Seminars 2018
Seminar
Tuesday, December 18, 2018
Curvatureinduced defect unbinding in active nematic
tori
Georgia Tech
Abstract
We will discuss our recent results with active nematic
liquid crystals on toroidal surfaces and show how,
despite the intrinsic activity and outofequilibrium
character of our system, we still observe remnants of
the expected curvatureinduced defect unbinding
predicted for nematics in their ground state. In our
experiments, however, the number of defects is far
larger than what one would expect for conventional
nematics. In addition, these defects move throughout
the toroidal surface and arrange in unexpected ways
when compared to conventional nematics. Comparing our
results to numerical simulations, we unravel the key
role of activity and how it affects the results via
defect creation and annihilation events.
The seminar will take place at 13:00 in classroom
2.3.B02 (Edificio Sabatini) Universidad Carlos III
Seminar
Tuesday, December 11, 2018
Projective symmetries and the Palatini formalism in
Einstein gravity and beyond
Universidad de Granada.
Abstract
In mathematics, the metric $g$ and the affine
connection $\Gamma$ of a manifold are two independent
fields, each with their own mathematical structure.
However, for every metric $g$, there is a special
connection $\mathring \Gamma$, called the LeviCivita
connection, which is entirely determined as a function
of $g$. In Riemannian geometry and standard General
Relativity, the geometrical properties of spacetime are
described using this LeviCivita connection. However,
one version of modified theories of gravity is to
consider other possible affine connections and
investigate their dynamics and their physical
influences. This class of theories are usually called
metricaffine gravities, the firstorder formalism or
the Palatini formalism. In this talk we will
investigate the Palatini formalism for the
EinsteinHilbert action, EinsteinDirac theory and the
GaussBonnet action and point out its relation to the
projective symmetries of these actions.
The seminar will take place at 12:00 in classroom
2.3.A04 (Edificio Sabatini) Universidad Carlos III
Seminar
Tuesday, November 13, 2018
Quantum versus Thermal annealing, the role of
Temperature Chaos
Universidad Complutense de Madrid
Abstract
Recent advances in quantum technology have led to the
development and manufacturing of experimental
programmable quantum annealing optimizers that contain
hundreds of quantum bits. These optimizers, commonly
referred to as `DWave' chips, promise to solve
practical optimization problems potentially faster than
conventional `classical' computers. Attempts to
quantify the quantum nature of these chips have been
met with both excitement and skepticism but have also
brought up numerous fundamental questions pertaining to
the distinguishability of experimental quantum
annealers from their classical thermal counterparts.
Inspired by recent results in spinglass theory that
recognize `temperature chaos' as the underlying
mechanism responsible for the computational
intractability of hard optimization problems, we devise
a general method to quantify the performance of quantum
annealers on optimization problems suffering from
varying degrees of temperature chaos: A superior
performance of quantum annealers over classical
algorithms on these would serve as a strong telltale
sign of quantum behavior. We utilize our method to
experimentally study the DWave Two chip on different
temperaturechaotic problems and find, surprisingly,
that its performance scales unfavorably as compared to
several analogous classical algorithms. We detect,
quantify and discuss several purely classical effects
that possibly mask the quantum behavior of the chip.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Friday, November 9, 2018
Vector dark matter at the end of inflation
Universidad de Granada.
Abstract
The nature of dark matter still remains a mystery.
While most of the experimental effort in the past was
aimed at detecting weakly interacting massive particles
(WIMPs), the lack of observation requires new
theoretical ideas and proposed search strategies to
cover as many alternatives as possible. In this
context, dark photons are among the alternative dark
matter candidates which have received increasing
attention in recent years. It has been already shown
that the longitudinal mode of a massive vector boson
can be produced by inflationary fluctuations and
account for the dark matter. Alternatively, in this
talk we discuss the possibility of instead producing
the transverse mode through its coupling to the
inflaton and the tachyonic instability to which it
leads. After production they behaves as radiation until
its wavelength stretches beyond their Compton
wavelength. After that, they become nonrelativistic
and behave like matter. We show that these transverse
vectors can account for the observed dark matter relic
density in the mass range from micra eV to hundreds
GeV.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Friday, October 19, 2018
Duality and universality in the 3state
antiferromagnetic Potts model
UC3M
Abstract
It is well known that universality does not hold in
general in antiferromagnetic (AF) models: the phase
diagram depends strongly on the microscopic structure
of the lattice. However, for the twodimensional
qstate Potts model, some sort of universal behavior
can be recovered, at least for some classes of lattices
and certain values of q. In this talk, I will introduce
some unusual and interesting properties of Potts AF
such as nonzero groundstate entropy density (without
frustration!) and the existence of zerotemperature
critical points and height representations for certain
models. These ideas will be needed in order to
understand a recent conjecture that predicts the
universality class of the 3state Potts AF on
(periodic) plane quadrangulations.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Friday, September 14, 2018
Coherence resonance in semiconductor superlattices at
room temperature
UC3M
Abstract
Random number generators (RNG) are key in many areas:
data security, numerical simulations, online games of
chance, etc. Classical generators can be either
insecure or slow. Recent solutions that avoid both
problems include doped weakly coupled semiconductor
superlattices which, operated at room temperature [1]
can produce spontaneous chaotic oscillations. This can
be exploited to generate true random numbers fast
enough and with little postprocessing. Under DC voltage bias, idealized superlattices [2] show selfsustained chaotic behavior for certain voltage intervals. If external and internal noise is taken into account, these voltage intervals can be made broader and the chaotic response enhanced. Thus, establishing superlattices as a robust physical entropy source. Another feature seen both in experiments [3] and numerical simulations [4] is coherence resonance: external band limited voltage noise of sufficient amplitude induces regular current selfoscillations in states that are stationary in the absence of noise. Furthermore, when a weak AC voltage signal is applied, coherence resonance triggers a stochastic resonance. That is, the current response is both phase locked to the AC signal and amplified, in the sense of a higher signal to noise ratio. [1] W. Li, I. Reidler, Y. Aviad, Y. Huang, H.
Song, Y. Zhang, M. Rosenbluth and I. Kanter, Phys. Rev.
Lett. 111 044102 (2013)
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Friday, June 8, 2018
Probing the microscopic dynamics of soft matter:
Simulations of colloidal Microrheology
Universidad de Almería
Abstract
Microrheology was introduced more than 20 years ago as
a novel technique to measure the viscosity at
microscopic scales. Here, a colloidal tracer is
introduced in the sample, and its dynamics is monitored
as it explores the bath. When no external force acts on
the tracer, socalled passive microrheology, it
diffuses freely and the system is in equilibrium; on
the other hand, when an external force drives the
tracer, the system falls out of equilibrium. In the
latter case, an effective friction coefficient can be
obtained from the steadystate relation
$F_\text{ext}=\gamma_\text{eff}\, \langle v \rangle$.
For a viscoelastic medium, this coefficient is constant
for small forces (within the linear regime) and
decreases as the force increases (forcethinning
regime). However, studying the dynamics of the tracer
provides much more information than just the friction
coefficient; it reflects the bath dynamics in the
linear regime, and a complicated interaction of the
pulled tracer and bath dynamics for large forces. In this talk, we will present simulations from Langevin dynamics simulations of quasihard colloidal spheres (see snapshot). We focus on the behaviour of the probe particle for increasing volume fraction of the host fluid, up to densities beyond the glass transition. The effective friction coefficient, as well as other dynamical quantities are studied and compared with a theoretical model based on the modecoupling approximation to describe the bath dynamics. Beyond the glass transition, a localized regime is found for small forces, where the tracer is trapped. The tracer breaks free beyond a critical force, entering an intriguing superdiffusive behaviour. Finally, we will study the connection between microrheology and bulk rheology using a "large" tracer.
The seminar will take place at 12:00 in classroom
2.1.A18 (Edificio Sabatini) Universidad Carlos III
Conference
Wednesday, May 30, 2018
The Physical Review at 125 years: Our past, our future,
and how you can help us shape it
Editor of Physical Review E (American Physical Society)
The conference will take place at 13:00 in the Salón de
Grados of the Padre Soler Building, Universidad Carlos
III, Leganés.
Seminar
Wednesday, April 25, 2018
La vida en la nanoescala
IMDEANanociencia
Abstract
El fenómeno de la vida envuelve varias escalas de
tamaño: desde las moléculas hasta los objetos que
manejamos con nuestras manos. Pero la vida va más allá
de las descripciones fenomenológicas o de las
explicaciones químicas, que cubren buena parte del
conocimiento básico que tenemos hasta hoy: la vida
obedece a las leyes de la Física, como cualquier otro
proceso en el universo. La vida se desarrolla fuera del
equilibrio termodinámico e implica el consumo de
energía química, principalmente, y la producción de
actividades diversas que van desde el trabajo mecánico
hasta el procesamiento de información. Por último, y no
menos relevante en nuestros días, la vida supone un
paradigma para la Nanotecnología puesto que la
maquinaria que hace funcionar a una célula o a un virus
es de sólo unos cuantos nanómetros. En esta charla nos adentraremos en el estudio de dicha maquinaria nanoscópica desde los puntos de vista del físico y del ingeniero, y presentaremos las metodologías experimentales que en los últimos veinte años nos están permitiendo manipular y medir la actividad de los constituyentes fundamentales de la materia animada. Exploraremos, a continuación, las nanopartículas, que posibilitan el control de fenómenos estímulorespuesta en medios fisiológicos y en escalas biomoleculares, y que, por tanto, resuenan en Nanomedicina. Por último, hablaremos del software en nuestras células, así como del hardware implicado en el procesamiento de la información genética. En lo segundo, nos centraremos en la mecanoquímica de los motores biomoleculares, que los dota de una eficiencia termodinámica muy por encima de la de los motores macroscópicos, y en los mecanismos de transferencia de información de alta fidelidad, que permiten conciliar el mantenimiento de la identidad genética con la evolución y adaptabilidad de las especies. El conjunto nos acercará a la imagen moderna que la Nanociencia proyecta sobre el fenómeno de la vida y a la investigación en Biología como fuente de inspiración en Nanotecnología.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, April 11, 2018
Selfassembly of active colloids
UCM
Abstract
Active matter represents a fundamentally new
nonequilibrium regime within statistical mechanics. In
contrast to traditional nonequilibrium systems, where
directional driving forces emerge as a result of global
changes in the thermodynamic variables (such as
temperature and pressure), active systems are
intrinsically out of equilibrium at the singleparticle
level. In our recent work, we have focused on studying
the collective behaviour and assembly in dilute
suspensions of spherical selfpropelled particles,
unraveling the role played by the particleparticle
interactions (whether shortrange or longrange,
isotropic or anisotropic) and by the hydrodynamics.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, March 21, 2018
Thermodynamics of mantle minerals from first principles
ICMM, CSIC
Abstract
In this talk I will present some recent results on the
thermodynamic properties of a series of minerals that
are important constituents of the Earth's mantle.
Beyond presenting those results and describing the
techniques used to obtain them, I will aim to
illustrate the fact that there are many interesting
questions relevant to the fields of geology and
geophysics that are ultimately materials questions,
questions that can begin to be addressed, if not fully
at least partially, with the techniques of
computational materials science.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, March 14, 2018
NUMERICAL SIMULATION AND OPTIMIZATION OF AIRFRAME
ASSEMBLY PROCESS
Saint Petersburg Polytechnic University
Abstract
During the airframe assembly process it is important to
control both gap between joined parts and stresses
caused by installed fixture elements. The main goal of
the presented work is to develop special tool for
numerical simulations of assembly process in order to
check and optimize the assembly technology.
The main challenge for simulation the assembly process is necessity to solve the contact problem for determination of deformed stress state of the assembly loaded by the forces from fastening elements. This contact problem has some peculiarities that were taken into account in order to derive efficient algorithm: The developed mathematical model combines dimension reduction with use of stateoftheart optimization algorithms for solving of derived quadratic programming problem. The described above algorithm was realized in software code and thoughtfully tested in the framework of joint project between Saint Petersburg Polytechnic University and Airbus Operations S.A.S. The verification of obtained results is made against analytic solution and number of physical experiments on real aircraft junctions. Simulation results have been already successfully implemented for optimization of Airbus assembly chain. Application examples and respective challenges are to be discussed during the presentation.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, March 7, 2018
Thermal diffusion segregation in granular mixtures
Universidad de Extremadura
Abstract
Among the different competing mechanisms involved in
granular segregation, thermal diffusion becomes the
most relevant one when an external energy input drives
the system into rapid flow conditions. In this regime,
granular matter flows like a fluid and kinetic theory
tools (conveniently adapted to account for the
inelastic character of collisions between grains) can
be quite useful to analyze thermal diffusion
segregation. Thermal diffusion is caused by the
relative motion of the components of a mixture due to
the presence of both gravity and a temperature
gradient. Due to this motion, a steady state is reached
where the separation effect arising from thermal
diffusion is balanced by the remixing effect of
ordinary diffusion. The aim of this contribution is
determine the socalled thermal diffusion factor
$\Lambda$ of a moderately dense granular binary mixture
(with coefficients of normal restitution $\alpha_{ij}$
for collisions between particles of species $i$ with
$j$) described by the (inelastic) Enskog kinetic
equation. A segregation criterion is derived from the
knowledge of $\Lambda$, which is explicitly obtained in
terms of the parameters of the system (masses and sizes
of particles, concentration, solid volume fraction and
coefficients of normal restitution) [1]. The sign of
$\Lambda$ determines the tendency of the large
particles to drift toward the cooler or warmer plate.
To test the i reliability of the theoretical
calculations, the factor $\Lambda$ is also obtained by
computer simulations [Monte Carlo (DSMC) and molecular
dynamics (MD) simulations] carried out for a granular
impurity (species 1) in a driven lowdensity granular
gas [2]. As an illustration, Fig. 1 shows the marginal segregation
curve ($\Lambda=0$) for a system with $\alpha_{22} =
0.9$ and $\alpha_{12} = 0.7$. It is quite apparent that
theory reproduces very well the phase diagram obtained
from simulations.
[1] Garzó, V. Phys. Rev. E, 78, pp. 020301
(R), 2008; Eur. Phys. J. E, 29, pp. 261274,
2009; New J. Phys., 13, pp. 055020, 2011.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, February 28, 2018
Selfassembling disorder in materials science: From
photonic crystals to photonic glasses
ICMM, CSIC
Abstract
Complex material structures are inherently interesting
for the curious scientific mind due to the challenge
their in depth understanding constitutes. Periodicity
greatly simplifies their understanding and adds
interesting new properties. Selfassembled
nanostructures usually develop ordered patterns in
three dimensions [1]. When disorder is dominant, some
interesting new phenomena may appear, as random lasing
[2]. Artificial opals are one of such possible
arrangements usually forming fcc structures with
promising photonic properties. Often, and undesirably,
unwanted defects are present spoiling the optical
properties of such nanostructures [3]. On the other
hand, and contrary to intuition, the introduction of
arbitrarily high amounts of disorder is, in some cases,
an equally difficult task but the resulting material
presents intriguing new optical properties. We have
grown novel nanophotonic materials, photonic
glasses, which are solid, disordered assembly of
monodisperse dielectric spheres [4], in which novel
devices such as resonant random lasing may be observed
[5].
In this talk, I will summarize our latest results regarding selfassembled photonic materials and, in particular, I will focused on the transition from order to disorder, and vice versa, when understanding selfassembly can be approached by studying colloidal crystallization. [1] J.F. GalisteoLópez, M. Ibisate, R. Sapienza,
L. FroufePérez, A. Blanco, and C. López, Adv. Mater.
23, 3069 (2011).
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, February 7, 2018
Thermal transistor based on stochastic switching
UC3M
Abstract
Fluctuations are strong in mesoscopic systems and have
to be taken into account for the description of
transport. We show that they can even be used as a
resource for the operation of a system as a device. We
use the physics of singleelectron tunneling to propose
a bipartite device [1,2] working as a thermal
transistor [3]. Charge and heat currents in a two
terminal conductor can be gated by thermal fluctuations
from a third terminal to which it is capacitively
coupled. The gate system can act as a switch that
injects neither charge nor energy into the conductor
hence achieving huge amplification factors. Nonthermal
properties of the tunneling electrons can be exploited
to operate the device with no energy consumption.
[1] R. Sánchez, M. Büttiker, Optimal energy quanta
to current conversion, Phys. Rev. B 83, 085428
(2011).
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Thursday, February 1, 2018
Investigando el camino de desplegamiento de proteínas
Universidad de Sevilla
Abstract
En esta charla revisamos algunos resultados teóricos
recientes que predicen el camino de desplegamiento de
proteínas simples, como sistemas modulares, como
función de la velocidad y la dirección de tracción.
Estos resultados teóricos se obtienen a partir de una
descripción mesoscópica, en que las extensiones de los
distintos módulos que componen la proteína obedecen
ecuaciones de evolución escolásticas (Langevin). Las
predicciones de este enfoque teórico se comparan tanto
con resultados experimentales como con simulaciones de
Dinámica Molecular.
The seminar will take place at 13:00 in classroom
2.1.A18 (Edificio Sabatini) Universidad Carlos III
Seminar
Tuesday, January 23, 2018
Estados termodinámicos en vidrios de espín en dimensión
finita
Universidad de Extremadura
Abstract
En esta charla se abordarán de una manera sencilla los
vidrios de espín en dimensión finita. Para ello
describiremos la obtención del modelo de
EdwardsAnderson a partir de primeros principios.
Posteriormente detallaremos las propiedades físicas de
las solución en el regimen de campo medio basada en la
rotura de la simetria de las réplicas y teorías
alternativas como el modelo de los droplets y la de los
pares caóticos. A continuación estudiaremos los
problemas que presenta el límite termodinámico en estos
modelos (por ejemplo, la definición de estados) y el
concepto de metaestado. Finalmente presentaré
resultados recientes sobre la construcción numérica del
metaestado y sus implicaciones sobre las diferentes
teorías que pretenden describir los vidrios de espín en
dimensión finita.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, January 17, 2018
Proliferation and Movement  Modelling Angiogenesis
University of Coimbra
Abstract
Angiogenesis  the growth of new blood vessels from a
preexisting vasculature  is key in both physiological
processes and on several pathological scenarios such as
cancer progression or diabetic retinopathy. In this
seminar we will present different mathematical
approaches to model sprouting angiogenesis. We will
focus on the role of cell migration and cell
proliferation in determining the morphology of the
resulting network. We will present mathematical models
that take into account the role of mechanics in
regulating endothelial cell proliferation and
migration. For the new vascular networks to be
functional, it is required that the growing sprouts
merge either with an existing functional mature vessel
or with another growing sprout. This process is called
anastomosis. We will present a systematic 2D and 3D
study of vessel growth in a tissue to address the
capability of angiogenic factor gradients to drive
anastomose formation. We will demonstrate that the
production of angiogenic factors by hypoxic cells is
able to promote vessel anastomoses events in both 2D
and 3D. We also verify that the morphology of these
networks has an increased resilience toward variations
in the endothelial cell's proliferation and chemotactic
response.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III
Seminar
Wednesday, January 10, 2018
The dynamics of a thin vibrated granular layer
(quasi2D geometry)
Univerdidad de Extremadura
Abstract
We review the main features of the dynamics of a
quasi2D thin granular layer. The system consists in a
densely packed set of identical spheres (metallic
spheres with a diameter $\sim$ 1mm) that is conned in a
vibrating box. The layer is horizontal and the shaking
is performed vertically. The box has a width $h \in
[1.25\sigma,1.9\sigma]$ so that the geometry forbids
vertical particle overlapping. We consider only
sineshaped vibration signals, with amplitude $A$ and
angular frequency $\omega$ so that the input
acceleration is typically greater than gravitational
acceleration: $\Gamma \equiv A\omega^2/g \gt 1$. As
energy input is gradually increased the system
undergoes over a series of phase transitions, including
a quasi2D hexaticlike phase that can be described in
the context of the KTHNY theory [1,2,3]. The observed
phase diagram changes considerably as a function of the
layer width $h$ and particle collision inelasticity. In
fact, we show that under the appropriate conditions a
suciently strong inelasticity may suppress any kind of
ordering/clustering in the system.
[1] J.M. Kosterlitz and D.J. Thouless, J. Phys. C
6 (1973) 1181.
The seminar will take place at 13:00 in classroom
2.0.D14 (Edificio Sabatini) Universidad Carlos III


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