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"Quasi-particle breakdown" in the quasi-one-dimensional Ising ferromagnet CoNb$_2$O$_6$

We present experimental and theoretical evidence that an interesting quantum many-body effect -- quasi-particle breakdown -- occurs in the quasi-one-dimensional spin-1/2 Ising-like ferromagnet CoNb$_2$O$_6$ in its paramagnetic phase at high transverse field as a result of explicit breaking of spin inversion symmetry. We propose a quantum spin Hamiltonian capturing the essential one-dimensional physics of CoNb$_2$O$_6$ and determine the exchange parameters of this model by fitting the calculated single particle dispersion to the one observed experimentally in applied transverse magnetic fields. We present high-resolution inelastic neutron scattering measurements of the single particle dispersion which observe "anomalous broadening" effects over a narrow energy range at intermediate energies. We propose that this effect originates from the decay of the one particle mode into two-particle states. This decay arises from (i) a finite overlap between the one-particle dispersion and the two-particle continuum in a narrow energy-momentum range and (ii) a small misalignment of the applied field away from the direction perpendicular to the Ising axis in the experiments, which allows for non-zero matrix elements for decay by breaking the $\mathbb{Z}_2$ spin inversion symmetry of the Hamiltonian.