Transport-Induced Dephasing (TID) in a Crystalline Environment

In some crystalline organic semiconductors, photon absorption can result in the creation of a pair of spin-entangled triplet excitons. The spin entanglement of a triplet pair persists, even as the triplet excitons individually diffuse throughout the crystal. Upon re-encounter, the triplets within the pair have acquired phase factors due to their time-evolved spin wavefunction, leading to oscillations in photoluminescence after pulsed excitation (fluorescence quantum beats).

In rubrene, an external magnetic field can be applied to tune the frequency of quantum beats via rotation of the applied field, except for an apparent lack of beats for field orientations along the ab plane. We now demonstrate that this beat suppression can be attributed to Transport-Induced Dephasing (TID). The differently-oriented molecules within the unit cell of rubrene have slightly different high-field energies, leading to three separate quantum beat frequencies. Triplet hopping between inequivalent sites causes the time-evolved wavefunction to flip between frequencies, with small phase factors continually introduced by variations in 2D hop time. These phase factors accumulate, and eventually cause total destructive interference in the ab plane (and indeed for most field orientations).