| 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. |
| Nobuyasu Ito (Tokyo) |
|---|
| 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. |
| Anuradha Jagannathan (Orsay) |
|---|
| 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. |
| Vladislav Popkov (Köln) |
|---|
| 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. |
| Alexander Saranin (Vladivostok) |
|---|
| 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 |
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