cfaed Seminar Series

The ability of a range of species such as birds, insects, and snails to navigate in Earth’s magnetic field is truly perplexing. How can a tiny magnetic field of order one Gauss induce a physiologically relevant reaction when Zeeman shifts are over a million times smaller than the thermal energy? The secret appears to lie in modifications to hyperfine interactions which become relevant because of the exceptionally long (tens of microseconds) spin coherence times of radical pairs. OLEDs provide a unique molecular proving ground to explore the subtle interplay between spin coherence, spin correlations and external fields by measuring spin-dependent transport and luminescence.

Spin-lattice relaxation in OLEDs is virtually independent of temperature and very slow. Spin dephasing on the microsecond timescale can be quantified by pulsed magnetic resonance and observed directly in transport (current) or luminescence. Slow spin dephasing gives rise to spin Rabi flopping of both electron and hole species, which, under suitable resonance conditions, couple to each other yielding spin beating. Such signals are, in principle, sensitive down to the individual 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 the emergence of metal-free triplet emitters, we can now even directly probe the singlet-triplet radical-pair mechanism under resonance simply by a color change in emission.

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John Lupton studied physics at Durham University, where he obtained his PhD in 2001. After a brief postdoctoral appointment in St. Andrews, he took on a position as spectroscopy group leader at the Max Planck Institute for Polymer Research in Mainz, subsequently moving to LMU Munich. Having turned down an offer for an associate professorship at Kiel University, he moved to the University of Utah in 2006. Since 2010 he holds a chair in experimental physics at Regensburg University. John received the Max Auwärter Prize of the Austrian Physical Society, a Packard Fellowship, a Research Corporation Scialog Fellowship, and an ERC consolidator grant.