Research articles

The observation of a superconductive current flowing through a topological insulator is considered the first step towards the observation of the elusive Majorana fermions. This is now achieved in a superconductor/topological insulator/superconductor junction in which direct evidence of Josephson supercurrents is reported.

Although (Ga,Mn)As is considered the model ferromagnetic semiconductor, the electronic structure of the charges — holes in this case — and its connection with the Curie temperature (T_C) are still unclear. Experiments now provide a direct link between T_C and the existence of an impurity band for the holes. Clarifying this issue is essential to designing other materials with potentially higher T_C.

In most unconventional superconductors, the superconducting phase is adjacent to a phase with some type of magnetic order. However, this is not a universal feature. For example, no magnetic order has so far been observed in Sr₂RuO₄. Now, low-energy muon relaxation experiments show the presence of a static magnetic order for this material, suggesting that this feature may in fact be universal.

The development of reliable diagnostic tools to investigate the performance of a battery in situ is required at present. Techniques based on magnetic resonance imaging are now shown to be able to non-invasively visualize and characterize the changes occurring in Li-ion battery electrodes and electrolyte.

Mesoporous colloidal gels with solid-like viscoelasticity formed from oil-in-water nanoemulsions are reported. Gelation is thermoreversible and occurs through interdroplet bridging of an end-functionalized oligomer. The gels can be photocrosslinked to encapsulate lipophilic biomolecules for their subsequent release through ultraviolet photolysis.

It is well known that to reduce dissipation in a superconductor it is necessary to introduce artificial pinning centres, that is, small regions in which superconductivity is suppressed. This is usually achieved by introducing small regions of non-superconducting phases. A new concept of pinning centres, the local suppression of superconductivity induced by strain, is now demonstrated.

Raman spectroscopy has already proved to be a powerful tool for studying the properties of single graphene layers. It is now shown that this technique can also provide information on the interaction between graphene sheets in multilayered graphene structures. In particular, a Raman peak corresponding to the interlayer shear mode, and probably linked to the interlayer coupling, is unveiled.

The realization of ultrastable, nanostructured glassy polymer films by pulsed-laser evaporation is reported. Compared with standard poly(methyl methacrylate) glass, these polymer glasses are 40% less dense and have a 40-degree-higher glass transition temperature. Their unique properties, which are a manifestation of their globular nanostructure, should make these glasses attractive for applications where weight and stability are critical.

Flexible electronics and other nanoscale devices require simple yet reliable assembly procedures. An optical welding technique for metal nanowires, based on surface plasmon resonances, is now used to fabricate interconnected nanowire networks with enhanced electrical properties for use as transparent electrodes in solar cells and other electrical devices.

The large-scale synthesis of single-walled carbon nanotubes (SWCNTs) with controlled chirality—which could find applications in fields such as electronics—remains a great challenge. It is now shown that the growth rates of SWCNTs are directly proportional to their chiral angles, suggesting a route towards selective synthesis based on kinetic control.

Lumped elements such as resistors, capacitors and inductors play a crucial role in electronic circuits. Now, inspired by metamaterials technology, the experimental realization of lumped circuit elements for optical frequencies provides a standardized platform for applications such as mixing and multiplexing of optical signals.

Interfaces between insulating oxides have revealed exotic electronic and magnetic properties. It is now shown that a complex magnetic structure can emerge in an oxide superlattice, and that specific interfaces can unexpectedly exhibit exchange bias. The observations reveal the induction of antiferromagnetism in a material that is usually paramagnetic.

It is demonstrated that graphene coatings do not alter the wetting behaviour of copper, gold or silicon surfaces. Such wetting transparency—shown to occur only for surfaces where surface–water interactions are dominated by van der Waals forces—and graphene’s ability to suppress copper oxidation result in a 30–40% increase in condensation heat transfer on copper. The findings have implications for graphene-based coatings with independently tunable electronic and wetting properties.

Mast cells act to enhance immune responses through the release of insoluble granules that contain inflammatory mediators. Now, submicrometre polymer particles are shown to replicate and enhance the functions of mast-cell granules in vivo, such as the targeting of draining lymph nodes and the timed release of encapsulated mediators. The particles can also polarize the resulting immune response.

Among other exotic properties graphene exhibits the highest thermal conductivity observed so far. This is true at least for graphene composed of only ¹²C atoms. However, it is now shown experimentally that regions of ¹³C atoms can substantially reduce the thermal conductivity. Aside from their fundamental importance, these results suggest that thermal conductivity can be tailored by varying the relative amounts of carbon isotopes used.

The growth of microcrystals can be controlled by various agents such as ions, small charged molecules and polyelectrolytes. However, their use is specific to the crystallizing material. It is now shown that oppositely charged nanoparticles can act as ‘universal’ surfactants for controlling the growth and stability of microcrystals of inorganic salts and of charged organic molecules.

A quantum critical point occurs when different stable phases of matter are in equilibrium at absolute zero temperature. Describing quantum criticality with a theoretical framework that unifies different types of transitions is highly desirable to understand how phenomena such as superconductivity and magnetism interact in correlated electron systems. A study now provides an indication of an underlying universality of quantum criticality, and highlights the role of dimensionality in such a unified theory.

Dissolution processes affect the performance of oxides in applications ranging from power generation to catalysis. A study on polyoxometalate ions, which are thought to model oxide surfaces, now suggests that dissolution is controlled by the stability of transient oxygen-stuffed structures.

In the quest for more efficient thermoelectrics, a common strategy has been to introduce nanostructures in bulk crystals, thus reducing the thermal conductivity without affecting the electrical transport properties. A route is now presented in which the aggregation of nanoplatelets creates nanostructured materials that have higher thermoelectric efficiencies than their bulk counterparts.