Letters in 2013

Geometrically frustrated spin-systems do not order magnetically even at absolute zero, forming instead a spin liquid or a glassy state. An organic conductor in which the charges, rather than spins, are frustrated now shows a similar absence of long-range order, resulting in a charge-cluster glass at low temperature.

An experiment now demonstrates the deterministic continuous-variable teleportation between two atomic ensembles at room temperature. This protocol makes it possible to teleport time-evolving quantum states from one ensemble to the other.

A study of the magnetic-field-induced superconductor–insulator transition shows that the insulating state is the electromagnetic dual of the superconducting state. However, the duality breaks down at low temperature, suggesting an extra insulating state—such as the proposed superinsulator.

Experiments with ultracold atomic gases can provide insight into more general phenomena, such as spin transport. A study of spin diffusion in a two-dimensional Fermi gas measured the lowest spin diffusion constant so far, approaching its quantum-limited value.

Quantum coherence has been extensively investigated in quantum optics, but less is known about its properties in massive particles. The higher-order many-body correlation functions have now been measured in an atom optics experiment, validating Wick’s theorem.

Different probes have found different superconducting pairing states in different iron-based high-temperature superconductors. Now transport measurements suggest that pressure drives the superconducting state in KFe₂As₂ from d-wave to s-wave.

Two indistinguishable single photons that simultaneously enter a beam splitter will always leave together, and this Hong–Ou–Mandel effect is now observed with microwave photons for the first time. Coherence between the beam-splitter output arms is demonstrated, enabling two-mode entanglement, which is useful for quantum communication processing at microwave frequencies.

Random lasing, where light is amplified through multiple scattering in a gain medium, could occur naturally in astrophysical environments. Experimental evidence for random lasing in a cloud of cold atoms may lead to a better understanding of these astrophysical lasers.

Metamaterials can negatively diffract optical-wavelength light; however, they suffer from high losses and only work over a narrow band of frequencies. Researchers now show how nonlinear optics in thin films of graphite can offer a solution. The negligible thickness of the layers reduces the losses, and the linear band structure of the material ensures broadband operation.

The Josephson effects that arise when two quantum states are coupled through a barrier are difficult to observe in optical systems because photon–photon interactions are so weak. Researchers have now demonstrated an optical realization of two such phenomena—macroscopic self-trapping and Josephson oscillations—using polariton condensates in overlapping microcavities.

Early specific-heat measurements of the archetypal spin ice Dy₂Ti₂O₇ showed a residual entropy at low temperatures similar to that found in water ice. A technique exploiting slow thermal equilibration now reveals an absence of this entropy—calling into question the nature of Dy₂Ti₂O₇ at low temperatures.

A Bose–Einstein condensate can exist in a superheated state well above the critical temperature if the interaction strength is tuned low. When the interactions are switched back on, the condensate boils away.

Free-flowing granular media can quickly become jammed above a critical density. Nonlinear dynamical systems analysis now suggests that jamming arises from the interaction between the density of instabilities and the propagation of disturbances throughout the material.

Graphene may be set to revolutionize electronics, but its small spin–orbit coupling limits its potential in spintronics. It is now shown, however, that adding hydrogen atoms can greatly enhance the magnetic properties of graphene. This then enabled the observation of the spin Hall effect, essential for controlling spin currents.

In traditional electron spin resonance techniques external magnetic fields are required. Now the electron spin can be manipulated in the absence of an applied magnetic field, by a technique that exploits the spin–orbit coupling of electrons travelling on surface acoustic waves.

In a topological insulator, the surface-state electron spins are ‘locked’ to their direction of travel. But when an electron is kicked out by a photon through the photoelectric effect, the spin polarization is not necessarily conserved. In fact, the ejected spins can be completely manipulated in three dimensions by the incident photons.

Networks competing for limited resources are often more vulnerable than isolated systems, but competition can also prove beneficial—and even prevent network failure in some cases. A new study identifies how best to link networks to capitalize on competition.

Data from the Cassini spacecraft identify strong electron acceleration as the solar wind approaches the magnetosphere of Saturn. This so-called bow shock unexpectedly occurs even when the magnetic field is roughly parallel to the shock-surface normal. Knowledge of the magnetic dependence of electron acceleration will aid understanding of supernova remnants.

Electrons can travel though very pure materials without scattering from defects. In this ballistic regime, magnetic fields can manipulate the electron trajectory. Such magnetic electron focusing is now observed in graphene. Although the effect has previously been seen in metals and semiconductors, it is evident in graphene at much higher temperatures—including room temperature.

When CaFe₂As₂ is lightly doped with Co an electronic liquid-crystalline state emerges, which becomes the ‘parent’ state of high-temperature superconductivity in this ferropnictide. A spectroscopic imaging study shows that the ‘nematic’ order is likely to be an artefact of the doping itself.