Research articles

Measurements of the local response and noise during the ageing of a polymer film show deviations from the expected fluctuation–dissipation theorem. Moreover, scaling behaviour suggests a separate universality class for fragile glass formers.

During galaxy formation, the condensing matter can swirl into a new star or feed a central black hole. But what favours one mechanism over the other? It may be that there exist two critical surface densities of the matter cloud: a lower limit above which star formation occurs, and a higher threshold above which black holes form.

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 structure that allows neutrons to be trapped in long-lived ‘whispering gallery’ states provides scientists with a potentially useful tool to study the interaction of neutrons with matter. It could also allow the development of quantum neutron optics.

A long-standing goal of experiments using cold atoms in optical lattices is to simulate the behaviour of strongly correlated electrons in solid-state systems. However, in these experiments, the atoms are confined to spatially inhomogeneous traps, whereas the desired information concerns homogeneous bulk systems. Theoretical work now proposes a way to connect the two types of system.

It is now shown that the semiconductor InSb becomes transparent to terahertz radiation when an appropriate magnetic field is applied. This effect has never been seen before, despite decades of research on InSb, and the phenomenon could find important applications in the burgeoning field of terahertz imaging.

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 spectroscopic technique that enables momentum-resolved probing of excitations of atomic gases in optical lattices allows the full band structure of such systems to be measured for the first time. The method should facilitate the comparison of quantum-gas phases with their condensed-matter counterparts.

When a Van Hove singularity exists near the Fermi energy of a solid’s density of states, it can cause a variety of exotic phenomena to emerge. Scanning tunnelling microscope measurements indicate that when graphite’s graphene sheets are rotated out of their usual alignment, it can generate low-energy Van Hove singularities for which the position is controlled by the angle of rotation.

The composition of integral quantum number particles such as protons and neutrons from the strong confinement of fractional quantum number particles such as quarks is well known in high-energy physics. Now, similar behaviour has been found in condensed-matter physics, in the excitation spectra of a weakly coupled spin-ladder compound.

X-ray sources such as free-electron lasers offer the potential to study matter at unprecedented spatial and temporal resolution. But that potential is limited by the poor quality of conventional X-ray optical elements. An in situ technique that corrects for wavefront aberrations and allows X-rays to be focused to a spot just 7 nm wide could provide a solution.

High-temperature superconductivity in the cuprates arises when charge carriers are added to an insulator. Between these states lies the so-called nodal liquid at low temperature. Photoemission spectroscopy suggests that superconductivity evolves smoothly from this nodal-liquid state.

A simple programmable quantum processor has been created using trapped atomic ions. The system can be programmed with 15 classical inputs to produce any unitary operation on two qubits. This trapped-ion approach is amenable to scaling up for creating more complex circuits.

The transition from a ferromagnetic to a paramagnetic state is observed directly as the density of carriers that mediate spin–spin coupling is varied. The measurement was performed on thin films of GaMnAs and was made possible by superconducting quantum interference devices (SQUIDS).

The Nernst effect—the generation of a transverse electric field in a system subject to a longitudinal temperature gradient and perpendicular magnetic field—is increasingly used as a probe of a material’s electronic structure. The discovery of an unexpected Nernst response in graphite establishes the role of dimensionality on this effect, and enables the individual contributions of bulk and surface to be distinguished.

There is considerable debate over the size and direction of the non-adiabatic component of the spin-torque generated when a current flows across a domain wall in a ferromagnet. Measurements of this property in a wall just 1–10 nm wide suggest its value is small, arising from purely magnetic dissipation mechanisms.

Measurements of the melting point of diamond at pressures of around 10 million atm suggest it could be present in crystalline form in the interiors of giant planets. At even higher pressures and temperatures about 50,000 K, diamond melts to form an unexpectedly complex, polymer-like fluid phase.

In a glassy system, a distribution of relaxation times indicates a system that continues to rearrange itself. Besides the main relaxations involved in the glass transition, there are faster dynamics associated with secondary relaxations, which are predicted to reconfigure structures that are stringy rather than tightly clustered.

Quantum oscillations in metals are a signature of electrons travelling in closed orbits in a magnetic field. Could such oscillations occur in the absence of closed orbits, as seems to be the case for the copper oxide superconductors that have arc-like segments instead of closed Fermi surfaces?

Gapless edge-state excitations known as one-dimensional chiral fermions explain many experimental observations of the behaviour of integer quantum Hall systems. But prevailing theory suggests the emergence of extra edge states as well. A new spectroscopic technique for probing the flow of energy in the edge channels of a quantum Hall device finds no loss of energy to such extra states.