Molecular conical intersections with odd electron number are realizations of the topological Yang monopole

Abstract

The two-level conical intersection of a molecule with odd electron number in the absence of magnetic field obeys time-reversal symmetry $T^{2}=-1$ (referred to as the $T^{2}=-1$ conical intersection), and has a five-dimensional branching space due to the Kramer degeneracy. Similar to how the conical intersection of a molecule in a magnetic field ($T^{2}=0$) behaves as the Dirac monopole, the $T^{2}=-1$ conical intersection behaves as the Yang monopole, an mathematical generalization of Dirac monopole with SU(2) gauge field and SO(5) symmetry. This implies that we can study topological properties of $T^{2}=-1$ conical intersections in chemistry based on what is known about Yang monopole in high energy physics. In this work, we present a few mathematical tools to study this connection. First, we show that geometric algebra and quaternion numbers together provides a natural way to utilize the $T^{2}=-1$ time reversal symmetry and the SO(5) symmetry in scaled coordinates, making it simple to derive eigenfunctions, Berry connection and Berry curvature of $T^{2}=-1$ conical intersection. In particular, this approach provides a simple proof that when viewed from the upper or lower states, the $T^{2}=-1$ conical intersection behaves as the self-dual or self-antidual Yang monopole with second Chern number $C_{2}=+\frac{1}{4}$ or $-\frac{1}{4}$ respectively. In addition, we propose a visualization method for the SU(2) Berry connection of $T^{2}=-1$ conical intersection. This is achieved by showing that the nonzero part of the Berry connection induces Hopf-fibration on the $S^{3}$ longitude space, and is further visualized through stereographic projection.