finally discuss the unitary relaxation from a non-equilibrium initial state, showing that both CFT and the exact solution of integrable models lead, contrarily to the ground state case to an extensive entanglement entropy. This can be understood in terms of causality.
Enrico Carlon
Thermodynamics of high density oligonucleotide microarrays
We analyze a series of controlled experiments on DNA microarrays produced by Affymetrix. In these experiments some genes are added in solution at known concentration according to a Latin square scheme. We show that the data can be fitted very well by a simple Langmuir model which takes into account 1) the hybridization (=binding) of sequences in solution with complementary sequences anchored at the microarray surface and 2) the hybridization of partially complementary sequences in solution. When appropriately rescaled the data collapse into a single master curve. Deviations from the theory, which are rarely observed, can also be explained.
Paolo de los Rios
Entropic pulling of polymers through membrane pores
Heat Shock Proteins 70 kDa (Hsp70) are multifunctional proteins that play a central role in the transport of proteins across cell membranes (e.g. the endoplasmic reticulum and the mitochondrial double membrane) and in the solubilization of protein aggregates. After a review of the basic biology and biochemistry of Hsp70s, and of current models for its functional mechanism, I will show how simple arguments from the statistical physics of polymers can provide a unified picture for the different cellular functions of Hsp70, and resolve current debates.
Andrea Gambassi
Finite-size scaling in the non-equilibrium critical behavior of the randomly driven lattice gas
The randomly driven lattice gas (RDLG) is a kinetic lattice gas model whose interacting particles are driven along one of the lattice axes by an external field E with randomly changing sign. For E = 0 the model reduces to the standard equilibrium lattice gas with a second-order phase transition point in the Ising universality class. Remarkably, the transition persists also in the non-equilibrium case of non-zero E, although it differs in nature from the equilibrium one. Within the field-theoretical (FT) approach we compute analytically the one-loop finite-size scaling (FSS) function for the finite-volume correlation length which we have recently investigated via Monte Carlo simulations in two dimensions [Phys. Rev. E 72, 056111 (2005)]. The FT prediction (a) highlights the influence of lattice shapes on the finite-size properties, (b) well describes the actual FSS behavior observed in numerical data, confirming the effectivness of the FT approach to a greater extent than previous studies -- primarily concerned with infinite-volume quantities such as critical exponents --, and (c) rules out a recent proposal of an alternative FT description of the critical properties of the RDLG.
Rob Hagemans
Dynamics of Integrable Spin Chains
Exactly solvable models in one dimension have been known for a long time. Although methods such as the Bethe Ansatz yield exact, non-perturbative expressions for the thermodynamics of such systems, their dynamics has remained inaccessible to these techniques. Building upon recently developed determinant representations, we have developed numerical methods to nonperturbatively calculate dynamical spin-spin correlation functions for integrable spin chains to very high precision. I will discuss these techniques and their relevance to neutron scattering experiments on antiferromagnetic compounds. Recently, we have applied these methods in combination with field theory and DMRG to improve understanding of the scaling and line shape of the correlation function in the small wave vector limit.
Rosemary Harris
Breakdown of Gallavotti-Cohen symmetry for stochastic dynamics
We consider the behaviour of current fluctuations in the one-dimensional partially asymmetric zero-range process with open boundaries. Significantly, we find that the distribution of large current fluctuations does not satisfy the Gallavotti-Cohen symmetry and that such a breakdown can generally occur in systems with unbounded state space. We also discuss the dependence of the asymptotic current distribution on the initial state of the system.
Yurij Holovatch
Entropy-induced osmosis of star polymers in a porous medium
We quantitatively study a model of osmosis decribing polymer stars in a solution where part of the space is occupied by a porous medium with quenched structural defects. To this end, we apply the field-theoretical renormalization group approach to study the influence of long-range correlated disorder on the scaling properties of f-arm polymer stars in a good solvent. As a result, we obtain numerical estimates for the set of star exponents governing scaling of the star partition function as well as the contact exponents that govern the star-star repulsion. We find that the solvent in the medium with correlated disorder is energetically less favorable and calculate quantitatively the relative equilibrium concentrations in and outside the porous medium. In collaboration with V. Blavats'ka (Lviv) and C. von Ferber (Krakow and Freiburg).
Ferenc Iglói
Strong Griffiths singularities in random systems and their relation to extreme value statistics
We consider interacting many particle systems with quenched disorder having strong Griffiths singularities, which are characterized by the dynamical exponent, $z$, such as random quantum systems and exclusion processes. In several $d=1$ and $d=2$ dimensional problems we have calculated the inverse time-scales, $\tau^{-1}$, in finite samples of linear size, $L$, either exactly or numerically. In all cases, having a discrete symmetry, the distribution function, $P(\tau^{-1},L)$, is found to depend on the variable, $u=\tau^{-1}L^{z/d}$, and to be universal given by the limit distribution of extremes of independent and identically distributed random numbers. This finding is explained in the framework of a strong disorder renormalization group approach\cite{im} when, after fast degrees of freedom are decimated out the system is transformed into a set of non-interacting localized excitations. The Fr\'echet distribution of $P(\tau^{-1},L)$ is expected to hold for all random systems having a strong disorder fixed point, in which the Griffiths singularities are dominated by disorder fluctuations.
Wolfhard Janke
Geometrical Picture of Phase Transitions
We discuss how suitably defined geometrical objects encode in their fractal structure thermal critical behaviour [1]. Emphasis will be placed on the two-dimensional Potts model for which two types of spin clusters can be defined. Whereas the Fortuin-Kasteleyn clusters describe the standard critical behaviour of the pure model, the geometrical clusters describe the tricritical behaviour that arises when including vacant sites in the pure Potts model. The close connection between the two models respectively the two cluster types can be explained by a ``dual map'' that conserves the central charge, so that both model/cluster types are in the same universality class. Similar considerations apply to the hulls respectively external perimeters of these clusters. The geometrical picture is supported by two conceptually different types of Monte Carlo simulations. In an outlook, further possible applications of the geometrical viewpoint to other systems [2,3] are briefly discussed. [1] W. Janke and A.M.J. Schakel, Nucl. Phys. {\bf B700}, 385 (2004); Comp. Phys. Comm. {\bf 169}, 222 (2005); Phys. Rev. {\bf E71}, 036703 (2005); Phys. Rev. Lett. {\bf 95}, 135702 (2005); and e-print cond-mat/0508734. [2] S. Wenzel, E. Bittner, W. Janke, A.M.J. Schakel, and A. Schiller, Phys. Rev. Lett. {\bf 95}, 051601 (2005). [3] E. Bittner, A. Krinner, and W. Janke, Phys. Rev. {\bf B72}, 094511 (2005).
Des Johnston
Continued Fractions and the Partially Asymmetric Exclusion Process
We note that a tridiagonal matrix representation of the matrix algebra of the partially asymmetric exclusion process (PASEP) lends itself to direct intepretation as the transfer matrix for weighted Motzkin lattice paths and allows a succint derivation of the normalisation and correlation lengths of the PASEP. A continued fraction (``J-Fraction'') representation of the lattice path generating function is particularly well suited to discussing the PASEP, which has height dependent weights. We use this as our principal tool in extracting the phase behaviour for $q<1$ and also discuss the $q \to 1$ limit.
Ralph Kenna
Scaling Relations for Logarithmic Corrections
Multiplicative logarithmic corrections to scaling are characteristic of a number of marginal scenarios, such as at the upper critical dimension, at the demarcation between transitions of first and second order and in certain diluted systems. Here, scaling relations between the exponents of such logarithms are established and confronted with a variety of results from the literature.
Vivien Lecomte
Dynamical phases in stochastic systems
On the macroscopic scale, and for most of their properties, systems in equilibrium can be described without prior knowledge of their dynamics. This is at variance with what occurs in out-of-equilibrium systems (with slow glassy dynamics, or in far from equilibirum steady-states) where the microscopic dynamics is the key to the systems' macroscopic features. We will import concepts of the theory of dynamical systems into the description of systems with Markov dynamics. These consist in focusing on the various histories (and their fluctuations) that the systems may follow. We will show on specific examples (the contact process and a kinetically constrained Ising model) how these tools can shed light onto the dynamical phases of such systems.
Sébastien Léonard
Activated aging dynamics and negative fluctuation-dissipation ratios
Despite decades of research our theoretical understanding of the glass transition remains incomplete. The microscopic origin of the dynamic slowing down can be explained by the presence of spatial heterogeneities which have been observed experimentally and numerically in various glass-formers and spin glasses. However very few studies of dynamic heterogeneity exist in the aging regime although all glasses are by definition out of equilibrium materials. In these systems, aging proceeds at large times via thermal activation. We show that this can lead to negative dynamical response functions and novel and well-defined violations of the fluctuation-dissipation theorem, in particular, negative fluctuation-dissipation ratios. Our analysis is based on detailed theoretical and numerical results for the activated aging regime of simple kinetically constrained models. The results are relevant to a variety of physical situations, such as aging in glass formers, thermally activated domain growth, and granular compaction.
Victor Martín-Mayor
Kosterlitz-Thouless transition in the 3D Heisenberg spin-glass?
Recent memory and rejuvenation experiments urge us to clarify the nature of the Heisenberg spin-glass. Here, we report the results of a Finite-Size Scaling study of the three dimensional Edwards-Anderson model with Heisenberg spins [1]. The combination of heat-bath with overrelaxation dynamics has allowed us to thermalize systems of unprecedented size (L=32) in spin glass studies. The presence of logarithmic corrections to scaling suggest that D=3 is extremelly close to (exactly equal?) the lower critical dimension for this system. [1] I. Campos, M. Cotallo, V. Martin-Mayor, S. Perez-Gaviro and A. Tarancon, manuscript in preparation.
Pierre Pujol
Zero-temperature Kosterlitz-Thouless transition in a two-dimensional quantum dimer model
In this talk we present a local interacting quantum dimer model on the square lattice, whose zero-temperature phase diagram is characterized by a line of critical points separating two ordered phases of the valence bond crystal type. On one side, the line of critical points terminates in a quantum transition inherited from a Kosterlitz-Thouless transition in an associated classical model. We also discuss the effect of a longer-range dimer interactions that can be used to suppress the line of critical points by gradually shrinking it to a single point. Finally, we propose a way to generalize the quantum Hamiltonian to a dilute dimer model in presence of monomers and we qualitatively discuss the phase diagram.
Heiko Rieger
Strong Disorder Renormalization Group Study of the Dissipative Random Transverse Field Ising Model
The interplay between disorder, quantum fluctuations and dissipation is studied in the random transverse Ising chain coupled to a dissipative Ohmic bath with a real space renormalization group. A typically very large lengths scale is identified above which the physics of frozen clusters dominates. Below this length scale a strong disorder fixed point determines scaling at a quasi-critical point. In a Griffiths-McCoy region frozen clusters produce already a finite magnetization resulting in a classical behavior of the susceptibility. These override the confluent singularities characterized by a continuously varying exponent and visible at energies larger than a cut-off that is exponentially small in the aformentioned length scale.
Raoul Santachiara
Effects of boundary conditions in non-Markovian Gaussian processes
We consider Gaussian signals, i.e. random functions with independent Gaussian Fourier modes, and compute their statistical properties in small windows. We determine moments of the probability distribution of the mean square width of these random functions in powers of the window size. We show that the moments, in the small-window limit, become universal, whereas they strongly depend on the boundary conditions for larger window size. Above a critical value of the exponent of the mode variance (alpha), in the small window limit, the probability distribution can be computed and we show that it is independent of alpha.
Christian von Ferber
Percolation in Complex Networks and Metropolis Public Transport
Empirical studies of many complex networks ranging from social networks to power grids and the Internet have revealed that in many cases properties like the node degree are power law distributed e.g. p(k)~ k^-l. This implies that the role of the nodes e.g. with respect to the connectivity differ considerably. A number of evolutionary growth schemes have been proposed that may reproduce these properties and explain the often non-equilibrium statistics. Our focus is on percolation phenomena in such networks. Here, percolation is defined by the birth of a giant connected component (incipient cluster). For a degree distribution p(k) as above the percolation critical exponents will depend on l. For a class of treelike networks we explicitly derive the transport properties of these networks and find the full density of states as well as scaling relations for the dynamical scaling exponents [1]. As a specific example of complex networks we analyze the public transport (PT) networks of a number of major cities of the world. While the primary network topology is defined by a set of routes each servicing an ordered series of given stations, a number of different neighborhood relations may be defined both for the routes and the stations. E.g. one either defines two stations as neighbors whenever they are serviced by a common route or only if one station is the successor of the other in the series serviced by this route. Previous studies of PT have mostly been restricted to much smaller networks and did not observe power law behavior for which we find clear indications in the larger of the networks that we analyze [2,3]. Removing nodes from the network we define paths to percolation. The corresponding behavior strongly depends on the path chosen, i.e. proceeding either by random removal or targeted attack of nodes with high centrality. Our findings for the relation between the topology and vulnerability of these networks is supported by simulations of a model for the evolution of PT networks that we propose. [1] F. Jasch, CvF, A. Blumen. PRE 68:051106 (2003). [2] CvF, Yu. Holovatch, V. Palchykov Condens. Matter Phys. 8:225 (2005). [3] CvF, T. Holovatch, Yu. Holovatch, V. Palchykov (2006, in preparation)
Tomasz Wydro
Finite-Size scaling at Yang-Lee singularities of 2D discrete spin models
We numerically study the Hamiltonian limits of the two dimensional (2D) Ising and 3-state Potts models in complex magnetic fields. From the Phenomenological Renormalization Group, we find the critical field values associated with the Yang-Lee singularity(YLS) in these models. We also determine the low-lying part of the excitation spectrum at the YLS. Finally, we compare the resulting patterns of energy levels to predictions for conformal non-unitary minimal models.
Philipp Aebi (Neuchatel)
ARPES on charge density wave compounds
A series of layered charge density wave materials are investigated with angle-resolved photoemission. Discussed are TaS2, TaSe2 , NbTe2, TiSe2 and TiTe2 with structures related to the so-called 1T polytype. Many of them undergo charge density wave transitions or exist with a distorted lattice structure. Attempts to explain the mechanism behind the structural reconstruction are given. Depending on the filling of the lowest occupied band a drastically different behavior is observed. Whereas density functional calculations of the electronic energy and momentum distribution reproduce well the experimental spectral weight distribution at the Fermi energy, the ARPES energy distribution curves reveal that for some of the compounds the Fermi surface is pseudogapped. Two different explanations are given, first based on density functional calculations accounting for the charge density wave induced lattice distortion and second relying on many body physics and polaron formation. Qualitatively both describe the observations well. However, in the future, in order to be selective, quantitative modeling will be necessary including the photoemission matrix elements.
Azzedine Bendounan (Wuertzburg)
Electronic structure of organic films on metal surfaces studied by high resolution UV- and resonant photoemission
I present high-resolution photoemission measurements on the electronic structure of organic/metal interfaces. My interest is focused on large p-conjugated planar molecules such as 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA). On noble metal surfaces, characterized by a flat density of states close to the Fermi energy, these molecules form highly ordered superstructures and grow in layer-by-layer mode. Ag(111), for example, is an ideal substrate since the molecule can diffuse over large distances and form ordered islands. After deposition of one monolayer of PTCDA, the Ag-Shockley surface state disappears and new features appear. One of these features is associated with the formation of a chemical bond resulting from an electron transfer between the substrate and the molecule. The other structures represent the highest occupied molecular orbital (HOMO) peaks, which are also modified by the bonding. We observe an important modification in the photoelectron intensity as function of the emission angle, which can be related to the orientation of the molecule on the substrate. By resonant photoemission on NTCDA/Ag(111) system, we are able to identify which Carbon bondings within the molecule are origin of the molecular orbital peaks observed in the photoemission spectra.
Véronique Brouet (Orsay)
Arpes studies of the electronic structure of new materials
Charge density waves (CDW) form a class of electronic instabilities particularly well suited for photoemission studies, as they are driven by the nesting of the Fermi Surface (FS) that can be directly imaged by this technique. Furthermore, they principally occur in low-dimensional materials, which greatly simplifies the analysis of photoemission spectra and allow much deeper investigation of their lineshapes. They most frequently take place in quasi-1D systems, where the possible formation of a Luttinger liquid, although interesting in its own right, interfere with the study of the CDW itself. For this reason, 2D CDW systems appear to offer a simpler opportunity to model the CDW behavior. The most extensively studied family of 2D CDW systems have been the transition-metal dichalgogenides (2H-NbSe2 , 1T-TaS2 ...). Another interesting family received attention more recently; it is formed by RTe3 and RTe2 materials (R=Y, La, Ce, Tb?), which are based on square planes of Te atoms. The CDW in these materials is characterized by a large gap (300 meV, one order of magnitude larger than in the transition metal systems) on some parts of the Fermi Surface, while other parts remain metallic. We will present results on two aspects : - the size of the gap along the FS, its connection to the nesting properties of the FS and the reconstruction of this FS in the CDW state - the lineshape of the spectra and what it can tell us about the nature of the metallic state and its excitations.
Harald Brune (Lausanne)
Magnetism of nanostructures at surfaces
Fundamental questions related to recording media of computer hard disks and to magnetic random access memories (MRAMs) can be addressed using surface science. One of these questions is the smallest unit to store one bit magnetically at room temperature, and the ultimate density such units may be placed without interaction. A question related to MRAMs is the tunnel-magneto-resistance (TMR) and its voltage dependence of junctions with nanometer dimension. We use self-assembly during atomic beam epitaxy to create metal islands on metal substrates and determine their morphology and composition with STM.In-situ Magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD) measurements enable a one–to–one correlation between magnetism and atomic morphology. This is used to identify the magnetic properties of the constituent atoms as a function of their lateral coordination The magnetic anisotropy energy K defines the stability of the orientation of the magnetic moment against thermal fluctuations and thereby the so-called superparamagnetic density limit. We find K to be strongly influenced by atomic coordination. Co step atoms on Pt(111) have 20 times the bulk value, and single adatoms even 200 times. Depending on the substrate symmetry, they can be inplane and out-of-plane magnetized. Superlattices of Co islands on Au(778) have uniaxial out-of-plane easy magnetization axes, they do not interact, and have the most uniform moments and anisotropies realized so far. They therefore represent model systems for the investigation of the fundamental density limit of magnetic recording. The anisotropy energy of an island with a given size can be increased up to four times in bi-metallic islands where the two elements Fe and Co form concentric shells or alloys. We finally show spin-polarized STM measurements showing a largely voltage independent contrast, which can reach an equivalent TMR of 850% thus approaching theoretical predictions for ideal tunnel junctions
Hervé Cercellier (Neuchatel)
Effect of local doping on a charge density wave
Among the quasi-2D transition-metal dichalcogenides (TMDC), TiSe2 has an intriguing behaviour, exhibiting a transition from a (1x1) room temperature phase to a (2x2x2) distorted phase at about 200 K accompanied by a maximum of its resistivity around this temperature. Different explanations have been given to account for the transition, among which a Jahn-Teller distortion and/or the formation of an excitonic insulator phase, as described by Kohn long ago (1967). Here we present new ARPES and STM/STS data above and below the transition temperature. Upon entering the distorted phase, backfolded bands appear in the photoemission spectra due to the new periodicity, and the shape of the bands changes. In STM a (2x2) superperiodicity is clearly observed, and the local density of states (LDOS) exhibits new features near the Fermi energy. The spectral function and density of states of an exciton condensate have been calculated, and are shown to agree very well with the spectroscopic data. These observations strongly support the excitonic insulator scenario. 1-T TiSe2 has a natural tendency for Ti-overdoping upon chemical synthesis. This nonstoechiometricity leads to the suppression of the transition when doping increases. STM data in the distorted phase show nanometer-scale inhomogeneities, wich are robust with respect to time and tunneling parameters. We present preliminary results that suggest that these inhomogeneities are due to local doping from Ti atoms below the surface.
Konstantin Eltsov (Moscow)
Use of molecular halogens to control structure and properties of solid state surfaces
Molecular halogens or halogen contained molecules are industrially important in microelectronics and heterogeneous catalysis due to their chemical activity to treat substrate materials or selectively interact with surface species. On atomic scale interaction of halogens with metallic or semiconductor substrates could be base for precise modification of surface properties. In the presentation, a few examples of such applications are demonstrated. First, submonolayer coverage of halogens on metals is attractive system to study structural phase transitions in chemisorbed layers because observed behavior is very close to processes in noble gases adsorbed at LHe temperatures (iodine on base planes of Cu). Secondly, we are able to study nucleation of halide film on metals and to understand the process on atomic scale (CuI on Cu). This knowledge could be applied to understand an early stage of semiconductors/semiconductors growth. Thirdly, under photon or electron flux, photosensitive halide film demonstrates new properties such as Surface Enhanced Raman Scattering (SERS) with k = 106÷107 (CuCl on Cu(111)). It gives a chance to determine structure of SERS active sites. Fourthly, selective interaction of halogens with binary semiconductors (A3B5) gives an promising opportunity to change the surface enrichment by one of the components and to control atomic structure very precisely (iodine on GaAs(001)). We are able to create both Ga-rich c(8x2), c(6x6) and As-rich c(2x8) by combination of iodine adsorption and thermal treatment.
Enrique Garcia Michel (Madrid)
Electronic and structural properties of Sn/Ge(111) below 30 K: observation of a Mott insulating ground state
The adsorption of 1/3 of a monolayer of Sn or Pb on Ge(111) or Si(111) gives rise to an ordered (√3x√3)R30º structure at room temperature. These systems have deserved widespread attention since the discovery in 1996 of a temperature-induced phase transition to a (3x3) structure, that is observed at temperatures below ~100 K (with the possible exception of Sn/Si(111)). The Sn/Ge(111) interface has been investigated with a host of experimental techniques, probing both the structural properties (STM, LEED; SXRD, photoelectron diffraction, helium atom scattering), and the electronic band structure (angle resolved photoemission, STM), but in all cases the lowest temperature reached was approx. 80 K. The results have enabled a detailed description of the properties of the (3x3) phase, which is due to a vertical distortion affecting mainly the Sn atoms (one out of three in the unit cell). This distortion makes the Sn adatoms nonequivalent and the surface becomes metallic. On the other hand, theoretical calculations have been performed in order to understand the origin of the (3x3) distortion and its competition with a flat (√3x√3)R30º structure. While the (3x3) structure is the ground state, it seems that the energetic difference with a flat (√3x√3)R30º phase would be very small. We report here an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approx. 200 K, reverts to a new (√3x√3)R30º phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition. The phase transition is fully reversible. Angle-resolved photoemission experiments show that concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The (√3x√3)R30º ground state is interpreted as the experimental realization of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice. The properties of the Mott insulating state are analyzed in detail, both with STM and angle resolved photoemission. The origin of the new ground state found is traced back to a change the delicate energetic balance between elastic and electronic energy in the (3x3) phase.
Marco Grioni (Lausanne)
High resolution ARPES of modulated two-dimensional structures
I will discuss the electronic structure of ordered monolayer and submonolayer structures obtained at the interfaces between a heavy metal overlayer (Pb or Bi) and the Ag(111) and Au(111) substrates. In the submonolayer surface alloys we have observed a very large energy- and momentum- spin-orbit splitting of the surface states. Remarkably, in mixed Pb-Bi alloys the splitting and the Fermi level position can be continuously tuned, in a rigid-band-like fashion. The morphology and electronic properties of the interfaces are radically different for a full monolayer, which exhibits a close-packed structure and a moiré modulation of the atomic positions. Here, we observe a selective breakdown of the surface electronic structure, though a surface state-mediated hybridization with the substrate. The resulting mixing with the bulk continuum leads to a striking energy broadening of the overlayer pz band.
Oliver Groening (Thun)
Low energy H-ion induced defects on graphite and single walled carbon nanotubes characterized by STM and STS
CNT have emerged as a kind of prototype nanomaterial due to out-standing mechanical, thermal, electronic and structural properties. With regard to the electronic properties CNT can be used as metallic or semiconductor nanowires or molecular Quantum dots. In this respect we are interested in the local modification of the electronic structure of CNT by mono-atomic defects and the possibility to control transport properties via such defects. An efficient way to create such defects is to expose the CNT to low energy (<10 eV kin. Energy) hydrogen ions. Taking HOPG graphite as model surface we will discuss the formation of single H chemisorption sites and single vacancies and the associated large-momentum scattering of electrons states at the Fermi energy of these defects. In the case of a high defect density coherent interference of the scattered electron waves will lead to a characteristic (√3x√3)R30° superstructure, which can be related to the Fermi-surface of graphite. In the case of CNT we will present results on Low-Temperature Scanning Tunneling Microscopy (LT-STM) and Scanning Tunneling Spectroscopy of single walled carbon nanotubes. As in the case of graphite the very characteristic (√3x√3)R30° superstructure can be found in the vicinity of the H-ion induced defects. On the positions of the defects strong modifications of the electronic structure can be observed. These modification can consist of the apparition of new intense electronic states in the band gap of semiconducting SWNT. In many cases we observe a pair of sharp electronic states symmetric with respect to the Fermi energy in the SWNT gap. Further we will present first indications of state quantization between defects.
Hervé Guyot (Grenoble)
Peierls transition, band structure and Fermi surface in two dimensional compounds
Some low dimensional compounds exhibit electronic instabilities towards charge density wave (CDW) states. One of the most investigated class of such compounds is a family of the transition metal oxides, that includes Mo4O11, the purple bronze KMo6O17 and the monophosphate tungsten bronze (PO2)4(WO3)8. The layered crystal structure and the confinement of d electrons in the middle of the layers make these bronzes quasi two-dimensional conductors. This anisotropy, associated to the presence of a large nesting of the Fermi surface and a strong electron-phonon coupling originate the charge density wave instabilities. We present a review of ARPES and STM experiments that reveal the topology of the Fermi surfaces and the nesting possibilities, determine the band structures and the Fermi vectors and characterize the modulations of the CDW. The results are discussed and compared with the theoretical predictions. Parts of the work were done at Lure (Orsay) in collaboration with M.C. Asensio and J. Avila.
Bertrand Kierren (Nancy)
From quantum box assembly to electronic superlattice in self organized metallic nano clusters on vicinal surfaces
We have investigated the electronic properties of self organized networks of Co and Ag nano dots deposited on Au vicinal surfaces. Thanks to the nanoscale patterning of the Au(788) and Au(23 23 21) surfaces, regular arrays of metallic dots can be obtained with a narrow size and shape distribution. We will point out the long range order of the system resulting from the preferential nucleation sites. Confinement of the Shockley surface state and band folding has been evidenced by both Scanning Tunneling Spectroscopy and Angle Resolved Photoemission. For Co deposition, the system can be regarded as a network of weakly coupled quantum boxes, whereas for Ag nano dots, a delocalized Bloch state description is appropriated.
Guy Le Lay (Marseille)
Physics of massively parallel silicon nanowires: atomic and electronic structures and passivation
We will present novel quantum silicon nanostructures with an extremely high aspect ratio obtained by condensing in situ under ultra-high vacuum silicon onto the clean, highly anisotropic, unreactive, Ag(110) surface. These are either massively parallel arrays of metallic quantized stripes or perfectly aligned Si nanowires. These nanostructures reveal striking aspects and astonishing features in scanning tunneling microscopy. They are characterized in great details by synchrotron radiation photoelectron spectroscopy of the valence bands and core-levels. Discrete quantum states are observed in the sp region of the valence band below the Fermi level and their dispersions followed along the high symmetry directions of the surface Brillouin zone. Fine features in low energy electron diffraction and extremely sharp Si core-levels reveal that up to macroscopic scales, the whole ensemble of massively parallel, quantized stripes is formed by atomically identical individual nanostructures [1,2]. Results of exposures of these new nano objects to atomic hydrogen or to oxygen so as to change their electronic properties from metallic to semiconducting or to insulating barriers will be further presented. These silicon nanostructures provide atomically precise new templates that could be eventually used, e.g., to align nanotubes or fix individual molecules as in a mould.
[1] Self-aligned silicon quantum wires on Ag(110) C. Leandri, G. Le Lay, B. Aufray, C. Girardeaux, J. Avila, M.E. Davila, M.C. Asensio, C. Ottviani and A. Cricenti, Surface Sci. 574 (2005) L9 [2] Temperature behaviour of silicon quantum wires on Ag(110) M.A. Valbuena, J. Avila, M.E. Davila, M.C. Asensio, C. Leandri, B. Aufray and G. Le Lay Appl. Surf. Sci. (2006) in press
Daniel Malterre (Nancy)
Reconstruction induced gaps and spectral weight distribution in Au(23 23 21)
The surface states of noble metals experience the modification of the potential induced by vicinality or nanostructuration at the surface. We show by ARPES and STM/STS that the Au reconstruction in vicinal surfaces leads to the formation of several small gaps and to spatial modulation of the electronic density. The values of the gaps and the phase of the electronic density obtained from these spectroscopic techniques allow to built the super-periodic potential associated with the reconstruction.
Friedrich Reinert (Wuerzburg)
Many-body effects in Shockley-type surface states
High-resolution photoemission spectroscopy allows to investigate small modifications of the band dispersion and photoemission lineshape of surface states, e.g. the Shockley-states on the (111) faces of noble metals. These modifications can be caused by many-body effects as e.g. electron-electron and electron-phonon coupling. Usually, in metallic systems these many body-effects are comparatively small (i.e. on the scale of 1 meV) and often blurred by side effects as e.g. the scattering at defects. However, recent results show that photoemission spectra on the quasi-two dimensional Shockley-states can be analysed quantitatively and theoretically described by ab initio methods.
Pascal Ruffieux (Thun)
Surface state scattering at large aromatic molecules
The adsorption of flat aromatic molecules on Cu(111) induces a prominent redistribution of the surface state density of states, as observed by low-temperature STM. Mapping of the local density of states reveals a pronounced localization of the surface state electrons near the molecule that sensitively changes with the shape of the molecule, as confirmed for different HBC-derived hydrocarbons. We attribute this observation to the scattering of the surface state at the molecule and present a multiple scattering simulation allowing the description of the charge redistribution of the investigated molecules. Analysis of the molecule distribution at submonolayer coverage evidences a repulsive intermolecular interaction at the Cu(111) surface. This substrate-mediated interaction is attributed to charge redistribution in the vicinity of the molecule due to surface state scattering. The balance between molecule-substrate interaction and molecule-molecule interaction and the resulting adsorption properties will be discussed for different functional groups on the HBC molecule.
Wolf Dieter Schneider (Lausanne)
Scanning tunneling spectroscopy of a two-dimensional solid: A Ce superlattice on Ag(111)
Low temperature scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms on Ag(111) reveals site-dependent characteristic features in differential conductance spectra and in spectroscopic images at atomic-scale spatial resolution. Using a tight-binding model, the overall spectral features are related to the scattering of Ag(111) surface-state electrons by the Ce adatoms, the site depencdence to the disorder induced by imperfections of the superlattice, and the opening of a gap in the local density of states to the observed stabilization of superlattices with adatom distances in the range of 2.3 to 3.5 nm.
Antonio Tejeda (Paris)
Fermi surface gapping and nesting in the surface phase transition of Sn/Cu(100)
Two-dimensional phase transitions triggered by a gain in electronic energy have deserved ample attention during recent years. One of the most important examples of this kind is the formation of a charge-density wave (CDW). The CDW state is an easily accessible, macroscopically coherent state with very interesting properties. As the CDW sets in, the lattice reorders slightly, giving rise to a periodic lattice distortion and a new supercell. In this work, we report angle-resolved photoemission spectroscopy (ARPES), low-energy electron diffraction (LEED), and surface x-ray diffraction (SXRD) measurements of 0.5 ML of Sn atoms on Cu(100). The use of ARPES allows us to directly probe the electronic band structure near the Fermi energy, while LEED and SXRD provide structural information. Above ~360 K, the surface presents a (√2x√2)R45° superstructure. A surface free-electron-like band defines a 2kF nesting vector equal to 1/3 of the √2 reciprocal lattice vector. In excellent agreement with this nesting vector, a reversible phase transition to a (√2x√2)R45° structure is observed at 360 K. The phase transition is associated with the partial gapping of the Fermi surface in areas coinciding with the 3Ö2 zone edge. We discuss the interpretation of this phase transition as the stabilization of a surface CDW.
Coriolan Tiusan (Nancy)
Spin polarized tunnel transport in magnetic tunnel junctions
The transport mechanisms in crystalline magnetic tunnel junctions (MTJ) gained the interest of the international scientific community after the publication of the theoretical work of Butler and al [Butler and al, J Appl. Phys. 81, 5518 (1997); MacLaren and al, Phys. Rev. B 56, 11827, (1997)]. They show that a realistic description of the band structure makes the mechanisms of transport impossible to describe within the free electrons model. Indeed, in crystalline systems the Bloch electrons are not any more distinguished according to their orbital character but are classified with respect to the symmetry of their associated electronic wave function. This determines a symmetry dependent wave function attenuation within the insulator [MacLaren and Al, Phys. Rev. B 59, 5470 (1999)]. Giant tunnel magnetoresistive effects, reaching several thousands of percents, are theoretically predicted in single-crystal MTJ employing bcc ferromagnetic electrodes and MgO insulating barriers [1,2]. A brief review of the standard theoretical techniques used to calculate spin-dependent transport techniques will be presented, from the free electrons to the fully ab-initio framework. Experimental results will be confronted to theory for single-crystal magnetic tunnel junctions, elaborated by Molecular Beam Epitaxy, employing bcc ferromagnetic electrodes respectively MgO(100) insulating barrier. We demonstrate that the interfacial chemical structure play a crucial role in the filtering efficiency. Roughness related chemical fluctuations or contamination of interface either by oxygen or by carbon even at sub-monolayer level reduce this efficiency and consequently the amplitude of the measured TMR effect. Moreover, our experiments emphasize the direct correlation between the symmetry conservation during the tunneling and the filtering efficiency. On the other hand, using spin polarized tunnel spectroscopy experiments we provide experimental evidence for the electronic interfacial resonance states contribution to the spin polarized tunnel transport [3].
[1]Butler and al, Phys. Rev. B 63, 054416 (2001)
[2]Mathon and Umerski, Phys. Rev. B 63, 220403 (2001)
[3]Tiusan, Phys. Rev. Lett. 93, 106602 (2004)
Jean-Yves Veuillen (Grenoble)
Scanning tunelling spectroscopy on 2D metallic islands on insulators
In this talk I shall present results obtained by scanning tunnelling microscopy and spectroscopy (STM/STS) on a genuine two dimensional metallic film (ErSi2) grown on an insulating substrate (actually: a Si(111) substrate). Previous angle resolved photoemission studies and ab-initio calculations had revealed two bands crossing the Fermi level in this 2D metallic monolayer. I shall firstly show that it is possible to recover the band structure of this material by studying the evolution with bias of the standing wave patterns due to confinement or to quantum interference effects. In a second part I shall present some results on the interaction between atomic size defects and the 2D metal, that lead to the occurrence of localized states split off from the 2D band. These points will be discussed in comparison with the well documented case of Shockley states at noble metal surfaces. Finally I shall discuss spectroscopic data obtained for disconnected silicide islands that show evidence for a hindered electronic transport parallel to the surface at low temperature.
Nadine Witkowski (Paris)
Investigation of molecule chemisorption on Si(001)-2x1 surfaces by means of surface reflectance spectroscopies.
Reflectance Spectroscopy (SDRS) and Reflectance Anisotropy Spectroscopy (RAS) are suitable tools to study the adsorption of molecules or atoms on the Si(001)-(2x1) surface. The capabilities of these techniques to investigate adsorptions is illustrated by several examples. In particular, quantitative information such as the number of surface Si atoms involved in the bonding, can be obtained by SDRS and RAS. Moreover, with SDRS it is possible to discriminate between adsorption on the Si dangling bond and adsorption inducing a breaking of the dimer.
Vladimir Yu. Yurov (Moscow)
Ag monolayer on Cu(111): new chemical properties and formation of "quantum-islands" at Cu deposition
At present, there is a great progress in creation of artificial materials on a base of quantum wells. Such artificial materials have a modified electronic characteristics and new physical and chemical properties. This technology allows creation of quantum wells of different chemical composition and thickness down to one atomic layer. The surface periodic superstructures as a template for the epitaxial growth of nanoobjects are very attractive for creating an ordered system in two dimensions. In our project we plan to grow a structured metal halide film under reaction of molecular halogens on network of the dislocation loops of Ag monolayer on Cu(111). As the first step we have found and studied an enlarged chemical activity of Ag/Cu(111) surface (in comparison with clean surfaces of Cu(111) and Ag(111) monocrystals) during reaction with Cl2. After Cu deposition at LT on monolayer of Ag on Cu(111) we have observed (by STM) islands stable even at RT, although the smallest of them includes about 20 atoms. The size of each island can be quantized according to the number of dislocation loops on the surrounding surface covered by the island. Simple ball model of island was originated which agrees with the model of Besenbacher and explains the observed shift in the dislocation loops rows on these islands and on the surrounding surface. The position of the islands (especially for small ones) corresponds to a center of dislocation loops on the surface. That give us an opportunity to get more similar sizes of islands and more perfect periodicity of it according to the dislocation lops network by adjusting properly a temperature of the substrate and evaporation rate of Cu.
Peter Zeppenfeld (Linz)
Optical reflection spectroscopy of nanostructured surfaces and thin films
Reflectance Difference Spectroscopy (RDS) provides a sensitive probe to the surface morphology and electronic structure. It is possible to correlate the RDS spectra with particular electronic states (such as surface states or quantum well states) and to characterize the growth of thin films and adlayers by monitoring these characteristic optical transitions. In addition, collective electronic excitations (plasmons) can be detected on nanostructured surfaces. This allows to characterize the formation and growth of metal clusters on surfaces as well as to study periodic surface gratings obtained during sputtering (ripple formation) or by means of nanoimprint lithography.

Communication par poster

Bertrand Berche (Nancy)
Logarithmic corrections and universal amplitude ratios in the 4-state Potts model in 2d
Monte Carlo (MC) simulations and series expansions (SE) data for the energy, specific heat, magnetization and susceptibility of the 4-state Potts model on the square lattice are analyzed in a vicinity of the critical points to estimate universal combinations of critical amplitudes. The role of logarithmic corrections is discussed. Accurate estimates of universal ratios $A_+/A_-$, $Gamma_+/Gamma_-$, $Gamma_T/Gamma_-$ and $R_c$ are given. The ratios of the susceptibility critical amplitudes obtained in our analysis differ from those predicted theoretically and supported by earlier SE and MC analyzes. The disagreement of our results might signal that the two-kink approximation used in the analytical estimates is not sufficient to describe with a reasonable accuracy the amplitudes of the 4-state model.
Christophe Brun (Marcoussis)
Charge-Density Waves and a Possible Extraordinary Surface Phase Transition at (100) Surface of NbSe3 Observed by STM.
Quasi-one dimensional (1D) metal NbSe3 undergoes two successive Peierls transitions at T1 = 144K and at T2 = 59K involving two different charge-density wave (CDW) vector Q1 and Q2. Early scanning tunneling microscopy (STM) studies of NbSe3 presented preliminary results due to the poor molecular resolution[1]. The identification of the chains and their relationship with the two CDW modulations were two open questions for STM measurement. In liquid helium Dai et al. finally identified both CDWs and the three different chains present in the surface unit cell. However a discrepancy remained between chains identification at 77K and 4K. We report here a systematic study of the CDW modulations at three temperature (5K, 62K and 78K) with molecular resolution under ultra-high vacuum (UHV) by STM. At 78K, the three different chains were carefully identified. Chain III carries most of the Q1 CDW. 2D Fourier Transform (FT) of the STM images shows clearly both the lattice and the Q1 superlattice spots in excellent agreement with the bulk reported values. Additionally we found that chain II, the closest to chain III, presents a small Q1 CDW modulation in phase with that on chain I, in good agreement with x-ray refinements[2]. At 5K, in addition to the coexistence of Q1 and Q2 superlattices, we observed additional satellite spots involving the two possible combinations of the two CDW wave vectors: Q1 + Q2 and Q1 - Q2, suggesting the existence of interaction between the two CDWs. Surprisingly at 78K we also found diffuse spots corresponding to the Q2 CDW. These are more elongated in the transverse direction than along the chain direction. At 62K, i.e. 3K above T2, the Q2 CDW is already well developed, leading to true superlattice spots in FT of the STM images. This coherent Q2 modulation affects mainly chain I, which is the chain mostly affected by the T2 transition as we observed it at 5K. The modulation amplitude observed of Q2 at 62K is larger than that of the Q1. We propose that it might be a new phase transition: an extraordinary surface CDW phase transition as predicted by Tossatti and Anderson in 1974[3].
[1] C. G. Slough et al. Phys. Rev. B. 39, 5496 (1989); Z. Dai et al., Phys. Rev. Lett. 66, 1318 (1991)
[2] S. van Smaalen et al., Phys. Rev. B 45, 3103 (1992)
[3] E. Tossatti and P.W. Anderson Solid State Comm. 14, 773 (1974).
Christophe Brun (Marcoussis)
Inhomogeneity of CDW Wave Vector at (-201) Surface of Rubidium Blue Bronze Rb_0.3 MoO_3
The blue bronze is one of the most studied quasi-one dimensional metals showing a Peierls transition at Tp=180K. Recently, using scanning tunneling microscopy under ultra-high vacuum at low temperature (UHV-LT-STM) on an in-situ cleaved (-201) surface of a rubidium blue bronze (Rb0.3MoO3) single crystal, molecular lattice and charge-density wave (CDW) superlattice were observed simultaneously at temperatures well below Tp [1]. The observed average surface CDW wave vector was in agreement with the bulk reported value projected onto the (-201) surface. The number N=Qb*/b* characterises the commensurability between the CDW and the lattice period along the chains. We found that for a given sample, optically distinct plateaus could yield different values of N. Moreover on the same plateau, N could significantly change with a tip displacement of only several microns, leading to true inhomogeneities of the CDW wave vector at surface. After cleavage, different local concentrations of surface Rb atoms may arise in particular from those located initially at equal distance from the layers. In particular it has already been shown that induced desorption of alkali ions modifies the surface electronic structure at the Fermi level [2]. First-principles density functional theory calculations were performed in order to understand the physics of the inhomogeneity. It is found that the surface density of Rb atoms plays a key role in determining the surface nesting vector. The calculated changes in the surface nesting vector induced by different realistic concentrations of surface Rb atoms are consistent with the observed inhomogeneities.
[1] C. Brun et al., Phys. Rev. B 72, 235119 (2005)
[2] K. Breuer et al., J. Vac. Sci. Technol. A 12(4) (1994).
Christophe Chatelain (Nancy)
Jarzynski relation in 2d Ising model
Clément Didiot (Nancy)
Confinement of Shockley states in self-organized nanostructures on Au(111) vicinal surfaces
We have studied the peculiar properties of the Shockley surface state in electronic 2D superlattices obtained by self ordering Ag nano structures on the reconstructed vicinal templates Au(788) and Au(23 23 21). Under appropriate conditions one can grow a close to perfect array of Ag nano islands with a very narrow size and shape distribution. The perfect ordering from nanometer to macroscopic scale allowed us to perform angle resolved photoemission spectroscopy to characterize the surface state band. We will show that the Shockley state is coherently scattered by the structure leading to complex standing wave patterns as revealed by the conductance maps recorded at 5K. We evidenced that the Ag nano dots behave as a collection of quantum wells which partially confine the Shockley surface state. Playing with the size of the Ag clusters, one can perfectly control the occupancy of the quasi discrete surface band leading to a fine tuning of the surface electronic properties at the scale of the sample. Scanning tunneling spectroscopy on the genuine Au vicinal surfaces will be also presented.
Sven Dorosz (Nancy)
Off equilibrium work distributions in quantum spin chains
We are studying the off-equilibrium state of a one dimensional quantum system. Primarily we consider the Ising chain of N spins initially in a canonical state($e^{-eta mathcal{hat{H}}}$), at constant inverse temperature $eta$. The system is driven out of equilibrium by a transverse magnetic field h(t).The evolution is calculated numerically by a Suzuki Trotter decomposition along the protocol h(t). At time $ au$ we determine the probability distribution $P(W)$ of the work transferred to the system. The distribution P(W) is analysed when the protocol h(t) is repeated until P(W) converges towards a stationnary distribution $P^infty(W)$. We will discuss the influence of parameters as the amplitude of the field and or the duration of the protocol $ au$ to the shape of the distribution.
Yannick Fagot-Revurat (Nancy)
Charge gap and magnetism across the metal-insulator transition of the quasi-1D compounds (Sr,Na)xV6O15
The vanadium bronzes b-AxV6O15 (A=Sr, Ca, Na, Li, Ag …), first discovered in the 80’s, are mixed-valence compounds V4+ (3d1)/V5+ (3d0) presenting very exciting physics such as low dimensionnal electronic properties, metal-insulator transition and superconductivity. The stoechiometric samples (Na,Sr)x=1V6O15 present a complex phase transition associating a metal-insulator transition (MIT), a structural transition and a charge ordering occuring at TMI=170 K for Sr1V6O15 and TMI=135 K for Na1V6O15. Superconducting properties have been also evidenced recently at high pressure below T=9 K in NaV6O15. At room-temperature, they present « metallic » properties (r//» 10-3 and r^/r//» 100) with a quasi-1D Fermi surface (from band structure calculations). Surprisingly, NaV6O15 has a metallic resistivity associated with a Curie-Weiss magnetic susceptibility whereas SrV6O15 is semiconducting with a Pauli-like paramagnetic suceptibility suggested delocalized carriers ! The ordering of the Sr (for x>0.9) and Na (for T<230 K) atoms lead to a doubling of the lattice parameter along the b-axis. Only one type of V4.83+ and V4.66+ site are observed respectively for NaV6O15 and SrV6O15 above the MIT. Below the Metal-insulator transition (MIT), a tripling of the lattice parameter is observed in both cases associated with a charge ordering. Nevertheless, magnetism is still present at low-T : a Curie-Weiss susceptibility is observed in the case of NaV6O15 whereas a spin-gap phase is revealed in the case of SrV6O15. A charge gap of 200 meV is evidenced below the TMI in Sr1V6O15 by angle-integrated photoemission measurements associated with a stabilization of the correlated d band. One of the key point to understand this complex phase transition could be the Fermi wave vector kF which can be extracted from ARPES measurements on single crystals.
Aleksandr Kapikranian (Nancy et Lviv)
Spin wave approximation in disordered XY model in 2d
The 2d XY model is known to possess in the thermodynamic limit at low temperatures a quasi-long-range order with zero magnetization and a power-law decay of the pair correlation function. We study influence of two factors: (i) dilution by a non-magnetic impurities and (ii) finite system size on the low-temperature phase of the 2d XY magnet. Both factors are important from the point of view of experimental realizations and MC simulations and lead to highly non-trivial effects. The presence of impurities changes the pair correlation function critical exponent, whereas the spontaneous magnetization is non-zero and well-defined for a finite system size. We give a quantitative description of the above phenomena using two complementary ways of analysis: an analytic approach and MC simulations. The analytical approach is based on the spin-wave approximation and on an expansion in the parameter which characterizes the deviation from completely homogeneous dilution. The quantities of interest (the spin-spin correlation function and the finite-size magnetization) are averaged over all the possible configurations of impurities and compared with the results of MC simulations. We discuss ranges of applicability and convergence properties of the expansions obtained.
Sven Krüger (Nancy)
Sodium Chloride on noble metal surfaces
One of the interests in surface science are systems of low dimenionality. We are interested by specific electronic states wich originates by quantum confinement in 1-D or 2-D nanostructures. When obtained by epitaxy on metallic surfaces, nanostructures usually show large electronic coupling with the substrate which strongly modify the genuine properties of the nanostructures. A solution can be found by growing an ultrathin insulating buffer on the substrate before growing the nanostructures. The thickness has to be limited to few atomic planes to ensure charge flow by tunnel transport as well for physical investigation (STM,STS, phtoemission, ...) as for technological applications.The growth properties are therefore challenging.Here an investigation of the growth properties of NaCl on Cu(111) and Au(111) are presented. LEED and STM/STS results are discussed and differences in structural properties of both systems are highlighted.
Pierre Mallet (Grenoble)
STM study of electron confinement and transport in 2D metallic nanoscale islands
Two-dimensional (2D) conducting systems lying at surfaces have recently gained attention, mainly because the quantum coherence of the 2D states can be probed using Scanning Tunneling Microscopy (STM). Most of the experiments reported so far have dealt with the (111) surface of noble metals, which supports a nearly 2D free electron gas, known as the Shockley state. Lateral confinement of the Shockley state within nanoscale structures have lead to new opportunities in nanophysics. Accordingly, experiments on real 2D metals on insulator surfaces, in which transport measurements in the surface layer are also possible, should receive increasing interest in the near future. In collaboration with a group from Madrid (UAM, Spain), we have focused on a promising system, which consists of one ErSi2 layer (of thickness ~3Å) on a Si(111) surface. Using a home made low temperature STM and ab initio calculation, we have shown that the ErSi2 layer behaves as a true 2D metal, in which electronic states at the Fermi level are not coupled with underlying Si bulk states. Our experiments will be presented here, with a careful analysis of lateral confinement of the 2D metal states within nano-constrictions and nanoscale islands. In the future, we believe that the ErSi2/Si(111) system will be a good candidate for studying mesoscopic physics in 2D conducting systems, combining STM and magneto-transport measurements.
Daniel Malterre (Nancy)
Fine tuning of spin-orbit splitting in Ag ultrathin films deposited on Au(111)
We have studied the modification of Au(111) surface state parameters with Ag deposition. We unambigously evidence a continuous decrease in the spin-orbit splitting of the two surface states subbands with increasing Ag coverage. An annealing leads to the formation of a chemically disordered Ag-Au alloy. This alloy formation is accompanied by an increase in the spin-orbit splitting. We established a quantitative correlation between the amplitude of this splitting and the relative amount of Au and Ag atoms probed by the surface state wave function proving the atomic character of the spin-orbit splitting. Then the control of both the morphology, and the chemical composition of the Ag-Au interface allows a continuous fine tuning of the surface state properties including the k-dependent spin-polarization.
Thierry Platini (Nancy)
Out of equilibrium processes in Ising quantum chains
Jean-Charles Walter (Nancy)
Stochastic formulation of quantum mechanics

Comité d'organisation

Bertrand Berche
Christophe Chatelain
Olivier Collet
Clément Didiot
Dragi Karevski
Bertrand Kierren
Daniel Malterre
Loic Turban

Nos partenaires

Université Henri Poincaré Institut Polytechnique de Lorraine
Comité National de la Recherche Scientifique Labo. de Physique des Matériaux
Société Francaise de Physique Conseil Régional de Lorraine
Grand Nancy

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