Towards a first-principles understanding of magnetoresistance in radical molecular junctions.

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Controlling molecules at the individual level is a formidable task, in which tremendous progress has been made in the past years, in particular in the fields of molecular electronics and spintronics. While switching between discrete states is a particularly dramatic example of such control, gradual modification of properties such as resistance via external parameters such as magnetic fields (magnetoresistance) is intriguing because it implies that the space of accessible properties of the system can be probed broadly, and because it is essential for functionalities such as sensing. Magnetoresistance of organic radicals, contacted by metal electrodes, has the additional allure of showing unexpected behaviors, many of which are not yet understood. Elucidating the underlying physical mechanisms could provide new insight not only into the inner workings of potential functional materials, but also into fundamental aspects of molecules under nonequilibrium conditions and how they interact with metal surfaces. Our goal is precisely to gain such insight from atomistic simulations. Since several mechanisms are plausible for magnetoresistance, the most conclusive theoretical evidence would come from first-principles approaches. First-principles methods also allow for an unbiased evaluation of how chemical structure affects physical behavior, as is essential for establishing structure-property relationships once the basic mechanisms are understood.The specific goal of this project is to shed light on the mechanism of currently puzzling magnetoresistance recently observed in single-molecule TEMPO-OPE radical junctions, and to suggest new radical—electrode systems promising (a) further insight into the fundamental underlying mechanisms of magnetoresistance and interface structures, and (b) potential for spintronics applications. Our major hypothesis to be scrutinized is that radical substituents may interact with gold electrodes directly, leading to magnetic-field dependent interface modifications, which then affect transport properties. Such substituent—electrode interactions could also lead to electron transport through the radical substituent. This could lead to Kondo features, and thus could help clarify whether the lack of Kondo signatures in the experiment points to the absence of such transport pathways through the radical substituent. Therefore, we will need to describe the Kondo properties of these radicals. This includes implementing and improving computational methodology. Finally, we aim at gaining new knowledge on general structure-property relationships for organic radicals and their interactions with metal surfaces and electrodes.