Showing papers by "Sergey Y. Savrasov published in 2021"

Exchange interactions and sensitivity of the Ni two-hole spin state to Hund's coupling in doped NdNiO 2

Abstract: The discovery of superconductivity in Nd${}_{1\ensuremath{-}x}$Sr${}_{x}$NiO${}_{2}$ has resulted in a flurry of experimental and theoretical work to understand the nature of nickelate superconductivity as compared to that of the cuprates and pnictides. Here, the authors use a combination of LDA+U and linear-response methods to study the magnetic exchange interactions in Nd${}_{1\ensuremath{-}x}$Sr${}_{x}$NiO${}_{2}$. The analysis reveals an underlying Mott insulating state comprised of Ni-3${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$/Ni-3${d}_{{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals with either an $S$=0 or $S$=1 two-hole ground state depending on the precise value of intra-atomic Hund's coupling.

Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet.

Abstract: The transverse voltage generated by a temperature gradient in a perpendicularly applied magnetic field, termed the Nernst effect, has promise for thermoelectric applications and for probing electronic structure. In magnetic materials, an anomalous Nernst effect (ANE) is possible in a zero magnetic field. We report a colossal ANE in the ferromagnetic metal UCo0.8Ru0.2Al, reaching 23 microvolts per kelvin. Uranium’s 5f electrons provide strong electronic correlations that lead to narrow bands, a known route to producing a large thermoelectric response. In addition, uranium’s strong spin-orbit coupling produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that in UCo0.8Ru0.2Al at least 148 Weyl nodes, and two nodal lines, exist within 60 millielectron volt of the Fermi level. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet

Abstract: Analogous to the Hall effect, the Nernst effect is the generation of a transverse voltage due to a temperature gradient in the presence of a perpendicular magnetic field. The Nernst effect has promise for thermoelectric applications and as a probe of electronic structure. In magnetic materials, a so-called anomalous Nernst effect (ANE) is possible in zero magnetic field. Here we report a colossal ANE reaching 23 $\mu$V/K in the ferromagnetic metal UCo$_{0.8}$Ru$_{0.2}$Al. Uranium's $5f$ electrons provide strong electronic correlations that lead to narrow bands, which are a known route to producing a large thermoelectric response. Additionally, the large nuclear charge of uranium generates strong spin-orbit coupling, which produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that at least 148 Weyl nodes and two nodal lines exist within $\pm$ 60 meV of the Fermi level in UCo$_{0.8}$Ru$_{0.2}$Al. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Electronic structure and topology across T c in the magnetic Weyl semimetal Co 3 Sn 2 S 2

Abstract: ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ 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.

Renormalized quasiparticles, topological monopoles, and superconducting line nodes in heavy-fermion Ce T X 3 compounds

Abstract: Noncentrosymmetric 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 Letter, we study the electronic properties of the family of inversion-broken $\mathrm{Ce}T{X}_{3}$ 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