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

在个体水平上控制分子是一项艰巨的任务，在过去的几年里已经取得了巨大的进步，特别是在分子电子学和自旋电子学领域。虽然在离散状态之间的切换是这种控制的一个特别戏剧性的例子，但是通过诸如磁场(磁阻)之类的外部参数来逐渐改变诸如电阻之类的属性是耐人寻味的，因为这意味着系统的可访问属性的空间可以被广泛地探测，并且因为它对于诸如传感之类的功能是必不可少的。与金属电极接触的有机自由基的磁电阻具有额外的诱惑力，表现出意想不到的行为，其中许多行为尚不清楚。阐明潜在的物理机制不仅可以为潜在的功能材料的内部工作提供新的见解，而且可以为分子在非平衡条件下的基本方面以及它们如何与金属表面相互作用提供新的见解。我们的目标正是从原子模拟中获得这样的洞察力。由于磁阻有几种可能的机制，最确凿的理论证据将来自第一性原理方法。第一性原理方法还允许公正地评估化学结构如何影响物理行为，这对于一旦了解基本机制后建立结构-性质关系是至关重要的。本项目的具体目标是阐明最近在单分子TEMPO-OPE自由基结中观察到的目前令人困惑的磁阻的机制，并提出新的自由基-电极系统，有望(A)进一步深入了解磁阻和界面结构的基本机制，以及(B)自旋电子学的潜在应用。我们要仔细研究的主要假设是，自由基取代基可能直接与金电极相互作用，导致磁场依赖的界面修饰，从而影响传输性质。这种取代基-电极相互作用也可能导致电子通过自由基转移。这可能导致近藤的特征，从而有助于澄清实验中缺乏近藤签名是否表明没有通过自由基取代基的运输途径。因此，我们需要描述这些自由基的近藤性质。这包括实施和改进计算方法。最后，我们的目标是获得关于有机自由基的一般结构-性质关系以及它们与金属表面和电极的相互作用的新知识。

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.