  Polarized superradiant emission and exciton delocalization in tetracene
single crystals are reported. Polarization-, time-, and temperature-resolved
spectroscopy evidence the complete polarization of the zero-phonon line of the
intrinsic tetracene emission from both the lower (F state) and the upper
(thermally activated) Davydov excitons. The superradiance of the F emission is
substantiated by a nearly linear decrease of the radiative lifetime with
temperature, being fifteen times shorter at 30 K compared to the isolated
molecule, with an exciton delocalization of about 40 molecules.
