Interfacial tension between immiscible phases is a well-known phenomenon, which manifests itself in everyday life, from the shape of droplets and foam bubbles to the capillary rise of sap in plants or the locomotion of insects on a water surface. More than a century ago, Korteweg generalized this notion by arguing that stresses at the interface between two miscible fluids act transiently as an effective, nonequilibrium interfacial tension, before homogenization is eventually reached. In spite of its relevance in fields as diverse as geosciences, polymer physics, multiphase flows, and fluid removal, experiments and theoretical works on the interfacial tension of miscible systems are still scarce, and mostly restricted to molecular fluids. This leaves crucial questions unanswered, concerning the very existence of the effective interfacial tension, its stabilizing or destabilizing character, and its dependence on the fluid's composition and concentration gradients. We present an extensive set of measurements on miscible complex fluids that demonstrate the existence and the stabilizing character of the effective interfacial tension, unveil new regimes beyond Korteweg's predictions, and quantify its dependence on the nature of the fluids and the composition gradient at the interface. We introduce a simple yet general model that rationalizes nonequilibrium interfacial stresses to arbitrary mixtures, beyond Korteweg's small gradient regime, and show that the model captures remarkably well both our new measurements and literature data on molecular and polymer fluids. Finally, we briefly discuss the relevance of our model to a variety of interface-driven problems, from phase separation to fracture, which are not adequately captured by current approaches based on the assumption of small gradients.

The optical properties of homoepitaxial non-polar (Zn,Mg)O/ZnO single quantum wells (QWs) grown by molecular beam epitaxy on a-plane (11-20) ZnO substrates are investigated by using reflectance and continuous wave photoluminescence spectroscopies. The reflectivity and CW-PL spectra measured at low temperature reveal strong in-plane optical anisotropies and clear reflectance structures. The signatures of confined excitons analogous to C-exciton and (A, B)-excitons in bulk ZnO, are detected using light polarized respectively along the c-axis and perpendicular to this axis. Temperature dependence of CW-PL has been investigated in the two polarizations. For electric field of the light perpendicular to the c-axis the total intensity of the PL lines decreases by about an order of magnitude when the temperature increases from 8 K to room temperature. At low temperature a line associated to an excitonic complex dominates the PL spectrum. When the temperature increases the intensity of this line decreases as the intensity of the line associated to the free exciton increases. In the other polarization for temperature above 100 K a new line appears in the spectrum. Its intensity increases with temperature. This feature is associated to the c-like free exciton. The degree of polarization of these lines is independent of temperature.

Superconducting hybrid junctions are revealing a variety of effects. Some of them are due to the special layout of these devices, which often use a coplanar configuration with relatively large barrier channels and the possibility of hosting Pearl vortices. A Josephson junction with a quasi-ideal two-dimensional barrier has been realized by growing graphene on SiC with Al electrodes. Chemical vapor deposition offers centimeter size monolayer areas where it is possible to realize a comparative analysis of different devices with nominally the same barrier. In samples with a graphene gap below 400 nm, we have found evidence of Josephson coherence in the presence of an incipient Berezinskii-Kosterlitz-Thouless transition. When the magnetic field is cycled, a remarkable hysteretic collapse and revival of the Josephson supercurrent occurs. Similar hysteresis are found in granular systems and are usually justified within the Bean critical state model (CSM). We show that the CSM, with appropriate account for the low-dimensional geometry, can partly explain the odd features measured in these junctions.

We report on the stability of the quantum Hall plateau in wide Hall bars made from a chemically gated graphene film grown on SiC. The ν=2 quantized plateau appears from fields B≃5  T and persists up to B≃80  T. At high current density, in the breakdown regime, the longitudinal resistance oscillates with a 1/B periodicity and an anomalous phase, which we relate to the presence of additional electron reservoirs. The high field experimental data suggest that these reservoirs induce a continuous increase of the carrier density up to the highest available magnetic field, thus enlarging the quantum plateaus. These in-plane inhomogeneities, in the form of high carrier density graphene pockets, modulate the quantum Hall effect breakdown and decrease the breakdown current.

Chemical vapor deposition has proved to be successful in producing graphene samples on silicon carbide (SiC) homogeneous at the centimeter scale in terms of Hall conductance quantization. Here, we report on the realization of co-planar diffusive Al/ monolayer graphene/ Al junctions on the same graphene sheet, with separations between the electrodes down to 200 nm. Robust Josephson coupling has been measured for separations not larger than 300 nm. Transport properties are reproducible on different junctions and indicate that graphene on SiC substrates is a concrete candidate to provide scalability of hybrid Josephson graphene/superconductor devices.

Electrical transport (resistivity and Hall Effect) have been studied in silicon doped aluminum nitride (AlN) thick epitaxial layers from 250 K up to 1000 K. The investigated samples, grown by molecular beam epitaxy were characterized by n-type conduction with an ambient temperature free carrier concentration of about ~ 1 x 10^15 cm^3. The donor level, situated about 250 meV below the conduction band edge, was found to be responsible for the experimentally observed increase of free carrier concentration withtemperature. The temperature dependence of carrier mobility has been analyzed in the framework of a multimode scattering model. In the investigated samples the main scattering mechanism is supposed to be dislocation scattering.

We derive exact analytical formulas for the radiative heat transfer between parallel slabs separated by vacuum and subject to arbitrary temperature profiles. We show that, depending on the derivatives of the temperature at points close to the slab-vacuum interfaces, the flux can exhibit one of several different asymptotic low-distance (d) behaviors, obeying either 1/d^2,1/d, or logarithmic power laws, or approaching a constant. Tailoring the temperature profile within the slabs could enable unprecedented tunability over heat exchange, leading for instance to sign-flip transitions (where the flux reverses sign) at tunable distances. Our results are relevant to the theoretical description of on-going experiments measuring near-field heat transfer at nanometric distances, where the coupling between radiative and conductive transfer could result in temperature gradients.