Distinguished Colloquium: Physics & Astronomy – Christoph Boehme

About the event

The  Department of Physics and Astronomy invites all to a distinguished colloquium featuring Christoph Boehme, Physics & Astronomy, University of Utah. Dr. Boehme will present their talk, “Spintronics of weakly spin-orbit coupled materials.”

Meet for refreshments before the lecture at 3:45 – 4:10 p.m. in the foyer on floor G above the lecture hall. Following the lecture, we will have a reception in the foyer on floor G above the lecture hall from 5 – 6:30 p.m.

Abstract:
Materials with weak spin-orbit coupling (SOC) of charge carrier states generally exhibit long charge carrier spin-relaxation times and thus long spin-diffusion lengths. Unfortunately though, many materials with weak SOC (e.g. organic semiconductors, amorphous silicon, etc.), are also structurally disordered, which causes low charge carrier mobilities and thus, short spin-diffusion lengths and in this competition between SOC and disorder, the latter typically wins. Spintronics applications using weak SOC have therefore been limited [1], albeit demonstrations of spin-transport effects such as the inverse spin-Hall effect using both organic based molecular magnetic injectors [2] as well as organic detectors [3] have been made. However, weak SOC also implies pronounced spin-selection rules which also cause pronounced spin-controlled magneto-optoelectronic effects [4-7] and thus, they provide alternative routes to spintronics, including spin memory [8, 9] and low-field magnetometry concepts [7].

In this talk, a brief survey of spin-selection rule based magneto-optoelectronic effects in organic semiconductor as well as their spintronics applicability is given. The physical parameters determining the dynamics of these transitions include spin-dipolar, spin-exchange, spin-orbit, as well as hyperfine interactions of charge carrier spins and the most direct way to explore these mechanisms is to carefully manipulate spin manifolds within these materials, e.g. by using magnetic resonance, and to then observe how macroscopic observables such as conductivity or optical transition rates respond. Measurements of spin-induced conductivity and optical changes after coherent spin resonant excitations are called pulsed electrically and optically detected magnetic resonance spectroscopies (pEDMR, pODMR, respectively). Several examples for pEDMR measurements and their implications for the observed spin-dependent processes will be discussed. In particular, the detection of permutation-symmetry dependent electron-hole recombination in conducting polymers allows for the observation of spin-coherence in complete absence of spin polarization, an effect that is in full analogy to the idea of using singlet/triplet qubits as information carriers. Furthermore, the ability to measure spin-resonance of Zeeman states whose splitting is far below the thermal energy allows to explore non-linear magnetic resonant effects under strong drive conditions, including the spin-Dicke effect and the Bloch-Siegert shift [10,11].

References

[1] C. Boehme and J. M. Lupton, Nature Nano.8, 612 (2013); [2] H. Liu at al., Nature Materials17, 308 (2018); [3] D. Sun et. al, Nature Materials15 (5), 863 (2016); [4] D. R. McCamey et al., Nature Materials7, 723 (2008). [5] D. R. McCamey et al., Phys. Rev. Lett.104, 017601(2010). [6] K. J. van Schooten, et al., Nature Commun.6:6688, 7688 (2015). [7] W. J. Baker et al., Nature Commun.3, 898 (2012); [8] W. J. Baker et al., Phys. Rev. Lett.108, 267601 (2012); [9] H. Malissa, et al., Science345 1487, (2014); [10] D. P. Waters, et al., Nature Physics11, (11) 910 (2015); [11] S. Jamali et al., Nano Letters17 (8) 4648 (2017).