Research articles

Two closely spaced two-dimensional systems can remain strongly coupled by electron–electron interactions even though they cannot physically exchange particles. Coulomb drag is a manifestation of this interaction—in which an electric current passed through one layer causes frictional charge flow in the other—now experimentally observed in bilayer graphene

Current shot-noise for a relativistic electron beam—proportional to the average current and frequency bandwidth of the beam—can be suppressed below the shot-noise limit at optical frequencies, through the exploitation of collective Coulomb interactions.

Entanglement is an important resource in quantum-enhanced technologies, but it is difficult to generate, especially in solid-state systems. An experiment now demonstrates the entanglement of two nuclear spins via a parity measurement of the electron spin in a nitrogen-vacancy centre in diamond.

Extreme ultraviolet and X-ray radiation can be generated when the high harmonics of incident laser light are reflected by a dense plasma, the so-called relativistically oscillating mirror mechanism. Theoretical studies have, however, predicted an alternative regime in which short-wavelength light is generated by dense electron nanobunches that form at the plasma–vacuum boundary. Signatures of this coherent synchrotron emission are now experimentally observed.

A super-elastic collision is one that results in an increase of kinetic energy in the colliding system. A probable occurrence of such a collision is shown in the huge, magnetized plasmas of two coronal mass ejections from the Sun.

The magnetic states found in iron-based superconductors are more complex than originally thought. This Review argues that the magnetism arises from both itinerant and localized electrons.

The so-called braking index calculated for the spin-down of rotating neutron stars, or pulsars, doesn’t tally well with observations. But a model accounting for a changing moment of inertia, as an increasing fraction of the stellar core becomes superfluid, can explain the rotational evolution of young pulsars.

Stable structures can self-assemble in plasmas flowing at supersonic speeds, as evident in many astronomical objects. But now it is also seen in the laboratory using two plasmas travelling in opposite directions, each created by ablating a plastic disc with high-power lasers.

A topological insulator has surface metallic states that are topologically protected by time-reversal symmetry. Tin telluride is now shown to be a ‘topological crystalline insulator’, in which the surface metallic state is instead protected by the mirror symmetry of the crystal.

Understanding the spin dynamics in magnetic nanostructures is important for spintronics, but so far it has been impossible to probe the spin dynamics directly. A neutron-scattering technique providing direct information about dynamical two-spin correlations in a molecular nanomagnet has now been demonstrated.

Sudden bursts of charged particles emitted from the surface of the Sun can disrupt the satellites orbiting Earth. However, the mechanisms that drive these so-called coronal mass ejections remain unclear. An advanced computer model now establishes a link between the onset of an ejection and the emergence of magnetic flux into the solar atmosphere.

The fractional alternating-current Josephson effect produces a series of steps in the current–voltage characteristics of a superconducting junction driven at radiofrequencies. This unusual phenomenon is now observed in a semiconductor–superconductor nanowire. What is more, a doubling in step size when a strong magnetic field is applied could be a possible signature of Majorana fermions, particles that are their own antiparticle.

A multicomponent gas of ytterbium atoms accommodates more entropy in its spin degrees of freedom than does its two-component analogue, leading to a lower effective temperature, and an easy route for cooling ultracold fermions towards a Mott-insulating state.

Atoms can be used as highly sensitive magnetic-field sensors. By exploiting the effects of electric fields on the optical transitions of excited Rydberg states, it is now demonstrated that it is also possible to probe very weak microwave electric fields with atoms.

Short nuclear spin–lattice relaxation times have long been a challenge for magnetic resonance imaging. The alternative of using long-lived nuclear spin states has so far required symmetry breaking, but a method of controlling these states without breaking the symmetry of the spin system has now been demonstrated.

A two-level quantum system driven by an electromagnetic field can oscillate between its two states. The effects of these so-called Rabi oscillations are usually obscured in many-body systems by the variation in properties of the particles involved. Now, however, coherent many-body Rabi oscillations are observed in a vapour made up of several hundred cold rubidium atoms.

Quantum gases are useful toy models for the study of quantum magnetism. Exquisite control of a spinor gas of fermionic atoms in an optical lattice has now been demonstrated, opening up the exploration of quantum magnetism with high spins.

Chirality is usually manifested by differences in a material’s response to left- and right-circularly polarized light. This difference is the result of the specific distribution of charge within chiral materials. A similar response has now been found to result from the chiral spin structure of an antiferromagnet.

Optical vortices exhibit a corkscrew-like shape as they travel. The study of this phenomenon, known as singular optics, is now extended to the high-power regime where high-harmonic processes become evident. This type of radiation could help illuminate novel attosecond phenomena in atoms and molecules.

An analogue of a magnetic monopole is now observed in a condensed state of light–matter hybrid particles known as cavity polaritons. Spin-phase excitations of the polariton fluid are accelerated along the cavity under the influence of a magnetic field—just as if they were single magnetic charges.