Date: Tuesday, September 15, 2015
Time: 11:00 am
Speaker: John Lupton, University of Regensburg
Title: Coherent Spin Oscillations in OLEDs
Location: 67-3111 Chemla Room
The ability of some animals to navigate using Earth’s magnetic field is truly perplexing. How can tiny fields of one Gauss induce physiologically relevant reactions when Zeeman shifts are over a million times smaller than kT? The secret appears to lie in field- induced modifications to the effect of hyperfine interactions which become relevant because of the exceptionally long spin coherence times of radical pairs. OLEDs provide an unrivaled proving ground to explore the interplay between spin coherence, spin correlations and external fields through spin-dependent transport and luminescence.
Spin-lattice relaxation in OLEDs is virtually independent of temperature and very slow. Spin dephasing over microseconds can be quantified by pulsed magnetic resonance using conventional echo schemes. Slow spin dephasing enables the direct observation of spin- Rabi flopping of both electron and hole species, which, under suitable resonance conditions, couple to each other by spin beating. Such signals are, in principle, sensitive down to the single carrier within the OLED, since the measurement reports on spin permutation symmetry rather than on thermal spin polarization. As the sole parameter determining the resonance condition is the g-factor, compact OLED-based low-frequency resonance circuits can be designed to serve as versatile magnetometers. With novel dual singlet-triplet emitters, singlet- triplet oscillations in the radical-pair can now also be probed directly by a color change in emission.
Recent highlights in exploiting coherent singlet-triplet oscillations in OLEDs include the demonstration of direct control of the hyperfine interaction by NMR, quantification of the zero-field splitting of intermolecular excited-state species, and the direct manifestation of the elusive ac-Zeeman and spin-Dicke effects.