Spin- and charge dynamics in ferromagnetic Josephson junctions

人们普遍认为，半导体器件的小型化面临着与散热和漏电流有关的基本物理限制，以及纳米器件的有限尺寸效应。因此，探索了可供选择的设备概念。最近，混合超导纳米结构引起了人们的极大兴趣，因为这项技术有可能克服这些限制，甚至有望为量子信息处理设备提供一种新的范式。然而，这种器件的实际实现需要完全了解超导和正常金属或铁磁电路元件之间的自旋和电荷电流的转移和动力学，以及这些系统中自旋和电荷自由度之间的耦合。在这个研究项目中，我们将研究超导体(S)/铁磁(F)杂化纳米结构在非平衡条件下的新的自旋和电荷输运现象。我们将集中在铁磁约瑟夫森结(FJJ)，其中两个超导体被铁磁势垒隔开。通过实验确定FJJ的电流相关系，可以直接获得它们的基本物理性质。到目前为止，对FJJ的研究大多纯粹是关于邻近效应，即交换分裂对库珀对穿透F层的影响。然而，铁磁体还具有有趣的动力学性质，可以用来产生、操纵和检测SF异质体系中的自旋流。在FJJ中，约瑟夫森超流与铁磁隔离物的磁化的动态耦合导致了新的输运现象。我们将阐明一些与竞争序参数的复杂相互作用有关的基本问题，特别是关于自旋、电荷和能量的非平衡分布的松弛机制的问题。最终目标是为下一代电子电路设计一种具有前所未有的开关速率的无耗散自旋电子器件的全新概念。在SF杂化系统中，通过引入具有非共线磁化的自旋活动界面，F中的三重态Cooper对所携带的长程超导对关联可以被建立起来。最近，理论预测暗示了在SF杂化系统中通过电子自旋与铁磁体的磁化的动态耦合来产生长程三重态配对的可能性。本项目旨在对这一新现象进行实验研究。目的是证明在具有长铁磁间隔的FJJ中存在超导对关联，该间隔具有通过铁磁共振引起的空间和时间上的非均匀磁化。在这种特殊情况下，在微波辐射下的输运实验中应该检测到明显的超流。

It is widely accepted that the downscaling of semiconductor devices faces fundamental physical limitations related to the heat dissipation and current leakage as well as a finite size effect in nanoscale devices. Therefore, alternative device concepts are explored. Recently, hybrid superconducting nanostructures attracted a tremendous interest, because this technology has the potential to overcome these limitations and even promises a new paradigm for quantum information processing devices. The practical realisation of such devices, however, requires a complete understanding of the transfer and the dynamics of spin and charge currents between superconducting and normal metal or ferromagnetic circuit elements as well as the coupling between spin and charge degrees of freedom in these systems. In this research project, we will investigate novel spin and charge transport phenomena in superconductor (S)/ferromagnet (F) hybrid nanostructures under non equilibrium conditions. We will focus on ferromagnetic Josephson junctions (fJJs), where two superconductors are separated by a ferromagnetic barrier. The experimental determination of the current phase relation of fJJs provides direct access to their fundamental physical properties. Most studies on fJJs so far have been concerned purely with proximity effects, i.e. the influence of exchange splitting on the penetration of Cooper pairs into the F layer. However, ferromagnets also possess interesting dynamic properties, which can be explored to generate, manipulate and detect spin currents in SF heterosystems. In fJJs the dynamic coupling of the Josephson supercurrent to the magnetisation of the ferromagnetic spacer leads to novel transport phenomena. We will elucidate some of the fundamental questions related to the complex interplay of competing order parameters and in particular the question of relaxation mechanisms of non equilibrium distributions with respect to spin, charge and energy. The ultimate aim is to devise a fundamentally new concept for dissipation less spin electronic devices with unprecedented switching rates for next generation electronic circuits. It has been established that long range superconducting pair correlations, carried by triplet cooper pairs in F, can be induced in SF hybrid systems via the introduction of spin active interfaces with a non collinear magnetisation. Recently, theoretical predictions hint to the possibility to generate long range triplet pairing in SF hybrid systems via a dynamic coupling of the electron spin to the magnetisation of the ferromagnet. This project aims to experimentally investigate this novel phenomenon. The objective is to show that superconducting pair correlations exist in fJJs with a long ferromagnetic spacer with spatially and temporally inhomogeneous magnetisation induced via a ferromagnetic resonance. In this special situation, a pronounced supercurrent should be detected in transport experiments under microwave irradiation.