Articles in 2011

It is widely believed that high-field superconductivity in heavy fermion metals is sustained only when the effective mass of its conduction electrons diverge. Measurements of magnetically driven changes in the electronic topology of URhGe suggest it is not divergence of the effective mass to infinity but a vanishing of the Fermi velocity to zero that supports this behaviour.

Superconductivity and magnetism have often been regarded as opposites. High magnetic fields usually destroy the superconducting state. But for superconductors constrained to two dimensions, a parallel magnetic field can actually enhance superconductivity.

At low temperatures and separated by sufficient distances, magnetic impurities embedded in non-magnetic metals lose their magnetic nature. But when two such atoms are brought close together, it reappears. By varying the distance between two cobalt atoms with a scanning tunnelling microscope, the quantum phase transition between these two states can be explored.

Electron spin in quantum dots are extensively studied as a qubit for quantum information processing. However, the coherence of electron spin is deleteriously influenced by nuclear spin. Quantum-dot holes are a potential alternative. Full control over hole-spin qubits is now achieved using picosecond lasers.

The robustness of edge states against external influence is a phenomenon that has been successfully applied to electron transport. A study now predicts that the same concept can also lead to improved optical devices. Topological protection might, for example, reduce the deleterious influence of disorder on coupled-resonator optical waveguides.

Laser-driven particle accelerators have the potential to be much cheaper than conventional accelerators. But so far, the reliability and energy spread of the beams they produce has been poor. A technique that decouples the particle-injection and acceleration stages of these devices could improve their performance.

That the final energy of an isolated system in contact with a heat bath follows the Gibbs distribution is a classical result of statistical physics. But the situation is different when the system is non-adiabatically driven out of equilibrium. Theoretical work now shows that in these cases the energy distribution is non-Gibbsian and that two qualitatively different regimes with a transition between them emerge.

Time-reversal symmetry makes massless Dirac fermions in topological insulators ‘gapless’. When a gap opens, it breaks this symmetry and confers mass to the fermions. But now a quantum phase transition has been observed in a three-dimensional topological insulator that allows these particles to acquire mass without symmetry breaking.

A strictly one-dimensional electron liquid or 'Luttinger liquid' may seem a purely theoretical construct. But measurements of the electronic structure of strings of gold atoms self-aligned on a germanium surface suggest this mythic state of matter is real, offering new possibilities to investigate and ultimately control its properties and behaviour.

Light can interact with the electrons in a crystalline solid, which in turn generates lattice vibrations or phonons. A related phenomenon was proposed 40 years ago in which it is the ions in the crystal rather than the electrons that mediate the interaction. This effect, known as ionic Raman scattering, is now observed experimentally.

The Tomonaga–Luttinger liquid model is the leading candidate for describing one-dimensional metallic conductors at low temperature. Yet, experimental evidence that it is valid is sketchy. Scanning tunnelling and photoemission spectra suggest that it does, in fact, describe the behaviour of chains of gold atoms self-assembled on the surface of germanium.

In a Mott insulator, repulsive interactions suppress conductivity. Such behaviour has been demonstrated, individually, for both bosonic and fermionic atoms in optical lattices. Now, a Bose–Fermi mixture is found to be Mott insulating too, even when the individual components are not.

The linear and hyperbolic electronic bands of single- and bilayer graphene give rise to quantum Hall effects that are different from those seen in previously studied 2D systems. The electronic structure of trilayer graphene includes both types of band, giving rise to even richer behaviour.

Equilibrium free-energy landscapes supply important information about complex molecules such as nucleic acids and proteins. But can equilibrium landscapes be calculated from measurements on a non-equilibrium system?

A single microwave photon in a superposition of two states of different frequency is now demonstrated using a superconducting quantum interference device to mediate the coupling between two harmonics of a resonator. Such quantum circuits bring closer the possibility of controlling photon–photon interactions at the single-photon level.

NASA's space shuttle programme is at an end — as may be the funding for its next space telescope.