Volume 7 Issue 9, September 2011

To first approximation, the dispersion relation around the Fermi energy of single-layer graphene is linear, making its charge carriers behave like massless relativistic subatomic particles. More careful inspection of its low-energy band structure suggests the picture is more complex, extending the analogy even further.

Surfaces inherently lack inversion symmetry. This property is now shown to promote the spontaneous formation of a lattice of spin vortices in a thin magnetic film, a finding that suggests a simple route towards new spintronics applications.

A comprehensive map of the spin fluctuations in high-temperature superconductors is emerging through the application of a novel experimental technique — and the surprising results are challenging theorists.

Phase information can be obtained from inelastically scattered X-rays by combining parametric down-conversion with tunable quantum interference. This is a step towards putting this nonlinear phenomenon to a practical use in the X-ray regime: investigating the optical response of chemical bonds at their electron-volt and subnanometre scales.

Bell’s theorem experiments, which test the completeness of quantum mechanics, have a number of loopholes. However, one type—detection loopholes—becomes smaller when the measurement has more possible outcomes. Bell’s inequality is now violated in tests with as many as 11 different results.

Similar to atoms in cold gases, exciton–polaritons in semiconductor microcavities can undergo Bose–Einstein condensation. Now, formation of a condensate in an excited orbital state has been observed in such a system, underlining the potential of exciton–polariton condensates to emulate condensed-matter physics.

Single electron spins have been detected before, but the methods used proved difficult to extend to multi-spin systems. A magnetic resonance imaging technique is now demonstrated that resolves proximal spins in three dimensions with nanometre-scale resolution. In addition to spatial mapping, the approach allows for coherent control of the individual spins.

Transferring graphene onto hexagonal boron nitride enables high-mobility multiterminal quantum Hall devices to be built. This makes it possible to study graphene's unique fractional quantum Hall behaviour more easily and more directly than previously.

Quantization of the current flowing across a nanometre-scale constriction in graphene is usually destroyed through charge-scattering from rough edges and impurities. But by using high-quality suspended samples and a constriction whose length is shorter than its width, conductance quantization in graphene has now been demonstrated.

Graphene’s linear dispersion relation makes its charge carriers behave as if they were massless. However, near the Dirac point where graphene’s valence and conduction bands meet, electron–electron interactions cause this relation to diverge, such that it becomes strongly nonlinear and the effective carrier velocity doubles.

Conventionally, the smallest object you can see with light at a certain wavelength λ is about λ/2 in size. Researchers have now broken this wavelength–resolution link. Combining ultraviolet and X-ray photons in a nonlinear process enabled the optical properties of diamond to be mapped down to a resolution of λ/380.

The chaotic nature of turbulence makes it difficult to develop simple rules of thumb to predict the behaviour of a turbulent flow. But analysis of the motion of three tracer particles with respect to a fourth suggests at least one: that the local alignment of turbulent rotation conserves angular momentum, similar to an ice-skater performing a pirouette.

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.

The combination of bulk momentum-space and local real-space probes shows that superconductivity and antiferromagnetism in an electron-doped copper oxide superconductor coexist and compete on a nanometre scale, with electronic spin excitations that are probably involved in the superconducting pairing mechanism.

In the copper oxide superconductors, spin fluctuations might be involved in the electronic pairing mechanism. The case for such magnetically mediated superconductivity is now strengthened by the discovery of high-energy magnetic excitations that are not affected by chemical doping levels within several cuprates.

Understanding how DNA bends and stretches provides insight into how the genetic information it contains is expressed. However, the role that the double-helical shape of DNA plays in determining its mechanical properties has not previously been investigated. Now, a model that incorporates DNA’s famous shape provides a better understanding of how the molecule reacts to large forces.