Observation of multi-mode spin-Rabi oscillation of strongly EPR driven polaron pairs in a π-conjugated polymer

Electron spin-Rabi oscillations due to electron paramagnetic-resonant (EPR) drive by a linearly polarized, radio-frequency domain oscillating magnetic field with amplitude B₁ in excess of both, a static magnetic Zeeman field B₀ and local random hyperfine fields, have been predicted to give rise to long-lived charge carrier spin pair states [1]. To test this, we performed strong-drive pulsed electrically detected magnetic (EDMR) resonance experiments at room temperature and low B₀ < 0.5mT. To observe EPR via electric current, we used spin-dependent polaron recombination currents in bipolar injection devices, essentially organic light emitting diodes (OLEDs), based on the perdeuterated π-conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] MEHPPV. Our experiments revealed that the Rabi nutation transient exhibited higher harmonics, whose number N increases linearly with B₁, following the relation |4B₁/B₀|, as predicted by theory. [1] R. K. Malla, M. E. Raikh, Phys. Rev. B 97, 035428 (2017).