We explore the influence of particle softness and internal structure on both the bulk andinterfacial rheological properties of colloidal suspensions. We probe bulk stresses byconventional rheology, by measuring the flow curves, shear stress versus strain rate, forsuspensions of soft, deformable microgel particles and suspensions of near hard-sphere-likesilica particles. A similar behaviour is seen for both kinds of particles in suspensions atconcentrations up to the random close packing volume fraction, in agreement with recenttheoretical predictions for sub-micron colloids. Transient interfacial stresses are measured byanalyzing the patterns formed by the interface between the suspensions and their solvent, dueto a generalized Saffman–Taylor hydrodynamic instability. At odds with the bulk behaviour,we find that microgels and hard particle suspensions exhibit vastly different interfacial stressproperties. We propose that this surprising behaviour results mainly from the difference inparticle internal structure (polymeric network for microgels versus compact solid for the silicaparticles), rather than softness alone.

Understanding and controlling growth of graphene on the carbon face (C-face) of SiC presents a significant challenge. In this work, we study the structural, vibrational, and dielectric function properties of graphene grown on the C-face of 4H-SiC by high-temperature sublimation in an argon atmosphere. The effect of growth temperature on the graphene number of layers and crystallite size is investigated and discussed in relation to graphene coverage and thickness homogeneity. An amorphous carbon layer at the interface between SiC and the graphene is identified, and its evolution with growth temperature is established. Atomic force microscopy, micro-Raman scattering spectroscopy, spectroscopic ellipsometry, and high-resolution cross-sectional transmission electron microscopy are combined to determine and correlate thickness, stacking order, dielectric function, and interface properties of graphene. The role of surface defects and growth temperature on the graphene growth mechanism and stacking is discussed, and a conclusion about the critical factors to achieve decoupled graphene layers is drawn.

The 13th edition of the International Summer School on Fundamental Problems in Statistical Physics took place from June 16 to 29, 2013 for the third consecutive time in “European Center La Foresta” in Leuven, Belgium. This special issue of Physica A publishes the proceedings of the school.

Hyper-Raman scattering is a non-linear inelastic spectroscopy sensitive to polar excitations of solids whatever the crystalline symmetry. This selection rule is very attractive in particular for centrosymmetric structures for which polar modes are inactive in Raman. HRS has been performed in the cubic relaxors PbMg1/3Nb2/3O3 (PMN) and PbMg1/3Ta2/3O3 (PMT), with particular attention to the low frequency region, down to about 2 cm-1. Spectra have been recorded over a wide temperature range (900K-30K in PMN) and the results compared to existing infrared and neutron scattering data. This complete set of vibrational information provides the framework for a detailed analysis of the polarization dynamics in these systems. In particular, in both materials, the soft mode response exhibits a doublet structure up to the highest temperature investigated [1,2] emphasizing therefore a common property of cubic relaxors. The lowest frequency component is assigned to the primary soft mode of symmetry F1u, while the second likely originates from a local disorder persisting until very high temperatures, and which lifts the cubic selection rules. On cooling, the soft F1u-mode of PMN becomes overdamped at the onset of the Burns temperature but very interestingly, it splits between 600K and 400K into two components which harden on decreasing further the temperature. This behavior likely highlights the onset of a local anisotropy of the polarization and provides therefore new insight about the relaxor nature.

Ferroelectric nano-objects display a rich variety of shapes, among them nanocubes (bulk or hollow),nanospheres, nanotori etc with fascinating morphogenesis mechanisms1. Structural and physicalconsequences of ferroelectric downsizing to form nanopowders or nanoceramics are very important bothfrom applicative and fundamental point of view (see ref. 2 for a good review). Indeed many interestingeffects have already been reported such as diminishing of permittivity and increase of diffusivity2,3,cancelation of dielectric relaxation in relaxors3,4, lowering of ground states, increase and rotation ofpolarization in relaxors and morphotropic compounds3,5 etc. In order to explain these experimental results,one must take into account not only size and microstrains effects but also existence of core-shellstructure2,5-8. Modeling also shows that there is optimal nanostructurations to maximize energy storage6 andelectrocaloric effect8. Calculations also show the possibility to form new phases in which vortices ofpolarization compete with homogeneous polarization8 or in which homogeneous hypertoroidization ofnanotori coexists with axial homogeneous toroidization10. After a survey of these effects, we will give newexperimental results observed in SrTiO3 showing at small size decoupling of oxygen octahedra tiltings anddistortion, strong shift of ferroelastic critical temperature11,12 and lowering of the tetragonal structuretowards orthorhombic ground state. Possible connection with a core-shell structure will also be discussed.