Letters in 2021

Active matter exhibits a plethora of collective phenomena in both biological and artificial systems. In a model system of colloidal rollers, polar states in active liquids can be controlled.

Twisted double bilayer graphene is predicted to be a topological insulator under certain conditions. Simultaneous bulk and edge measurements now show metallic transport with a bulk bandgap, suggestive of this prediction.

The performance of superconducting devices can be degraded by quasiparticle generation mechanisms that are difficult to identify and eliminate. Now, a small superconducting island can be kept quasiparticle free for seconds at a time.

A semiconductor platform for experimentally investigating the multiorbital Bose–Hubbard model with long-range interactions is demonstrated. The interactions between the excitons are strong enough to reach the Mott insulator regime.

Accurate measurements of the ohm require high magnetic fields to support the quantum Hall effect. Now, high precision is achieved by using the quantum anomalous Hall effect in a low magnetic field, making the measurement much more accessible.

Although magnons in the quantum Hall regime of graphene have been detected, their thermodynamic properties have not yet been measured. Now, a local probe technique enables the detection of the magnon density and chemical potential.

Topological states that are created from strong electron–electron interactions at half-integer superlattice fillings are observed at zero magnetic field.

Propagating spin waves known as magnons are expected to carry a dipole moment in the quantum Hall regime. Now, this moment has been detected, demonstrating that the degrees of freedom of spin and charge are entangled in quantum Hall magnons.

Stacking and twisting two-dimensional materials has led to the observation of a variety of electronic phenomena. Now, magnetic behaviour that is distinct from anything seen in individual layers is induced by a moiré pattern in double bilayer chromium triiodide.

Experiments on cell monolayers on corrugated hydrogels reveal the effects of local curvature on the shape of cells and nuclei. A vertex model lends support to the idea that the modulation of tissue thickness may enable curvature sensing.

Form factors encode the structure of nucleons. Measurements from electron–positron annihilation at BESIII reveal an oscillating behaviour of the neutron electromagnetic form factor, and clarify a long-standing photon–nucleon interaction puzzle.

Twisted bilayer graphene hosts flat electronic bands, but their relationship to the observed correlated phases is still debated. Here, it is shown that electron–electron interactions can help to flatten the bands and generate the correlated phases.

Interactions between atoms in a Bose–Einstein condensate cause quantum fluctuations and the creation of additional correlations between pairs of atoms. These effects have now been directly observed, confirming long-standing theoretical predictions.

As tissues grow, a small fraction of cells can give rise to a large fraction of the tissue. A model borrowed from forest fires suggests that this can occur spontaneously in development as a collective property of the cell interaction network.

The evolution of many-body magnetic spin systems is influenced by many factors, including inhomogeneity and the presence of interfaces. These effects have now been measured in a far-from-equilibrium binary mixture of ultracold gases.

Through chemical design, the spins in molecular nanomagnets may be used as electrically tunable qubits. Electrical control of molecular distortion enables manipulation of the quantum spin state while suppressing decoherence from magnetic fields.

A study of growing apples shows that the singular cusp at the stalk has a universal form that arises due to the differential growth of a soft solid. Although the cusps are usually symmetric, they can lose stability to form lobes that depend on the geometry of the fruit.

The development of glands involves cylindrical branches transforming into spherical alveoli. Now there is evidence to suggest that this process can be understood as a budding instability driven by a decrease in tension anisotropy in the tissue.

In addition to the broken time-reversal symmetry that typifies Chern insulators, twisted bilayer graphene hosts a set of topological states with broken translational symmetry.

Accurate mass measurements of the indium isotopes adjacent to the doubly magic ¹⁰⁰Sn provide critical benchmarks for ab initio theory, which withstands the challenge.