# A single nodal loop of accidental degeneracies in minimal symmetry: triclinic CaAs$_3$

## TL;DR

This paper reports that triclinic CaAs₃, with minimal inversion symmetry, naturally hosts a topologically nontrivial nodal loop of accidental degeneracies, serving as a fundamental example of such phenomena in low-symmetry crystals.

## Contribution

It demonstrates the existence of a topological nodal loop in CaAs₃ with minimal symmetry, without the need for tuning, highlighting a new fundamental case in topological materials.

## Key findings

- CaAs₃ has a topological nodal loop centered on a Brillouin zone face.
- The loop is very flat in energy and intersects the Fermi level four times.
- Spin-orbit coupling lifts the degeneracy, resulting in trivial Kramers pairs.

## Abstract

The existence of closed loops of degeneracies in crystals has been intimately connected to associated crystal symmetries, raising the question: what is the minimum symmetry required for topological character, and can one find an example? Triclinic CaAs$_3$, in space group $P{\bar 1}$ with only a center of inversion, has been found to display, without need for tuning, a nodal loop of accidental degeneracies with topological character, centered on one face of the Brillouin zone that is otherwise fully gapped. The small loop is very flat in energy, yet is cut four times by the Fermi energy, a condition that results in an intricate repeated touching of inversion related pairs of Fermi surfaces at Weyl points. Spin-orbit coupling lifts the fourfold degeneracy along the loop, leaving trivial Kramers pairs. With its single nodal loop that emerges without protection from any point group symmetry, CaAs$_3$ represents the primal "hydrogen atom" of nodal loop systems.