The spin-Dicke effect in OLED magnetoresistance

Pairs of charge-carrier spins in organic semiconductors constitute four-level systems that can be driven electromagnetically(1). Given appropriate conditions for ultrastrong coupling(2)-weak local hyperfine fields B-hyp, large magnetic resonant driving fields B-1 and low static fields B-o that define Zeeman splitting-the spin-Dicke effect, a collective transition of spin states, has been predicted(3). This parameter range is challenging to probe by electron paramagnetic resonance spectroscopy because thermal magnetic polarization is negligible. It is accessed through spin-dependent conductivity that is controlled by electron-hole pairs of singlet and triplet spin-permutation symmetry without the need of thermal spin polarization(4). Signatures of collective behaviour of carrier spins are revealed in the steady-state magnetoresistance of organic light-emitting diodes (OLEDs), rather than through radiative transitions. For intermediate B-1, the a.c.-Zeeman effect appears. For large B-1, a collective spin-ensemble state arises, inverting the current change under resonance and removing power broadening, thereby offering a unique window to ambient macroscopic quantum coherence.