Fig. 1. 1D chain structure in [Ni(chxn)₂Br]Br₂, schematics of excitonic states and possible radiation from an interference of exciton wavefunctions in a 1D Mott insulator.

a 1D Ni-Br chain. Red arrows show electron spins. b Schematic of electron excitations from a Br ion to the left and right Ni ion ($\mid L⟩$ and $\mid R⟩$). $U$ is the on-site Coulomb repulsion. c Schematic of electron wavefunctions of the odd-parity exciton $\mid {\phi }_{\mathrm{o}}⟩$ and the even-parity exciton $\mid {\phi }_{\mathrm{e}}⟩$. d Time evolutions of the real part of the wavefunction of the system, $\mathrm{Re}\mid \psi ⟩$, at $\left(i+1\right)$ and $\left(i-1\right)$ sites (the black lines). The green curve shows an expected oscillation of the averaged electron density with the frequency of $\left({\omega }_{\mathrm{e}}-{\omega }_{\mathrm{o}}\right)$ corresponding to the low-frequency component in the expected value of the polarization, $P=e⟨\psi \mid x\mid \psi ⟩$, from which an electromagnetic wave with the electric field $E_{\mathrm{r}}\left(\propto \frac{{\partial }^{2}P}{\partial t^2}\right)$ is emitted.