The role of spin-orbit interaction in organic semiconductors

First HiFi users

First users on the new HiFi muon instrument at ISIS.(left to right) Dr Alan Drew, Dr Laura Nuccio, Leander Schulz, Maureen Willis (Queen Mary University of London) and Dr Iain McKenzie (ISIS).
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Traditional electronics relies on the charge of an electron to carry information. But the demand for smaller, faster electronics has caused scientists to look at another property of electrons – spin. The field of spintronics (“spin transport electronics”) is developing rapidly, already responsible for significant advances in data storage. Now scientists are looking to organic semiconductors, which have the potential for cheaper, more versatile devices. However, the fundamental physics is not fully understood.

A group of scientists have been using the HIFI instrument at ISIS to understand the mechanism of electron spin relaxation in organic semiconductors. They were the first group to use HIFI when it came online in 2009. Their results have just been published in the journal Physical Review Letters.

Organic semiconductors potentially have several advantages over their inorganic counterparts, that could lead to large area and low cost applications. In spintronics a property of particular interest is the electron spin relaxation time, which can be several orders of magnitude longer than that observed in inorganic semiconductors. Spin relaxation leads to loss of spin polarisation, limiting the magnitude of magnetoresistance that can be achieved in spintronic devices such as spin valves.

So far efforts to understand electron spin relaxation in organic semiconductors have concentrated on the hyperfine interaction, with the role of spin-orbit interaction largely ignored. This study combined muon spin spectroscopy with time-resolved photoluminescence measurements and found evidence for a significant contribution from spin-orbit interaction to the relaxation time.

The group tested the theory by systematically modifying the spin-orbit interaction by chemically substituting atoms in the organic semiconductor with heavier ones. Two series of molecular semiconductors were studied at both HIFI and the ALC instrument at the Paul Scherrer Institute.

Molecular structures

Molecular structures of the Xq3 series and the TES series. Hydrogen atoms not shown for clarity.
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Spin polarised muons have been shown to be a sensitive probe for detecting electron spin relaxation rate. Spin-polarised muons are implanted into the target material. Some of the implanted muons capture electrons from the material, forming muonium, a hydrogen-like species. If a magnetic field is applied parallel to the initial direction of spin of the muons, near certain characteristic fields depolarisation of the muon spin occurs, due to cross relaxation effects. This can be detected by Avoided Level Crossing (ALC) resonances. As the spins of the muons and electrons are coupled in the muonium, relaxation of the electron spin correspondingly causes relaxation of the muon spin, which is observed in the ALC resonance.

The results showed a strong dependence of ALC amplitude on temperature and atomic mass, implying a thermally activated spin-orbit based spin relaxation mechanism.

The research was part of an EPSRC funded grant, EP/G054568/1. Laura Nucchio was the postdoctoral research associate on the grant. She says, “With this grant, we set out to understand the intrinsic charge and spin transport mechanisms in organic seminconductors through muon spin relaxation. What we have found for the first time is direct evidence that besides the hyperfine interaction, spin-orbit interaction also makes a significant contribution to the electron spin relaxation time.”

Dr Drew was the Principal Investigator on the project. He has now been awarded a €1.5m European Research to upgrade the HIFI spectrometer. Dr Drew says,"This was one of the first experiments I performed in this theme of my research, which eventually led to me getting a grant to upgrade the HiFi spectrometer with a laser, enabling laser-pump muon-probe experiments to take place. This new technique will enable muons to measure the dynamics of excited electronic states in molecules, which are relevant for many biological and chemical processes. These areas are not traditionally explored with muons. Examples of areas this new muon method could investigate include DNA repair, photosynthesis, catalysis and REDOX reactions, as well as more traditional areas for muons, such as magnetism or semiconductors."

Sara Fletcher

Research date: May 2013

Further Information

Importance of Spin-Orbit Interaction for the Electron Spin Relaxation in Organic Semiconductors, L. Nuccio, M. Willis, L. Schulz, S. Fratini, F. Messina, M. D’Amico, F. L. Pratt, J. S. Lord, I. McKenzie, M. Loth, B. Purushothaman, J. Anthony, M. Heeney, R. M. Wilson, I. Hernández, M. Cannas, K. Sedlak, T. Kreouzis, W. P. Gillin, C. Bernhard, and A. J. Drew, Phys. Rev. Lett. 110, 216602 (2013)

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