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