Letters in 2011

Toothpaste, mayonnaise and other systems are soft particle glasses. In these, the soft particles are jammed so that the glasses behave like weak solids at rest but at sufficient stress flow like liquids. This has made their theoretical understanding difficult. A new micromechanical model is now able to predict the rheology of these soft particle glasses.

Self-assembled monolayers of thiols have applications ranging from surface coatings to nanomechanical sensors, where they transmit analyte-induced stress to a cantilever detector. For gold nanocrystals it is now shown that the adsorption of propanethiol alone can induce large chemical stress, with different directionality on curved and flat surfaces.

Iron-based superconductors all share the same building blocks. So why do local magnetic properties vary from one compound to another? A new theoretical model explains the variation in physical properties and links it to the structural differences, providing a description for a wide range of materials.

The use of flexible polymer substrates not only reduces weight and fabrication costs of solar cells, but their bendability also enables new applications. A careful design of Cu(In,Ga)Se₂ solar cells grown on polymer substrates now solves earlier fabrication issues, leading to conversion efficiencies matching those grown on rigid substrates.

Crystallization of a liquid usually starts at a solid surface — for instance, that of impurities or of a container's walls — and surface roughness is known to enhance crystal nucleation rates. It is now shown with polymer films patterned with spherical nanopores 15–120 nm in size that the shape of the pores can either enhance or hinder crystal nucleation.

Josephson junctions have been intensely studied from a fundamental and technological point of view. It is now shown how by using ferromagnetic insulators for the barrier it is possible to strongly affect the superconducting current and in particular its magnetic and spin properties.

The fabrication of composite microfibres with tunable topography and chemical composition is now possible with a microfluidic method that mimics the fibre-spinning process of spiders. The method allows for the synthesis of a variety of structurally and spatially coded fibres for multiple applications, such as directional water harvesting and the co-culture of encapsulated cells.

The interaction between electron and nuclear spins in quantum dots is often seen as detrimental for the use of electron spin for quantum information processing. It is now shown, however, that such interaction can be used to coherently control the polarization of tens of thousands of nuclear spins, opening the way to experiments using nuclear rather than electron spin.

In vitro experiments of a biomaterial's degradability rarely predict its in vivo behaviour. It is now shown that tracking the hydrolytic and enzymatic erosion of model materials by non-invasive fluorescence imaging allows the prediction of in vivo erosion from in vitro data. The approach should enable rapid screening of erodable biomaterials.

The coupling between electron spins and phonons could lead to a new typology of electronic devices. The effects of such coupling are now experimentally demonstrated by injecting sound waves into a magnetic strip. The results also help to explain the origin of the spin Seebeck effect, which has been controversial for a while.

The arrangement of defects in solid-state phases has an enormous influence on material properties. It is here shown that powerful X-rays can be used to change the properties of an oxide superconductor, thus effectively writing superconducting regions within an insulating matrix. The results open the way to the manipulation of superconductors and potentially other phases.

A suspension of magnetic colloidal particles confined at a liquid–liquid interface and energized by an external periodic magnetic field self-assembles into star-shaped structures that can be magnetically manipulated to capture and transport smaller non-magnetic particles.

Single-walled carbon nanotubes are shown to template the self-assembly of graphene nanoribbons inside the tubes on electron-beam irradiation of a random mixture of precursors containing C and S atoms. The nanoribbons, which adopt a helical shape, are stabilized by sulphur-terminated dangling bonds.

Superparamagnetic nanoparticles under an external magnetic field align in the field’s direction to minimize magnetic-dipole interactions. By modulating and fixing the alignment of magnetic nanoparticles in polymeric microcomponents through photopolymerization, magnetic nanocomposite microactuators were programmed to undergo complex motion, such as anisotropic bending and crawling.

Solid electrolytes can improve the safety of the next generation of high-energy batteries, but they still suffer from low ionic conductivities and stability. Li₁₀GeP₂S₁₂ exhibits high lithium ionic conductivity at room temperature and should be practically advantageous with regard to device fabrication, stability and safety.

Three-dimensional photonic devices are of interest as light emitters, detectors or waveguides. However, so far their fabrication has remained a challenge. The template-directed epitaxy of three-dimensional semiconductor structures now offers a new strategy for the realization of photonic devices, demonstrated by the realization of a three-dimensional photonic crystal light-emitting diode.

Acoustic rectifiers are of relevance for applications such as biomedical ultrasound imaging. In these systems, amplification increases gradually with signal amplitude. A new approach based on bifurcation in chaotic systems now enables a sharp switching between rectification states that could be used in nonlinear acoustic devices.

The combined magnetic and thermoelectric properties of nanostructures have recently attracted considerable attention. It is now demonstrated that the Seebeck coefficient in a magnetic tunnelling junction is strongly dependent on the magnetic configuration.

Graphite remains stable at pressures higher than those of its equilibrium coexistence with diamond. This has proved hard to explain, owing to the difficulty in simulating the transition with accuracy. Ab initio calculations using a trained neural-network potential now show that the stability of graphite and the direct transformation of graphite to diamond can be accounted for by a nucleation mechanism.

The energy and power density of lithium-ion batteries depends to a large extent on storing lithium by incorporation in the crystal structure of the cathode. The reason that LiFePo₄ functions as a cathode at a reasonable rate is now explained theoretically by the availability of a single phase-transformation path at low overpotential.