My problem is your problem

Nature 464, 1025–1028 (2010)

It all happened so quickly. At 3:25 on 28 September 2003, an electrical power line running across the Swiss Alps failed. Two minutes later, most of Italy was dark. A disastrous cascading effect had taken its course, involving not only the power grid, but the internet communication network as well — with the first power stations, communication nodes failed, causing further power stations to fail in turn. A national blackout ensued.

Sergey Buldyrev and colleagues present a general framework to analyse such catastrophic cascades in interdependent networks. The robustness of complex networks has been studied before, but Buldyrev et al. show that the situation changes significantly when accounting for the dependence of one network on another.

For example, single networks are less susceptible to random failure when the degree of its nodes — that is, the number of connections a given node has to other nodes — is broadly distributed. In interdependent networks, the opposite behaviour is found. As the model applies also to scenarios in which there are more than two interdependent networks, Buldyrev and colleagues' results should provide insight into a wide class of real-world situations.

Supersolid gas

Nature doi: 10.1038/nature09009 (2010)

The Dicke model describes a two-level system of spin-1/2 particles coupled to an electromagnetic field such that each spin is coupled with equal strength to all the other spins. For a phase transition to occur in such a system with infinitely long-range interactions, the coupling strength and the energy separation of the two ground states should be comparable — a condition usually impossible to satisfy, given the high energy scales of optical transition frequencies. However, Kristian Baumann and co-workers have managed it, using a driven Bose–Einstein condensate of 87Rb atoms coupled to an optical Fabry–Pérot cavity, for which the energies of the optical Raman transitions are much lower.

A pump laser, which drives the whole system, is coherently scattered by the atoms into the optical cavity. This gives rise to two-photon processes that lead to the long-range interactions. At a critical driving frequency, the Rb atoms self-organize into even or odd sites of a chequerboard pattern, which further improves their ability to scatter coherently. By design, light scattering causes atoms to be preferentially attracted to one of the sites, which then leads to more light scattering into the cavity in a positive feedback cycle that heralds a quantum phase transition into a macroscopically populated state that breaks Ising symmetry. Given the non-trivial diagonal long-range order, the state is conceptually equivalent to a supersolid.

It's elementary

Phys. Rev. Lett. 104, 142502 (2010)

Claims and falsifications have often dogged the quest to synthesize superheavy elements, but the heaviest element accepted at this time (of which only a few atoms have ever been created) has atomic number Z = 118. Elements up to Z = 116 are also known. Now, Yuri Oganessian and colleagues are able to complete that line up, with their synthesis of element 117.

Using the heavy-ion cyclotron at the Joint Institute for Nuclear Research in Dubna, Russia, they identified two isotopes, 293117 and 294117, in the fusion of calcium-48 projectiles with a berkelium-294 target. Both isotopes decay — by means of α-decay, through elements 115 and 113 — to arrive at roentgenium-281 and dubnium-270, respectively; those nuclei then undergo spontaneous fission. The details of the decay chains have revealed more of the properties of neutron-rich isotopes, underlining the importance in models of the concepts of nuclear shells and of the 'island of stability' for superheavy elements.

One-way flow

Nature Mater. 9, 413–417 (2010)

Credit: © 2010 NPG

A small volume of water forms a droplet when placed on a surface. Surface tension determines the exact shape of this drop, but it is generally symmetric. Kuang- Han Chu, Rong Xiao and Evelyn Wang have now structured a surface so that it forces water to flow preferentially in just one direction.

Surface patterning, it is known, offers some control over wettability — grooves, for example, can lead to anisotropically shaped drops. But such approaches have, in the past, always created droplets that maintain mirror-image symmetry: they are the same on the left as they are on the right.

Chu et al. patterned a silicon substrate into a grid of pillars, 500–750 nm across and 6–10 μm tall. One microlitre of water on this surface formed an axially symmetric droplet. But when a thin layer of gold was coated on one side of the pillars, the built-in stress made the nanostructures bend. On this engineered surface, the water spread in the direction of the bend (pictured). Such an effect could be incorporated into microfluidic systems for another level of control.

Ever decreasing circles

Phys. Rev. Lett. 104, 133002 (2010)

Neutral gas atoms are more difficult to manipulate or detect than ions. A way to solve this, for detection purposes at least, is to ionize them using a strong electric field. Carbon nanotubes are well suited to this task, particularly as they are small (so large fields can be generated in small volumes using relatively moderate voltages). Nanotube-based gas ionization sensors built so far have used fields generated at the tips of forests of vertical nanotubes: Anne Goodsell and colleagues demonstrate a simple device that exploits a radial field around a single, horizontally suspended, carbon nanotube.

When a sufficiently large voltage is applied to a suspended nanotube and a beam of neutral atoms allowed to pass nearby, the field generated by the nanotube can polarize atoms in the beam. The polarized atoms are attracted into a spiral orbit of tighter and tighter circles around the nanotube. The closer they get to the nanotube, the stronger the field they experience, until eventually they are ionized. The ions are then repelled by the nanotube's field, and ejected outwards to be picked up by a conventional ion detector.