Since the first experimental evidence of photoluminescence of semiconducting single-walled carbon nanotubes (SWNTs), studies have been conducted to investigate the optical properties of these nano-structures, motivated by possible applications in the fields of quantum information, biological labeling, opto-electronics or laser technology. The experimental investigation of the intrinsic optical properties of SWNTs is challenging since they are extremely sensitive to the influence of their physico-chemical environment. In this context, samples with reduced perturbations, such as suspended SWNTs, are particularly interesting.
In this talk, I will present an experimental study of the magneto-optical properties of suspended SWNTs through time-resolved photoluminescence spectroscopy. Measurements are performed at the single-object level using a home-built confocal optical microscope with a large numerical aperture (NA = 0.95) operating at cryogenic temperature (down to 2K) and high magnetic field (up to 7T). The evolution of the photoluminescence spectra and decay signals with increasing magnetic fields shows the influence of the Aharonov-Bohm effect on the two lowest-energy singlet excitons, namely the ground exciton which is optically inactive (dark exciton) and an exciton lying a few millielectronvolts higher in energy which is optically active (bright exciton). A model based on rate equations and including the Aharonov-Bohm coupling between these two excitons enables to determine separately the excitons lifetimes and to derive quantitative information on the energy relaxation from the photo-excited higher levels. The energy relaxation following the photo-excitation of the S22 transition leads to a bright state population efficiency four times lower than that of the dark state, but it significantly increases when energy relaxation occurs from the KK’ excitonic levels. Thanks to a good signal to noise ratio, the photoluminescence spectra also reveal the presence of a very weak intrinsic zero-field coupling between the dark and the bright excitons, as well as an excitonic mobility preserved at liquid helium temperature in suspended SWNTs.

The mechanical properties of epithelial tissues, which are typically composed of a single layer of tightly bound cells, play an important role in development and disease. Therefore, our goal is to develop theoretical and computational tools to help explain global mechanical behavior and cell migration patterns in these tissues. In this talk, I will focus on connections between tissue rigidity and collective migration using two different techniques in two different geometries. In the first project, we develop a hydrodynamic model for bulk tissues that couples cell shapes, tissue rigidity, and cell-substrate interactions by assuming that cells exert traction forces preferentially along gradients in the in-plane tissue stiffness, in accord with recent experiments. Analyzing the steady states of the model, we identify a new “morphotactic” parameter that our model predicts will control pattern formation in real tissues. In the second project, we developed a new active vertex model and employed it to simulate so called “wound-healing” experiments in which a tissue is allowed to expand into free space. These simulations indicate that tissues are unable to sustain a mechanical stress in the absence of propulsive forces generated by cell-substrate interaction. Further, our results suggest that polar order in these propulsive forces qualitatively changes the observed stress profile, and may thereby promote cell proliferation.

Strongly coupled theories exhibiting walking dynamics provide a scenario for beyond the Standard Model physics, in which electro-weak symmetry is broken dynamically and the large hierarchy between the electro-weak and Planck energy scales is naturally generated. In this talk, we discuss the construction of walking theories within the context of gauge-gravity duality, focusing in particular on the circumstances under which a parametrically light composite state, the dilaton, is present in the spectrum. The examples we provide come both from bottom-up models as well as top-down models embedded in M-theory and type-IIB supergravity, and include the baryonic branch of the Klebanov-Strassler field theory.

Si l'on force des particules discrètes à s'écouler dans un canal étroit ou à travers une constriction, des effets assez surprenants peuvent se produire. J'aborderai quelques exemples de tels écoulements.
Dans le domaine de la microfluidique, la mise au point de laboratoires sur puce demande de transporter des émulsions très concentrées à travers des microconduits. Or, expérimentalement, cet écoulement confiné peut fortement s'écarter des prévisions faites sur la base de la rhéologie macroscopique de ces matériaux [1]. De semblables écarts ont été relevés pour d'autres types de solides mous, comme les pâtes colloïdales. Les modèles élasto-plastiques que nous avons développés clarifient l'origine de ces déviations sous confinement. En effet, ces modèles génériques, qui décrivent un ensemble de régions mésoscopiques alternant entre un comportement élastique et une phase de relaxation plastique, nous ont aidés à faire la part des choses entre la contribution du chargement hétérogène et la rhéologie spécifique près des parois [2].
Pour en venir à des situations d'écoulement non uniforme, je parlerai ensuite d'écoulements à travers une constriction étroite, pour lesquels des bouchons peuvent se produire et interrompre le débit. Pour des écoulements granulaires, ces bouchons peuvent être détruits à l'aide de vibrations, mais ils ne cèdent pas spontanément : ils ont des temps de vie très largement distribués. Je présenterai un modèle simple qui rend compte de ces statistiques anormales. D'un autre côté, quand les particules sont des piétons franchissant une porte, les bouchons sont résolus naturellement. Néanmoins, des similitudes avec leurs homologues granulaires existent. [4-5].

[1] Goyon, J., et al. "Spatial cooperativity in soft glassy flows." Nature 454.7200 (2008).
[2] Nicolas, Alexandre, and Jean-Louis Barrat. "Spatial cooperativity in microchannel flows of soft jammed materials: A mesoscopic approach." Physical review letters 110.13 (2013).
[3] Nicolas, Alexandre, Garcimartín, Ángel and Zuriguel, Iker, "A trap model for clogging and unclogging in granular hopper flows". Submitted (2017)
[4] Zuriguel, Iker, et al. "Clogging transition of many-particle systems flowing through bottlenecks." Scientific reports 4 (2014): 732
[5] Nicolas, Alexandre, Sebastián Bouzat, and Marcelo N. Kuperman. "Pedestrian flows through a narrow doorway: Effect of individual behaviours on the global flow and microscopic dynamics." Transportation Research Part B: Methodological 99 (2017).

Interfaces of theoretical physics with other fields vary along two axes: whether exhaustive microscopic knowledge is available to construct models, and whether physics provides substance or only tools. I will argue that even interfaces that are remote on both counts, e.g. with economics, ecology and evolution, can open important new questions in our field, while answering these questions is likely to happen at more controlled interfaces, e.g. with cell biology.

From kinetic theories of economic interactions to spin glass phases in ecological dynamics, I will discuss some examples in my experience that either resonated or not with scientists in other fields, and sometimes pushed our tools in unexpected directions, to ask when physics can have a meaningful two-way interaction with these other theoretical corpora.
I will in particular propose that statistical physics can give a sharp meaning to the notion of biological function, which other sciences have largely failed to theorize. In turn, this notion poses definite mathematical and empirical questions in systems such as transport and metabolic networks, which can provide a testing ground for theoretical ideas across interfaces.

Graphene is a key material for nanoelectronics. Nevertheless, its zero gap makes it unsuitable for applications needing semiconductors with sizeable energy gaps. Therefore, a lot of efforts are being made to develop materials with non-zero gap compatible with the hexagonal lattice of graphene. In this framework, small pieces of graphene such as graphene nanoribbons and quantum dots have a lot of assets. Thanks to the so-called bottom-up synthesis a precise control of the size, shape and edges of these objects is achievable [1]. This opens the way to a precise tailoring of their physical properties.

In this presentation, I will first present the different research topics of the group. Then I will show our recent results on the optical properties of both graphene nanoribbons and quantum dots synthesized by bottom-up chemistry [2-4].


References:

[1] R. Rieger and K. Müllen, "Forever young: polycyclic aromatic hydrocarbons as model cases for structural and optical studies", J. Phys. Org. Chem. 23, 315-325 (2010).

[2] Shen Zhao et al, “Optical Investigation of On-Surface Synthesized Armchair Graphene Nanoribbons”, Physica Status Solidi b 254, 1700223 (2017)

[3] Shen Zhao et al, “Fluorescence from graphene nanoribbons of well-defined structure” Carbon 119, 235 (2017)

[4] Shen Zhao et al, “Single photon emission from graphene quantum dots at room temperature”, in preparation

In general, single-walled carbon nanotubes (SWCNTs) are synthesized by
chemical vapor deposition (CVD) at 700-900ºC using Fe, Co and Ni catalysts. However,
due to Ostwald ripening of catalyst particles, the uniformity of the SWCNT diameter is
not sufficient, which is not preferable for electronics applications. In addition, reduction
of the growth temperature is desired for realization of SWCNT devices compatible with
LSI manufacturing processes. In this study, we performed SWCNT growth by alcohol
catalytic CVD using platinum-group metal catalysts (Ru, Rh and Pt), since these metals
have high-melting points (> 1700ºC), and reduction of Ostwald ripening is expected. By
optimizing the growth condition, SWCNTs were grown from these catalysts at the growth
temperature between 500 and 700ºC. Irrespective of the catalyst metals, the diameter and
chirality distribution became narrower, as the growth temperature decreased. In particular,
the diameters of most SWCNTs grown from Pt catalysts were below 1 nm and their
diameter distribution was quite narrow [1]. On the other hand, SWCNTs were grown from
Rh catalysts even at 300ºC, which is the lowest temperature record of SWCNT growth
[2]. We will also discuss growth mechanism of SWCNTs from these platinum-group
metal catalysts.
[1] T. Maruyama et al. Carbon 96 (2016) 6.
[2] T. Maruyama et al. Carbon 116 (2017) 128.

Recent studies have shown that the presence of boundaries can strongly affect the dynamics of physical systems.
For example, negative mobility and fluid recirculation occurs when an electrolyte is driven in a varying-section channels, rectification occurs active particles such as molecular motors or active swimmers moving in inhomogeneous environments and off--diagonal terms in the mobility tensor appears for binary mixtures of hard sphere confined between corrugated plates.
Clearly the interplay between the confined systems and the confining walls is maximized when the typical length scales of the confined system match with the size of the confining walls.
In this contribution I will discuss the general mechanisms at the basis of the interplay between the confined system and the confinement. In particular, by means of a few example I will show how analytical insight into the possible regimes can be attained.

Firstly, I will discuss how the electrostatic interaction between a tracer particle and the channel walls affect particle dynamics. In such a scenario, novel dynamical regimes such as negative mobility and asymmetric passage times can arise due to the interplay between the electrostatic interactions and the local entropic drive induced by the varying--section channel.

Secondly I will show how the insight gained by such studies can be transferred to the problem of polymer translocation across varying--section channels.
In particular, I will show that, under suitable approximation, it is possible to reduce the problem of polymer translocation across varying--section channels to that of a single point--like particle under an effective potential that can be derived from the equilibrium local free energy of the polymer. Interestingly, the model predicts a non-monotonous dependence on the translocation time of the polymer across the pore appears. By comparing with Brownian dynamics simulations I will discuss the quantitative reliability of the point--like approximation as well as its regime of validity.

Finally, when the interactions between the confined systems and the confinement are non-conservative, as it happens for hydrodynamic interactions, the interplay between the confined system, say an active swimmer, and the confinement is more involved. I will, briefly discuss how such a scenario can be described on the basis of the experienced gained with the conservative forces.