Quantum Cafe: Rafael Fernandes
Title: Topological Properties of Altermagnets
Abstract: The properties of a magnetic state depend on which symmetries of the lattice leave the state unchanged when combined with time-reversal, i.e., with flipping all the magnetic moments. In a ferromagnet, no such symmetry exists, resulting in a nonzero magnetization and a uniform Zeeman splitting of the spin-up and spin-down bands. In contrast, this type of symmetry is present in a collinear antiferromagnet, since a lattice translation or inversion “undoes” the flipping of the spins, leading to degenerate spin-up and spin-down bands with no Zeeman splitting. Between these two types of magnetic states, however, lies a broad range of systems for which the symmetry that relates configurations of flipped spins is a rotation (proper or improper). Called altermagnets, these states have no magnetization, like an antiferromagnet, yet their bands display a nodal Zeeman splitting, resembling a “d-wave” (or higher-order) ferromagnet. In this talk, he will discuss the various connections between altermagnets and phenomena of interest in correlated electronic systems, such as multipolar order and Pomeranchuk instabilities. He will then show how spin-orbit coupling endows altermagnets with interesting and nontrivial topological properties, including mirror-protected nodal lines, Chern bands, and Weyl nodal lines in the electronic spectrum.
Fernandes received his Ph.D. in physics at the University of Campinas, Brazil, in 2008. After a postdoc in Ames Lab and a joint postdoc in Columbia University/Los Alamos National Lab, he joined the faculty at the University of Minnesota, and then moved to the University of Illinois Urbana-Champaign, where he is currently a Professor. Fernandes is a condensed matter theorist working on strongly correlated electronic many-body systems. He is particularly interested in clean and disordered systems in which the collective behavior of the electrons gives rise to quantum states that break different symmetries of the system, such as superconductivity, magnetism, nematic ordering, and orbital