Nodal-chain metals

TL;DR

This paper introduces a new topologically distinct fermionic excitation called a nodal chain in metals, exemplified by IrF$_4$, with unique surface states and anomalous magnetotransport properties.

Contribution

The discovery and topological characterization of nodal-chain fermions in metals, expanding the understanding of quasiparticles beyond previously known excitations.

Findings

Nodal chain is topologically distinct from other fermionic excitations.

IrF$_4$ hosts nodal chain fermions with specific surface states.

Presence of nodal chains leads to anomalous magnetotransport properties.

Abstract

The band theory of solids is arguably the most successful theory of condensed matter physics, providing the description of the electronic energy levels in a variety of materials. Electronic wavefunctions obtained from the band theory allow for a topological characterization of the system and the electronic spectrum may host robust, topologically protected fermionic quasiparticles. Many of these quasiparticles are analogs of the elementary particles of the Standard Model, but others do not have a counterpart in relativistic high-energy theories. A full list of possible quasiparticles in solids is still unknown, even in the non-interacting case. Here, we report on a new type of fermionic excitation that appears in metals. This excitation forms a nodal chain -- a chain of connected loops in momentum space -- along which conduction and valence band touch. We prove that the nodal chain is topologically distinct from any other excitation reported before. We discuss the symmetry requirements for the appearance of this novel excitation and predict that it is realized in an existing material IrF$_4$, as well as in other compounds of this material class. Using IrF$_4$ as an example, we provide a detailed discussion of the topological surface states associated with the nodal chain. Furthermore, we argue that the presence of the novel quasiparticles results in anomalous magnetotransport properties, distinct from those of the known materials.