# Groupe de Physique Statistique

## Equipe 106, Institut Jean Lamour

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### Statistical Physics and Low Dimensional Systems 2007

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

## mercredi 23 mai 2007 - vendredi 25 mai 2007

#### Programme de l'atelier

mercredi 23 mai 2007 Conférences plénières 14:15 - Opening 14:30 - Ingo Peschel, Entanglement evolution in a chain after a local quench 15:30 - Enrique Ortega, Electronic states in self-assembled metallic nanostructures

Physique StatistiqueSystèmes de basse dimension
mercredi 23 mai 2007
16:30 - Coffee Break
17:30 - Jean-Marc Debierre
18:00 - Ferenc Iglói
18:30 - Poster Session
19:30 - Loic's Party
jeudi 24 mai 2007
09:00 - Welcome Coffee
09:30 - Gunter Schütz
10:00 - Vladislav Popkov
10:30 - Andrea Gambassi
11:00 - Coffee Break
11:30 - Nobuyasu Ito
12:00 - Viktor Eisler
12:30 - Lunch break
14:30 - Ralph Kenna
15:00 - Lev Shchur
15:30 - Paolo Butera
16:00 - Coffee Break
16:30 - Anuradha Jagannathan
17:00 - Stéphane Attal
17:30 - Ian Campbell
18:00 - Poster Session
20:00 - Party
vendredi 25 mai 2007
09:00 - Welcome Coffee
09:30 - Martin Evans
10:00 - Paolo de los Rios
10:30 - Kyung-Myriam Kroll
11:00 - Coffee Break
11:30 - Des Johnston
12:00 - Yurij Holovatch
12:30 - Lunch break
14:30 - Satya Majumdar
15:00 - Leticia Cugliandolo
15:30 - Ernesto Medina
16:00 - Coffee Break
16:30 - Rosemary Harris
17:00 - Sudhir Jain
17:30 - Wolfhard Janke
mercredi 23 mai 2007
16:30 - Coffee Break
17:30 - Andres Santander
18:00 - Hervé Cercellier
18:30 - Poster Session
19:30 - Party
jeudi 24 mai 2007
09:00 - Welcome Coffee
09:30 - Corsin Battaglia
10:00 - Enrique Garcia Michel
10:30 - Friedrich Reinert
11:00 - Coffee Break
11:30 - Wolf Dieter Schneider
12:00 - Pascal Ruffieux
12:30 - Lunch break
14:30 - Amina Taleb Ibrahimi
15:00 - Pierre Mallet
15:30 - Guy Le Lay
16:00 - Coffee Break
16:30 - Muriel Sicot
17:00 - Stefano Rusponi
17:30 - Poster Session
18:00 - Poster Session
20:00 - Party
vendredi 25 mai 2007
09:00 - Welcome Coffee
09:30 - Tristan Cren
10:00 - Alexander Schneider
10:30 - Maxime Berthe
11:00 - Coffee Break
11:30 - Konstantin Eltsov
12:00 - Boris Andrushechkin
12:30 - Lunch break
14:30 - Vincent Fournée
15:00 - Alexander Saranin
16:00 - Coffee Break

#### Orateurs

Physique StatistiqueSystèmes de basse dimension
Stéphane Attal (Lyon)
From repeated quantum interactions to quantum noises
We consider the general situation of a simple quantum system interacting with a chain of independent copies of another quantum system. We show that the Hamiltonian evolution of this repeated interaction converges, in the continuous interaction limit, to a quantum Langevin equation. In this talk, we will explain what the quantum noises are and how they appear spontaneously from this deterministic model of repeated quantum interactions
Paolo Butera (Milano)
Universality in spin systems
High-temperature expansions for various lattice spin models of the XY class in two and three dimensions have been extended or computed from the beginning. An analysis of these data makes fairly accurate estimates of the models' critical properties possible and confirms the validity of universality in the appropriate conditions.
Ian Campbell (Montpellier)
Scaling rules, critical exponents, and dynamics in Spin Glass systems
Systematic rules, inspired by high temperature series results, are proposed for optimizing the normalizations of the leading critical terms for thermodynamic observables in ferromagnets and in spin glasses. The "extended scaling" formulae are used to analyze high precision numerical data from the critical region up to infinite temperature in canonical ferromagnets and spin glasses. Campbell, Hukushima and Takayama, PRL 97 (2006) 117202 and cond-mat/0612665
Leticia Cugliandolo (Paris)
Coarsening in two dimensions
We consider the statistics of the areas enclosed by domain boundaries (hulls') during the curvature-driven coarsening dynamics of a two-dimensional nonconserved scalar field from a disordered initial state. We show that the number of hulls per unit area that enclose an area greater than $A$ has, for large time $t$, the scaling form $N_h(A,t) = 2c/(A+\lambda t)$, demonstrating the validity of dynamical scaling in this system, where $c=1/8\pi\sqrt{3}$ is a universal constant. Domain areas (regions of aligned spins) have a similar distribution up to very large values of $A/\lambda t$. Identical forms are obtained for coarsening from a critical initial state, but with $c$ replaced by $c/2$.
Paolo de los Rios (Lausanne)
Configuration Space Networks: how to represent rugged energy landscape
Jean-Marc Debierre (Marseille)
Instabilities of vicinal surfaces induced by electromigration
Two aspects of the instabilities driven by a constant electrical field E on a vicinal surface (surface electromigration) are presented. For E perpendicular to the steps, a step bunching instability may occur : the adatom advection due to evaporation/deposition is shown to induce some stability inversions like those observed experimentally for Si(111). For E parallel to the steps,, the in-phase meandering instability is studied by solving a nonlinear equation derived from the Burton-Cabrerea-Frank model.
Viktor Eisler (Berlin)
Fluctuations in subsystems of the zero-temperature XX chain: emergence of an effective temperature
The zero-temperature XX chain is studied with emphasis on the properties of a block of spins inside the chain. We investigate the quantum fluctuations resulting from the entanglement of the block with the rest of the chain. It is shown that the rest of the chain can be considered as a thermal environment and an effective temperature can be introduced to describe the fluctuations. I will also show that our description is robust in the sense that several independent definitions yield the same effective temperature in the limit of large block size.
Martin Evans (Edinburgh)
An Exclusion Process for modelling Fungal Hyphal Growth
A simple model for mass transport within a growing fungal filament is proposed. Inspired by the role of microtubule-transported vesicles, we embody the dynamics of mass along a quasi-one-dimensional hypha with mutually excluding particles hopping on a growing one-dimensional lattice. The model is a generalisation of the asymmetric exclusion process (ASEP) to a dynamically extending lattice. We discuss mean-field and improved mean-field equations and present a phase diagram of the model's steady state behaviour which generalises that of the ASEP. In particular we identify a region in which a shock in the density travels forward more slowly than the tip of the lattice and thus moves away from both boundaries. We discuss our results in the context of filamentous fungus, {\it Neurospora crassa}.
Andrea Gambassi (Stuttgart)
Dynamic crossover in the persistence properties of critical systems
Rosemary Harris (Saarbrücken)
Modelling the influence of tau-proteins on intracellular transport
Intracellular transport involves the directed stepwise motion of molecular motors along a cellular filament network. This can be modelled by variants of the asymmetric exclusion process that allow for the finite processivity of motors, i.e., their attachment and detachment from the filament. Motivated by experimental results on the interplay between molecular motors and tau-proteins we discuss the influence, in such a minimal model, of a second species of particles which does not affect the dynamics on the filament but alters locally the attachment rates of the motor proteins. Numerical and analytical approaches indicate that transport is degraded by a high concentration of tau but relatively robust to smaller concentrations. This may be relevant to the disruption of intracellular transport observed in diseases such as Alzheimers.
Yurij Holovatch (Lviv)
Phase transition in the frustrated spin systems with noncollinear order at space dimension d=3.99
Using the field-theoretical renormalization group (RG) approach we analyze the controversial question about the nature of the phase transition in frustrated spin systems with noncollinear order. Currently, there exists substantial disagreement between the theoretical predictions of the order of the phase transition in these systems: whereas both the non-perturbative approach and the epsilon-expansion predict a first-order phase transition for order parameter components number n=2,3 at space dimension d=3, the RG analysis performed directly at fixed d=3 by perturbative means brings about the existence of the stable reachable fixed point of the RG equations and hence predicts a second-order scenario. We resolve this discrepancy by performing the RG study at fixed space dimension close to d=4 and in the region 3 < d < 4. Our analysis suggests that the fixed point found at d=3 persists even at d=4 and is an unphysical one. Consequently, the phase transition in the magnets under consideration is of the first order. In collaboration with Bertrand Delamotte, Dmytro Ivaneyko, Dominique Mouhanna, and Matthieu Tissier.
Ferenc Iglói (Budapest)
Entanglement entropy of inhomogeneous quantum systems
We study the entanglement entropy of blocks of contiguous spins in two types of inhomogeneous quantum spin systems. For quenched disorder we show that in the 2d random quantum Ising (QI) model there is a double-logarithmic multiplicative correction to the area law. For aperiodic modulation of the couplings in 1d Heisenberg, XX, and QI models we study the behavior of the effective central charge for different irrelevant, marginal and relevant type of perturbations. For strong aperiodic modulation of the couplings we obtain exact results through renormalization.
Nonequilibrium simulation study on transport phenomena
Fourier-type heat conduction phenomena are studied numerically using nonequilibrium computer simulations. It was confirmed that particle models with hard-core or Lennard-Jones interaction show normal conduction in three-dimensional systems, but their thermal conductivities diverge anomalously in lower dimensional systems. Nonlinear lattice models shows anomalous divergence irrespective of their lattice dimensionality. Some results on electoric conduction, Newtonean fluid, and application to interface transport are to be given. Ref. T. Shimada, T. Murakami, S. Yukawa, K. Saito and N. Ito, J. Phys. Soc. Jpn. vol.69 (2000) p.3150 T. Murakami, T. Shimada, S. Yukawa and N. Ito, J. Phys. Soc. Jpn. vol.72 (2003) p.1049 T. Ishiwata, T. Murakami, S. Yukawa and N. Ito, Intern. J. Modern. Phys. C vol.15 (2004) p.1413. F. Ogushi, S. Yukawa and N. Ito, J. Phys. Soc. Jpn. vol.74 (2005) p.827. F. Ogushi, S. Yukawa and N. Ito, J. Phys. Soc. Jpn. Vol.75 (2006) 073001. F. Ogushi, N. Ito and B. Li, in preparation. H. Shiba, S. Yukawa and N. Ito, J. Phys. Soc. Jpn. vol.75 (2006) 103001. T. Yuge, A. Shimizu and N. Ito, J. Phys. Soc. Jpn. vol.74 (2005) p.1895.
Quantum fluctuations as a function of the local environment in 2D quantum antiferromagnets
I consider the Heisenberg S=1/2 antiferromagnet on a variety of structures, where the environments can vary from site to site. I consider first bipartite structures with Néel type order in the ground state, where quantum fluctuations dress the classical value of the local order parameter to varying degrees depending on, for example, the coordination number of the site. As I will illustrate with calculations by linear spin wave theory and Quantum Monte Carlo, it turns out that quantum fluctuations behave in the opposite way to what one expects from a naive mean field type argument. Examples of periodic as well as quasiperiodic structures will be shown, along with an explanation in terms of a simple cluster model. Some effects of including frustration will be discussed.
Sudhir Jain (Birmingham)
Persistence and the Random Bond Ising Model
We study the zero-temperature persistence phenomenon in the random bond ± J Ising model on a square lattice via extensive numerical simulations. We find strong evidence for ‘blocking’ regardless of the amount disorder present in the system. The fraction of spins which never flips displays interesting non-monotonic, double-humped behaviour as the concentration of ferromagnetic bonds, p, is varied from zero to one. The peak is identified with the onset of the zero-temperature spin glass transition in the model. The residual persistence is found to decay algebraically and the persistence exponent θ (p) ~ 0.9 over the range 0.1 ≤ p ≤ 0.9. Our results are completely consistent with the result of Gandolfi, Newman and Stein for infinite systems that this model has ‘mixed’ behaviour, namely positive fractions of spins that flip finitely and infinitely often, respectively. [Gandolfi, Newman and Stein, Commun. Math. Phys. 214, 373, (2000).] Ref: S. Jain and H. Flynn, Phys Rev E73, R025701 (2006)
Wolfhard Janke (Leipzig)
Exploring Free-Energy Landscapes of Peptide Folding and Aggregation
We first discuss characteristic free-energy landscapes relevant for the folding process of short petides. The results are obtained from multicanonical Monte Carlo simulations of a coarse-grained hydrophobic-polar continuum model [1]. Then, to study peptide aggregation processes, this model is generalized to describe also interacting heteropolymers of finite length. For the data analysis, we propose the use of the microcanonical interpretation [2]. Along this line, we find that the microcanonical entropy behaves convex in the transition region, leading to a negative microcanonical specific heat. As this effect is also seen in first-order-like transitions of other finite systems, our results provide clear evidence for recent hints that the characterisation of phase separation in first-order-like transitions of finite systems profits from this microcanonical view. [1] S. Schnabel, M. Bachmann, and W. Janke, Phys. Rev. Lett. 98, 048103 (2007); J. Chem. Phys. 126, 105102 (2007). [2] Ch. Junghans, M. Bachmann, and W. Janke, Phys. Rev. Lett. 97, 218103 (2006).
Des Johnston (Edinburgh)
Frustration with Fat Graphs
After briefly reviewing the solution and simulation of a ferromagnetic Ising model on planar random graphs (which was first investigated because of the interest to conformal field theory and string theory), we consider the behaviour of antiferromagnets on such graphs. From the statistical mechanical point of view such models investigate the effects of connectivity disorder, rather than the usually considered case of bond disorder. Various different phenomena are observed: while an annealed average over graphs in which the graph connectivity changes on the same timescale as the spins allows the dynamical emergence of a Neel ordered phase, this is not possible for a quenched average, where a zero-temperature spin-glass phase appears instead.
Ralph Kenna (Coventry)
Logarithmic scaling relations: latest developments
Kyung-Myriam Kroll (Lille)
Analysis of background noise in DNA microarrays
Satya Majumdar (Orsay)
Largest eigenvalue of a random matrix
The statistical properties of the largest eigenvalue of a random matrix are of interest in diverse fields ranging from disordered systems to string theory. In this talk I'll discuss some recent developements on the theory of extremely rare fluctuations of the largest eigenvalue and its various applications.
Ernesto Medina (Caracas)
Antiresonances: a new route to Decoherence
Decoherence in transport is generally understood by invoking Buttiker's voltage probe model or other mechanisms involving phase scrambling of the wavefunction when contacts to a reservoir are considered. Such models are phenomenological and provide no direct mechanism for information loss. Here we show that phase information loss in the context of transport can be explained in terms of a generic instability of quasi 1D systems due to Antiresonances, or zeros of the transmission[1]. Such an instability, combined with a limited resolution is shown to lead to the appropriate decoherent transport with the only condition that the reservoir' typical energy spacing is smaller than the energy resolution of the experiment. [1] L. Foa Torres, H. M. Pastawski, and E. Medina Europhysics Letters 73, 164 (2006).
Ingo Peschel (Berlin)
Entanglement evolution in a chain after a local quench
In a quench, one changes the Hamiltonian of a system abruptly, which then leads to a time evolution of its initial state. The behaviour of the entanglement after a global quench has been the topic of several recent studies. I will report on results for a chain of free electrons (XX model) where a local scattering potential is suddenly removed. The entanglement entropy then shows a logarithmic increase followed by a slow universal decay. This will be compared to the situation for a typical global quench.
Boundary driven phase transitions of the first order for systems of conservation laws
We argue that driven systems with two particle species and hardcore interactions generically undergo at least two different types of boundary-driven first order phase transitions. One observes them in succession, bringing the system from a low density state to a fully jammed state, by keeping one boundary fixed and changing gradually conditions on the other boundary. As in one-component systems, the phase transitions are caused by shocks motion.
Gunter Schütz (Jülich)
Boundary-induced phase transition in single-file diffusion of two species of particles
We study two-component single-file diffusion inside a narrow channel that at its ends is open and connected with particle reservoirs. Using a two-species version of the symmetric simple exclusion process as a model, we propose a hydrodynamic description of the coarse-grained dynamics with a self-diffusion coefficient that is inversely proportional to the length of the channel. The theory predicts an unexpected nonequilibrium phase transition for the bulk particle density as the external total density gradient between the reservoirs is varied. The individual particle currents do not in general satisfy Fick's first law. These results are confirmed by extensive dynamical Monte-Carlo simulations for equal diffusivities of the two components.
Lev Shchur (Chernogolovka)
Finite-size effects in harmonic measure estimation of DLA clusters
We discuss issue of multifractality and multiscaling of DLA clusters. We report results of simulations in which variable size of probing particles introduced. Our approarch dramatically increase accuracy of fractal dimension estimation.
Boris Andrushechkin (Moscow)
Chlorine on Ag(111): A low temperature STM study
Chlorine adsorption on Ag(111) has been intensively studied for many years since the 1970s. The interest to this system is closely related with important industrial reaction of ethylene epoxidation. Several attempts to determine atomic structure of chlorinated silver (111) surface have been made by LEED, EXAFS, XSW, STM. However, to date the problem of the accurate determination of the structures formed by chlorine on Ag(111) remained unsolved in many respects due to the high mobility of chlorine monolayer at room temperature. Here we present a first-time low temperature (5 K) STM study of Cl/Ag(111) system. In particular, we have found that at <1/3 ML chlorine atoms do not form well ordered structures even at T< 130 K, however local order is present: atoms form small randomly distributed islands with (√3×√3)R30º structure, which coexist with rings of atoms in different adsorption sites. At 1/3 ML (√3×√3)R30º structure is formed. Further increase of coverage leads to the uniaxial compression of chlorine lattice, similar to the case of Cl,I/Cu(111) and I/Ag(111). The mechanism of this continuous phase transition implies formation of the striped domain walls. We found also that after some critical coverage uniaxially compressed phase is replaced by complex structure (17×17). Also we detected formation of small AgCl clusters on top of (17×17) structure. Thus, we have shown that application of the low-temperature STM opens a new page in the investigation of halogen/metal systems. Our results allow us to revise several structural models proposed on the base of the LEED and EXAFS data and suggest new mechanisms of the phase transitions in the adsorbed monolayers.
Corsin Battaglia (Neuchatel)
Stabilization of silicon honeycomb chains by trivalent adsorbates
Self-assembled arrays of atomic chains on Si(111) represent a fascinating family of nanostructures with quasi-one-dimensional electronic properties. These surface reconstructions are stabilized by a variety of adsorbates ranging from alkali and alkaline earth metals to noble and rare earth metals. Combining the complementary strength of dynamical low-energy electron diffraction, scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we show that besides monovalent and divalent adsorbates, trivalent adsorbates are also able to stabilize silicon honeycomb chains. Consequently silicon honeycomb chains emerge as a most stable, universal building block shared by many atomic chain structures.
Maxime Berthe (Lille)
Running current through a single non resonant quantum state in silicon
Due to its ability to resolve geometric structures on the atomic scale, scanning tunnelling microscopy (STM) is a technique well suited to identify individual point defects on a surface. In principle, its versatility should allow to spectroscopically characterize the coupling of tunnelling electrons to the electronic states and the nuclear motions of a defect. Such a measurement, which has recently been achieved on isolated molecules adsorbed on a thin insulating layer still needs to be demonstrated for other systems, particularly semiconductor systems. We have studied the transfer of electrons through the localized dangling bond state of an isolated Si adatom lying in a passivated silicon surface at 5 Kelvin. While the state is electronically decoupled from the silicon bulk states, a strong electron-vibration coupling is evidenced by measurement of the inelastic current and supported by density functional calculations.
Hervé Cercellier (Neuchatel)
Spectroscopic properties of an excitonic insulator: The case of 1T-TiSe2
Among the quasi-2D transition-metal dichalcogenides (TMDC's), TiSe2 shows a special behaviour, with a transition at 202 K from a (1x1) to a distorted (2x2x2) charge density wave (CDW) phase, accompanied by an unusual behaviour of its resistivity with temperature. Though it has been studied for several decades now, the nature of the transition remains unclear, the most likely scenarios being a band Jahn-Teller mechanism and/or the onset of an excitonic insulator phase, a phase which has never been directly evidenced so far. We present high resolution ARPES and STM/STS measurements of TiSe2 for various temperatures. In the RT phase the band structure consists in a Se 4p-derived valence band near the Gamma point and a slightly occupied Ti 3d-derived conduction band near L. Upon entering the distorted phase, the spectral function shows backfolded bands with the CDW superperiodicity, and the band dispersions near the Fermi energy change. The backfolded bands carry an unusually large spectral weight, which is a signature of many-body interactions in the system. In STM the (2x2) periodicity is clearly observed at the surface, and the differential conductivity spectra exhibit previously unobserved features near the Fermi energy. The experimental data are compared to theoretical calculations of the spectral function and density of states of an excitonic insulator. The agreement between theoretical predictions and experimental data is astonishingly good, thus giving a strong evidence of an excitonic insulator ground state in TiSe2.
Tristan Cren (Université Paris 6)
Probing the superconducting condensate on a nanometer scale
Superconductivity is a rare example of a quantum system in which the wavefunction has a macroscopically measurable quantum effect, due to the unique condensate of electron pairs. The amplitude of the condensate wavefunction is directly related to the pair density but both the amplitude and the phase enter the Josephson current, the tunneling of pairs between two superconductors. Very sensitive devices exploit the superconducting state, however a better understanding of the condensate on the local scale is needed. In a fundamental way, such a study is necessary to understand the unconventional high-Tc cuprates, and the origin of the pseudogap, as well as multiple gap, or gapless superconductors. We present a new technique to probe the superconducting state on the local scale: Scanning Tunneling Spectroscopy (STS) with superconducting tips. The talk is divided in two parts. In the first one, we report on Josephson STS, based on tunnelling of Cooper pairs where the condensate is directly probed by measuring the local Josephson current (JC) between a superconducting tip and sample. Since few years, Josephson STS is an experimental challenge: It requires that the tip be close to atomic contact in order to measure the JC. We demonstrate how this difficulty can be overcome and present the first spatial mapping of the JC on the nanometer scale. The case of an MgB2 film, subject to a normal magnetic field, is considered. In the second part, we address the question of probing the supercurrents in superconducting (SC) samples using quasiparticle SIS tunneling. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle (QP) spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe2, reveals both the vortex cores, on the scale of the SC coherence length, and the supercurrents, on the scale of the London penetration length. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.
Konstantin Eltsov (Moscow)
Atomic scale control of surface structure of GaAs(001) with molecular iodine
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 metal or semiconductor substrates could be base for precise modification of surface structure and its properties. For binary semiconductors (A3B5) selective interaction of halogens with one of the elements (A or B) could give an opportunity to change the surface enrichment and surface reconstruction, respectively. In the presentation, we report a process of selective interaction of molecular iodine with gallium atoms on GaAs(001)- 4x2. As a result, we are able to create not only Ga-rich 4x2 but nx6 and As-rich 2x4 structures if to combine molecular iodine adsorption in range of coverage 0.1÷1.0 ML and thermal treatment at moderate temperatures 250÷300 °C. At low coverage (t < 0.2 ML), iodine is adsorbed as pairs in vacancy rows between As atoms on charge features usually called as "ghosts". At higher coverage, iodine atoms occupy adsorption sites above Ga atoms in sp2 states and Ga dimers. Starting at t < 0.7 ML, 4x2 structure is destroyed and thermal desorption (TD) spectra demonstrate As2 species in temperature range 220÷550 C. At iodine coverage t < 0.7 ML only GaI species is found as two peaks at 200 and 250°C in TD spectra. We established that these two TD peaks are due to desorption of GaI from Ga sp2-state (200 °C position) and from Ga dimers (250 °C position). If iodine coverage is less than 0.4 ML the thermal heating gives the creation of nx6 structures with different quality and ratio of As-As and Ga-As dimers in upper rows. At 0.4 < t < 0.7 ML, new unknown surface reconstructions are found after removing of reaction products by heating. If to remove iodine species at t > 0.7 ML, perfect 2x4 is formed. All the treatments and measurements have been done in the same UHV setup equipped with STM (Sigma Scan Ltd.) and standard surface analysis facilities as AES, LEED, TDS.
Vincent Fournée (Nancy)
New phenomena in epitaxial growth: solid films on quasicrystalline substrates.
A quasiperiodic arrangement of atoms has only been realized in binary or ternary alloys, known as quasicrystals. These are complex intermetallics with long-range aperiodic order and non-crystallographic rotational symmetry (usually five-fold or ten-fold symmetry), first discovered by D. Shechtman et al.[1]. The physical properties arising from the quasiperiodic arrangement of the metal atoms significantly depart from that of periodic alloys and have attracted a broad interest. The most surprising feature is probably the fact that quasicrystals are alloys of metallic elements, many of them containing about 70 at. % of Al, but behaves like poorly metallic systems. A long standing issue has been to understand the relative influence of the quasiperiodic order on the physical properties of quasicrystals, independently from the complex chemistry associated with such alloys. This has been the starting point of recent attempts to grow new quasiperiodic systems by using quasicrystalline surfaces as templates to force a quasiperiodic structure in metal thin films deposited on such substrates [2, 3]. Here I will give an overview of the research conducted in the field of solid film growth on quasiperiodic surfaces. An atomistic description of quasicrystalline surfaces will be presented and discussed in relation to bulk structural models. Then the various phenomena occurring during thin film growth on quasiperiodic surfaces will be outlined. Emphasis will be placed on the nucleation mechanisms of the solid films, on their growth modes in relation to the nature of the deposited metals, on the possibility of intermixing or alloying at the interface, and on the epitaxial relationships at the crystal-quasicrystal interfaces. We will also describe situations where the deposited elements adopt a quasiperiodic structure, which opens up the possibility of extending our understanding of the relation between quasiperiodicity and the physical properties of such structurally and chemically complex solids. [1] D. Shechtman, I. Blech, D. Gratias, and J. W. Cahn, Phys. Rev. Lett. 53 (1984) 1951. [2] V. Fournee and P. A. Thiel, J. Phys. D: Applied Phys. 38 (2005) R83. [3] R. McGrath, J. Ledieu, and R. D. Diehl, Progress in Surface Science 75 (2004) 131.
Guy Le Lay (Marseille)
A grating with pitch at the molecular scale
By self-assembly of individual, straight, silicon nanowires (Si-NWs) grown on an anisotropic silver (110) surface we fabricate at macroscopic sizes a grating with a molecular-scale pitch (center to center distance) of only 1.6 nm. Characteristics of these Si-NWs studied by scanning tunnelling microscopy and spectroscopy as well as high-resolution synchrotron radiation photoelectron spectroscopy will be addressed in this talk together with their mutual arrangements.
Pierre Mallet (Grenoble)
Electron states of mono- and bilayer graphene on SiC probed by STM
Since the pioneering works of Professor Geim (University of Manchester) and Professor Kim (Colombia University) in year 2005, graphene has become one of the hottest topics in the condensed matter field, in particular due to its very exciting and promising electron properties. For these experiments, graphite exfoliation was achieved in order to isolate single graphene layers. It has been shown that graphene can also be grown by graphitization of silicon carbide substrates. For both methods, decoupling of the graphene wave functions from the neighbouring environment is a fundamental issue. I will present a scanning tunneling microscopy (STM) study of a gently-graphitized 6H-SiC(0001) surface in ultra high vacuum. From an analysis of atomic scale images, we have identified two different kinds of terraces, which we unambiguously attribute to mono- and bilayer graphene capping a C-rich interface. At low temperature, both terraces show (sqrt(3) × sqrt(3)) quantum interferences generated by static impurities. Such interferences are a fingerprint of pi-like states close to the Fermi level. We conclude that the metallic states of the first graphene layer are almost unperturbed by the underlying C rich interface, which acts as a charge buffer layer. Our results are in deep agreement with recent photoemission measurements (A. Bostwick et al., Nature Physics 3, 36 (2007)). .
Enrique Garcia Michel (Madrid)
Molecular self-assembly and surface electronic structure:
Supramolecular chemistry is an interesting pathway to functionalise surfaces for a number of applications. It is still an open question what is the relative importance of molecule-molecule vs. molecule-substrate interaction in the formation of ordered structures of molecules on metal surfaces. Planar aromatic hydrocarbon molecules like pentacene lack the ability to form hydrogen bonds. Despite this fact, it has been proved that pentacene molecules interact with each other to form well-ordered structures on Cu(110). There has been an attempt to explain this behaviour as the result of molecule-substrate interaction mediated by charge-density waves [1] We investigate whether the process of pentacene self-assembly on Cu(110) depends on the shape of the molecule and its detailed atomic structure, or whether it can be forced by the morphology and/or the electronic structure of the Cu substrate. Angle-resolved photoemission with synchrotron radiation, scanning tunnelling microscopy, and low-energy electron diffraction are combined to provide us with a complete data set on the behaviour of pentacene molecules in the low to intermediate coverage range. We have observed that pentacene tends to form 1D wires running along the [1-10] direction even for very low coverages. Increasing the coverage results in an ordered array of molecular rows evenly spaced. The adsorption process modifies dramatically the Cu(110) Shockley surface state, which resides at the surface Y point on the clean Cu(110) surface. Additional modifications of the surface electronic structure are detected along the surface Gamma-X direction. From the STM and angle resolved photoemission results we discuss the role of the surface electrons in the molecule-molecule interaction, and the way it affects the self-assembling process
Enrique Ortega (San Sebastian)
Electronic states in self-assembled metallic nanostructures
Nanostructured Au, Ag, and Cu noble metal surfaces are excellent playgrounds to realize self-assembled metallic nanostructures and probe their extended electronic states. Using angular photoemission, namely the sophisticated version of the photoelectric effect, we test surface electron bands in step arrays and dislocation networks. In step lattices, we observe 1D quantum well energy levels of electrons confined in terraces, and are able of probing their electron wave functions in a rather straightforward manner. The surface state dimensionality can be tuned by varying the density of steps. I will show different examples, which range from the clear-cut 1D and 2D cases observed in 1D step arrays with different lattice constant, to a mixture of 1D and 2D states in more complex faceted surfaces. 2D dislocation networks may exhibit Fermi surface nesting of the Shockley-type surface state. In such cases, the free-electron like band is modified around the Fermi energy, triggering the interplay between the geometry and the electronic structure. I will discuss such situation in two different systems.
Friedrich Reinert (Wuerzburg)
Electronic States in Thin Epitaxial Organic Films: Interface and Confinement
The electronic properties of organic molecules in thin films is influenced significantly by the intermolecular interaction and by the interaction with the substrate at the interface. Due to its surface sensitivity, angle-resolved photoemission spectroscopy (ARUPS) is an ideal and direct tool for the investigation of the electronic structure of such systems. Furthermore, because of the high energy resolution achieved today, one is able to resolve details in the photoemission line shape and bandstructure, giving information about many-body effects on the scale of a few meV. This talk summarizes the results of recent photoemission experiments on electronic states in thin organic films, e.g. monolayers of PTCDA and NTCDA on the (111) faces of noble metals.
Pascal Ruffieux (Thun)
Site-selective adsorption of molecules on the Ag/Pt(111) strain-relief pattern and investigation of the local electronic properties
An important precondition for constructing functional molecular assemblies at the surface is the selective positioning of molecular building blocks on predefined adsorption sites. This is required for the sequential building of supramolecular structures where a first molecule is anchored and defines the base unit for further molecules that will be attached via attractive intermolecular interactions. This requires, therefore, the preparation of appropriate nano-structured template surfaces for the site-selective adsorption of a molecular building block. We investigate the adsorption behavior of large polycyclic aromatic hydrocarbons (PAHs) on various template surfaces. Here, we present the successful site-selective anchoring of PAHs on two monolayers of Ag on Pt(111) forming a two-dimensional strain-relief network. Molecules preferably adsorb individually on the corners of the discommensuration triangle around the hcp1 region, indicating that the local increase of the binding energy exceeds attractive intermolecular interactions. In order to understand the selective adsorption behavior we have investigated the local electronic properties of the strain-relief pattern with scanning tunneling spectroscopy. With respect to the surface state we find a significant variation of the onset in the different stacking regions and a large variations in the unoccupied density of states. Furthermore, we find a pronounced energy shift of the image potential states (IPS) for the different stacking regions indicating a local variation of the surface potential. Modeling the IPS for the local Coulomb potential allows the assignment of the local surface potential to the different stacking areas.
Stefano Rusponi (Lausanne)
Magnetism of two-dimensional nanostructures: pure versus alloyed composition
From a fundamental point of view, only three issues define the ultimate density limit of magnetic information storage. First, the magnetization direction of the particles composing a bit has to be stable against thermal fluctuations. This stability is given by the magnetic anisotropy energy K (MAE). Second, the distributions of magnetic moment and magnetic anisotropy energy of the bits have to be narrow, ideally monodisperse. Perfectly uniform magnetic properties would allow single particle bits. In current media the heterogeneity forces one to use at least 300 particles (grains) per bit. Third, mutual magnetic interactions between adjacent bits have to be minimized, which is achieved for out-of-plane magnetization. Two-dimensional nanostructures at surfaces are ideally suited to address such issues on model systems. We report record values of the density of non-interacting particles (26 Teradots/in2) and of the width of their K–distributions (17% HWHM) for Co nanodots on Au(788) [1]. However, their blocking temperature remains at 50 K. Bimetallic nanoparticles represent a potential candidate for room temperature blocked particles. Nanoparticles have significantly larger orbital moments than in bulk, therefore also a higher anisotropy of the orbital moment leading to strong MAE owing to spin-orbit coupling [1]. Consequently, the anisotropy of two-dimensional Co islands on Pt(111) is principally determined by the edge atoms [2,3]. In addition to the coordination effect, the MAE is strongly influenced by compositional effects and lattice distortions. While the MAE of ferromagnetic transition metals in their cubic structures is of the order of some tens of eV/atom, structurally distorted alloys, such as FePt in L10 phase [4] or FeCo in a bct phase [5] may have MAE values close to 1 meV/atom. Thus, low-dimensional bimetallic nanostructures should allow combining the unquenched orbital moment arising from the reduced dimensionality with the anisotropy produced by alloying. Results are presented for Co1-xPtx and Co1-xFex islands on Pt(111) substrate for different alloy compositions. A substantial increase of the perpendicular anisotropy is observed for the alloyed with respect to the pure elements-based islands. References [1] N. Weiss, T. Cren, M. Epple, S. Rusponi, G. Baudot, S. Rohart, A. Tejeda, V. Repain, S. Rousset, P. Ohresser, F. Scheurer, P. Bencok, and H. Brune, Phys. Rev. Lett. 95, 157204 (2005) [2] P. Gambardella, S. Rusponi, M. Veronese, S.S. Dhesi, I. Cabria, R. Zeller, P.H. Dederichs, A. Dallmeyer, C. Grazioli, K. Kern, C. Carbone, and H. Brune, Science 300, 1130 (2003) [3] S. Rusponi, T. Cren, N. Weiss, M. Epple, P. Buluschek, L. Claude, and H. Brune, Nature Materials, 2, 546 (2003) [4] B. M. Lairson and B. M. Clemens, Appl. Phys. Lett. 63, 1438 (1993) [5] T. Burkert, L. Nordström, O. Eriksson, and O. Heinonen, Phys. Rev. Lett. 93, 027203 (2004).
Andres Santander (Université Paris Sud)
Anisotropic self-energy and antiferromagnetic correlations in slightly overdoped electron-doped cuprate Sm(0.84)Ce(0.16)CuO4 studied by ARPES
In cuprate superconductors, superconductivity competes with various ground states, the most notorious being antiferromagnetic (AF) order. More precisely, at low carrier concentrations, the ground state of cuprates is AF, irrespective of the nature (holes or electrons) of the charge carriers. Understanding the effects of AF correlations in cuprates is therefore important for the comprehension of the remarkable properties of these materials. In the electron-doped cuprates of formula R2-xCexCuO4 (R=La, Pr, Nd, Sm, Eu), as the Ce concentration is changed, one obtains a superconducting (SC) phase that is adjacent to, and might even overlap with, the AF phase. Electron-doped cuprates offer thus a good opportunity to study the effect of AF correlations on the normal and SC states of these materials. We performed angle-resolved photoemission (ARPES) measurements on slightly overdoped Sm2-xCexCuO4 (x = 0.16 ± 0.1). The resulting Fermi surface presents suppressed spectral weight at the “hot-spots”, where the Fermi surface crosses the AF-zone boundary. The dispersions and line-widths along the zone-diagonal and zone-edge show a kink, distinctive of the interaction of the electrons with other excitation(s) in the system. However, the energy of the kink, the spectral line-shapes and the energy dependence of the line-widths are very different along these two directions. We will discuss these results and their relation to AF correlations in this system.
Ordered Nanostructures on Semiconductor Surfaces
Self-organization of atoms adsorbed on atomically-clean semiconductor surfaces in ultra-high vacuum has been used to to form low-dimensional nanostructures upon adsorption on Si(111), Si(100) and Ge(100) surfaces. Highly-ordered arrays of the identical-size nanoclusters (i.e., magic-cluster 2D crystals) have been successfully fabricated with Group-III adsorbates on Si surfaces. In the case of In/Si(100), the perfectly-ordered 4×3 can be formed. It has been found that each 4×3-In pyramid-like cluster formed by 6 In and 7 Si atoms can be modified by further In deposition, namely, central Si atom in the cluster can be replaced by two In atoms, thus forming In8Si6 cluster. This modification of the magic-cluster composition leading to the changeover of its electronic properties (i.e., cluster doping) has been demonstrated [1]. It has been found that dynamic behavior of the doped cluster in In/Si(100) system opens a prospect for using the cluster as an atomic-scale memory cell [2]. We demonstrate that adding the second adsorbate (In) to the well known Si(111)α-√3×√3-Au structure alters the domain wall structure. Upon annealing at 600°C, the domain-walls are eliminated and highly-ordered almost defect-free homogeneous Si(111)√3×√3-(Au,In) develops. Plausible mechanism of stabilization of the domain-wall-free surface is the stress relieving caused by In adsorption, which in turn affects electronic properties of the surface phase [3]. The following temperature-induced reversible structural phase transitions have recently been found in the surface phases developed on the Si(111), Si(100) and Ge(100) surfaces: Transition Transition temperature System Ref. √7×√3 ↔ √7×√7 240 K In/Si(111) [4] 2×1 ↔ (6,1)×(0,6) 100 K Tl/Si(100) [5] 2×1 ↔ c(12×14) 120 K Tl/Ge(100) [6] Possible mechanisms of these phase transitions will be discussed. Formation of ordered nanostructures on the various modified Si surfaces will also be discussed. [1] V. G. Kotlyar, A. V. Zotov, A. A. Saranin, T. V. Kasyanova, E. N. Chukurov, I. V. Pisarenko and V. G. Lifshits, Phys. Rev. Lett. 91, 026104 (2003). [2] A. A. Saranin, A. V. Zotov, I. A. Kuyanov, M. Kishida, Y. Murata, S. Honda, M. Katayama, K. Oura, C. M. Wei and Y. L. Wang, Phys. Rev. B 74, 125304 (2006). [3] D. V. Gruznev, I. N. Filippov, D. A. Olyanich, D. N. Chubenko, I. A. Kuyanov, A. A. Saranin, A. V. Zotov and V. G. Lifshits, Phys. Rev. B 73, 115335 (2006). [4] A. A. Saranin, A. V. Zotov, I. A. Kuyanov, V. G. Kotlyar, M. Kishida, Y. Murata, H. Okado, I. Matsuda, H. Morikawa, N. Miyata, S. Hasegawa, M. Katayama and K. Oura, Phys. Rev. B 71, 165307 (2005). [5] A. A. Saranin, A. V. Zotov, M. Kishida, Y. Murata, S. Honda, M. Katayama and K. Oura, Surf. Sci. 601, 595 (2007). [6] A. A. Saranin, A. V. Zotov, M. Kishida, Y. Murata, S. Honda, M. Katayama, K. Oura, D. V. Gruznev, A. Visikovskiy, and H. Tochihara, Phys. Rev. B 74, 035436 (2006).
Alexander Schneider (Erlangen)
Lifetimes of Surface-State Electrons in Co Nanostructures probed by STS
The dynamical properties of electrons in states localized at the surface of a solid are important in determining charge transfer processes of e.g. adsorbate systems. Scanning tunnelling spectroscopy of such states allows the determination of the phase coherence length of surface-state electrons and is an ideal tool to study locally the energy dependence of the lifetime of the electrons. By example of surface-state electrons of Co islands on Cu(111) I will discuss the various aspects of the method and of its application to electrons confined to nanostructures. The results obtained for the lifetime of the majority-spin electrons of the Co islands show that additional lifetime limiting mechanisms are active in that system.
Wolf Dieter Schneider (Lausanne)
Subgap Structure in Asymmetric Superconducting Tunnel Junctions
Conductance measurements between two superconducting electrodes with different gap energies D1 and D2 were performed with a low-temperature scanning tunneling microscope. The temperature dependence and tip-sample distance dependence of the spectra show a pronounced subgap structure which is interpreted as multiple Andreev reflections (MARs). Low temperature conductance peaks not seen in symmetric superconducting tunnel junctions arise at energies ±‡D1 - D2‡when the superconducting gaps are sufficiently different. We propose an explanation of these findings by extending the full counting statistics of MARs to describe these asymmetric junctions [1]. [1] M. Ternes, W.-D. Schneider, J. C. Cuevas, C. P. Lutz, C. F. Hirjibehedin, and A. Heinrich, Phys. Rev. B 74, 132501 (2006).
Muriel Sicot (Eindhoven)
Selective tuning of electronic properties of Co nanoislands
Using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), we have studied effects of H2 adsorption at 5 K on the electronic properties of nanometer-scale triangular Co islands prepared by molecular beam epitaxy on Cu(111). Before H2 adsorption, two surfaces states are observed. However, after adsorption both are quenched. Local spectroscopy and spatial maps of the conductance are analyzed. We found that total removal of the adsorbate from the surface of the islands can be induced by the STM tip. Moreover, the tip affects only one selected island without perturbing surrounding islands. This process can be used to control individually the electronic properties of the Co nanoparticles.
Amina Taleb Ibrahimi (Paris)
Future Capacities at SOLEIL for photoelectron spectroscopy: Applications to low dimensional systems

#### Communication par poster

Florian Baumann
Ageing in the bosonic contact and pair-contact processes
Ageing phenomena and scaling behaviour have been considered in many systems with detailed balance such as simple magnetic systems. Therefore it is an interesting question to extend these studies to systems without detailed balance. Two examples of such systems are presented here, and the exact results for the ageing exponents and the scaling functions are given. In a different approach the theory of local scale-invariance (LSI) is used to derive predictions for the form of the scaling functions, and these predictions are compared to the exact results.
Luis Cardenas (Nancy)
Temperature dependence of the correlated sp-type surface state observed in the semi-conducting surface K/Si(111)-B
The segregation of boron (B) atoms at the silicon (111) surface leads to a 3x3R30° surface reconstruction. The adsorption of potassium adatoms (K), electron donor, on preferential sites, leads to the formation of a dangling bond-type surface state in the semi-conducting gap. The amount of K controls the filling such as at a coverage of 1/3 of mono-layer, the surface band is half-filled. The surface-enhanced electronic correlations have been shown to induce a metal-insulator transition with the opening of a Mott gap on sp-type bands ten years ago [1]. Such a Mott transition has been recently reported in Sn/Ge(111) at low temperature and is known to exist in other semi-conducting surface presenting a triangular pattern [2]. The formation of a frustrated magnetic triangular network at surface has been proposed with a possible magnetic ordering at low temperature in a non-colinear Néel phase [3]. Experimental results are strongly needed to evidence the possibility to observe magnetism at low-T (may be superconductivity) since these semi-conducting surfaces are good candidates to form hole-doped frustrated magnets. We report here on Auger spectroscopy, LEED and STM results to characterize and control the K/Si(111)3x3R30°-B interface. Then, ARPES is used to probe the effect of strong correlations in the surface state properties. Here we report two dispersive features characterizing the surface band whose temperature dependence has been measured [4]. An attempt to analyze the photoemission spectral function A(k,) in term of electron-magnon and/or electron-phonon interaction is presented.[1] H.H. Weitering et al., Phys. Rev. Lett. 78, 1331(1997); [2] R. Cortes et al., Phys. Rev. Lett. 96 126103 (2006) and references therein; G. Santoro et al., Phys. Rev. B 59, 1891 (1999);[3] L.O. Manuel et al., Phys. Rev. B 69, 184407 (2004). [4] L. Cardenas et al., to be published (2007).
Clément Didiot (Nancy)
Reconstruction-induced multiple gaps in the weak coupling limit : the surface band of Au(111) vicinal surfaces
C. Didiot, Y. Fagot-Revurat, S. Pons, B. Kierren, C. Chatelain and D. Malterre : Nano-structuration of surfaces is a powerful way to drive electronic properties at nanometric scale and test basic solid state physics concepts. Ideal one-dimensional systems can be elaborated by using the anisotropic properties of vicinal surfaces. Indeed, the confinement of surface electrons by steps has been already obtained on Au vicinal surfaces leading to quantum well surface states [1]. We evidence here that parallel to the steps, the one-dimensional surface electron gas feels the weak super-periodic potential induced by the Au surface reconstruction. Therefore, a careful analysis of angle-resolved photoemission spectroscopy (ARPES) data evidences the opening of several gaps in the surface band structure of Au(232321) and Au(788) surfaces. More, scanning tunneling spectroscopy (STS) allowed us to probe the corresponding energy dependence of the surface electronic density through the gaps. From a methodological point of view, the values of the gaps and the phase of the electronic density allow to estimate the amplitude and the shape of the reconstruction potential which yields the electronic Bragg diffraction, the gap formation and the modulation of the electronic density. A quantitative analysis is derived in the framework of a simple pseudo-potential approach in agreement with our experimental data. The redistribution of ARPES spectral weight over the different Brillouin zones is also discussed. The combination of ARPES, STM and STS technics allows a full understanding of the influence of a one-dimensional superperiodic potential on the band structure in the weak coupling limit. [1] Mugarza et al., Phys. Rev. Lett. 87, 107601 (2001); [2] Didiot et al., Phys. Rev. B 74, 081404(R)
Clément Didiot (Nancy)
Electronic properties of self organized Ag nano dots on Au(788) and Au(23 23 21). A STM/STS and ARPES investigation
Yannick Fagot-Revurat (Nancy)
Photoemission study of a K doped Si(111)-B surface
Emmanouil Frantzeskakis (Lausanne)
Photoemission ...
Emilie Gaudry (INPL - Nancy)
X-ray absorption spectroscopy applied to the determination of relaxations
to appear...
Oleksandr Kapikranian (Nancy - Lviv)
Influence of disorder on the 2D XY model properties
We present an analytic approach to study concurrent influence of quenched non-magnetic site-dilution and finiteness of the lattice on the 2D XY model. Two significant deeply connected features of this spin model are: a special type of ordering (quasi-long-range order) below a certain temperature and a size-dependent mean value of magnetisation in the low-temperature phase that goes to zero (according to the Mermin-Wagner-Hohenberg theorem) in the thermodynamic limit. We focus our attention on the asymptotic behaviour of the spin-spin correlation function and the probability distribution of magnetisation. The analytic approach is based on the spin-wave approximation valid for the low-temperature regime and an expansion in the parameters which characterise the deviation from completely homogeneous configuration of impurities. We further support the analytic considerations by Monte Carlo simulations performed for different concentrations of impurities and compare analytic and MC results. We present as the main quantitative result of the work the exponent of the spin-spin correlation function power law decay. It is non universal depending not only on temperature as in the pure model but also on concentration of magnetic sites. This exponent characterises also the vanishing of magnetisation with increasing lattice size.
Guy Le Lay (Luminy)
Growth of Si nanostructures on Ag(001)
The first stages of the growth of silicon on Ag(001) at moderate temperatures start by the formation of a p(3 x 3) superstructure, which continuously evolves with increasing coverages toward a more complex superstructure. In this work, the atomic arrangement of the p(3 x 3) and of the "complex" superstructure have been investigated using scanning tunnelling microscopy, surface X-ray diffraction and low energy electron diffraction. The atomic model retained for the p(3 x 3) reconstruction consists in four silicon atoms (tetramers) adsorbed near hollow and bridge sites of the top most Ag(001) surface layer. For higher coverages, i.e., when the "complex" superstructure starts to develop, the silicon overlayer forms periodic stripes, most probably bi-layers, with a graphitic like structure
Thierry Platini (Nancy)
Relaxation in the XX and Ising quantum chains
We present the results obtained concerning the relaxation behaviour of a non equilibrium $XX$ and Ising quantum chain in a transverse field. We use an inhomogeneous initial state obtained by setting in contact several sub-systems, initially equilibrated at a given temperature. First we consider an initial thermal kink-like state where half of the chain is thermalized at a very high temperature $T_b$ while the remaining half, called the system, is set at a lower temperature $T_s$. From this initial state, we derive analytically the Green function associated to the dynamical behaviour of the transverse magnetization. Depending on the strength of the magnetic field and on the temperatures of the systems, in the XX model, different regimes are obtained for the magnetic relaxation. In particular, with an initial droplet-like state, where a finite part of the chain is thermalized at a temperature $T_s$ in contact at both ends with an infinite temperature environnement, we derive analytically the behaviour of the time-dependent system magnetization.
Jean-Yves Veuillen (Grenoble)
Charge transport to nanoscale metallic islands grown on a semiconductor substrate probed by STM.
In an attempt to identify the transport mechanisms of the charges injected by an STM tip on a semiconductor surface we have performed an STM/STS study of nanoscale two dimensional metallic islands (made of ErSi2) grown on a Si(111) substrate. The experiment consists in taking tunnelling spectra were taken on metallic islands for different tunnelling resistances. The relative efficiency of the different possible charge transport channels from the island to the bulk substrate can be modified by changing several experimental parameters: the substrate temperature, the substrate doping and the structure of the Si(111) surface in between the islands (by selective oxygen adsorption). For clean (non O2 dosed) samples the transport takes places parallel to the surface, being ohmic at low fields. For oxygen dosed samples the transport is perpendicular to the surface (across the metal-semiconductor interface) and has a rectifying behaviour. In the later case the characteristics of the nanoscale junction at 300 K can be derived from STS data (at 300 K)
Jean-Charles Walter (Nancy)
Aging of the fully-frustrated two and three states clock model: Preliminary results
We study the aging properties of the fully-frustrated clock model (FFCM) with two and three states (these models are equivalent to the fully-frustrated Ising model (FFIM) and the fully-frustrated three-state Potts model (FFPM)). A model is said to be fully-frustrated when each plaquette is frustrated. This is the case on a square lattice when each plaquette contains three ferromagnetic (resp. antiferromagnetic) bonds and one anti-ferromagnetic (resp. ferromagnetic) bond. We restricted ourselves the coupling configuration called Zig-Zag. The FFIM has been solved exactly by Villain [1]. The phase diagram of the FFPM has been studied numerically by Foster et al. [2]. Both models display a paramagnetic phase at non-vanishing temperature and undergo a phase transition to the ferromagnetic phase at zero temperature. We study these two models by means of Monte-Carlo simulations with Glauber dynamics on a square lattice (lattice size 192x192 and periodic boundary conditions). The system is initially prepared in the paramagnetic phase and then quenched at the critical temperature. In a such a situation, homogeneous ferromagnets display aging due to the competition between the different ordered phases [3]. We study the large-time behaviour of the spin-spin autocorrelation functions, the response to a magnetic field and the fluctuation-dissipation ratio. We observed the existence of aging. We give estimates of the exponents λ/z and ac= 2β/νz and of the asymptotic value Xinf of the fluctuation-dissipation ratio. These quantites are believed to be universal [4]. While the two models belong to two well-distinct universality classes in the homogeneous case, our preliminary data seem to indicate a common behaviour in presence of frustration.
[1] J. Villain (1977), J. Phys. C, 10, 1717.
[2] D.P. Foster, C. Gerard and I. Puha (2001), J. Phys. A, 34, 5183.
[3] A. J. Bray (1994), Adv. Phys., 43, 357.
[4] C. Godrèche and J. M. Luck (2000), J. Phys. A, 33, 9141.

#### Comité d'organisation

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

#### Nos partenaires

Nous remercions le Centre de Recherche INRIA (Institut National de Recherche en Informatique et en Automatique) de Nancy Grand Est d'avoir mis à disposition ses locaux pour les conférences plénières.