Articles in 2011

Transforming a quantum system with high fidelity is usually a trade-off between an increase in speed—thereby minimizing decoherence—and robustness against fluctuating control parameters. Protocols at these two extreme limits are now demonstrated and compared using Bose–Einstein condensates in optical traps.

An optical technique based on Doppler velocimetry reveals important aspects of the physics underlying the propagation of spin polarization in a two-dimensional electron gas. The spin mobility is shown to track the high electron mobility, but coherent spin precession is lost at temperatures near 150 K, posing a challenge for future spintronics devices.

Laser-driven proton accelerators could enable more effective cancer treatment, but to fulfil this function proton beams with a higher energy and narrower energy spread will need to be produced. Discovery of a laser–plasma acceleration mechanism that generates 20 MeV proton beams with a 1% spread is a promising step.

The controlled creation of one-dimensional conductive channels at the cores of topological defects in the multiferroic material BiFeO₃ demonstrates that such defects can drive metal–insulator phase transitions, and might provide a route towards high-density information storage.

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.

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.

So-called topological properties can make quantum systems robust to a wide class of microscopic perturbations. Theoretical work now shows that topological features and phenomena occur not only in closed systems, but also in open quantum systems with appropriately engineered dissipation.

Experiments that exploit non-classical properties of light promise to provide unique information about many-body systems. The limited availability of non-classical light sources, however, makes their implementation challenging. A method to calculate the quantum-optical response of a material from signals measured by using coherent-light excitation might provide an alternative route.

Understanding the origin of colossal magnetoresistance in the manganites has proved to be one of the more difficult challenges in condensed-matter physics. An unexpected discovery of polarons in the metallic ground state of bilayer manganites could be an important clue.

A single quantum system comprising a nitrogen-vacancy in diamond is now coupled to a nanowire cantilever. Magnetic fields can then couple the nitrogen-vacancy spin and the oscillator enabling read-out of the nanometre-scale motion.

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.

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.

After decades of research, the microscopic details of the superconductor–insulator transition in two-dimensions, which is driven by the presence of disorder, are revealed by simulations. These include a phase transition from a gapped superconductor to a gapped insulator, for example.

Skyrmions are topologically protected field configurations that appear as solutions of continuous quantum-field theories. Recently, they have been observed in magnetic bulk alloys, where a lattice of skyrmions is stabilized by an external magnetic field. In contrast, this study finds evidence for a skyrmion lattice as a spontaneous ground state, encoded into a magnetic spin texture on the atomic scale.

Mechanical deformations in graphene have been shown to be associated with ‘fictitious’ magnetic fields. Theoretical work now suggests that these fields can give rise to an analogue of the Aharonov–Bohm effect, a phenomenon that might be used to sensitively detect small deformations of the graphene sheet.