Research articles

Orbital order is important to many correlated electron phenomena, including colossal magnetoresistance and high-temperature superconductivity. A study of a previously unreported structure transition in KCuF₃ suggests that direct interorbital exchange is important to understanding such order.

Helical Dirac fermion states in topological insulators could enable dissipation-free spintronics and robust quantum information processors. A study of the influence of disorder on these states shows that although they are resilient against backscattering by magnetic impurities, fluctuations caused by charge impurities could cause problems for such applications.

Unconventional superconductivity is usually associated with a layered system. But how thin can a layered superconductor be and continue to be superconducting? Painstakingly grown superlattices of the heavy-fermion superconductor CeCoIn₅ suggest it could be as thin as a single layer.

Disorder-induced Anderson localization usually causes conducting materials to become insulating at low temperature. Graphene is a notable exception. But by increasing the carrier density in one graphene layer, a metal–insulator transition can be induced in an isolated second layer stacked above it.

Constraint-satisfaction problems are among the computationally hardest tasks: solutions are efficiently checkable, but no efficient algorithms are known to compute those solutions. Fresh insight might come from physics. A study mapping optimization hardness onto the phenomena of turbulence and chaos suggests that constraint-satisfaction problems can be tackled using analog devices.

So-called topological properties can make quantum systems robust to a wide class of microscopic perturbations. Theoretical work now shows that topological features and phenomena occur not only in closed systems, but also in open quantum systems with appropriately engineered dissipation.

The electronic properties of graphene depends on how many layers are involved. Monolayer graphene is a zero-gapped semi-metal. Bilayer graphene is a small-gapped semiconductor. Magnetotransport measurements indicate trilayer graphene can be both, depending on its stacking.

Monolayer graphene has no electronic band gap. Bilayer graphene does, and can be controlled by an electric field. And for trilayer graphene, infrared transmission measurements indicate both situations are possible depending on the stacking of the layers.

Soon after the isolation of graphene, it was discovered that the charge carriers in monolayer and bilayer sheets exhibit exotic Berry phases of π and 2π respectively. Now, magnetotransport measurements suggest the sequence continues in trilayer graphene, with charge carriers that exhibit a Berry phase of 3π.

Experiments that exploit non-classical properties of light promise to provide unique information about many-body systems. The limited availability of non-classical light sources, however, makes their implementation challenging. A method to calculate the quantum-optical response of a material from signals measured by using coherent-light excitation might provide an alternative route.

The radiation produced when an intense laser interacts with a solid target could provide a cheaper source of X-rays to synchrotrons and free-electron lasers. But they can also produce short bursts of gamma rays, whereas synchrotrons do not.

At the nanoscale, the conductance of a coherent conductor is reduced by the back-action of the circuit in which it is inserted. The effect has been primarily studied for cases where it is small, but these authors explore the regime of strong back-action—with conductance reductions of up to 90%—and propose a generalized expression for the conductance of quantum channels embedded in linear circuits.

An open quantum system loses its ‘quantumness’ when information about the state leaks into its surroundings. Researchers now control this so-called decoherence in a single photon. By rotating an optical filter, the information flow between the photon and its environment can be tuned. This concept could be harnessed for future quantum technologies.

Understanding the origin of colossal magnetoresistance in the manganites has proved to be one of the more difficult challenges in condensed-matter physics. An unexpected discovery of polarons in the metallic ground state of bilayer manganites could be an important clue.

‘Squeezed light’ enables quantum noise in one aspect of light to be reduced by increasing the noise, or more accurately the quantum uncertainty, of a complementary aspect. This has now been used to push the detectors at the heart of the GEO600 gravitational wave observatory to unprecedented levels of sensitivity.

When the insulators lanthanum aluminate and strontium titanate are brought together, the interface between them forms a two-dimensional superconductor. Moreover, magnetic imaging of this interface shows that superconductivity and ferromagnetism coexist in separated nanoscale domains.

A single quantum system comprising a nitrogen-vacancy in diamond is now coupled to a nanowire cantilever. Magnetic fields can then couple the nitrogen-vacancy spin and the oscillator enabling read-out of the nanometre-scale motion.

Lanthanum aluminate and strontium titanate are insulators, but when you bring them together, the interface between them becomes a two-dimensional superconductor. Even more surprising, magnetometry and transport measurements show that this superconducting state coexists with magnetic order.

It is widely believed that high-field superconductivity in heavy fermion metals is sustained only when the effective mass of its conduction electrons diverge. Measurements of magnetically driven changes in the electronic topology of URhGe suggest it is not divergence of the effective mass to infinity but a vanishing of the Fermi velocity to zero that supports this behaviour.

Superconductivity and magnetism have often been regarded as opposites. High magnetic fields usually destroy the superconducting state. But for superconductors constrained to two dimensions, a parallel magnetic field can actually enhance superconductivity.