Identifying Band Structure Changes of FePS across the Antiferromagnetic Phase Transition.

Magnetic 2D materials enable interesting tuning options of magnetism. As an example, the van der Waals material FePS, a zig-zag-type intralayer antiferromagnet, exhibits very strong magnetoelastic coupling due to the different bond lengths along different ferromagnetic and antiferromagnetic coupling directions enabling elastic tuning of magnetic properties. The likely cause of the length change is the intricate competition between direct exchange of the Fe atoms and superexchange via the S and P atoms.

Influence of temperature and crack-tip speed on crack propagation in elastic solids.

I study the influence of temperature and the crack-tip velocity of bond breaking at the crack tip in rubber-like materials. Bond breaking is considered as a stress-aided thermally activated process and results in an effective crack propagation energy, which increases strongly with decreasing temperature or increasing crack-tip speed. This effect is particularly important for adhesive (interfacial) crack propagation but less important for cohesive (bulk) crack propagation owing to the much larger bond-breaking energies in the latter case.

Dry and lubricated sliding friction for rubber on concrete: the role of surface energies.

We study the influence of lubricant fluids (water-glycerol mixtures) on rubber sliding friction for two different rubber tread compounds on a concrete surface. We find that for the lubricated contacts the sliding friction below a critical velocity is similar to that of the dry contact, but for > the friction drops fast with increasing sliding speed. We discuss the origin of this effect and show that it is not a "normal" mixed lubrication effect but depends on surface (or interfacial) energies.

Role of strain softening and viscoelastic memory for the rolling friction of two tire tread compounds.

Rolling friction is of great importance for many applications, such as tires and conveyor belts. We study the rolling friction for hard cylinders rolling on flat rubber sheets. The rolling friction depends on the number of rolling cycles, the rolling speed, and the temperature.

Ice breakloose friction.

We discuss the origin of the breakloose (or static) friction force when an ice block is slid on a hard randomly rough substrate surface. If the substrate has roughness with small enough amplitude (of order a 1 nm or less), the breakloose force may be due to interfacial slip and is determined by the elastic energy per unit area, Uel/A0, stored at the interface after the block has been displaced a short distance from its original position. The theory assumes complete contact between the solids at the interface and that there is no elastic deformation energy at the interface in the original state before the application of the tangential force.

Sliding friction on ice.

We study the friction when rectangular blocks made from rubber, polyethylene, and silica glass are sliding on ice surfaces at different temperatures ranging from -40 to 0 °C, and sliding speeds ranging from 3 μm/s to 1 cm s-1. We consider a winter tire rubber compound both in the form of a compact block and as a foam with ∼10% void volume. We find that both rubber compounds exhibit a similar friction on ice for all studied temperatures.

Migration Kinetics of Surface Ions in Oxygen-Deficient Perovskite During Topotactic Transitions.

Oxygen diffusivity and surface exchange kinetics underpin the ionic, electronic, and catalytic functionalities of complex multivalent oxides. Towards understanding and controlling the kinetics of oxygen transport in emerging technologies, it is highly desirable to reveal the underlying lattice dynamics and ionic activities related to oxygen variation. In this study, the evolution of oxygen content is identified in real-time during the progress of a topotactic phase transition in La Sr MnO epitaxial thin films, both at the surface and throughout the bulk.

Self-reduction of the native TiO (110) surface during cooling after thermal annealing - in-operando investigations.

We investigate the thermal reduction of TiO in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions.

Local surface conductivity of transition metal oxides mapped with true atomic resolution.

The introduction of transition metal oxides for building nanodevices in information technology promises to overcome the scaling limits of conventional semiconductors and to reduce global power consumption significantly. However, oxide surfaces can exhibit heterogeneity on the nanoscale e.g.

Quasi-One-Dimensional Metal-Insulator Transitions in Compound Semiconductor Surfaces.

Existing examples of Peierls-type 1D systems on surfaces involve depositing metallic overlayers on semiconducting substrates, in particular, at step edges. Here we propose a new class of Peierls system on the (101[over ¯]0) surface of metal-anion wurtzite semiconductors. When the anions are bonded to hydrogen or lithium atoms, we obtain rows of threefold coordinated metal atoms that act as one-atom-wide metallic structures.

Graphene-multiferroic interfaces for spintronics applications.

Graphene and magnetoelectric multiferroics are promising materials for spintronic devices with high performance and low energy consumption. A very long spin diffusion length and high carrier mobility make graphene attractive for spintronics. The coupling between ferroelectricity and magnetism, which characterises magnetoelectrics, opens the way towards unique device architectures.

Realization of a vertical topological p-n junction in epitaxial Sb2Te3/Bi2Te3 heterostructures.

Three-dimensional (3D) topological insulators are a new state of quantum matter, which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p-n junction made of a heterostructure of two different binary 3D TI materials Bi2Te3 and Sb2Te3 epitaxially grown on Si(111).