Articles in 2020

A general approach to derive direction-dependent complex refractive indices close to zero produces infinite families of time-reversible and infinite families of time-irreversible electromagnetic materials, without invoking the concept of topology.

Topological defects in the nematic order of actin fibres in a regenerating organism are shown to be tied to key feature formation. Fibre alignment sets the regenerated body axis and defect sites form organizing centres for the developing body plan.

Spin currents are generated from an antiferromagnet/heavy-metal heterostructure using optical excitation on picosecond timescales. This will have applications in antiferromagnetic spintronics.

Energy–momentum phase-matching enables strong interactions between free electrons and light waves. As a result, the wavefunction of the electron exhibits a comb structure, which was observed using photon-induced near-field electron microscopy.

Active matter particles self-propel but controlling their direction of motion can be challenging. Here the authors place motile bacteria inside microdroplets and control their propulsion by exploiting the asymmetric director structure of the surrounding liquid crystal.

Stacking a monolayer and bilayer of graphene, with a small twist angle between them, creates a tunable platform where the physics of both twisted bilayer graphene and twisted double bilayer graphene can be realized.

Scale-invariant magnetic anisotropy in RuCl₃ has been revealed through measurements of its magnetotropic coefficient, providing evidence for a high degree of exchange frustration that favours the formation of a spin liquid state.

Very long baseline interferometry is used to compare two optical clocks located in Japan and Italy through the observation of extragalactic radio sources. This approach overcomes limitations of the performance of satellite transfer techniques.

Animal behaviour is characterized by repeated movements which can be difficult to analyse quantitatively. Here, the authors apply a data-driven framework based on theory of dynamical systems to characterize nematode behaviour and explain its complexity through deterministic chaotic dynamics.

Spatiotemporal waves appear during collective cell migration and are affected by mechanical forces and biochemical signalling. Here the authors develop a biophysical model that can quantitatively account for complex mechanochemical patterns, and predict how they can be used for optimal collective migration.

Inertial dynamics are observed in a ferromagnet. Specifically, a nutation is seen on top of the usual spin precession that has a lifetime on the order of 10 picoseconds.

The phrase ‘arrow of time’ refers to the asymmetry in the flow of events. A machine learning algorithm trained to infer its direction identifies entropy production as the relevant underlying physical principle in the decision-making process.

Many-body localization—a phenomenon where an isolated system fails to reach thermal equilibrium—has been studied with a programmable quantum processor, which reveals the crucial role played by the initial energy on the onset of localization.

The authors investigate the role of spherical confinement and curvature-induced topological defects on the crystallization of charged colloids. They conclude that crystallization in spherical confinement is due to a combination of thermodynamics and kinetic pathways.

Einstein–Podolsky–Rosen entanglement between a millimetre-size mechanical membrane oscillator and a collective atomic spin oscillator formed by an ensemble of caesium atoms is achieved, although the two systems are spatially separated by one metre.

A time–information uncertainty relation in thermodynamics has been derived, analogous to the time–energy uncertainty relation in quantum mechanics, imposing limits on the speed of energy and entropy exchange between a system and external reservoirs.

A new form of superradiance is predicted that ‘in contrast to the standard effect’ arises even for vanishing numbers of particles per wavelength. This finding may enable coherent emission in plasma accelerators.

A weak-to-strong quantum measurement transition has been observed in a single-trapped-ion system, where the ion’s internal electronic state and its vibrational motion play the roles of the measured system and the measuring pointer.

Softness, a machine-learned structural quantity, has been recently identified as a parameter that characterizes glassy dynamics. Here, the authors observe devitrification in 3D soft colloidal glasses and find that softness may indicate regions predisposed to crystallization.

The radiation emission rate from gravity-related wave function collapse is calculated and the results of a dedicated experiment at the Gran Sasso laboratory are reported, ruling out the natural parameter-free version of the Diósi–Penrose model.