Magnetic Weyl semimetals are a newly discovered class of topological materials that may serve as a platform for exotic phenomena, such as axion insulators or the quantum anomalous Hall effect. Here, we use angle-resolved photoelectron spectroscopy and ab initio calculations to discover Weyl cones in CoS2, a ferromagnet with pyrite structure that has been long studied as a candidate for half-metallicity, which makes it an attractive material for spintronic devices. We directly observe the topological Fermi arc surface states that link the Weyl nodes, which will influence the performance of CoS2 as a spin injector by modifying its spin polarization at interfaces. In addition, we directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS2 cannot be a true half-metal.

The ultrafast optical response of two nodal-line semimetals, ZrSiS and ZrSiSe, was studied in the near-infrared using transient reflectivity. The two materials exhibit similar responses, characterized by two features, well resolved in time and energy. The first transient feature decays after a few hundred femtoseconds, while the second lasts for nanoseconds. Using Drude-Lorentz fits of the materials' equilibrium reflectance, we show that the fast response is well-represented by a decrease of the Drude plasma 1 arXiv:2005.04248v1 [cond-mat.mtrl-sci] 8 May 2020 frequency, and the second feature by an increase of the Drude scattering rate. This directly connects the transient data to a physical picture in which carriers, after being excited away from the Fermi energy, return to that vicinity within a few hundred femtoseconds by sharing their excess energy with the phonon bath, resulting in a hot lattice that relaxes only through slow diffusion processes (ns). The emerging picture reveals that the sudden change of the density of carriers at the Fermi level instantaneously modifies the transport properties of the materials on a timescale not compatible with electron phonon thermalization and is largely driven by the reduced density of states at the nodal line.

Here we report an ultrafast optical spectroscopic study of the nodal-line semimetal ZrSiTe. Our measurements reveal that, converse to other compounds of the family, the sudden injection of electronic excitations results in a strongly coherent response of an A 1g phonon mode that dynamically modifies the distance between Zr and Te atoms and Si layers. "Frozen phonon" density functional theory calculations, in which band structures are calculated as a function of nuclear position along the phonon mode coordinate, show that large displacements along this mode alter the material's electronic structure significantly, forcing bands to approach and even cross the Fermi energy. The incoherent part of the time-domain response reveals that a delayed electronic response at low fluence discontinuously evolves into an instantaneous one for excitation densities larger than 3.43 × 10 17 cm −3 . This sudden change of the dissipative channels for electronic excitations is indicative of an ultrafast Lifshitz transition, which we tentatively associate with a change in topology of the Fermi surface driven by a symmetry-preserving A 1g phonon mode.

Some moral theorists argue that being an innocent beneficiary of significant harms inflicted by others may be sufficient to ground special duties to address the hardships suffered by the victims, at least when it is impossible to extract compensation from those who perpetrated the harm. This idea has been applied to climate change in the form of the beneficiary‐pays principle. Other philosophers, however, are quite sceptical about beneficiary pays (both in general and in the particular case of human‐induced climate change). Our aim in this article is to examine their critiques. We conclude that, while they have made important points, the principle remains worthy of further development and exploration. Our purpose in engaging with these critiques is constructive — we aim to formulate beneficiary pays in ways that would give it a plausible role in allocating the cost of addressing human‐induced climate change, while acknowledging that some understandings of the principle would make it unsuitable for this purpose.

Square-net materials are well positioned to lead optical spectroscopic explorations into the electronic structure, photoinduced dynamics, and phase transitions in topological semimetals. Hundreds of square-net topological semimetals can be prepared that have remarkably different electronic and optical properties despite having similar structures. Here we present what has been gleaned recently from these materials with the whole gamut of optical spectroscopies, ranging from steady-state reflectance and Raman investigations into topological band structures, electronic correlations, and equilibrium phase transitions to time-resolved techniques used to decipher ultrafast relaxation dynamics and nonequilibrium photoinduced phase transitions. We end with a discussion of some major remaining questions and possible future research directions.