Unique Electromagnetic Responses of Unconventional Topological Semimetals

Elucidating the physical manifestations of nontrivial band topology in materials is of great interest in quantum materials science. Recent advances in the classification of topological band crossing have led to the discovery of various new kinds of low-energy excitations. Yet, the unique physical properties that qualitatively distinguish them from Weyl and Dirac fermions or conventional metals are rarely known. In this talk, we present our theoretical discovery of unique electromagnetic response properties in unconventional topological point-node semimetals [1,2]. First, we reveal the pattern of the divergent nonlinear responses as the chemical potential approaches the energy of an overtilted (i.e. the so-called type-II) nodal point, with the leading divergence determined by the dispersion and dimensionality [1]. Then, we show that chiral multifold nodal points feature the unique circular dichroism that is absent for Weyl or Dirac fermions [2]. Our finding demonstrates that new types of excitations indeed possess new responses, motivating further exploration of other unconventional fermions whose unique response properties are elusive.<br/><br/>This work was supported by the Center for Advancement of Topological Semimetals (CATS), an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, through the Ames Laboratory under contract No. DE-AC02-07CH11358.<br/><br/>[1] Junyeong Ahn, Topological enhancement of nonlinear transport in unconventional point-node semimetals, Phys. Rev. B 107, L201112 (2023).<br/>[2] Junyeong Ahn and Barun Ghosh, Topological Circular Dichroism in Chiral Multifold Semimetals, Phys. Rev. Lett. 131, 116603 (2023).