Letters in 2013

The friction of surfaces in relative motion and separated by a few nanometres is thought to be dominated by electronic effects. It is now found that the friction sensed by an AFM tip oscillating above a NbSe₂ surface takes the form of giant dissipation peaks, and that the peaks are related to a hysteresis cycle where the oscillating tip locally pumps 2π slips in the phase of a charge-density wave.

Understanding the thermal transport properties of superlattice structures is relevant to a number of possible practical applications. Now, the scattering of phonons in oxide superlattices is shown to undergo a crossover from an incoherent to a coherent regime, which in turn strongly alters their thermal behaviour.

Bose–Einstein condensates of exciton–polaritons have been stabilized in a range of crystalline systems. Now, polaritons are shown to condense at room temperature using a microcavity within an organic polymer.

The interplay between the electronic and magnetic degrees of freedom in multiferroic materials offers promise for a range of applications. Now, a technique for imaging the magnetoelectric domains directly is developed, and demonstrated on the hexagonal manganite ErMnO₃.

A requirement for the reversible mechanical actuation of liquid-crystal elastomers is macroscale alignment. However, current processing techniques do not achieve reliable and robust alignment, which limits the practical use of these materials as actuators and artificial muscles. It is now shown that by introducing polymers with exchangeable covalent bonds, liquid-crystal elastomers can be easily processed and aligned, and subsequently remodelled.

The photoluminescent properties of electron spins at nitrogen–vacancy (NV) centres are promising for use in quantum information and magnetometry. It is now shown that the coherence times of NV centres in nanodiamonds can be engineered to be comparable to those of bulk diamond.

Lanthanide-doped nanocrystals can be used to upconvert infrared radiation into visible light, and are thought to be promising for a range of photonic and biological imaging applications. It is now shown that the upconversion efficiency can be improved by appropriately clustering the lanthanide ions on different structural sublattices.

The generation and manipulation of single photons is important for quantum information and metrology. Highly bright and stable single-photon sources are now identified in silicon carbide, a wide-bandgap semiconductor widely used for photonic and electronic devices.

Oxide ion conductors are technologically relevant for applications in electrochemical devices such as sensors, separation membranes and fuel cells. Magnesium doping in Na_0.5Bi_0.5TiO₃—a piezoelectric material that suffers from high leakage conductivity—now results in a family of ionic conductors that could prove significant not only for dielectric-based applications but also for intermediate-temperature solid-oxide fuel cells.

Size effects and geometry can significantly modify the properties of nanoparticles with direct impact on their biocompatibility and chemical reactivity. Using high-resolution electron microscopy it is now shown that strain gradients induced in the oxide shell of cuboid Fe nanoparticles can lead to oxide domain formation and shape evolution of the particles.

Compared with their rigid counterparts, thin-film solar cells grown on flexible substrates usually display lower power-conversion efficiencies. Now, the application of a post-deposition alkaline treatment that modifies the chemical composition of the surfaces of Cu(In,Ga)Se₂ thin films reduces optical losses in these flexible photovoltaic architectures. Furthermore, efficiencies comparable to solar cells based on polycrystalline silicon are achieved.

Although metals cannot be ferroelectric in the strict sense of the term, it has long been predicted that they can undergo structural transitions that share similarities with ferroelectricity. LiOsO₃ is now shown to be an experimental realization of such a ferroelectric-like metal.

Clathrate materials have been the subject of intense investigation because of their beneficial properties, in particular their low thermal conductivities. Now, improved thermopower at high temperatures arising from strong electron correlation effects has been achieved in a type-I clathrate containing cerium guest atoms.

The emergence of conductivity at the {001} interface of LaAlO₃ and SrTiO₃ is one of the more celebrated examples of interface engineering. Using a microscopy approach based on a sensitive magnetometry probe, it is now shown that narrow paths of enhanced conductivity occur along the crystallographic axes of the oxide structures.

Hard biological materials such as diatoms and sea sponges can inspire the design of structural materials that are mechanically robust yet lightweight. Hollow titanium nitride lattices have now been fabricated that mimic the length scales (from 10 nm to 100 μm) and hierarchy of biological materials. These lattices attain tensile strengths of 1.75 GPa without failing (even after multiple deformation cycles) because of the low probability of pre-existing flaws.

For alloys displaying diffusive transport behaviour, understanding the electrochemical factors that control dealloying-induced morphologies could prove important for battery development. Composition, particle size and dealloying rate are now shown to affect morphology evolution in the Li–Sn system, and dealloying is found to be governed by both percolation-dissolution and solid-state-diffusion mechanisms.

Sodium cobaltate has latterly received attention due to its appealing thermoelectric properties. By combining inelastic X-ray and neutron scattering results with detailed first-principles calculations, it is now shown that low-energy rattling modes of sodium ions within multi-vacancy clusters play a central role in determining the low thermal conductivity of this material.

The nonlinear response of a weak electrolyte to an applied electric field is known as the Wien effect. This is now simulated on a lattice Coulomb gas, therefore providing a platform for investigating system-specific corrections to the firmly established theory accounting for it.

The interplay between magnetism and superconductivity in copper oxide superconductors has been a topic of intense research. Now, a systematic resonant inelastic X-ray scattering study of strontium-doped lanthanum cuprate shows that high-energy magnetic excitations persist over a wide doping range.

The emergence of superconductivity of insulating oxide interfaces has raised a number of intriguing theoretical challenges. Now, the critical temperature of strontium-doped lanthanum cuprate bilayer samples is shown to remain unchanged over a wide doping range, implying that changes in the carrier density cannot be the origin of the enhanced critical temperatures seen with respect to single-phase samples.