Volume 7 Issue 4, April 2011

The term 'high-temperature superconductor' used to refer only to copper-based compounds — now, iron-based pnictides have entered the frame. The comparison of these two types of superconductor is revealing, and suggestive of what might be needed to achieve even higher transition temperatures.

For turbulent flows, the energy transfer between large and small whirls depends on the dimension of the fluid. Imposing large-scale shear on a three-dimensional system can unexpectedly induce two-dimensional behaviour.

A microscope that can both resolve individual atoms in an optical lattice and control their spin states should enable the exploration of many-body physics.

One of the main uncertainties in the burn-up of X-ray bursts from neutron stars has been removed with the weighing of a key nucleus, ⁶⁵As, at a new ion storage ring.

'Mottness' or a reconstructed Fermi surface? Nodes and arcs, or pockets? Both approaches to the pseudogap state of cuprate superconductors receive some support from specific heat data obtained up to 45 T. Can the two be reconciled?

Dilute magnetic semiconductors such as gallium manganese arsenide could be key to the development of spintronics. But the relationship between electronic transport and magnetic properties has been hotly debated. Data indicating the preservation of the non-magnetic character of the host material provide startling new insight.

The ability to coherently switch a state between two systems is a key requirement for quantum information processing. Such control is now demonstrated by shifting the quantum state of a microwave photon between any one of three superconducting-circuit resonators: in analogy to the classic three cups and a ball game.

For the iron pnictide superconductors, a first-principles calculation of the magnetic state shows that correlations are important if we are to understand both the paramagnetic and magnetic phases. Moreover, the pnictides are fundamentally different from the cuprate superconductors in terms of spin and orbital physics.

Above the superconducting temperature for a given material, correlations between pairs of electrons are already present. Many experiments indicate that such correlations may exist up to 100 K above the transition. However, a temporal probe of the superconducting fluctuations suggests that correlations only exist within a narrow temperature range.

In thin magnetic films with confined geometry, the magnetization can adopt vortex-shaped arrangements. Such vortex states have been studied intensely in ferromagnetic films, but this paper reports the first direct observation of them in an antiferromagnetic system.

Molecular hydrogen exists in two forms, which differ in the relative orientation of their nuclear spins. Interconversion between the two isomeric forms is extremely rare, unless there is an interaction breaking the symmetry between the two nuclei. Magnetic surfaces are known to act as such a catalyst, but this study finds that electric fields can also induce the spin flips necessary for the interconversion.

The use of microwaves to read and write information in superconducting qubits usually requires magnetic components that are difficult to integrate with chip-based circuits. A cascade of parametric amplifiers is now proposed instead, which could provide a more versatile and noise-free alternative.

Power-law scaling of critical phenomena has been most powerful for predictions near a critical point. By averaging the noise emitted by avalanches of a given duration, however, universal scaling functions can extend the predictive power of scaling far from the critical point.

When a thick fluid, such as the Earth’s atmosphere, is driven simultaneously by a large-scale two-dimensional vortex and small-scale three-dimensional turbulence, experiments show that the large-scale flow dominates. Turbulence is thus confined to two dimensions, giving rise to an upscale energy cascade that powers the intermediate and large-scale flows.

The mystery of the missing bound states within a superconducting vortex in a pnictide superconductor has been solved. Not only are bound states present, they also provide information on the gap structure of Ba_0.6K_0.4Fe₂As₂.

Quantum oscillations in a copper-oxide superconductor are observed using a thermodynamic probe. Surprisingly, these oscillations lie on a background signal that is consistent with d-wave superconductivity in the vortex state, in a magnetic field up to 45 T.

Microwave radiation has a dramatic effect on the magneto-resistance of two-dimensional electron systems, even reducing it below zero. It is thought that this is the result of the formation of distinct current domains. Direct experimental evidence for these domains is now presented for the first time.

The magnetic properties of GaMnAs could be useful in the development of spintronic devices. Yet the precise origin of these properties has been hotly debated. Resonant-tunnelling spectra obtained from GaMnAs devices of superlative quality could finally resolve this issue.

The coupling of spin and orbital motion of electrons in carbon nanotubes has been demonstrated before, but a study now shows that the strength and sign of the spin–orbit coupling can be tuned by a gate voltage, and that, importantly for future applications, the effect survives in the presence of disorder.

A micrometre-scale device that exploits the piezoresistive characteristics of silicon acts like an engine, converting heat into mechanical work in one mode of operation, and, in another, like a refrigerator, suppressing mechanical fluctuations.

Magnetic reconnection has long been implicated in the acceleration of electrons to relativistic speeds in the Earth’s magnetosphere. Satellite observations and simulations indicate it is just part of the story, the rest of which involves a second process known as betatron acceleration.