We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed λ. For such models to be viable, the dark sector should contain an additional component responsible for the late-time accelerated expansion rate (X). We explore the departure of the expansion history of these models from the concordanceΛ cold dark matter (ΛCDM) model. For a large class of our models, the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with wX>−1 will eventually contract and we derive an analytical expression for the scale factor a(t) in the neighborhood of its maximal value. We find also the scale factor for models ending in a Big Rip in the regime where dustlike matter density is negligible compared to λ. We address further the viability of such models, in particular when a high H0 is taken into account. While we find no decisive evidence for a nonzero λ, the best models are obtained with a phantom behavior on redshifts z≳1 with a higher evidence for nonzero λ. An observed value for h substantially higher than 0.70 would be a decisive test of their viability.

When a microparticle is trapped at a fluid interface, particle's electrical charge and weight combine to deform the interface. Such deformation is expected to affect the particle diffusion via hydrodynamics boundary conditions. Using available models of particle-induced electrostatic deformation of the interface and particle dynamics at the interface, we are able to analytically predict particle diffusion coefficient values in a large range of particle's contact angle and size. This might offer a solid background of numerical values to compare with for future experimental studies in the field of particle diffusion at a fluid interface.

Supramolecular associations provide a promising route to functional materials with properties such as self-healing, easy recyclability or extraordinary mechanical strength and toughness. The latter benefit especially from the transient character of the formed network, which enables dissipation of energy as well as regeneration of the internal structures. However, recent investigations revealed intrinsic limitations in the achievable mechanical enhancement. This manuscript presents studies of a set of telechelic polymers with hydrogen-bonding chain ends exhibiting an extraordinarily high, almost glass-like, rubbery plateau. This is ascribed to the segregation of the associative ends into clusters and formation of an interfacial layer surrounding these clusters. An approach adopted from the field of polymer nanocomposites provides a quantitative description of the data and reveals the strongly altered mechanical properties of the polymer in the interfacial layer. These results demonstrate how employing phase separating dynamic bonds can lead to the creation of high-performance materials.

Antiferromagnetic materials are promising platforms for next-generation spintronics owing to their fast dynamics and high robustness against parasitic magnetic fields. However, nanoscale imaging of the magnetic order in such materials with zero net magnetization remains a major experimental challenge. Here we show that non-collinear antiferromagnetic spin textures can be imaged by probing the magnetic noise they locally produce via thermal populations of magnons. To this end, we perform nanoscale, all-optical relaxometry with a scanning quantum sensor based on a single nitrogen-vacancy (NV) defect in diamond. Magnetic noise is detected through an increase of the spin relaxation rate of the NV defect, which results in an overall reduction of its photoluminescence signal under continuous laser illumination. As a proof-of-concept, the efficiency of the method is demonstrated by imaging various spin textures in synthetic antiferromagnets, including domain walls, spin spirals and antiferromagnetic skyrmions. This imaging procedure could be extended to a large class of intrinsic antiferromagnets and opens up new opportunities for studying the physics of localized spin wave modes for magnonics.

We perform a detailed analysis of electronic polarizability of graphene with different theoretical approaches.From Kubo’s linear response formalism, we give a general expression of frequency and wave-vector dependentpolarizability within the random phase approximation. Four theoretical approaches have been applied to thesingle-layer graphene and their differences are on the band overlap of wave functions. By comparing with theab initio calculation, we discuss the validity of methods used in literature. Our results show that the tightbinding method is as good as the time-demanding ab initio approach in calculating the polarizability of graphene.Moreover, due to the special Dirac-cone band structure of graphene, the Dirac model reproduces results of thetight-binding method for energy smaller than 3 eV. For doped graphene, the intraband transitions dominateat low energies and can be described by the Lindhard formula for two-dimensional electron gases. At zerotemperature and long-wavelength limit, with the relaxation time approximation, all theoretical methods reduceto a long-wave analytical formula and the intraband contributions agree to the Drude polarizability of graphene.Effects of electrical doping and temperature are also discussed. This work may provide a solid reference forresearches and applications of the screening effect of graphene.

In order to analyze why the CIGS (CuInGaSe2) - based solar cells efficiency decrease for wide band gap (high Ga content), we have performed a series of samples by PVD using the three stage process. The average compositions of our solar cells range from x = 0 to x = 0.88, as measured by X-ray diffraction. An important feature of this process is to create a double Ga gradient into the absorber, which contributes to improve efficiencies, and this has a major impact on the determination of the sample composition from x-ray diffraction data. We have developed a model in order to assess this impact and question the validity of the compositions extracted from x-ray data. This model allows to get some information about gradient shapes. Using our model, we have obtained some insights on the evolution of the gallium gradient in samples with increasing Ga content, and we determine that this gradient is less pronounced when increasing the amount of gallium. It is a well known fact that this gradient assists the extraction of photocreated carriers, and the modifications of the gallium profile that we have determined may explain, for some part, the degradation of solar cells efficiency for high gallium compositions.

We propose a novel model for a glass-forming liquid which allows to switch in a continuous manner froma standard three-dimensional liquid to a fully connected mean-field model. This is achieved by introducingk additional particle-particle interactions which thus augments the effective number of neighbors of eachparticle. Our computer simulations of this system show that the structure of the liquid does not changewith the introduction of these pseudo neighbours and by means of analytical calculations, we determine thestructural properties related to these additional neighbors. We show that the relaxation dynamics of thesystem slows down very quickly with increasing k and that the onset and the mode-coupling temperaturesincrease. The systems with high values of k follow the MCT power law behaviour for a larger temperaturerange compared to the ones with lower values of k. The dynamic susceptibility indicates that the dynamicheterogeneity decreases with increasing k whereas the non-Gaussian parameter is independent of it. Thus weconclude that with the increase in the number of pseudo neighbours the system becomes more mean-field like.By comparing our results with previous studies on mean-field like system we come to the conclusion that thedetails of how the mean-field limit is approached are important since they can lead to different dynamicalbehavior in this limit.