Letters in 2014

In crystalline topological insulators, the combination of an insulating bulk with conducting surface states is due to particular crystal symmetry. The associated Dirac cones—linear crossings in the electronic band structure—exhibit non-trivial orbital textures that have now been probed by means of scanning tunnelling spectroscopy.

High-T_c superconducting cuprates exhibit gap nodes. Recent experiments have revealed the existence of a node-free superconducting-like energy gap in deeply underdoped cuprates. Now it is argued that such systems are topological superconductors with antiferromagnetic order.

Jammed systems are typically thought of as being amorphous. Simulations of packings with varying disorder reveal a crossover from crystalline behaviour, which suggests the physics of jamming also applies to highly ordered systems—providing a new framework for understanding amorphous solids.

The dissipation-less flow of supercurrent through a wire is a well-known property of superconductors. But in some cases, a normal current can flow in the presence of superconductivity. This may be due to non-equilibrium physics.

A cosmological model treating dark matter as a coherent quantum wave agrees well with conventional dark-matter theory on an astronomical scale. But on smaller scales, the quantum nature of wave-like dark matter can explain dark-matter cores that are observed in dwarf galaxies, which standard theory cannot.

From the manner of its discovery in 2012, it was apparent that the 125 GeV Higgs boson couples to bosons, but does it couple to fermions too? Yes, says the CMS Collaboration at CERN, who present combined evidence of Higgs decay to pairs of bottom quarks and pairs of tau leptons.

Feshbach resonances provide a powerful tool for engineering interactions in ultracold atomic gases. The strong exciton–photon coupling in semiconductor microcavities facilitates the demonstration of a polaritonic Feshbach resonance with promising implications for manipulating polariton quantum fluids.

When a water drop bounces back from a hydrophobic surface, its initial, spherical shape is usually restored. Now, experiments with a specially engineered superhydrophobic surface made from micrometre-sized tapered pillars covered with copper oxide ‘nanoflowers’ show that droplets can bounce back with a flat, pancake-like shape.

Developing a theory that describes rotating turbulence has so far proved challenging. Now, experiments show signatures of inertial waves in rotating turbulence, implying that such flow can be thought of as resulting from interacting inertial waves—solutions of the linearized rotating Navier–Stokes equation.

Excitons — electron–hole pairs held together by the Coulomb force — are quasiparticles that are created when light interacts with matter. In metals, exciton generation is hard to detect; indeed, holes are usually not associated with metals. Now, using femtosecond laser pulses triggering three-photon photoemission processes, excitonic response is reported for silver surfaces.

The origin of the large magnetic fields observed in the interior of the supernova remnant Cassiopeia A is still unclear. Laboratory experiments of laser-produced shocks provide new insights into the mechanisms of magnetic field amplification.

Nanoscale metallic tips are a useful source of electrons for material characterization. It is now shown how terahertz radiation can provide precision control and enhancement of photoelectron emission from these sources. The approach can shape the spectrum of the electron pulse, which could pave the way to improvements in ultrafast electron diffraction and transmission electron microscopy.

An active galactic nucleus is the brightest source of electromagnetic radiation in the Universe, believed to be powered by a supermassive black hole at its core. There are two main types of active galactic nuclei, though the differences may be down to varying viewing angles. Or are they?

Random lasers generate the optical feedback required for stimulated emission by scattering light from disordered particles. Their inherent randomness, however, makes controlling the emission wavelength difficult. It is now shown that this problem can be remedied by carefully matching the pump laser to the specific random medium. The concept is applied to a one-dimensional optofluidic device, but could also be applicable to other random lasers.

Gamma-ray bursts are among the most luminous explosions in the cosmos, but the mechanism behind the energetic radiation remains unclear. ‘Fast cooling’ electrons in a decaying magnetic field may offer an explanation.

When the charge density wave state in TiSe₂ is suppressed by hydrostatic pressure or chemical doping, superconductivity appears. This suggests the presence of a quantum critical point. Yet a high pressure X-ray study unexpectedly finds that the quantum critical point is nowhere near the superconducting dome.

An argument by contradiction shows that the pseudogap state in the high-temperature superconducting cuprates is due to the superconducting pairing rather than being an independent or even competing state.

Bismuth selenide is a prototypical 3D topological insulator; its electronic spectrum features a Dirac cone populated by surface states. Now, it is experimentally and numerically shown that a bandgap forms beyond a certain critical compressive strain, destroying the surface states.

Bad metals, such as the copper oxide superconductors, do conduct electricity but the origin of their poor conductivity is unclear. A study of disordered rare-earth nickelates now provides microscopic insights into bad-metal behaviour

Superconductivity in iron pnictide compounds occurs near a magnetic phase and magnetic spin fluctuations are prime candidates for the superconducting pairing mechanism. What does this mean now that a second magnetic phase, next to another superconducting phase, is found at higher doping levels?