Letters in 2014

Nuclear magnetic resonance measurements reveal two separate relaxation channels—one associated with a Fermi liquid state and the other with a non-Fermi liquid state—coexisting near a quantum phase transition in YbRh₂Si₂.

Dipole-forbidden vibrational transitions in molecular ions are very weak and difficult to characterize. The sympathetic cooling provided by a Coulomb crystal is shown to allow interrogation times long enough to observe them.

Nonlinear inertial flows usually influence the motion of swimming organisms, but most studies focus on the tractable case of swimmers too small to feel such effects. A mechanistic principle now unifies the varied dynamics of macroscopic swimmers.

The electrons associated with the conducting surface states of topological insulators are described by a two-component wavefunction. Experiments on Bi₂Se₃ now show that the structure of Landau levels reflects this two-component nature.

Connecting complex networks is known to exacerbate perturbations and lead to cascading failures, but natural networks of networks are surprisingly stable. A theory now proposes that network structure holds the key to understanding this paradox.

Two concentric carbon nanotubes don’t need to have a common finite unit cell. Absorption spectra of such incommensurate double-walled carbon nanotubes reveal strong hybridization of the electron wavefunctions — unusual for van der Waals-coupled structures. The observations can be rationalized by zone folding the electronic structure of twisted-and-stretched graphene bilayers.

The interaction of a quantum system with its surroundings is usually detrimental, introducing decoherence. Experiments now show how such interactions can be harnessed to provide all-optical control of the spin state of a quantum dot.

Repeatedly probing a quantum system restricts its evolution, providing a route for state engineering. Such confinement, described by quantum Zeno dynamics, has now been implemented to generate superposition states in a multi-level Rydberg atom.

Strontium titanate is a common substrate for growing oxide heterostructures—from superconductors to interfaces that support several phases of matter. But in an all-strontium-titanate device with a liquid electrolyte and metal-oxide gate, the results are anything but common.

Electrons in graphene have a pseudospin, but controlling this degree of freedom is challenging. Evidence now suggests that the moiré superlattices arising in two-dimensional heterostructures can be used to electrically manipulate pseudospins.

Quantized resistivity values for 2D electron systems don’t necessarily result from an external magnetic field as in the ‘normal’ quantum Hall effect; they can arise due to a material's intrinsic ferromagnetism too—the quantum anomalous Hall effect. Experiments with a ferromagnetic topological insulator now establish how the anomalous states can be mapped onto the normal states.

Hybridized systems offer a promising route for developing quantum devices, but inhomogeneous broadening limits the practical use of large spin ensembles. Suppression of the decoherence induced by such broadening has now been demonstrated for a superconducting cavity coupled to an ensemble of nitrogen–vacancy centres in diamond.

Fetching an object by means of sending a wave—impossible? Not necessarily. As now demonstrated experimentally, generating waves on a water surface using a set of plungers can cause a floating particle to move counter to the general direction of wave propagation. The effect originates from vorticity creation by steep 3D waves.

Electron energy-loss spectroscopy uses inelastically scattered electrons to provide information about a material’s chemical composition. It is now shown that localized plasmonic excitations can lead to nonlinear scattering, significantly enhancing the signals arising from inelastic electrons.

Majorana fermions, which are their own antiparticles, are expected to exist in topological superconductors. A study using superconducting leads in contact with a quantum well reveals the presence of supercurrents along one-dimensional sample edges of a quantum spin Hall state. These edge supercurrents are topological.

Numerical evidence now supports the idea that a liquid–liquid transition forms a generic feature of tetrahedrally coordinated liquids. This result establishes the physical validity of such a transition and provides a possible explanation for the anomalous behaviour of liquid water.

In a Josephson junction, a current flows from one superconductor to another through a barrier without any voltage being applied. SQUIDs, for example, are based on this phenomenon. Now, an iron-based multi-band superconductor shows signs of intrinsic Josephson junctions, opening up prospects for applications.

Characterizing an unknown quantum state typically relies on analysing the outcome of a large set of measurements. Certain quantum-processing tasks are now shown to be realizable using only approximate knowledge of the state, which can be gathered with exponentially fewer resources.

The spin Hall effect, which arises from the spin–orbit interaction, is expected to be energy dependent, but experiments typically only characterize electrons near the Fermi surface. A tunnelling spectroscopy method has now been developed to probe the energy dependence.

Under certain conditions electrons in confined systems can solidify into Wigner crystals. Nuclear magnetic resonance is used to probe the local electron density of a two-dimensional system in quantum Hall regimes, revealing the role of quantum and thermal fluctuations in Wigner crystallization.