# Topological quantum phase transition in the Ising-like antiferromagnetic spin chain BaCo$_2$V$_2$O$_8$

## TL;DR

This paper reports the experimental observation of a topological quantum phase transition in BaCo$_2$V$_2$O$_8$, driven by a magnetic field, where two dual topological excitations compete, revealing complex solitonic dynamics.

## Contribution

It provides the first experimental realization of a transition between two different topological excitations in a quasi-one-dimensional antiferromagnet.

## Key findings

- Neutron scattering shows a drastic change in quantum excitations at the critical field.
- The transition is between two types of solitonic topological objects.
- Theoretical calculations support the experimental identification of the transition.

## Abstract

Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations are at the heart of our understanding of a whole novel class of phase transitions. In most of the cases, those transitions are controlled by a single type of topological objects. There are however some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realization of such cases is thus of considerable interest. We show here that this situation occurs in BaCo$_2$V$_2$O$_8$, a spin-1/2 Ising-like quasi-one dimensional antiferromagnet when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological objects, which are captured by different components of the dynamical structure factor.