Letters in 2013

Chemical substitution often mimics the effects of applied pressure on a compound, and ‘doping’ is a standard way to reach a quantum critical point from a given phase. However, CeCoIn₅ is a natural quantum critical superconductor, and Cd-doping tunes the system away from criticality. Applied pressure reverses the effect of doping, but although superconductivity is restored, quantum criticality is not.

Frequency combs provide a broad series of well-calibrated spectral lines for highly precise metrology and spectroscopy, but this usually involves a trade-off between power and accuracy. A comb created by adjusting the time delay between two optical pulses now enables both. This so-called Ramsey comb could probe fundamental problems such as determining the size of the proton.

CeCoIn₅ is a d-wave heavy-fermion superconductor. By tuning the coupling between magnetic and superconducting order, a phase with inhomogeneous p-wave superconductivity can be detected, which coexists with d-wave superconductivity and spin-density-wave order.

The creation of Feshbach molecules by exploiting engineered spin–orbit coupling in a spin-polarized Fermi gas advances the experimental study of topological superfluidity in ultracold gases.

In open quantum systems the correlations between the system and its environment play an important role. A trapped-ion experiment demonstrates that these correlations can be detected without accessing or knowing anything about the environment or its interactions.

Magnetic monopoles continue to be elusive. However, an experiment now shows that the interaction of an electron beam with the tip of a nanoscopically thin magnetic needle—a close approximation to a magnetic monopole field—generates an electron vortex state, as expected for a true magnetic monopole field.

Microgravity experiments on a dust bed in a ‘drop tower’ set-up reveal the ability of martian soil to act as an efficient gas pump when heated by the Sun.

Networks that fail can sometimes recover spontaneously—think of traffic jams suddenly easing or people waking from a coma. A model for such recoveries reveals spontaneous ‘phase flipping’ between high-activity and low-activity modes, in analogy with first-order phase transitions near a critical point.

Confined within a porous aerogel, superfluid ³He loses its long-range order owing to random microscopic disorder, and becomes a glassy superfluid. Intriguingly, this effect can be switched off and the superfluidity restored.

Being able to sense nuclear spin dimers is an important next step towards single-molecule structural analysis from NMR measurements. Now the sensing of a single ¹³C–¹³C nuclear spin dimer near a nitrogen–vacancy centre in diamond is reported, together with a structural characterization at atomic-scale resolution.

Every metal has an underlying Fermi surface that gives rise to quantum oscillations. So far, quantum oscillation measurements in the superconductor YBCO have been inconclusive owing to the structural complexities of the material. Quantum oscillations in a Hg-based cuprate—with a much simpler structure—help to establish the origin and universality of the oscillations.

When a paramagnetic molecule is placed on a superconducting surface the lifetime of its spin excitations increases dramatically. This effect, caused by the depletion of the electronic states within the energy gap at the Fermi level, could find application in coherent spin manipulation.

Femtosecond pulses from X-ray free-electron lasers offer a powerful method for observing the coherent dynamic of phonons in crystalline materials, it is now shown. This time-resolved spectroscopic tool could provide insight into low-energy collective excitations in solids and how they interact at a microscopic level to determine the material’s macroscopic properties.

Ultracold atoms in optical lattices are used to study various phenomena in condensed-matter physics, such as magnetism. A lattice-shaking technique can induce a strong effective spin-interaction, leading to the formation of ferromagnetic domains.

An action generates an equal and opposite reaction. If it were possible, however, for one of the two bodies to have negative mass, they would accelerate each other. A situation analogous to this is now realized in an optical system. Solitons moving in an optical mesh lattice exhibit either an effective positive or negative mass, thus enabling observation of self-acceleration.

The intensity of optically-pumped fluorescence generated from a single atomic defect in diamond can be reduced by 80% in just 100 ns by applying infrared laser light. This result demonstrates the possibility of using these so-called nitrogen–vacancy centres to create optical switches that operate at room temperature.

BiTeCl is a topological insulator with strong inversion asymmetry, which exhibits bulk charge polarization and pyroelectricity. Such a long-sought topological insulator paves the way for applications involving natural p–n junctions and spintronics.

Real-world networks are rarely isolated. A model of an interdependent network of networks shows that an abrupt phase transition occurs when interconnections between independent networks are added. This study also suggests ways to minimize the danger of abrupt structural changes to real networks.

The Van Allen radiation belts are two rings of charged particles encircling the Earth. Therefore the transient appearance in 2012 of a third ring between the inner and outer belts was a surprise. A study of the ultrarelativistic electrons in this middle ring reveals new physics for particles above 2 MeV.

An ab initio study suggests that a known oxide superconductor, BaBiO₃, can be doped into a topological insulating state. This would simplify topological insulator–superconductor structures for applications.