Letters in 2010

Regardless of what the origin of superconductivity is in the recently discovered iron-based superconductor, it would be useful to know how good these materials are for applications. Sophisticated experiments now show that SmFeAs_0.75F_0.25 exhibits a high and nearly isotropic critical current, a potentially important result for their use in applications.

An important component of spintronics devices is the magnetic electrode, which is usually made from an inorganic alloy. However, an organic-based spin polarizer is now demonstrated, opening new possibilities for developing organic/inorganic hybrid spintronics devices.

The simplest iron-based superconductor is the chalcogenide Fe_1+yTe_1−xSe_x. Previous work suggested a different magnetic origin of superconductivity owing to differences in its electronic states of this material and the iron pnictides, or at least in their parent compounds —the undoped and non-superconducting versions. The differences are now reconciled by showing a modification of the Fe_1+yTe_1−xSe_x states when the Se content is increased.

Materials with perpendicular anisotropy receive considerable attention owing to their potential in being employed in efficient memory devices. It is now shown that a type of magnetic tunnel junction widely studied for in-plane magnetic anisotropy has all the properties necessary to realize stable and efficient devices based on perpendicular magnetic anisotropy.

Fibres are typically used as passive devices, whether in fibre-optical cables used in telecommunciations or as yarns for clothing. The demonstration of polymer-based piezoelectric fibres that can be drawn to tens of metres in length, and whose acoustic response can be actively controlled, suggests possible applications in, for example, medical imaging or acoustic sensing.

What happens to a crystal placed under a huge pressure? In the case of aluminium, it is now shown that the standard, low-pressure close-packed structure transforms into an open one, with incommensurate host–guest arrangement. The findings could have important implications for a wider range of elements.

Mucosal diseases are becoming more prevalent and needle-free vaccines could be instrumental in combating this. A nanometre-sized hydrogel consisting of a cationic type of cholesteryl group bearing pullulan has now been used as an intranasal vaccine-delivery system.

The search for active semiconductor photocatalysts that split water directly under visible-light irradiation remains challenging for solar applications. An orthophosphate semiconductor, Ag₃PO₄, which is capable of harnessing visible light to oxidize water as well as decompose organic contaminants in aqueous solution is now reported.

Topological insulators have been predicted and recently demonstrated experimentally in a series of binary alloys. It is now show theoretically that ternary half-Heusler alloys have electronic properties similar to those of the experimentally verified topological insulators, and represent a platform for observing quantum topological phenomena.

For metal–organic frameworks to be used for applications such as gas storage it is necessary to direct their assembly. Here, thin crystalline films of metal–organic frameworks are fabricated on a solid surface with structural growth control over both in-plane and out-of-plane orientations relative to the substrate.

Topological insulators have been predicted and recently demonstrated experimentally in a series of binary alloys. It is now show theoretically that about 50 Heusler compounds show features similar to those of the confirmed topological insulator HgTe, which considerably expands the possibility of realizing quantum topological phenomena.

When a superconductor is shrunk to the nanoscale, quantum size effects are predicted to strongly influence superconductivity. This is now demonstrated in Sn nanoparticles in which a reduction in size leads to a substantial enhancement of the superconducting gap.

Bulk metallic glasses (BMGs) show good compressive mechanical properties that make them attractive for applications. However, BMGs tend to fail under tensile strain. Through secondary phases these problems can be remedied to some degree. A mechanism is now demonstrated where BMGs show enhanced tensile ductility though the deformation-induced precipitation of nanocrystals.

A biomedical application of a nanoconjugate is now shown in vivo. Sealed carbon nanotubes filled with a radionuclide are functionalized with carbohydrate molecules without prompting cargo release. The stability and biocompatibility of the capsule together with the radioactive payload enables in vivo imaging of the system and delivery of a high-density radiodose.

The ability to propagate heat in a film should improve with increasing thickness. However, graphene has a higher thermal conductivity than graphite, despite having a smaller thickness. The crossover from two-dimensional to bulk graphite is now studied experimentally and explained theoretically. The results may pave the way to thermal management applications in nanoelectronics.

Resistive nanowire arrays are intensively pursued as easy-to-fabricate memory technology, where data can be written and read through simple voltage–current operations. However, problems encountered in achieving stable switching independent of external influences has hampered their progress. The complementary, antiserial arrangement of two memory elements is now shown to lead to the desired stability.

So far, the realization of negative-refractive-index materials has required the use of resonating metallic structures, leading to an inherently narrowband operation around those resonances. Here, negative-refractive-index materials are proposed that consist of single coaxial waveguide layers, with a negative refractive index at a broad range of visible wavelengths.

Quantum transport phenomena have been widely investigated in semiconducting compounds, but extending these studies to oxides is not simple owing to their low mobilities. It is now demonstrated that SrTiO₃ films can be grown by molecular beam epitaxy; the films show very high electron mobility, opening the way to oxide heterostructures with excellent transport properties.

Solid-state materials showing giant caloric effects near room temperature could provide an alternative to cooling devices based on gas cycles. Strong emphasis has so far been dedicated to caloric effects induced by a magnetic field. It is now demonstrated that a small pressure applied to the compound Ni—Mn–In gives rise to a giant caloric response.

Approaches for controlling surface wettability and liquid spreading are numerous and diverse, but introducing directionality to the control of these phenomena is far from trivial. Nanostructured surfaces are now used to allow the propagation of a liquid in a single direction, while constraining it in the other three directions.