Étude ab initio des phonons cohérents dans le Bismuth
le vendredi 20 décembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : En physique des matériaux, les expériences de type pompes-sondes ont permis d’étudier des phénomènes ultra-rapides comme les transitions de phases photo-induites ou la génération de phonons cohérents. Dans ce domaine, le bismuth apparaît comme un système modèle. En effet, dans ce matériau, une impulsion ultra-courte excite de façon sélective un phonon en centre de zone de symétrie A1g, ce qui provoque de fortes variations de la réflectivité. Récemment, les expériences de diffraction X résolues en temps sont venues enrichir les mesures de réflectivité en permettant d’accéder directement aux déplacements atomiques liés au phonon. Cependant, ces résultats expérimentaux s’avèrent insuffisants si l’on souhaite comprendre en détail les mécanismes de génération des phonons cohérents.

Pour étudier ces phénomènes plus en détail, nous avons travaillé sur une modélisation du système permettant de confronter expérience et calcul ab initio. Nous avons développé un modèle thermodynamique, inspiré du modèle à deux températures, pour décrire l’évolution simultanée du réseau ionique en présence d’un phonon cohérent et du système électronique. Ce modèle a été paramétré par le calcul ab initio, et permet de décrire les différents échanges d’énergie entre les sous systèmes, ainsi que l’évolution de la température électronique et de la température du réseau. Les résultats sont en très bon accord avec les expériences de diffraction X résolues en temps. Ils ont permis, entre autre, de reproduire les variations de la fréquence des oscillations observées expérimentalement et de montrer que les oscillations des atomes s'accompagnent d'une oscillation de la température électronique. Nous avons ensuite enrichi notre modèle afin de le confronter aux expériences de type pompe sonde. Nous avons ainsi déterminé à partir du calcul ab initio les propriétés optiques du bismuth en fonction de la température et des oscillations du phonon cohérent afin de reproduire les mesures de réflectivité.

Maximal height of N non-intersecting Brownian excursions: from Yang-Mills theory to interfaces in disordered media
le vendredi 13 décembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Non-intersecting random walkers (or 'vicious walkers') have been studied in
various physical situations, ranging from polymer physics to wetting and melting
transitions and more recently in connection with random matrix theory or
stochastic growth processes in the Kardar-Parisi-Zhang (KPZ) universality class.
In this talk, I will present a method based on path integrals associated to free
Fermions models to study such statistical systems. I will use this method to
calculate exactly the cumulative distribution function (CDF) of the maximal
height of N non-intersecting Brownian excursions. I will show that this CDF is
identical to the partition function of 2d Yang Mills (YM) theory on a sphere
with the gauge group Sp(2N). I will show that, in the large N limit, the CDF
exhibits a third order phase transition, akin to the Douglas-Kazakov transition
found in 2d YM. I will also show that the critical behavior, close to the
transition point, is described by the Tracy-Widom distribution for $eta = 1$,
which describes the fluctuations of the largest eigenvalue of Random Matrices
belonging to the Gaussian Orthogonal Ensemble.

Electronic transport as a tool to investigate the microscopic structure of a density-wave state
le lundi 9 décembre 2013 à 14h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Louis Weil E424, Bat E, Institut Neel

Résumé : The electronic transport is very sensitive to the
microscopic structure of the density-wave state and may serve as a
powerful tool for its investigation. In my talk I present several
examples of how electronic transport reveals the microscopic structure
of the DW state. In some cases the theory of these effects is evident;
sometimes they are less trivial, being a long-standing puzzle.
After a brief review I will talk about two recent examples of an
unusual influence of a density wave on the electronic transport: (i)
Spontaneous breaking of isotropy observed in the in-plane conductivity
of rare-earth tritellurides, and (ii) the phase inversion of the
Shubnikov-de Haas oscillations after passing a transition to a
density-wave state observed in organic metals.

Locally self-similar phase diagram of a disordered Potts model on a hierarchical lattice
le vendredi 6 décembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The Potts model with bond disorder is studied in the limit of a infinite number of states on a hierarchical lattice.
This model is "academical" since neither the hierarchical lattice nor the infinite number of states are really
physical. However the final result, ie the phase diagram of the model obtained by renormalization, is unusual and
can justify this study.

Majorana and Andreev bound states in topological wires in the proximity of superconductors
le vendredi 29 novembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : We study one-dimensional topological SN and SNS long junctions
obtained by placing a topological insulating nanowire in the proximity
of either one or two SC finite-size leads. Using the Majorana
Polarization order parameter we find that for a finite-size SN
junction the ABS spectrum exhibits a zero-energy extended state which
carries a full Majorana fermion, while the ABS of long SNS junctions
with a phase difference of pi transform into two zero-energy states
carrying two Majorana fermions with the same MP. We also study the
effects of finite SC penetration depths in such junctions, as well as
the effects of uniform phase gradients. Last but not least we analyze
a more realistic model for the coupling between a superconducting
substrate and a topological wire, the resulting proximity effect, and
the role played by the Andreev bound states in the topological wire in
such a setup.

Localized waves in granular crystals
le vendredi 22 novembre 2013 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Granular crystals consist of a collection of masses (typically steel beads) arranged on a regular lattice and interacting nonlinearly by contact. These systems display different types of nonlinear wave phenomena, such as the formation of localized waves (solitary waves or breathers) after an impact. The wave dynamics is strongly influenced by lattice properties (type of discrete elements, existence of confining potentials, precompression), which opens interesting possibilities to control stress waves. Granular crystals can be modeled by different types of lattice differential equations depending on their structural properties. In particular, one-dimensional
granular chains can lead to the Fermi-Pasta-Ulam (FPU) model with Hertzian potential, mixed FPU-Klein-Gordon lattices or the discrete p-Schrödinger equation, a new asymptotic model obtained when confining potentials are present.
We will illustrate the rich properties of localized waves in these models through numerical simulations and analytical results.

On the Growth and Phase Transitions of Self-Assembled Quantum Dots -- Theoretical Studies
le vendredi 15 novembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The talk discusses first some earlier analytic approaches
on the growth and phase transitions of self-assembled
quantum dots formed on semiconductor surfaces.
Furthermore, current related research activity will be
presented regarding a remarkable bulk morphological transition
taking place in a layered prototype system. Our related
Monte Carlo and Cahn-Hilliard simulations reproduced
all the experimentally observed morphological
phases including the growth of pinholes,
the formation of a percolation network and
its breaking up into isolated quantum-dots.
The simulated dot sizes were in a quantitative
agreement with the experimental values.
The robustness of the results, i. e. the insensitivity
to temperature and to the details of atomic exchange
mechanisms and binding interaction types undoubtedly
demonstrates the topological nature
of this nanocapillarity forces driven transition.

Universal superfluid transition and transport properties of two-dimensional dirty bosons
le vendredi 8 novembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The interplay of disorder and interactions has been at the center of intense
theoretical and experimental activity the past decades. It sustains rich phase
diagrams, relevant to a wide range of condensed-matter systems. Recent
evelopments with ultracold atoms spark a renewed interest and open new
challenging issues. For instance, in interacting systems on the continuum, the
intervening phases are largely debated and still in a conjectural stage. In
this talk I will discuss the phase diagram of disordered and interacting
ultracold atoms in two dimensions, that we have obtained thanks to accurate,
large-scale Quantum Monte Carlo simulations.[I] I will show that the superfuid
transition is strongly protected against disorder, up to the zero-temperature
Bose-glass transition. Most of its critical properties can be understood in terms
of an universal BKT description with a simple scaling of the critical
temperature versus the disorder strength. I will then address the strongly
disordered regime at finite temperature where the possible existence of a
(many-body) localized phase constitutes a challenging open question. Thanks to a
taylored methodological improvement, we have gained direct access to the
conducting properties. I will show that the finite-temperature insulating phase
merging at large disorder strength is well described by a thermally activated
behavior of the Arrhenius type.

Bose-Einstein condensation of interacting particles and the quantum de Finetti theorem
le vendredi 18 octobre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The observation of Bose-Einstein condensation in dilute atomic gases twenty years ago has given a new impetus to the theoretical study of large bosonic systems. Much of our current understanding of this physics is based on the mean-field approximation, which in this context roughly amounts to assuming that all particles behave independently of one another. That this approximation is a sensible one for a great variety of large bosonic systems is a remarkable fact, and I will argue that it can be seen as following from a very special structure property of the set of bosonic states, the quantum de Finetti theorem. I shall discuss the original theorem along with recent variants and applications to interacting bosonic systems. This is joint work with Mathieu Lewin and Phan Thành Nam.

Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts
le jeudi 10 octobre 2013 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Quantum point contacts, elementary building blocks of
semiconductor-based quantum circuits, are narrow one-dimensional
constrictions usually patterned in a two-dimensional electron system,
e.g. by applying voltages to local gates. It is one of the paradigms of
mesoscopic physics that the linear conductance of a point contact, when
measured as function of its channel width, is quantized in units of GQ
= 2e²/h. However, the conductance also exhibits an unexpected shoulder
at ∼ 0.7 GQ, known as the "0.7-anomaly", whose origin is still
subject to debate. Proposed theoretical explanations have evoked
spontaneous spin polarization, ferromagnetic spin coupling, the
formation of a quasi-bound state leading to the Kondo effect, Wigner
crystallisation and various treatments of inelastic scattering. However,
explicit calculations that fully reproduce the various experimental
observations in the regime of the 0.7-anomaly, including the zero-bias
peak that typically accompanies it, are still lacking. Here we offer a
detailed microscopic explanation for both the 0.7-anomaly and the
zero-bias peak: their common origin is a smeared van Hove singularity in
the local density of states at the bottom of the lowest one-dimensional
subband of the point contact, which causes an anomalous enhancement in
the Hartree potential barrier, magnetic spin susceptibility and
inelastic scattering rate. We present theoretical calculations and
experimental results that show good qualitative agreement for the
dependence of the conductance on gate voltage, magnetic field,
temperature, source-drain voltage (including the zero-bias peak) and
interaction strength. We also clarify how the low-energy scale governing
the 0.7-anomaly depends on gate voltage and interactions. For low
energies we predict and observe Fermi-liquid behaviour similar to that
known for the Kondo effect in quantum dots. At high energies, however,
the similarities between 0.7-anomaly and Kondo effect cease.

Theory of complex transport in magnetic metals and alloys
le vendredi 4 octobre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : In this talk we will review a recent progress in the first-principles study of
transport properties of magnetic metals and alloys based on the Kubo-Greenwood
approach as formulated in the framework of the dirac version of the
tight-binding linear muffin-tin orbital method. A possible disorder in studied
systems is described by the coherent potential approximation.
Specifically, we will study: (i) The spin-disorder resistivity (SDR) of
transition metal ferromagnets, rare-earth metals, and Ni-based Heusler alloys.
We identify the SDR at the Curie temperature with the residual resistivity of
the corresponding system evaluated in the framework of the disordered local
moment (DLM) model [1];
(ii) the anisotropic magnetoresistance (AMR) and the anomalous Hall effect (AHE)
of chosen Ni-based transition metal alloys [2] as well as ordering Pd-rich PdFe
alloys with complex lattice [3];
and the AHE in chosen half-metallic Heusler alloys with native disorder [4].
Results of theoretical calculations will be compared with available experimental
data.

(*) In collaboration with V. Drchal, I.Turek, S. Khmelevskiy, J. Glasbrener, and
K. Belashchenko

References:
[1] Phys.Rev. B85 (2012) 144423 and Phys.Rev. B86 (2012) 214405
[2] Phys. Rev. B 86 (2012) 014405
[3] Phys. Rev. B 84 (2011) 214436
[4] Phys. Rev. B 88 (2013) 014422

Discrete-time, discrete-space quantum theory
le vendredi 27 septembre 2013 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Physics usually formulates its laws in the language of Partial Differential Equations. But in order to solve these equations PDEs numerically, we usually have to discretize both time and space, thereby obtaining a discrete numerical model of the physical phenomenon that we are interested in. The broad category of computer models that are obtained in this way is called Cellular Automata. Thus, it could be said that we end up formulating physics phenomena in the language of computer programs. In this talk, we will see how much of that can be done with Quantum Physics. We will mention results about: the mathematical structure of discrete time discrete space models of quantum theory; the notion of simulation in those Quantum Cellular Automata; how to formulate some quantum physics phenomenon in the language of quantum computers programs.

Nonlinear optics from ab-initio by means of the dynamical Berry-phase
le vendredi 20 septembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle MAG 1, Maison des Magistères

Résumé : We present an ab-initio real-time based computational approach to nonlinear
optical properties in Condensed Matter systems. The equation of motions, and in
particular the coupling of the electrons with the external electric field, are
derived from the Berry phase formulation of the dynamical polarization. The
zero-field Hamiltonian includes crystal local field effects, the renormalization
of the independent particle energy levels by correlation and excitonic effects
within the screened Hartree- Fock self-energy operator. The approach is
validated by calculating the second-harmonic generation of SiC and AlAs bulk
semiconductors: an excellent agreement is obtained with existing ab-initio
calculations from response theory in frequency domain. We finally show
applications to the second-harmonic generation of CdTe, MoS2, h-BN and the
third-harmonic generation of Si.

References:
Real-time approach to the optical properties of solids and nanostructures:
Time-dependent Bethe-Salpeter equation, Phys. Rev. B 84, 245110 (2011).
Nonlinear optics from ab-initio by means of the dynamical Berry-phase,
C. Attaccalite and M. Gruning, Phys. Rev. B in press.

DFT modeling of the covalent functionalization of graphene: from ideal to realistic models
le vendredi 6 septembre 2013 à 15h00

Séminaire théorie

Personne à contacter :

Lieu : Salle Weil E424, 3eme Etage, Batiment E, Insitut Neel

Résumé : The production of multiple types of graphene, such as free standing, epitaxial
graphene on silicon carbide and metals, graphene in solution, chemically grownand metals, graphene in solution, chemically grown
graphene-like molecules, various graphene nanoribbons, and graphene oxide with
different levels of reduction and various chemical composition, demonstrate the
need for additional investigation beyond the basic principles of graphene
functionalization for avoidance of occasionally contradictions between the
predictions from first-principles simulations and experimental results. Herein,
I report the current state of modeling of the different types of graphene using
density functional theory (DFT) methods. The main focus is on the static
(substrate, shape, curvature, strain and doping) and dynamic (starting point of
functionalization, migration barriers and stability of configurations) aspects
that provide a more correct and selective modeling of the chemisorption of
various chemical species on the graphene scaffold. Based on the recent modeling
of experimentally realized functionalization of different types of graphene we
can conclude that the formation of uniform one- or two-sided functionalized
graphene discussed in earlier studies is an exception to the typical scenarios
of graphene chemistry. The presence of different substrates, defects and lattice
distortions, such as ripples and strain, results in the formation of clusters or
lines from the functional groups. Several configurations of the chemical species
on the graphene substrate have been found to exist with ideal models but are
only stable for graphene functionalized under special conditions. And finally
employments of realistic models of graphenes for description of unexpected
properties of graphene such as low dimensional ice formation or efficient
catalysis of various reactions are also reported.

Phase diagram of the frustrated spin-1/2 XY and Heisenberg models on the honeycomb lattice
le vendredi 6 septembre 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : In this talk I present the phase diagram of the frustrated spin-1/2 XY and
Heisenberg models on the honeycomb lattice, obtained by using accurate
correlated wave functions and Variational Monte Carlo simulations. Upon
increasing the frustration, these models show a very rich sequence of
spin-ordered phases and a spin-liquid state is energetically favorable in a
small region of intermediate frustration.1 In my investigation, I consider an
unprecedented broad variety of spin (spiral) waves. These ordered phases are
represented by classically ordered states supplemented with a long-range Jastrow
factor, which includes relevant correlations and dramatically improves the
description provided by the purely classical solution of the models. The
construction of the spin-liquid state is based on a spin decomposition in terms
of fermions, experiencing a Gutzwiller projection and long-range Jastrow
correlations. In comparison with the classical phase diagram, the quantum
fluctuations prolong the stability of the Néel antiferromagnet and favor a
stripe order for intermediate and quite strong frustration. The spiral waves are
ground state for strong frustration and the 120th-order becomes the
lowest-energy phase for very strong frustration. I also discuss connections with
experiments on magnetically frustrated systems.

J. Carrasquilla, A. Di Ciolo, F. Becca, V. Galitski, and M. Rigol,
arXiv:1307.2267 (2013); A. Di Ciolo, J. Carrasquilla, F. Becca, M. Rigol, and V. Galitski, in preparation.

Two-dimensional nanosheets for opto-electronic applications
le vendredi 12 juillet 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Two-dimensional nanosheets for opto-electronic applications

After the discovery of graphene, new emerging two-dimensional
materials including layers of BN, hybridized graphene-BN (CBN) and
dichalcogenides (MoS_{2},MoSe_{2},WS_{2} etc.) have been the focus of strong
interest for their tunable opto-electronic properties, their high
carrier mobility and the envisaged possibility to use them in
photovoltaic devices.
In this talk, I will focus on the electronic and optical properties of
CBN hybridized monolayers [1-2] studied by ab-initio density
functional theory and many-body perturbation theory (GW and
Bethe-Salpeter) calculations.
Then based on our recent first-principles simulations, I will present
candidate interfaces and device architectures to implement CBN [3] and
other monolayer semiconductors [4] into efficient and photostable
excitonic solar cells, alternative to those based on conjugated
polymers and small molecules.

[1] M. Bernardi, M. Palummo, J. C. Grossman Phys. Rev. Lett. 108, 226805 (2012)
[2] L. Ci, et al., Nature Mater. 9, 430 (2010)
[3] M. Bernardi, M. Palummo, J. C. Grossman ACS NANO 2012, 6 (11), pp 10082-10089
[4] M. Bernardi, M. Palummo, J. C. Grossman submitted to Nano Letters

Biquadratic exchange in Fe pnictides from band structure calculations
le mardi 9 juillet 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Most undoped Fe arsenides, parent compounds for Fe based superconductors,
undergo a transition into a collinear state with stripe-like magnetic order
in which anti-ferromagnetic (AFM) Fe chains are ferromagnetically ordered
along the direction perpendicular to the chains. Two such collinear magnetic
structures, characterized by ordering vectors (π,0) or (0,π), are
connected by infinite number of non-collinear states with two AFM
sublattices of second Fe neighbors rotated by an arbitrary angle with
respect to each other. In the classical Heisenberg model all these states
are degenerate. Band structure calculation show, however, that the
degeneracy is lifted already at the mean field LSDA level and that in Fe
arsenides (π,0) and (0,π) magnetic orders are separated by an energy
barrier comparable to the energy difference between Neel and stripe AFM
orders. The shape of the barrier can be reproduced by adding a biquadratic
term to the Heisenberg model. We discuss the microscopic origin of the
barrier and show that it is related to the peculiar band structure of Fe
pnictides and nesting properties of their Fermi surfaces. The results for Fe
arsenides are compared to BaMn_{2}As_{2} and hypothetical KFe_{2}Se_{2} for
which we found that a non-collinear 90-degree spin arrangement is more
favorable than collinear ones. A doping dependence of the barrier is also
discussed.

First-passage statistics and search strategies
le vendredi 28 juin 2013 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : How long does it take a "searcher" to reach a "target" for the first time? This
first-passage time is a key quantity for evaluating the kinetics of various
processes, and in particular chemical reactions involving "small" numbers of
particles such as gene transcription, or at larger scales the time needed for
animals to find food resources.
I will present recent results which enable the evaluation of the distribution
of first-passage time for a wide range of random search processes evolving in
a confined domain. This approach reveals a general dependence of the
first-passage time distribution on the geometry of the problem, which can become
a key parameter that controls the kinetics of the search process. I will show
how these results apply to transport in disordered and fractal media, and
highlight their implications in transcription kinetics and other search
processes at larger scales.

Hybrid Potential Simulation Methods for Studying Enzyme Catalysis
le vendredi 21 juin 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : An important goal of computational and theoretical biochemistry is
helping elucidate how enzymes achieve their catalytic efficiency. The
differing length and time scales of processes that contribute to
catalysis, however, makes this a challenging task for molecular
simulation techniques. An approach that has proved particularly
powerful for the investigation of the chemical steps in enzymatic and
other condensed phase reaction processes is the use of hybrid quantum
chemical and molecular mechanical potentials. This talk will describe
the types of hybrid potentials developed and implemented in the
author's group and illustrate their use by a presentation of some
recent applications to a variety of enzyme systems.

Frustrated magnetism and resonating valence bond physics in 2D kagome-like magnets with inequivalent loops
le vendredi 14 juin 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Using a combination of exact diagonalization, analytical strong-coupling theories and resonating valence bond
approaches we determine the phase diagram and the low-energy physics of three kagome-like Heisenberg
antiferromagnets with inequivalent resonance loops. At weak coupling the lattices become effectively bipartite,
while at strong coupling heavily frustrated nets emerge. Competing tunneling amplitudes result in the intermediate
coupling regime in short-ranged spin correlations, the presence of a manyfold of low-lying singlets and the stabilization
of valence bond crystal and spin-nematic phases.

Exciton Condensates are Super!
le lundi 10 juin 2013 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Electronic systems can have a type of order in which coherence is spontaneously
established between two distinct groups of electrons. So far this
(particle-hole or exciton condensate) type of order has been found only in
double-layer two-dimensional electron gas systems, and only in certain strong
magnetic field limits. I will review some of the surprising superfluid
transport effects that have already been observed in double-layer exciton
condensates, and speculate on others that may be observable in the future.

Correlated Electron-Ion Dynamics (CEID): An efficient method to model electronic (de)coherence from an atomistic point of view
le vendredi 7 juin 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Quantum coherence between electronic and nuclear dynamics, as observed
experimentally in organic semiconductors, is the object of an intense
theoretical and computational effort. To simulate this kind of quantum coherent
dynamics, an efficient numerical scheme based on Correlated Electron-Ion
Dynamics (CEID) has been recently devised [1]. In this talk, I describe a
further generalization of CEID [2] and its practical numerical implementation
[3]. To illustrate the capability of this extended CEID scheme, an atomistic
model of the electronic decoherence of a short conjugated oligomer is presented.
Finally, I discuss convergence and scaling properties of the extended CEID
scheme along with its applicability to larger systems, e.g., to investigate the
non-radiative relaxation of photo-excited conjugated polymers [4].

[1] L. Stella et al., J. Chem. Phys. 127, 214104 (2007)
[2] L. Stella et al., J. Chem. Phys. 134, 194105 (2011)
[3] https://bitbucket.org/lstella/polyceid
[4] E.J. McEniry et al., Eur. Phys. J. B 77, 305–329 (2010)

Normal fluid phases of He 3 in two dimensions
le vendredi 31 mai 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : I will present ongoing work, based on quantum Monte Carlo simulations,
on the normal fluid phase of ³He in two dimensions, both for the strictly
2D case and for more realistic models of monolayers adsorbed on different
substrates. We find close agreement with the experiment for both the static
spin susceptibility and the dynamic structure factor. For weak enough
alkali metal substrates, we predict a gas-liquid phase transition not
found for strictly 2D ³He.

The new resonating valence bond method for ab-initio electronic simulations
le vendredi 24 mai 2013 à 11h00

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Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The Resonating Valence Bond theory of the chemical bond was introduced soon after the discovery of quantum mechanics and has contributed to explain the role of electron correlation within a particularly simple and intuitive approach, where the chemical bond between two nearby atoms is described by one or more singlet electron pairs.
We revisit the Pauling's resonating valence bond theory of the chemical bond
within a new formulation, introduced
by P.W. Anderson soon after the discovery of High Tc superconductivity.
It is shown that this intuitive picture
of electron correlation becomes now practical and efficient,
and allows us to perform realistic simulations with correlated wavefunctions
corresponding to several hundred atoms.
Few examples will be given: i) in the Beryllium dimer we
show the accuracy of the method for a particularly difficult case
where single determinant approaches (DFT or Hartree-Fock) miserably fail, ii) recent finite temperature realistic simulations of liquid hydrogen and liquid water.

Topological Matter and Why You Should Be Interested
le vendredi 3 mai 2013 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In two dimensional topological phases of matter, processes depend on gross topology rather than detailed geometry. Thinking in 2+1 dimensions, particle world lines can be interpreted as knots or links, and the amplitude for certain processes becomes a topological invariant of that link. While sounding rather exotic, we believe that such phases of matter not only exist, but have actually been observed in quantum Hall experiments, and could provide a uniquely practical route to building a quantum computer. Possibilities have also been proposed for creating similar physics in systems ranging from superfluid helium to strontium ruthenate to semiconductor-superconductor junctions to quantum wires to spin systems to cold atoms.

Spiral Luttinger liquids: helical nuclear spin order, Rashba nanowires, and their conductance
le vendredi 26 avril 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : When taking into account the nuclear spins, a quantum wire is a finite size Kondo lattice system. In such a system, the RKKY interaction can result in an ordered state at low temperatures, in which the Kondo lattice spins form one or several helices. These helices in turn induce a partial ordering of the electrons, and open up gaps in their spectrum. The helical order is a relatively stable phenomenon that persists even for multiple electronic subbands.

The electrons in this state form a quasi-helical (or "spiral") Luttinger liquid, equivalent to a Rashba nanowire in a magnetic field. (Quasi-) helical Luttinger liquids have applications as spin filters or Cooper pair splitters, and are a crucial ingredient for topological wires with Majorana end states. We discuss experimental signatures of quasi-helical Luttinger liquids (both for Rashba nanowires and ordered Kondo lattices), and find, for example, a non-universal conductance that deviates from 1 e^2/h. This underlines that quasi-helical systems differ from their ideal counterparts in experimentally important quantities.

Quantum Many Body Physics with Strongly Interacting Light-Matter Systems
le lundi 8 avril 2013 à 15h30

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : In the physics of strongly correlated quantum systems the electromagnetic
radiation has traditionally assumed the role of a spectroscopic probe
and thus treated as a classical field. In recent years an increasing control
over light-matter interactions at the genuine quantum level
has been achieved due to experimental developments in quantum optics and
quantum electronics. This has brought forth a novel class of many body
systems where elementary excitations are made by single quanta of light
and matter. These hybrid setups are currently attracting a great experimental
and theoretical interest, for the unique features they offer to explore quantum
many body physics in novel far from equilibrium regimes.
Motivated by the experimental effort, currently ongoing at Princeton, to realize
these correlated systems of photons and atoms using superconducting circuits, in
this talk I will discuss the physics of large arrays of microwave resonators
coupled to superconducting qubits via the elementary Rabi non-linearity. I will
argue that the very nature of photon field and its interaction with matter-like
excitations allows to stabilize finite-density quantum phases of correlated
photons out of the vacuum. I will discuss the properties of these phases and the
quantum phase transition occurring between them and highlight the differences
with the physics of interacting massive quantum particles.

Quantum Brachistochrone
le vendredi 5 avril 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : We investigate the application of optimal control of a single-qubit coupled to an ohmic heat bath. For the weak bath coupling regime, we derive a Bloch-Redfield master equation describing the evolution of the qubit state parameterized by vectors in the Bloch sphere. By use of the optimal control methodology we determine the field that generates a single qubit rotation. We use the techniques of automatic differentiation to compute the gradient for the cost functional. We consider also the concept of Quantum Brachistochrone. Here the problem naturally arises of determining the minimal transition time between an initial state and a final state. The optimal control is of bang-bang type and switches from the upper to the lower value of the control bounds.

Scale Invariance in Atomic Physics: from Efimov states to Fermions at Unitarity
le vendredi 22 mars 2013 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : The talk will provide an introduction to two
examples in ultracold atom physics where
scale invariance plays an important role:
three-body Efimov states of bosonic atoms
and Fermions at infinite scattering length.
We discuss the issue of an apparently 'universal'
three-body parameter in the Efimov context
and both thermodynamics and transport properties
of the unitary Fermi gas.

Nanoscale Nonlinear Thermoelectricity - Cooling, Catastrophes and Carnot
le vendredi 15 mars 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : I start by summarizing thermoelectric effects, and how we might be able
to use them for refrigeration, perhaps to cool nanoscale systems to previously
unreachable temperatures (as low as a few mK). However quantum effects
cannot be ignored in such low temperature nanoscale systems. Thus, I develop a
quantum theory of thermoelectric effects, which is capable of dealing with the
highly non-linear effects necessary for efficient refrigerators.

I apply the theory to refrigeration by point-contacts including Hartree-type
interaction effects,
and predict a discontinuity in the cooling response (a ``fold-catastrophe'' in
mathematics). I then turn to arbitrary such quantum systems, and show that
there are certain fundamental bounds on heat-flow.
Some of these bounds are thermodynamic in nature,
such as Carnot's thermodynamic bounds on heat engines and refrigeration,
while others are purely quantum.

Visualizing electron correlation in nano-objects using a scanning tunneling microscope: Molecules, quantum dots, carbon nanotubes
le vendredi 8 mars 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Scanning tunnelling spectroscopy (STS) visualizes electron states in both
extended systems and nano-objects, such as quantum dots, molecules, carbon
nanotubes. Whereas extended quantum states are insensitive to electron number
fluctuations, an energy gap opens each time a new electron is injected by the
STS tip into a nano-object. This gap originates from the interaction of the next
incoming electron with the others already present in the system. Under this
Coulomb blockade condition, STS maps the wave function modulus of the electron
injected by the tip into the nano-object. The obtained image is routinely
interpreted as the atomic-like or molecular orbital of the added electron, that
experiences the mean field of the other electrons already populating the system.
A fundamental question is whether features of the tunnelling map may appear due
to electron-electron correlation beyond mean field [1]. In this talk I will
demonstrate that the answer is positive, focusing on planar molecules with metal
centres [2], semiconductor quantum dots [3], quantum wires and carbon nanotubes [4].

[1] M. Rontani and E. Molinari, Phys. Rev. B 71, 233106 (2005); M. Rontani,
Nature Mat. 10, 173 (2011).
[2] D. Toroz, M. Rontani, and S. Corni, J. Chem. Phys. 134, 024104 (2011); Phys.
Rev. Lett. 110, 018305 (2013).
[3] G. Maruccio, M. Janson, A. Schramm, C. Meyer, T. Matsui, C. Heyn, W. Hansen,
R. Wiesendanger, M. Rontani, and E. Molinari, Nano Lett. 7, 2701 (2007).
[4] A. Secchi and M. Rontani, Phys. Rev. B 85, 121410(R) (2012).

Many Particle Models of Stochastic Transport
le vendredi 1er mars 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Active transport is critical for cellular organization and function. Much of the
intracellular long-distance transport is carried out by specialised proteins,
so-called molecular motors.
The molecular motors are connected to cargo like vesicles or cell organelles,
and moving along the filaments of the cytoskeleton. Molecular motors of the
kinesin and dynein
family move in opposite direction along microtubule-filaments. Some of the
cargo is even moved by kinesin and dynein motors, and frequently changes its
directionality.
In my talk I will discuss a few coorperative transport phenomena that are
related to motor driven intracellular transport. In particular variants of
stochastic many particle models
of transport by molecular motors are discussed, which show a strong tendency to
form macroscopic clusters on static lattices. Inspired by the fact that the
microscopic tracks for
molecular motors are dynamical, the influence of different types of lattice
dynamics on stochastic bidirectional transport will be examined.

Liquid and crystal phases of dipolar fermions in two dimensions
le vendredi 22 février 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Quantum degenerate gases interacting with long-range dipolar forces have become a fascinating new research direction in the field of ultracold atoms. In the seminar I will briefly review some of the recent experimental and theoretical progresses on this topic and then I will focus on the properties of dipolar fermions in two spatial dimensions. I will report on results obtained using quantum Monte Carlo methods concerning the equation of state of the liquid and crystal phase at zero temperature which correspond, respectively, to the regime of low and high density. Results on the critical density of the liquid to solid quantum phase transition are presented and the possible existence of a stripe phase close to the freezing density is discussed. Preliminary results on a bilayer system with a dipolar impurity interacting with a system of dipolar fermions will also be discussed.

Mechanistic aspects of ultrafast photoprocesses in bioorganic systems
le vendredi 15 février 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Excited-state dynamics of molecular systems plays a fundamental role in several
fields, such as reactive scattering, hot chemical reactions, transport
processes, photochemistry, and photophysics. In the last decade, computational
simulations have become an important tool to unveil reaction mechanisms in these
processes. In particular, semi-classical nonadiabatic dynamics simulations have
revealed complex scenarios, where multiple reaction pathways are in constant
competition among them and whose output is deeply dependent on details of a
manifold of potential-energy surfaces. In this talk, I will deliver an overview
of recent achievements in this field, including a critical appraisal of the
strengths and limitations of the available simulation methods. Special focus
will be laid on the deactivation dynamics of UV-excited nucleobases, a
phenomenon that may have played a central role for life evolution on Earth.

Dynamics in one dimension: from integrability to inelastic neutron scattering and beyond
le vendredi 8 février 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : Over the last few years, integrability has become a method of choice for the calculation of equilibrium dynamical correlation functions of systems such as spin chains and interacting atomic gases, its main strength being its ability to go beyond low-energy effective theories. A brief review will be given of results on simple and more elaborate observables relevant to experiments such as inelastic neutron scattering, resonant inelastic x-ray scattering and their equivalents in cold atomic systems. Recent applications to out-of-equilibrium physics in cold atoms will also be discussed.

Structural features underlying the dynamics of supercooled dynamics
le vendredi 1er février 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : We apply instantaneous shear deformations to supercooled liquids to investigate the correlations between t
he soft modes of the inherent structure belonging to the initial and final configurations and the isoconfi
gurational Debye-Waller factors of the initial thermal configuration, as a function of the temperature and
of the strain amplitude. The spatial distributions of non-affine displacements (NAD) characterizing such
response are correlated to the dynamical heterogeneities of the supercooled liquid, suggesting that partic
les in regions of large NADs are likely to be more mobile than those belonging to small values of NADs.
Moreover, our normal mode analysis shows that cooperative regions in NAD are strongly correlated to the lo
w energy soft modes of the inherent structure of the supercooled liquid, responsible for the onset of plas
ticity in the amorphous solid. In addition, we also observe a well-defined critical deformation amplitude,
above which these correlations are lost.

Breaking the flux limit: A novel atom laser using time-dependent adiabatic potentials
le vendredi 25 janvier 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Atom laser - coherent beams of matter originating from Bose-Einstein condensates (BEC) - are one of the most prominent demonstrations of matter wave optics. They have been proposed as coherent atom sources for matter-wave interferometry and direct atom lithography. Atom lasers are generated by coupling atoms from a trapped BEC into free-space using either a weak RF field [1] or a weak Bragg beam [2]. In this talk I will present a novel type of output coupling of an atom-laser from a BEC, which uses a strong RF field to create a time-varying adiabatic potential (TAP) in a magnetic Ioffe-Pritchared trap. In combination with gravity, the TAP opens a small hole is created in the very bottom of the trap from which the atom-beam is allowed to escape. The TAP atom laser avoids the flux limits of the traditional laser based on weak coupling. This allowed us to demonstrate an increase in flux by more than one order of magnitude to 7 x 10^{7} atoms/s, whilst preserving some of the lowest divergences reported so far (6 mrad) [3]. The TAP also allowed us to generate thermal atom beams with record temperatures as low as 300 nK at a peak-flux of up to 3 x 10^{8} atoms/s. In is talk will discuss the generation and limits of the TAP atom laser.

The electromagnetic vacuum of random media
le lundi 21 janvier 2013 à 11h00

Séminaire LPMMC

Personne à contacter :

Lieu : Amphithéâtre, Maison des Magistères

Résumé : In this talk I present an analytical approach to the study of several phenomena related to the vacuum field in dielectric random media. In the electric dipole approximation, exact expressions for the dipole emission rate and the van der Waals energy are derived as a function of the electrical susceptibility. Approximate expressions for the total vacuum energy are given, they all free of divergences. The role of local field factors is explained. The difference amongst the spectra of fluctuations which enter each of the aforementioned quantities is clarified. The results are compared with those obtained in the effective medium approximation.

Static fluctuations of a thick 1D interface in the 1+1 Directed Polymer formulation
le vendredi 18 janvier 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The one-dimensional Kardar-Parisi-Zhang (KPZ) equation is at the crossroad
between a wide range of theoretical models and experimental systems such as
roughening phenomena and stochastic growth, the Burgers equation in
hydrodynamics or the 1+1 Directed Polymer, and the very definition and
implications of the KPZ universality class have been expanding since the 1980',
both in physicists and mathematicians communities.

We have tackled the interplay between thermal fluctuations and a correlated
disordered energy landscape, from the vantage point of static 1D elastic
interfaces in a random-bond disorder. The experimental realizations of such
interfaces always exhibit a non-zero disorder correlation length or thickness
(typically tens of nanometers for domain walls in ferromagnetic thin films), and
turn out to display a low-temperature regime where thermal fluctuations have not
totally erased the imprint of the disorder correlation. This low-temperature
regime is experimentally relevant for ferromagnetic domain walls, but thanks to
the KPZ connections it might also describe the high-velocity steady-state
dynamics of interfaces in nematic liquid crystals (even though without
impurities).

Title: Phonon-enhanced coherent scattering in a driven quantum dot
le vendredi 11 janvier 2013 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The recent experimental characterisation of exciton-phonon interactions in a coherently-driven semiconductor
quantum dot (QD) [1, 2], and their interpretation in terms of a two-level system in contact with a bosonic
environment, have demonstrated that QDs offer a natural platform in which to explore dissipative dynamics in
the solid-state. In particular, the interplay between laser-driven coherent excitonic oscillations and incoherent
phonon-induced processes leads to a rich dynamical behaviour, which can also have a profound effect on the dot
photon emission characteristics. In this talk, I shall explore the crucial role played by the solid-state environment
in determining the photon emission properties of a driven quantum dot [3]. In fact, I shall show that such
environmental interactions can lead to quantum dot emission characteristics that deviate fundamentally from
the well-established quantum optical behaviour of driven atoms. Specifically, for resonant driving, the coherently
emitted radiation field can actually increase with driving strength due to the quantum nature of the phonon bath.
This behaviour is in stark contrast to the conventional (quantum optical) expectation of a monotonically
decreasing fraction of coherent emission with stronger driving, and should be observable in experimentally
achievable regimes.

References
[1] A. J. Ramsay et al., Phys. Rev. Lett. 104, 017402 (2010)
[2] A. J. Ramsay et al., Phys. Rev. Lett. 105, 177402 (2010)
[3] D. P. S. McCutcheon and A. Nazir, arXiv:1208.4620

Partial control of information and correlations in scattering media
le lundi 7 janvier 2013 à 14h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Random matrix theory as well as microscopic theory predict that the
transmission eigenvalues of a disordered dielectric material have, in the
diffusive regime, a bimodal distribution peaked around 0 and 1 that gives rise
to the concept of closed and open eigenchannels. However, in a typical optical
experiment where only a small fraction of the channels are excited or measured,
the distribution of the transmission eigenvalues is not the bimodal but the
Marchenko-Pastur law. We propose an analytical theory that quantitatively
describes the transition between these two distributions. In particular, we show
that the reduction of the number of controlled input/output channels abruptly
suppresses the open eigenchannels and then gradually yields to an effective loss
of the correlations contained in the scattering matrix. This effect is
illustrated with the study of the information capacity of a disordered
waveguide. Finally, we show how the abrupt loss of the open eigenchannels can
dramatically reduce the effect of coherent enhancement of absorption.