Experimental tests of spin transport models in organic semiconductors*

It has been a long-standing question whether spin-transport and charge-transport in organic materials occur via the same electronic processes [1, 2], i.e. via charge excitations such as polaron states. If so, then the spin current that is produced by spin-pumping through Ferromagnetic resonance (FMR) excitation in an inverse spin-Hall-effect (ISHE) experiment on a NiFe/Alq₃/Pt device [3] must be diminished under electron paramagnetic resonance (EPR) excitation of the charge carriers [4]. We have executed such experiments and do not observe such quenching, which shows that spin-transport is neither due to propagation of charge carriers nor due to spin-transfer between weakly coupled charge carriers. However, these observations are consistent with the idea of exchange mediated transport. We discuss the implications of our experimental results as well as various control experiments that have been done to scrutinize our results. [1] M. Groesbeck et al. (unpublished); [2] Z. Yue et al., Phys. Rev. B 92, 045405 (2015); [3] S.W. Jiang et al., Phys. Rev. Lett. 115, 086601 (2015); [4] D. R. McCamey et al., Nature Mat. 7, 723 (2008).