Volume 6 Issue 3, March 2010

Funding of the arXiv preprint server must now be shared by more of its users.

The moment of conception of the geometric phase can be pinpointed precisely, but related ideas had been formulated before, in various guises. Not less varied were the ramifications that became clear once the concept was identified formally.

During the 50 years since its discovery, the Aharonov–Bohm effect has had a significant impact on the development of physics. Its arguably deepest implication, however, has been virtually ignored.

Can the hole left by ionization in an N₂ molecule be imaged with both ångström-scale spatial and subfemtosecond temporal resolution?

It is 50 years since the discovery of the Aharonov–Bohm effect, and 25 years since that of the Berry phase. A celebration of this double anniversary at the University of Bristol made evident that these discoveries still offer much food for thought.

Even simple creatures, such as cockroaches, are capable of complex responses to changes in their environment. But robots usually require complicated dedicated control circuits to perform just a single action. Chaos control theory could allow simpler control strategies to realize more complex behaviour.

Most materials either absorb or transmit X-rays. This is useful for imaging but makes it notoriously difficult to build mirrors for reflective X-ray optics. A demonstration of the high X-ray reflectivity of diamond could provide a timely solution to make the most of the next generation of free-electron lasers.

Research on synchronization of coupled oscillators has helped explain how uniform behaviour emerges in populations of non-uniform systems. But explaining how uniform populations engage in 'chimera states' — states of sustainable non-uniform synchronization — may prove to be just as fascinating.

The fundamental symmetries of parity and time are now being exploited to enable the spatial guiding and selection of propagating radiation, and could ultimately underpin a new generation of sophisticated, integrated photonic devices.

Plasmas, like most fluids, usually become more homogeneous when subjected to turbulence. But in the Earth's magnetosphere, and in an unusual device whose confining field is generated by a levitated half-tonne superconducting magnet, precisely the opposite sometimes happens.

Measurements of the magnetic-field-dependent polarization of a one-dimensional organic quantum magnet suggest its ferroelectric behaviour is mediated by a spin–Peierls instability. Such behaviour could provide a promising new approach to the design of spin-driven ferroelectrics.

Conventionally, the states of a two-dimensional quantum ring in a high magnetic field have a well-defined spatial structure. But Coulomb repulsion between individual orbits causes oscillations in the size of this structure each time a magnetic flux-quantum enters or leaves the system. This effect has now been measured experimentally in semiconducting quantum rings.

A neutron scattering study reveals that the magnetic fluctuations in an iron arsenide superconductor behave according to the conventional theories of metals, unlike the cuprate superconductors. Moreover, the magnetic spin-excitation energies are sufficient to mediate the Cooper pairs that form the superconducting state.

A neutron scattering study shows that the spin excitations in both pnictide- and copper-oxide-based superconductors have the same four-fold symmetry. If these excitations do indeed mediate the superconductivity, the two families of materials may be more similar than previously thought.

The Ruderman–Kittel–Kasuya–Yosida interaction indirectly couples the moments of magnetic atoms through conduction electrons. Using a spin-polarized scanning tunnelling microscope, the direction and strength of this interaction between pairs and triplets of isolated atoms on a surface has been imaged directly.

A photonic system that shows behaviour similar to that of a violation of parity–time symmetry provides a convenient test bed to explore this and related phenomena. It could also lead to a new class of optical materials with exotic properties that exploit non-reciprocal light flow.

Hard-X-ray mirrors usually rely on total external reflection at grazing incidence, owing to the high-penetration and low X-ray reflectivity of most materials. A demonstration of the almost perfect reflectance of hard X-rays from diamond at near-normal incidence could allow the development an entirely new class of X-ray optics.

Advances in X-ray imaging are allowing the investigation of molecular dynamics on an attosecond timescale and at angstrom-scale spatial resolution. It is now even possible to reconstruct images of the molecular orbitals, which provides us with a better understanding of how molecules respond to intense fields.

Turbulence usually makes plasmas more homogeneous. But in an unusual device for which the confining field is generated by a levitated half-tonne superconducting magnet, a study finds that turbulent fluctuations can actually increase the density of a plasma by driving diffusion against a density gradient.

Back-action, the effect of a measurement on the subject system, limits precision when determining position. This is of particular importance in nanomechanical oscillators, which could soon enter the quantum regime. A technique that avoids back-action by coupling the oscillator to a microwave resonator has now been demonstrated.

A microfluidic valve that amplifies the pressure in a fluid channel enables the realization of static microfluidic digital control logic. This in turn could enable more versatility and integration in the control of flows in ‘lab-on-a-chip’ systems.

Mimicking even the simplest of animal behaviour, such as walking along uneven terrain, is a challenging task. A study finds that incorporating a simple but inherently chaotic pattern generator into the control system of an autonomous robot allows it to show adaptive behaviour, enabling it to successfully navigate through a complex environment.