Emergent D_8^(1) spectrum and topological soliton excitation in CoNb_2O_6

Quantum integrability emerging near a quantum critical point (QCP) is manifested by exotic excitation spectrum that is organized by the associated algebraic structure. A well known example is the emergent E_8 integrability near the QCP of a transverse field Ising chain (TFIC), which was long predicted theoretically and initially proposed to be realized in the quasi-one-dimensional (q1D) quantum magnet CoNb_2O_6. However, later measurements on the spin excitation spectrum of this material revealed a series of satellite peaks that cannot be described by the E_8 Lie algebra. Motivated by these experimental progresses, we hereby revisit the spin excitations of CoNb_2O_6 by combining numerical calculation and analytical analysis. We show that, as effects of strong interchain fluctuations, the spectrum of the system near the 1D QCP is characterized by the D_8^(1) Lie algebra with robust topological soliton excitation. We further show that the D_8^(1) spectrum can be realized in a broad class of interacting quantum systems. Our results advance the exploration of integrability and manipulation of topological excitations in quantum critical systems.