Topological Hybrids of Magnons and Magnon Bound Pairs

TL;DR

This paper explores topological phenomena arising from hybridization of magnons and magnon bound pairs in quantum magnets lacking particle-number conservation, revealing exotic edge states with mixed spin character.

Contribution

It introduces a new topological mechanism based on particle-number sector hybridization in quantum magnets, distinct from classical topological magnons.

Findings

Hybridization causes topological anticrossings in the spectrum.

Chiral edge excitations are superpositions of different particle sectors.

Quantum effects vanish in the classical limit.

Abstract

We consider quantum condensed matter systems without particle-number conservation. Since the particle number is not a good quantum number, states belonging to different particle-number sectors can hybridize, which causes topological anticrossings in the spectrum. The resulting spectral gaps support chiral edge excitations whose wavefunction is a superposition of states in the two hybridized sectors. This situation is realized in fully saturated spin-anisotropic quantum magnets without spin conservation, in which single magnons hybridize with magnon bound pairs, i.e., two-magnon bound states. The resulting chiral edge excitations are exotic composites that carry mixed spin-multipolar character, inheriting spin-dipolar and spin-quadrupolar character from their single-particleness and two-particleness, respectively. In contrast to established topological magnons, the topological effects discussed here are of genuine quantum mechanical origin and vanish in the classical limit. We discuss implications for both intrinsic anomalous Hall-type transport and beyond-spintronics computation paradigms. We conclude that fully polarized quantum magnets are a promising platform for topology caused by hybridizations between particle-number sectors, complementing the field of ultracold atoms working with a conserved number of particles.