-
Cécile Appert, (Paris, France)
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.
Ref: Phys. Rev. Lett. 95 010601 (2005)
-
Michael Bachmann, (Leipzig, Germany)
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, (Erlangen, Germany
and Nancy, France)
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, (Leipzig, Germany)
The evaporation/condensation
transition of Ising
droplets
In recent analytical work,
Biskup {\it et al.} [{\it Europhys.\ Lett.}
{\bf 60} (2002) 21] 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, (Strasbourg, France)
The influence of phonons on low dimensional magnetic
systems
In this talk, I will present results on
-
Pasquale Calabrese, (Amsterdam,
Netherlands)
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, (Polytech'Lille and
Interdisciplinary Research Institute, France)
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, (Lausanne, Swiss)
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.
-
Martin Evans, (Edinburgh, Scottland)
Condensation Transitions in
Nonequilibrium Steady States
Systems driven out of equilibrium can often
exhibit behaviour not seen in
systems in thermal equilibrium - for example phase transitions in
one-dimensional systems. In this talk I will review a very simple model
of a nonequilibrium systems known as the the `Zero-Range Process'.
This model
involve particles hopping stochastically on a lattice
and enjoys product measure states.
Using the product measure, I shall show how a phase transition occurs
wherein a finite fraction of the particles condenses onto one site. More
generally I
shall discuss a necessary and sufficient condition
for a general class of mass
transport models to exhibit a
product measure steady state and review a recent canonical analysis of the
condensation transition. The condensation can be understood in the
context of sums of positive Levy random variables.
M. R. Evans and T. Hanney J. Phys. A vol 38 R195 (2005)
S. N. Majumdar, M.R.Evans and R. K. P. Zia Phys. Rev. Lett vol 94
180601 (2005)
-
Christian von Ferber, (Krakow, Poland)
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-λ.
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 λ. 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)
-
Andrea Gambassi, (Stuttgart, Germany)
Finite-size scaling in the non-equilibrium critical behavior of the
randomly driven lattice gas: The field-theoretical approach
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, (Amsterdam, Netherlands)
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, (Juelich, Germany)
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, (Lviv, Ukraina)
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, (Budapest, Hungary)
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, (Leipzig, Germany)
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, (Edinburgh, Scottland)
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, (Coventry, England)
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, (Orsay, France)
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,
(Montpellier, France)
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.
Activated aging dynamics and negative fluctuation-dissipation ratios
P. MAYER, S. LEONARD, L. BERTHIER, J. P. GARRAHAN, P. SOLLICH
Physical Review Letters, 96, 030602 (2006)
-
Victor Martín-Mayor, (Madrid, Spain)
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, (Lyon, France)
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, (Saarbruecken, Germany)
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, (Strasbourg, France)
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.
- Tomasz Wydro (Metz, France)
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, Switzerland)
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, Germany)
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, France)
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 Switzerland)
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 don’t 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, Switzerland)
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, Russia)
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.
-
Marco Grioni, (Lausanne, Switzerland)
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, Switzerland)
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, France)
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, France)
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 Lelay, (Marseille, France)
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, France)
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.
-
Enrique Garcia Michel, (Madrid, Spain)
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.
-
Enrique Ortega , (Madrid, Spain) (cancelled)
Vicinal surfaces as model systems to study electronic states in
low-dimensional systems with angle-resolved photoemission.
Vicinal noble metal surfaces with one-dimensional arrays of steps are
excellent playgrounds for testing the electronic properties in lateral
superlattices [1]. Vicinal surfaces usually self-assemble forming a 1D
step superlattice, due to strong step-step interaction. The terrace
width d is the lattice constant, which is tuned macroscopically by
changing the miscut angle of the surface with respect to the [111]
direction. Interestingly, in noble metal surfaces it is possible to
tailor surface states by simply changing d, thereby switching from 2D
step superlattice bands for narrow terraces to 1D confinement and
quantum well states in wide terraces.
The analysis of such simple systems during the past few years, has
allowed us to develop a general framework to understand photoemission
data in more complex 1D superstructures, like two-phase systems. These
are produced by altering the equilibrium shape of the vicinal surface
with surface reconstructions and adatom adsorption. We have searched
into different systems by means of STM and high resolution
angle-resolved photoemission. Vicinal Au(111) surfaces display faceting,
i.e., two-phase separation of wide terraces and step bunches, with a
finite number of steps tuned by miscut angle. In photoemission, we
observe a split-off of the surface band into 1D and 2D surface states.
Ag adsorption on vicinal Cu (111) leads to different types of
structures, depending on the miscut angle. In all cases, the surface
state band also split into distinct Ag-like and Cu-like states. The
latter are of particular interest, since they exhibit complex quantum
size effects and varied scattering properties, depending on the
particular structure.
References
[1] A. Mugarza and J. E. Ortega, J. Phys. Cond. Mat. 15, S3281 (2003).
-
Friedrich Reinert, (Wuerzburg, Germany)
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, Switzerland)
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.
-
Antonio Tejeda, (Paris, France)
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.
-
Wolf Dieter Schneider, (Lausanne, Switzerland)
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.
-
Coriolan Tiusan, (Nancy, France)
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, France)
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, France)
Investigation of molecule chemisorption on Si(001)-2x1 surfaces by means
of surface reflectance spectroscopies.
Surface sensitive optical spectroscopies such as Surface Differential
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, Russia)
Ag monolayer on Cu(111): new chemical properties
and formation of "quantum-islands" at Cu deposition
Abstract
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, Austria)
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.
|
