Observation of excitonic n-strings in a quasi-one-dimensional charge-transfer crystal

Collective excitations, such as plasmons, rotons, electron-hole liquid, and excitonic molecules, have been extensively studied in condensed matter. Wannier excitons in inorganic semiconductors are bound by the exchange interactions between two electrons of the exciton, and the bound state of more than two excitons is not possible. We demonstrate here a new type of collective excitations, bound states of multiple charge-transfer (CT) excitons. Coulomb interactions in one dimension are shown to bind a train of several (more than two) CT excitons. Experimental evidence for these new type of elementary excitations is reported in a quasi-one-dimensional CT crystal of anthracene PMDA. High density excitation by femtosecond light pulses generates multi-exciton chains, which we refer to as excitonic n-strings with n = 1, 2, 3, etc., along the stack axis of the crystal. Both the n = 2 excitonic string (biexciton) and the n = 3 string (tri-exciton) are observed. This report provides evidence for an n>2 exciton chain in this system. The stability of the n-string exciton is supported by our theoretical calculations based on the extended Hubbard Hamiltonian in one dimension.