Letters in 2012

Although fundamentally intriguing, iron-based superconductors have not been seriously considered for applications because of the limited superconducting current that has so far been observed in wires made from these materials. It is now shown that by following a specific synthesis procedure it is possible to achieve superconducting currents that are close to commercial requirements.

Conventional sensors generate a signal that is directly proportional to the concentration of the target molecule. Now, by means of an enzyme that controls the growth of silver nanocrystals on plasmonic transducers, a nanosensor with sensitivity that is inversely proportional to concentration and that can detect ultralow concentrations of the cancer biomarker prostate-specific antigen in whole serum is demonstrated.

Although intrinsic superconductivity in graphene has not been demonstrated yet, superconductivity in this material can be induced by the proximity effect. The deposition of metallic nanoparticles on a graphene layer allows the status of graphene to be tuned from insulating to superconducting. This metal–graphene hybrid material can therefore be seen as a model system to elucidate the properties of inhomogeneous superconductors.

Liquid-crystalline order can be templated in a material by refilling a photopolymerized liquid-crystal cast with the material after the non-polymerized portion has been washed out. This approach has now been used to template, in achiral liquid crystals, chiral three-dimensional blue phases with unprecedented thermal stability that are suitable for narrowband mirrorless lasing and switchable electro-optic devices.

The so-called pseudogap is a feature of high-T_c superconductors that has puzzled scientists since its discovery. It is of widespread opinion that this feature is associated with a structural symmetry breaking. Now, a highly sensitive scanning tunnelling microscopy experiment shows that a specific structural symmetry is not, as many believed, at the origin of the pseudogap state.

Geometrically frustrated magnets are systems where it is impossible for all magnetic interactions to occur simultaneously. The discovery of frustrated magnetism in a system where the magnetic moments are situated across clusters of transition-metal elements instead of individual ions promises a new approach for controlling such magnetic states.

Light absorption across the bandgap in semiconductors is exploited in many important applications such as photovoltaics, light-emitting diodes and photocatalytic conversion, but whether coloured metals can be used in such applications is unclear. A red metallic oxide Sr_1-xNbO₃ is now shown to be effective under visible light to photocatalyse the oxidation of methylene blue, and the oxidation and reduction of water.

The self-assembly of colloidal particles functionalized with complementary DNA strands into crystalline structures has been hampered by kinetic trapping into disordered aggregates, which effectively limits the temperature window where crystallization occurs. A strategy to design DNA-functionalized colloids with a broadened crystallization window is now proposed, and is supported by theory and simulations.

Although sodium is an abundant element that can be electrochemically and reversibly extracted from and inserted into layered materials, the resulting reversible capacity for storing energy remains low. A manganese–iron–sodium-based electrode is now shown to exhibit a reversible capacity of 190 mAh g⁻¹ due to electrochemically active Fe³⁺/Fe⁴⁺ redox reactions.

Understanding how heat is transferred across interfaces is important for the efficiency of micro- and nanoscale electronic devices. Here, it is shown that there is a direct link between the bonding character of an interface and the thermal transport across it.

The substitution of oxygen by hydride anions in oxide materials to form oxyhydrides has been difficult to achieve because it requires highly reducing conditions without transferring an electron from the hydride. An oxyhydride of BaTiO₃ that is electronically conducting, stable in air and water at ambient conditions, and exchangeable with hydrogen gas at 400 °C, has now been prepared.

Ultrafast and intense optical pulses have been used to study spin-density-waves in pnictide compounds, which are known to exhibit unconventional superconductivity. The results show that the magnetic order follows lattice motion, which suggests that a spin–phonon coupling may play an important role in the formation of spin-density-waves and superconductivity.

Electromagnetic waves propagating on the surface of materials are used in a variety of applications such as on-chip photonics. The demonstration now of a nearly 100% efficient coupling of these surface waves to freely propagating waves promises to improve photonic applications such as surface–plasmon couplers, antireflection coatings and many more.

The maximum imaging resolution in classical optics is limited to approximately the wavelength of light used, and subwavelength resolution can only be achieved by advanced imaging schemes. The appeal of the super-oscillatory lens optical microscope described here is that it enables subwavelength imaging with, in principle, unlimited resolution using a modified conventional microscope.

Even though phase-change materials are used in optical as well as electronic information storage applications, some issues, such as their fast crystallization kinetics, remain poorly understood. The use of ultrafast differential scanning calorimetry now reveals that the fast kinetics is based on properties similar to those of fragile liquids.

A common route to obtain efficient thermoelectrics is to optimize the ratio between electrical and thermal conductivity. Typically, materials with a complex, glass-like phonon structure and therefore a very low thermal conductivity are studied. Now, a route showing that solid ions in a liquid-like state can have a low enough thermal conductivity to compete with the best existing thermoelectrics is proposed.

Although the superior electrochemical performance of supercapacitors capable of rapidly storing electrical energy is due to reversible ion adsorption in porous carbon electrodes, the molecular origin of this phenomenon is still poorly understood. A quantitative picture of the structure of an ionic liquid adsorbed inside realistically modelled microporous carbon electrodes is now proposed.

Magnetic tunnel junctions play an important role in controlling electron spin in spintronic devices. The reversible, remanent switching of electron-spin polarization in multiferroic tunnel junctions now enables significant technological possibilities for spin electronics.

The slow decay of photoconductivity in amorphous oxide semiconductors hampers their use in photosensor arrays with viable frame rates. A gated sensor architecture now provides direct control over the Fermi-level position in the semiconductor layer, and eliminates persistent photoconductivity by accelerating electron recombination with ionized oxygen vacancy sites.

Oxide materials show a versatile range of phenomena that in many cases can be controlled by growing thin films of oxides next to each other. The observation now that electrical conductance of domain walls in a ferroelectric can be tuned simply through the domain-wall orientation offers a flexible way of controlling functionality in complex oxides.