Title: OLED “Magnetoreception”

Time: 2023-10-11 15:30

Lecturer:  Prof. John Lupton

University of Regensburg

Venue: Lecture Hall, No.3 Building,TAN KAH KEE Laboratory,Xiang’an Campus

Room 202, Lecture Hall, Lu-Jiaxi Building, Siming Campus

Abstract：

OLEDs are everywhere now. Were it for their inventors, this may not have been so. The original OLED patent explicitly excluded light emission from triplet excitations – phosphorescence – and laid claim exclusively to fluorescence from singlets. Three out of four charge recombination events end up in triplets, so losing this energy clearly diminishes technological appeal. Overcoming this limitation requires spin mixing to coax luminescence from otherwise dark states. The simplest way to do this is by spin-orbit coupling through the heavy-atom effect. Provided that spin-coherence times are sufficiently long, spin precession in weak magnetic fields may already suffice to convert triplets to singlets. 

OLEDs work by spin-dependent recombination of electrically injected charges of opposite sign – the solid-state analogue of radical-pair-based magnetic-field-dependent chemical reactions, which are also responsible for the navigational ability of birds. Resistance and brightness of OLEDs are sensitive to magnetic fields of nanotesla strength. At geomagnetic field strengths, the magnetoresistance can show anisotropies of 35 % – an OLED inclinational compass. As in classic experiments on disorienting birds in resonant RF fields, this compass is suppressed under resonant drive. Coherent motion of spins can be probed directly in the time domain by pulsed current-detected EPR, allowing hyperfine and spin-orbit coupling effects to be discriminated from dipolar and exchange interactions in the carrier pair. Current-detected NMR even permits isotopic fingerprinting of the OLED material.

Since OLED spin states are effectively two-level systems, analogues of quantum-optical phenomena such as superradiance emerge under resonant drive. Because magnetic resonances persist at very small fields, an intriguing regime of light-matter interaction opens up: the condition of “deep-strong” drive, where the frequency at which the system is driven exceeds its natural frequency. The change in OLED current under resonance can be accurately computed with quantum mechanics, revealing signatures of coherently dressed states along with exotic transitions such as multiphoton and half-field resonances. It is interesting to speculate whether such exotic states may arise quite naturally at low static fields due to intrinsic molecular dynamics which, because of the motion of nuclear magnetic moments, result in time-varying magnetic fields that constitute effective resonant drive fields.

Bio of Prof. John Lupton:

John Lupton holds a chair in experimental physics at the University of Regensburg, Germany, where he presently serves as head of department. He is also Research Professor of Physics at the University of Utah, US, where he maintains a small research activity. John studied physics at the University of Durham, UK, and held postdoctoral appointments at the University of St Andrews, at the MPI in Mainz and at LMU Munich. Distinctions include a Packard Fellowship, a Research Corporation Scialog award, and an ERC Starting Grant. His research interests span single-molecule spectroscopy of pi-conjugated macromolecules, spin physics of molecular materials, and the optics of semiconductor and metallic nanostructures.