At the beginning of 2021, eight Physical Review journals began publishing Letters which are intended for the accelerated publication of important new results targeted to the specific readership of each journal.

Despite its ubiquity in the characterization of the electronic structure of solids, the interpretation of angle-resolved photoemission spectroscopy measurements is by no means a trivial exercise. Here, the authors implement a combined experimental and computational methodology to exemplify this problem and its solutions in application to the canonical Fe-based superconductor LiFeAs. This work acts to reconcile many years of conflicting experimental studies, while also presenting a viable route forwards in the rapidly growing field of topological superconductivity.

Magnetic frustration in a material may drive spin-spiral order, spontaneously breaking the translation and chiral symmetries of the crystal. The underlying hierarchy of magnetic interactions is reflected in the magnon excitations, which can be probed by inelastic neutron scattering. Here, the authors study the dispersions of magnons in the helimagnet FeP and compare the experimentally observed spectra with simulations. Upon examination of alternative models, the authors are able to correctly describe the spin excitations in FeP and unveil the corresponding frustration mechanism.

Edge excitations of (2+1)$d$ topological phases are usually described using continuum field theories. But the boundaries of some (2+1)$d$ topological phases can also be described using latticelike edge theories that have a finite-dimensional Hilbert space for a finite-sized boundary. Here, the authors construct such finite-dimensional edge theories for a family of Abelian topological phases. The most interesting examples are “ungappable”: they have the property that they cannot be gapped by any local interaction.

Twisted bilayer graphene exhibits flat bands, for which $C_3$ rotational symmetry is responsible. Therefore, to address the question whether other symmetries allow flat bands, the authors investigate here a twisted bilayer model with $C_4$ rotational symmetry, composed of the square lattice with $\pi$ flux per plaquette. It turns out that the model shows stable flat bands at arbitrarily small twist angles, and in particular, the gapped model allows many flat bands within the gap, which are reminiscent of the Landau levels.

The topology of typical Chern insulators is rooted in the system periodicity along two directions of real space. Here, the authors depart from this paradigm and demonstrate that a generic non-Hermitian photonic waveguide periodic along a single direction can be regarded as a topological system with a synthetic dimension. The gap Chern number of the extended system is identical to the number of bands below the gap and controls the number of Tamm state branches at an interface with another waveguide.

Thermalization is usually difficult to avoid in large complicated quantum systems. However, it was recently discovered that some models with dynamical constraints can fail to thermalize due to quantum many-body scarring. To explore the connection between dynamical constraints and scarring, the authors here construct a class of spin-chain models in which the strength of the constraints can be tuned. It is observed that the scarring becomes more extreme as the constraint is weakened.

The authors employ entanglement entropy as the means to reliably predict the presence of Majorana zero modes (MZMs) in experimentally relevant setups. Connecting with quantum transport calculations of the conductance spectra, they present ways to reliably signal the presence of topological MZMs that possess true nonlocal correlations, thereby distinguishing topological phase transitions from trivial ones. The detection of MZMs is a contentious issue, and the models presented here set the stage for looking beyond conductance spectra for their reliable identification.

A geometrically frustrated lattice provides an ideal platform to search for novel electromagnetic phenomena, since it is proven to yield exotic topological electronic states as well as novel magnetic ground states. This paper reports magnetotransport properties across the spin-ice-like magnetic order and liquid-gas-type magnetic transitions in the well-characterized pyrochlore semimetal Pr${}_2$Ir${}_2$O${}_7$. Remarkable field and pressure dependence of the anomalous Hall conductivity points to the dramatic change of the Weyl point locations in momentum space. These findings open up a new direction for topological band engineering in hybrid quantum materials with relativistic conduction electrons and localized magnetic moments.

The mechanism responsible for the generation of optically induced spin currents upon femtosecond laser-pulse excitation of ultrathin ferromagnetic layers is still a heavily debated topic. In this paper, the authors study the phase of spin-current induced terahertz spin waves in noncollinear bilayer structures to probe spin currents in Fourier space. They measure a significant laser-pulse energy dependence of the spin-wave phase, which is quantitatively consistent with a spin current that is proportional to the time derivative of the magnetization.

Nowadays, control of light based on the manipulation of surface plasmon polaritons (SPPs) in plasmonic crystals becomes a cornerstone of photonics. The authors study the SPP-driven enhancement of linear and nonlinear magneto-optical effects in magnetoplasmonic crystals (MPCs) based on corrugated gold and ferromagnetic films. A signature of the considered MPCs is the magnetic anisotropy mediated by the surface profile of a thin ferromagnetic layer, which allows to intertwine plasmonic effects with nonuniform magnetization, providing an extra venue for control of magneto-optical effects.

Multichannel transport may mask the intrinsic topological Hall effects and give rise to artificial topological Hall effects in van der Waals heterointerfaces. Here, the authors systematically investigate the multichannel transport in van der Waals ferromagnetic-ferromagnetic and ferromagnetic-metal heterostructures. Interestingly, both positive and negative topological Hall effects can be mimicked. The observations have been made with a proven lack of relation to chiral spin textures and are attributed to multichannel Hall effects occurring at the heterointerfaces.

Supercurrent transport requires vortices in superconductors to be pinned by material defects. Vortices are trapped in minima of the pinning potential landscape, and one has to determine what fraction of this landscape comes with a positive curvature. Such a curvature analysis is done by studying the Hessian matrix of the landscape. The authors find the stable area fraction to be unexpectedly low (yellow color in the figure). In particular, for Gaussian random landscapes it is $(3-\sqrt{3})/6\approx 21\%$. The results offer new hints for optimizing pinning landscapes. Their transdisciplinary application to natural landscapes exhibits a surprising universality.

Energy dissipation in current microelectronic devices is an inefficient process due to its non-Fourier behavior. By using thermoreflectance imaging, the authors measure the temperature distribution around nanoscale heaters of different sizes and shapes on a silicon substrate to test state-of-the-art modeling. The results indicate that hydrodynamiclike thermal transport equations successfully predict the local system response at temperatures ranging from 100 to 300 K. This is contrasted with kinetic interpretations based on the suppression of long mean free path phonons, which are shown not to become manifest in the experiments.

The anomalous Nernst effect (ANE), in which a thermal gradient drives a transverse charge current, can have both extrinsic and intrinsic origins. The intrinsic ANE should be enhanced when the Fermi level is in close proximity to Weyl points in the band structure. Magnetic Heusler alloys allow for tuning both the topology of the band structure and the position of the Fermi level. Here, the authors show that this tunability can be used to enhance the intrinsic ANE in epitaxial Heusler alloy thin films.

APS Editor in Chief, Michael Thoennessen, discusses a new opportunity for communicating authors to include their pronouns together with their contact email in order to promote a more respectful, inclusive, and equitable environment.