Articles in 2012

Topological insulators are now shown to be protected not only by time-reversal symmetry, but also by crystal lattice symmetry. By accounting for the crystalline symmetries, additional topological insulators can be predicted.

A time-dependent study of the effective temperature of carriers in impurity-free graphene now indicates that a disorder-assisted mechanism is responsible for cooling hot electrons. Observation of this so-called supercollision contradicts the idea that electron–phonon interactions dominate cooling.

Charge transport is usually limited by collisions between the carriers, impurities and/or phonons. Collisions involving three bodies are generally much rarer. A study now reveals, however, that such supercollisions can play an important role in the properties of graphene.

The electronic properties of graphene are spatially controlled from metallic to semiconducting by patterning steps into the underlying silicon carbide substrate. This bottom-up approach could be the basis for integrated graphene electronics.

Photonic crystals efficiently control wave propagation on a wavelength scale, but this means they can become very large when long wavelengths are involved. Metamaterials made of resonant unit cells can confine and guide waves even at scales far below their wavelength.

Different experimental probes have found different bosonic modes in the iron-based superconductors. A scanning tunnelling spectroscopy study of two separate superconductors now links the tunnelling mode with the ‘neutron resonance’, both of which vanish when superconductivity disappears.

An increase in diffusion beyond the ballistic-transport regime is now demonstrated. This so-called hyper-transport is observed in an optical experiment, but it might also be evident in other systems with time-varying disorder.

Topological entanglement entropy provides a robust measure for detecting the long-range entanglement that characterizes quantum ground states displaying topological order. A new method for calculating this entropy isolates minimally entangled states from the ground states of a topological phase—offering a reliable test for identifying topological spin liquids.

New data, backed up by simulations, support the existence of Majorana fermions in the one-dimensional topological superconductor that is induced by placing an aluminium superconductor close to an indium-arsenide nanowire.

Fast particles propagating through a classical medium give rise to shock waves. Calculations now uncover the surprising behaviour of particles in one-dimensional quantum fluids: a fast particle will never come to a full stop, and a supersonic particle will propagate through the medium undergoing long-lived oscillations.

Enhanced control of the nuclear spin orientation of rare isotopes has now been demonstrated. This technique is considerably more efficient than traditional methods and significantly broadens the domain of accessible nuclei, promising insights in nuclear physics and applications in material science.

Two closely spaced two-dimensional systems can remain strongly coupled by electron–electron interactions even though they cannot physically exchange particles. Coulomb drag is a manifestation of this interaction—in which an electric current passed through one layer causes frictional charge flow in the other—now experimentally observed in bilayer graphene

A super-elastic collision is one that results in an increase of kinetic energy in the colliding system. A probable occurrence of such a collision is shown in the huge, magnetized plasmas of two coronal mass ejections from the Sun.

Understanding the spin dynamics in magnetic nanostructures is important for spintronics, but so far it has been impossible to probe the spin dynamics directly. A neutron-scattering technique providing direct information about dynamical two-spin correlations in a molecular nanomagnet has now been demonstrated.

Sudden bursts of charged particles emitted from the surface of the Sun can disrupt the satellites orbiting Earth. However, the mechanisms that drive these so-called coronal mass ejections remain unclear. An advanced computer model now establishes a link between the onset of an ejection and the emergence of magnetic flux into the solar atmosphere.

A multicomponent gas of ytterbium atoms accommodates more entropy in its spin degrees of freedom than does its two-component analogue, leading to a lower effective temperature, and an easy route for cooling ultracold fermions towards a Mott-insulating state.

Atoms can be used as highly sensitive magnetic-field sensors. By exploiting the effects of electric fields on the optical transitions of excited Rydberg states, it is now demonstrated that it is also possible to probe very weak microwave electric fields with atoms.

Short nuclear spin–lattice relaxation times have long been a challenge for magnetic resonance imaging. The alternative of using long-lived nuclear spin states has so far required symmetry breaking, but a method of controlling these states without breaking the symmetry of the spin system has now been demonstrated.

Quantum gases are useful toy models for the study of quantum magnetism. Exquisite control of a spinor gas of fermionic atoms in an optical lattice has now been demonstrated, opening up the exploration of quantum magnetism with high spins.

The ability to modify a material’s magnetization with an electric field could enable lower-power electronic devices. Such ‘magnetoelectric’ behaviour is usually only seen at the interface between magnetostrictive and electrostrictive materials, but has now been observed in the bulk of single-component rare-earth ferrites.