Fig. 1

Photoexcited states in a one-dimensional (1D) Mott insulator of bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane (ET-F₂TCNQ). a Schematics of a ground state, exciton, and biexciton in a half-filled 1D Mott insulator. The intersite Coulomb repulsive energies for the nearest, second-nearest, and third-nearest two sites are denoted by \(V_1\), \(V_2\), and \(V_3\), respectively. The binding energy of biexciton is \(\left( {V_1 - 2V_2 + V_3} \right)\), which is equal to \(V_1/3\) in the case that the Coulomb repulsion energies are inversely proportional to the distance between two electrons, \(V_2 = V_1/2\) and \(V_3 = V_1/3\). b Molecular structures and 1D molecular stacks along the a axis of ET-F₂TCNQ. c Reflectivity spectra of ET-F₂TCNQ polarized parallel to the a axis (E//a) and perpendicular to the a-axis (E\(\bot\)a)