Experimental observation of quantum many-body excitations of E8 symmetry in the Ising chain ferromagnet CoNb2O6

Close to the quantum critical point of the transverse-field Ising spin-chain model, an exotic dynamic spectrum was predicted to emerge upon a perturbative longitudinal field. The dynamic spectrum consists of eight particles and is governed by the symmetry of the $E_8$ Lie algebra. Here we report on high-resolution terahertz spectroscopy of quantum spin dynamics in the ferromagnetic Ising-chain material ${\mathrm{CoNb}}_2{\mathrm{O}}_6$. At 0.25 K in the magnetically ordered phase we identify characteristics of the first six $E_8$ particles, ${\mathrm{m}}_1$ to ${\mathrm{m}}_6$, and the two-particle $({\mathrm{m}}_1+{\mathrm{m}}_2)$ continuum in an applied transverse magnetic field of $B_c^{\text{1D}}=4.75 \mathrm{T}$, before the three-dimensional magnetic order is suppressed above $B_c^{\text{3D}}\approx 5.3 \mathrm{T}$. The observation of the higher-energy particles $({\mathrm{m}}_3 \mathrm{to} {\mathrm{m}}_6)$ above the low-energy two-particle continua features quantum many-body effects in the exotic dynamic spectrum.