Spectroscopic detection of Dirac and Weyl fermions in real materials is vital for both, promising applications and fundamental bridge between high-energy and condensed-matter physics. While the presence of Dirac and noncentrosymmetric Weyl fermions is well established in many materials, the magnetic Weyl semimetals still escape direct experimental detection. In order to find a time-reversal symmetry breaking Weyl state we design two materials and present here experimental and theoretical evidence of realization of such a state in one of them, YbMnBi₂. We model the time-reversal symmetry breaking observed by magnetization and magneto-optical microscopy measurements by canted antiferromagnetism and find a number of Weyl points. Using angle-resolved photoemission, we directly observe two pairs of Weyl points connected by the Fermi arcs. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties.

An essential ingredient for the production of Majorana fermions for use in quantum computing is topological superconductivity^1,2. As bulk topological superconductors remain elusive, the most promising approaches exploit proximity-induced superconductivity³, making systems fragile and difficult to realize^4–7. Due to their intrinsic topology⁸, Weyl semimetals are also potential candidates^1,2, but have always been connected with bulk superconductivity, leaving the possibility of intrinsic superconductivity of their topological surface states, the Fermi arcs, practically without attention, even from the theory side. Here, by means of angle-resolved photoemission spectroscopy and ab initio calculations, we identify topological Fermi arcs on two opposing surfaces of the non-centrosymmetric Weyl material trigonal PtBi₂ (ref. ⁹). We show these states become superconducting at temperatures around 10 K. Remarkably, the corresponding coherence peaks appear as the strongest and sharpest excitations ever detected by photoemission from solids. Our findings indicate that superconductivity in PtBi₂ can occur exclusively at the surface, rendering it a possible platform to host Majorana modes in intrinsically topological superconductor–normal metal–superconductor Josephson junctions.

(Ba,K)BiO₃ constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature T_c of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO₃ with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO₃. The parent compound BaSbO_3−δ shows a larger charge density wave gap compared to BaBiO₃. As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to T_c = 15 K. This value is lower than the maximum T_c of (Ba,K)BiO₃, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO₃ indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors.

$M$CrS${}_{2}$ compounds ($M=\mathrm{Li}$, Na, K, Cu, Ag, and Au) with triangular Cr layers show a large variety of magnetic ground states, ranging from 120${}^{$\circ${}}$ antiferromagnetic order of Cr spins in LiCrS${}_{2}$ to double stripes in AgCrS${}_{2}$, helimagnetic order in NaCrS${}_{2}$, and, finally, ferromagnetic Cr layers in KCrS${}_{2}$. On the base of ab initio band structure calculations and an analysis of various contributions to exchange interactions between Cr spins, we explain this tendency as originating from a competition between antiferromagnetic direct nearest-neighbor $d$-$d$ exchange and ferromagnetic superexchange via Sulfur $p$ states, which leads to a change of sign of the nearest-neighbor interaction, depending on the radius of an $M$ ion. It is shown that other important interactions are the third-neighbor interaction in a layer and interlayer exchange. We suggest that strong magnetoelastic coupling is probably responsible for the multiferroic properties of at least one material in this family, namely, AgCrS${}_{2}$.

The presence of both inversion (P) and time-reversal (T) symmetries in solids leads to a double degeneracy of the electronic bands (Kramers degeneracy). By lifting the degeneracy, spin textures manifest themselves in momentum space, as in topological insulators or in strong Rashba materials. The existence of spin textures with Kramers degeneracy, however, is difficult to observe directly. Here, we use quantum interference measurements to provide evidence for the existence of hidden entanglement between spin and momentum in the antiperovskite-type Dirac material Sr₃SnO. We find robust weak antilocalization (WAL) independent of the position of E_F. The observed WAL is fitted using a single interference channel at low doping, which implies that the different Dirac valleys are mixed by disorder. Notably, this mixing does not suppress WAL, suggesting contrasting interference physics compared to graphene. We identify scattering among axially spin-momentum locked states as a key process that leads to a spin-orbital entanglement.