EXCITON STRINGS IN AN ORGANIC CHARGE-TRANSFER CRYSTAL

COLLECTIVE excitations resulting from many-body Coulomb interactions h ave been studied extensively in the solid state1: for example, the exc hange interaction between the electrons in two excitons (bound electro n-hole pairs) can bind the excitons together, forming a biexciton. At the other extreme, if the number of excitons is sufficiently large (ap proximately 10(6)), they can condense into a degenerate 'liquid' phase known as an electron-hole drop. But in conventional semiconductors, i ntermediate bound states, consisting of more than two excitons, are no t formed. We show here, both theoretically and experimentally, that bo und states of multiple excitons can form in the organic charge-transfe r solid anthracene-(pyromellitic acid dianhydride). Coulomb interactio ns along the one-dimensional stacks of this material can stabilize tra ins of several charge-transfer excitons, and we refer to the resulting collective excitations as exciton strings.