The spin-Dicke effect in OLED magnetoresistance

In organic semiconductors, pairs of charge-carrying spins can behave as four-level systems. It is now shown that in the regime of ultrastrong coupling, the collective behaviour of these spins gives rise to a spin-Dicke effect. 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 0 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.