Monolayer transition metal dichalcogenides (TMDs) have recently emerged as a new material system in which coupled spin-valley physics can be explored. In this talk I will discuss the optical and magneto optical properties of excitons in mono and tri – layer transitions metal dichalcogenides (TMDs).
First, I will show that optical spectroscopy in high magnetic fields B < 70 T reveals the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In mechanically exfoliated monolayer WSe2, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe2 where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model. Additionally, in the monolayer material valley polarization is induced by magnetic field for a charged exciton demonstrating it’s inter valley character [1]. Furthermore I will compare the optical and mechanical properties of the exfoliated and large area single layer grown by Chemical vapor deposition. The results of the magneto optical studies suggests, surprisingly, that the 0.2% tensile strain, which is naturally present in large area CVD grown Mo based TMDs on sapphire, does not represent any serious impediment for device applications [2]. Finally, I will show our very recent results of the optical properties of the trilayer structures such as MoS2/MoSe2/MoS2 obtained by a combination of CVD growth and mechanical transfer. First, our results demonstrate a new, simple, way to encapsulate monolayer TMDS toward better optical/electrical properties. The optical properties of MoSe2 improve dramatically after embedding it into MoS2. Furthermore, while optically pumping in MoS2, the charge transfer between the layers is observed in tri layer structures leading to valley polarization of MoSe2. These results demonstrate that the spin is preserved during the charge transfer [3]. In the end, the measurements of the valley polarization of MoSe2 obtained for resonant excitation as close as 10 meV suggest that inter valley scattering is dominated by exchange interaction rather than by phonons. The experimental results are further supported by a simple model [4].


[1] A. Mitioglu, PP et al, Nano Letters 15, 4387 (2015)
[2] A. Mitioglu, PP et al, PRB 93, 165412 (2016)
[3] A. Surrente, PP et al under preparation
[4] M. Baranowski, PP et al submitted

Photonic crystals have reshaped light-based technologies during the last few decades. Remarkably, some years ago Raghu and Haldane discovered that the light propagation in photonic crystals may depend on some global topological properties determined by the manner how the light states are entangled in the spectral domain. The standard topological classification of photonic crystals relies on the fact that these structures have a periodic structure. The periodicity is of vital importance to guarantee that the underlying wave vector space is a closed surface with no boundary.
In this talk, I will show that it is possible to calculate topological invariants of a wide class of bianisotropic nonreciprocal electromagnetic continua with no intrinsic periodicity. The nontrivial topology of the relevant materials will be linked to the emergence of topological edge states immune to back reflections. Furthermore, I will discuss rather general conditions under which a photonic platform can support propagation channels insensitive to backscattering, showing that systems invariant under the action of a PTD operator-the composition of the parity, time-reversal, and duality transformations-may enable bidirectional waveguiding totally insensitive to deformations, analogous to the spin-Hall effect, but for light waves. Finally, time permitting, I will discuss the consequences of a topological transport of light in the context of thermally generated radiation in a closed cavity at a thermodynamic equilibrium.

Colour centres in large bandgap materials are leading candidates for a great variety of applications ranging from biophysics to quantum information. Their optical properties and ability to optically control their electronic and spin degrees of freedom is at the core of their potential for metrology. Therefore, understanding their optical properties is crucial for the successful implementation of these applications. In this talk, I will present an example of how the fluorescence stability of nitrogen-vacancy colour centres in diamond can be used for the fluorescent marking of a biological specimen, and also discuss on the large variety of emission spectra that a large bandgap material can host. In particular, we will discuss the cases of the nitrogen-vacancy and the silicon-vacancy colour centres in diamond and show how symmetry rules the interaction between the electronic degrees of freedom and phonons resulting in dramatically different emission spectra even though their atomic configuration differ just by one atom.

Biologists have recently come to appreciate that eukaryotic cells are home to a multiplicity of non-membrane bound compartments, many of which form and dissolve as needed for the cell to function. These dynamical “liquid droplets” enable many central cellular functions – from ribosome assembly, to RNA regulation and storage, to signaling and metabolism. While it is clear that these compartments represent a type of separated phase, what controls their formation, how specific biological components are included or excluded, and how these structures influence physiological and biochemical processes remain largely mysterious. I will discuss recent experiments on phase separated droplets both in vitro and in vivo, and will present theoretical results that highlight a novel “magic number” effect relevant to the formation and control of two-component phase separated liquid droplets.

Our group has been working for several years on fracture, or rather on crack propagation in soft materials. We developed several original methods and tools to characterize the damage taking place at the tip of a crack. In this seminar I will introduce the topic of fracture of soft materials and its important unsolved scientific questions, then present our picture of the structure of a typical crack from a materials point of view and from available characterization techniques such as optical microscopy and SEM, infrared microscopy, Small and Wide angle X-ray scattering and last but not least the incorporation of mechanoluminescent and fluorescent molecules in the materials.

Porous materials are at the heart of numerous important applications: adsorption (gas sensing, pollutants entrapment, drug delivery, chromatography), energy (porous materials are used for hydrogen storage, fuel cells and batteries), environment (phase separation, water treatment, nuclear waste storage), Earth science (the soil porosity ensures exchange between the soil and the atmosphere), etc. In this colloquium, I will present the state-of-the-art on adsorption, phase transition and transport in multiscale porous materials which possess pore sizes spanning several orders of magnitude (from the nm scale to the macroscale). I will first discuss the different adsorption regimes encountered when a fluid is set in contact with a porous material: from irreversible adsorption/capillary condensation for the largest pores to reversible/continuous filling for the smallest pores. We will see that simple thermodynamic modeling allows rationalizing these different regimes provided that a description of the shift of the critical point of the confined fluid is taken into account. Then, I will show how transport in such media can be described without having to rely on macroscopic concepts such as hydrodynamics (which is known to breakdown at the nm scale). Using parameters and coefficients available to experiments, we will see how transport coefficients can be rigorously obtained from simple models in the framework of statistical mechanics. While these results mostly pertain to basic science, they will be discussed in connection with important examples such as hydrocarbon recovery from shale gas plants.

Large-scale force generation is essential for biological functions such as cell motility, embryonic development, and muscle contraction. In these processes, active forces generated at the molecular level by motor proteins are transmitted by disordered fiber networks, resulting in large-scale contractile stresses. I will present a comprehensive theoretical study of force transmission in these networks. While the linear response to small forces is remarkably simple, taking into account the nonlinear properties of the filaments yields strikingly counter-intuitive effects such as the reversal of extensile forces into contractile ones. These forces are furthermore amplified as they induce buckling on large scales in the network, resulting in large tensile stresses at the macroscopic scale.  Our predictions are quantitatively consistent with experiments on reconstituted tissues and actomyosin networks, and shed light on the role of the network microstructure in shaping active stresses in cells and tissue.

The ubiquity of collective motions observed at all scales in biological systems has driven a surge of scientific activity. Within physics, important theoretical progress was achieved by studying microscopic point-particles models and their continuous descriptions. Among the landmark results are the possibility of a true long-range polar ordered collective motion as well as of a Motility Induced Phase Separation (MIPS). The robustness of these observations against the numerous factors integrated out in the above effective models is a matter of crucial importance.

This is where human-designed model experimental systems have a key role to play. Janus colloids, swimming droplets or walking grains are amazing experimental realization of self propelled particles. They are far more simple than their biological inspiration, and already contain important realistic factors, such as hydrodynamics effects and pairwise force interactions, which, at least in principle, can be controlled.

In the present talk, I will illustrate that matter in the case of two remarkable experimental systems, namely rolling colloids and walking grains.

The structure and dynamics of liquids at very high temperature have been little explored because they are too hot to handle, difficult to contain, and chemically too aggressive to warrant the survival of most examination tools. Accordingly, containerless levitation techniques have been developed for such investigations. I will review some of the levitation techniques and outline their exotic advantages: (1) creating a “naked” very hot liquid to be investigated by different probes, (2) undercooling liquids down to several hundred degrees below their equilibrium freezing point and (3) exploring the domains of two co-existing liquids. A special attention will be given to levitation techniques combined with neutron and x-ray techniques as well as experimental results obtained on various materials.