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${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ is a magnetic Weyl semimetal, in which ferromagnetic ordering at 177 K is predicted to stabilize Weyl points. We perform temperature and spatial dependent angle-resolved photoemission spectroscopy measurements through the Curie temperature (${T}_{c}$), which show large band shifts and renormalization concomitant with the onset of magnetism. We argue that ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ evolves from a Mott ferromagnet below ${T}_{c}$ to a correlated metallic state above ${T}_{c}$. To understand the magnetism, we derive a tight-binding model of Co-$3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals on the kagome lattice. At the filling obtained by first-principles calculations, this model reproduces the ferromagnetic ground state, and results in the reduction of Coulomb interactions due to cluster effects. Using a disordered local moment simulation, we show how this reduced Hubbard $U$ leads to a collapse of the bands across the magnetic transition, resulting in a correlated state, which carries associated characteristic photoemission signatures that are distinct from those of a simple lifting of exchange splitting. The behavior of topology across ${T}_{c}$ is discussed in the context of this description of the magnetism.

Using the density-functional-based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd1-xSrxNiO2. Our calculated nearest-neighbor exchange constant J1=82 meV is large, weakly affected by doping, and is only slightly smaller than that found in the sister compound CaCuO2. However, we find that the hole doping significantly enhances the interlayer exchange coupling as it affects the magnetic moment of the Ni-3d3z2-r2 orbital. This can be understood in terms of the small hybridization of Ni-3d3z2-r2 within the NiO2 plane, which results in a flat band near the Fermi level, and its large overlap along the z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level do not affect the magnetic exchange interactions, and thus they may not participate in the superconductivity of this compound. Whereas many previous works emphasized the importance of the Ni-3dx2-y2 and Nd-5d orbitals, we analyze instead the solution of the Ni-3dx2-y2/Ni-3d3z2-r2 minimal model using dynamical mean field theory. It reveals an underlying Mott insulating state that, depending on the precise values of the intra-atomic Hund's coupling smaller or larger than 0.83 eV, selects upon doping either S=0 or 1 two-hole states at low energies, leading to very different quasiparticle band structures. We propose that trends upon doping in the spin excitation spectrum and the quasiparticle density of states can be a way to probe the Ni 3d8 configuration. © 2021 American Physical Society. X.W. is supported by the NSFC (Grants No. 11834006, No. 11525417, No. 51721001, and No. 11790311), National Key R&D Program of China (Grants No. 2018YFA0305704 and No. 2017YFA0303203), and by 111 Project. X.W. also acknowledges the support from the Tencent Foundation through the XPLORER PRIZE. V.I., G.R., and S.Y.S. are supported by NSF DMR Grant No. 1832728. I.L. acknowledges support by the Russian Foundation for Basic Research (Project No. 18-32-20076). The DMFT electronic structure calculations were supported by the state assignment of Minobrnauki of Russia (theme “Electron” No. AAAA-A18-118020190098-5).

Non-centrosymmetric superconductors have recently attracted much attention, since the lack of inversion symmetry mixes spin singlet and triplet pairing states, which may allow the realization of topological superconductivity. In this work, we study the electronic properties of the family of inversion-broken CeTX$_3$ heavy-fermion superconductors, finding topological nodal lines as well as Dirac and Weyl points, which are renormalized closer to the Fermi energy by correlations. We find that the Weyl nodal lines have a substantial effect on the Fermi surface spin structure of the normal state, and lead to line nodes in the superconducting phase., Comment: 6 pages, 4 figures

Although topological invariants have been introduced to classify the appearance of protected electronic states at surfaces of insulators, there are no corresponding indexes for Weyl semimetals whose nodal points may appear randomly in the bulk Brillouin Zone (BZ). Here we use a well-known result that every Weyl point acts as a Dirac monopole and generates integer Berry flux to search for the monopoles on rectangular BZ grids that are commonly employed in self-consistent electronic structure calculations. The method resembles data mining technology of computer science and is demonstrated on locating the Weyl points in known Weyl semimetals. It is subsequently used in high throughput screening several hundreds of compounds and predicting a dozen new materials hosting nodal Weyl points and/or lines., Comment: 6 pages, 4 figures, 1 table, supplementary info (1 table, 13 figures)