Volume 8 Issue 5, May 2012

Two experiments have measured an all-important number in neutrino physics. Going by the innocuous name of 'θ₁₃', this parameter's value has significant implications for our understanding of the Universe.

Biological systems can adapt to changes in their environment over a wide range of conditions, but responding quickly and accurately is energetically costly. A study pins down the relationship between energy, speed and accuracy.

An experimental demonstration that the expansion of ultracold atoms in three dimensions can be frozen by disorder provides fertile ground for studies of metal–insulator transitions in disordered systems — including those with interacting particles.

The energy gap associated with Cooper pair formation in unconventional superconductors can fall to zero along lines of the Fermi surface. Differences in the shape and location of these lines bear information on the interaction that triggers Cooper pair formation.

A cross-validation study comparing experimental findings obtained with a system of ultracold fermions with the results of a method based on computing contributions from millions of Feynman diagrams underlines the potential of the so-called bold diagrammatic Monte Carlo technique for solving problems in the area of strongly correlated quantum matter.

The Cooper pairs of conventional superconductors exhibit a nodeless s-wave symmetry, and most unconventional superconductors, including cuprates and heavy-fermion materials, exhibit nodal d-wave pairing. In contrast to both, angle-resolved photoemission spectroscopy measurements indicate that the iron-based superconductor BaFe₂(As_0.7P_0.3)₂ exhibits an unusual nodal s-wave pairing.

An outstanding question about the iron-based superconductors has been whether or not their magnetic characteristics are dominated by itinerant or localized magnetic moments. Absolute measurements and calculations of the magnetic response of undoped and Ni-doped BaFe₂As₂ indicate the latter.

It is well known that graphene deposited on hexagonal boron nitride produces moiré patterns in scanning tunnelling microscopy images. The interaction that produces this pattern also produces a commensurate periodic potential that generates a set of Dirac points that are different from those of the graphene lattice itself.

Conventional approaches to optomechanics control and monitor the motion of nanoscale mechanical resonators by coupling it to a high-quality photonic cavity. An all-mechanical implementation is now demonstrated by creating a so-called phonon cavity from different oscillating modes of the resonator. This idea opens a route to using solid-state systems to investigate physics not accessible in their analogous, but better developed, quantum-optics counterpart.

Commutation relations define the limit to which two complementary properties can be simultaneously known—Heisenberg’s uncertainty principle. Yet it is thought that these canonical relations might be different in the quantum gravity regime. Researchers now show how quantum-optics experiments might provide a direct route for studying these effects.

An experimental study of three-dimensional localization of ultracold atoms in controlled disorder provides evidence for behaviour that is consistent with Anderson localization, but incompatible with classical trapping.

The magnetic character of the cuprates is suspected by many to be involved in the emergence of unconventional superconductivity. The discovery of a second distinct magnetic excitation in HgBa₂CuO₄ supports a multiband picture of the magnetic structure of these materials.

Spin transfer torque—the transfer of angular momentum from a spin-polarized current to a ferromagnet’s magnetization—has already found commercial application in memory devices, but the underlying physics is still not fully understood. Researchers now demonstrate the crucial role played by the polarization of the laser light that generates the current; a subtle effect only evident when isolated from other influences such as heating.

A demonstration of the ability to coherently control the collective attosecond dynamics of relativistic electrons driven through a plasma by an intense laser represents an important step in the development of techniques to manipulate and study extreme states of matter.

It is well known that organisms profit from adapting to their environment. A study of stochastic adaptation dynamics shows that this comes at the expense of adaptive speed and accuracy—providing a framework for understanding adaptation in noisy biological systems.

Small-world topologies characterize many natural and human-built networks. Yet, how such networks organize their link weights is not fully understood. These authors report an organization scheme that captures important features of real-world systems, and identify learning rules that allow evolving networks to obtain such weight organizations based on their history.