Articles in 2011

In his tragically short life, Alan Turing helped define what computing machines are capable of, and where they reach inherent limits. His legacy is still felt every day, in areas ranging from computational complexity theory to cryptography and quantum computing.

Fibre networks inspire an update to a century-old criterion for mechanical integrity — showing that a little bending resistance can go a long way.

Defects in diamond crystals possess rare physical properties that can enable new forms of technology. Unlocking this potential requires rapid quantum-state measurement, a 'quantum snapshot', which has now been achieved.

The behaviour of molecules and solids is governed by the interplay of electronic orbitals. Superfluidity, in contrast, is typically considered a single-orbital effect. Now, a combined experimental and theoretical study provides evidence for a multi-orbital superfluid, with a complex order parameter, occurring in a binary spin mixture of atoms trapped in an hexagonal optical lattice.

Glass-forming liquids are generally thought to relax through a collective rearrangement of domains, correlated over a length scale that increases with decreasing temperature. A numerical study now reveals a surprising twist to the story, claiming that relaxation depends non-monotonically on temperature.

In fibre networks, mechanical stability relies on the fibres’ bending resistance—in contrast to rubbers, where entropic stretching is the key. The extent to which the mechanics of fibre networks is controlled by bending is, however, an open question. The study of a general lattice-based model of fibrous networks now reveals two rigidity critical points, one of which controls a rich crossover from stretching-dominated to bending-dominated behaviour.

An in-plane magnetic field usually destroys the isotropic fractional quantum Hall states of two-dimensional electron systems, and gives rise to anisotropic liquid-crystal-like states. An unexpected observation of the coexistence of both states at once suggests the emergence of a new quantum phase of matter.

A quantum particle can tunnel through an energy barrier that it would otherwise be unable to surmount. This phenomenon has an important role in atomic processes such as ionization. Researchers now use an attosecond ‘clock’ to take a precise look at the dynamics of this process and identify the trajectory taken by the escaping electron.

In strong magnetic fields, clean two-dimensional electron systems support fractional Hall states that exhibit isotropically vanishing longitudinal resistance. At low field these states disappear and an anisotropic stripe phase emerges. And in between, contrary to expectation, these states can coexist.

Multiple valleys in the electronic structure of certain crystal lattices could enable the development of so-called valleytronic devices. But to do so, the degeneracy of these valleys must be lifted. Measurements of the anisotropic magnetoelectric response of bismuth suggest that its three-fold valley degeneracy breaks spontaneously at low temperatures and high fields.

Orbital order is important to many correlated electron phenomena, including colossal magnetoresistance and high-temperature superconductivity. A study of a previously unreported structure transition in KCuF₃ suggests that direct interorbital exchange is important to understanding such order.

Heavy electrons, formed through the quantum mixture of localized and itinerant electrons, can pair to create unconventional superconductivity in a two-dimensional lattice that is just one-unit-cell thick.

Helical Dirac fermion states in topological insulators could enable dissipation-free spintronics and robust quantum information processors. A study of the influence of disorder on these states shows that although they are resilient against backscattering by magnetic impurities, fluctuations caused by charge impurities could cause problems for such applications.

Unconventional superconductivity is usually associated with a layered system. But how thin can a layered superconductor be and continue to be superconducting? Painstakingly grown superlattices of the heavy-fermion superconductor CeCoIn₅ suggest it could be as thin as a single layer.

Disorder-induced Anderson localization usually causes conducting materials to become insulating at low temperature. Graphene is a notable exception. But by increasing the carrier density in one graphene layer, a metal–insulator transition can be induced in an isolated second layer stacked above it.

The 2011 Nobel Prize in Physics has been awarded to Saul Perlmutter, Brian Schmidt and Adam Riess, "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".