David Waters Thesis Defense 08/012/14

Thesis Defense

David Waters

Tuesday, August 12, 2014
10:00AM (110 INSCC)

Title: Magnetoresistance of polymer diodes in presence of oscillating magnetic fields at radio frequency

The work presented focused on the study of low-magnetic field ( < 10mT) magnetoresistance effects of organic polymer diodes based on the p-conjugated polymer poly[2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) in the presence of oscillating (ac) magnetic fields in the radio frequency (RF) range. In this regime, the combination of static and ac fields can magnetic resonantly influence the electron-spin degree of freedom of localized charge carrier states. As long as bipolar (electron and hole) injection conditions influence the net current of the polymer diode, magnetic resonance changes of the charge carrier spin state can affect spin-dependent charge carrier recombination rates and therefore the material’s conductivity. Since the observed spin-dependent recombination currents are governed by the charge carrier pair’s spin-permutation symmetry, magnetoresistance measurements under ac drive allow for the electrical detection of magnetic resonance under very low magnetic field conditions where traditional, inductive magnetic resonance detection schemes fail due to a lack of thermal spin polarization.

The exploration of a magnetic resonance regime where the driving field /B/_1 approaches the same magnitude as the static magnetic field /B/_0 : Usually, magnetic resonance spectroscopy is conducted with /B_0 / >> /B_1 /, which allows for the application of the rotating frame approximation for the description of magnetic resonance effects. This approach breaks down when /B_1 /* *approaches /B_0 /. A regime where magnetic resonance effects become non-linear emerges and interesting collective spin-phenomena occur, including spin-cooperativity, where the resonantly-driven spin ensemble assumes a macroscopically collective state, analogous to the superradiant Dicke regime for resonantly excited electric dipoles. In this thesis, experiments are presented that tested and confirmed previous theoretical predictions. When /B_1 /* ~ */B_0, /the emerging spin-cooperativity of recombining polaron pairs in organic semiconductors can be observed through magnetoresistance measurements. The experimental results presented confirmed the theory in all aspects and demonstrated the emergence of the spin-Dicke effect.

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