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; these phase factors lead to oscillations in the fluorescence signal recovered after pulsed illumination (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. For certain field orientations, we observe a rapid decay in the beat amplitude of the global triplet pair population, even as individual triplet pairs retain their spin coherence. We now demonstrate that this rapid decoherence effect for certain field orientations can be attributed to Transport-Induced Dephasing (TID). The differently-oriented molecules within the unit cell of rubrene have slightly different Hamiltonians, due to their different relative field angles. These molecular Hamiltonians share eigenstates, but have slightly different eigenenergies, 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 two dimensional hop time. These phase factors accumulate, and eventually cause total destructive interference. Our TID model explains the lack of applied-field beats in most of the ab crystal plane, and can be used to quantify two-dimensional hopping times by analyzing the rate of beat amplitude decay.