Copy of Andreev Reflection in Superconducting Spin Polarised Devices

固体中电子之间的相互作用是现代物理学研究中许多最有趣和最令人兴奋的物理现象的原因。该项目旨在研究和利用两种现象之间的相互作用，这两种现象与电子角动量或自旋的量子性质密切相关，乍一看似乎是不相容和互补的。通过使用最新的实验技术来创建纳米级结构并在低于三分之一度的温度下进行测量，这项工作将发展我们对最基本的相互作用的理解，这些相互作用将成为电子学下一个时代的核心。这项工作建立在我在设备材料方面的经验基础上，运用对材料科学的理解来帮助研究低温和凝聚态物理学中的问题，然后这些问题可以应用于有用的电子或磁性设备。电子可以用一组属性来描述，例如能量，动量和角动量，量子力学限制在某些特定的值。对于角动量或自旋，有两个值;“向上”和“向下”。超导性是一种材料在低温和磁场下导电而无电阻的特性，在金属中与具有相反自旋的电子配对有关，一个“向上”，一个“向下”。铁磁性，一种在临界温度以下具有永久磁性的材料，与电子在一个方向上的排列有关。电子自旋也可以被认为是使电子充当小的条形磁铁;因此对齐电子自旋使所有单独的磁矩加在一起。特别感兴趣的是电子从铁磁体（平行取向）转移到超导体（反平行取向）的过程。传统上，进入超导体的单个电子在一个称为Andreev反射的过程中这样做-它需要带着第二个电子，该电子必须具有相反的自旋才能在超导体中形成一对。如果电子来自铁磁体，由于自旋的平行排列，可能没有足够的合适电子。还有另一种可能：大多数真实的磁性材料不会形成所有电子磁矩都沿同一方向排列的单畴。相反，具有不同取向的多个畴被畴壁分开，其中磁矩从一个取向扭转到另一个取向。如果进入超导体的两个电子来自不同的畴，那么电子配对可能会更容易-这一过程称为交叉Andreev反射。要做到这一点，两个畴需要放置在超导相干长度内-电子配对发生的距离。这个距离通常在10 s到100 s的nm范围内，这也是许多材料中畴壁宽度的长度尺度。然而，这种效应已经被证明，至少在理论上，形成操纵和控制电子自旋取向的设备的基础。利用电子的自旋和电荷来携带信息是当前许多研究的主题，所谓的“自旋电子学”的发展被广泛认为是开辟电子学新时代的关键，就像晶体管的发展一样重要。该项目既依赖于又补充了这项研究。在超导器件中结合自旋电子元件将使人们对自旋电子学研究中的材料优化和器件设计有更深入的了解，并阐明超导和磁性材料相互作用的基本物理。实质上，这将等同于从被动磁性器件到自旋主动器件的超导器件的转变。

Interactions between electrons in solids are responsible for many of the most intriguing and exciting physical phenomena studied in modern physics. This project aims to investigate and exploit interactions between 2 such phenomena, intimately connected to the quantum nature of an electron's angular momentum, or spin, that at first sight appear to be incompatible and complementary. By using the latest experimental techniques for creating nanometre scale structures and carrying out measurements at temperatures less than a third of a degree, this work will develop our understanding of the most fundamental interactions that will be at the heart of the next epoch in electronics. This work builds on my experience in device materials, applying an understanding of materials science to help study problems in low temperature and condensed matter physics that can then be applied in useful electronic or magnetic devices.Electrons can be described in terms of a set of properties, for example, energy, momentum and angular moment, that quantum mechanics limits to certain specific values. For angular momentum or spin there are 2 values; 'up' and 'down'. Superconductivity, the property where a material will conduct electricity at low temperatures and magnetic fields without resistance, is associated in metals with pairing of electrons with opposite spins, one 'up' and one 'down'. Ferromagnetism, a material's having permanent magnetism below a critical temperature, is associated with the alignment of electrons in one direction. Electron spin can also be thought of as making the electrons act as little bar magnets; so aligning electron spins makes all the individual magnetic moments add up together.Of particular interest are the processes where electrons are transferred from a ferromagnet (parallel orientation) to a superconductor (anti-parallel orientation). Conventionally, a single electron entering a superconductor does so in a process known as Andreev reflection - it needs to take a second electron with it that must have opposite spin to form a pair in the superconductor. If the electrons are coming from a ferromagnet, there may not be enough suitable electrons due to the parallel alignment of spins.There is another possibility: most real magnetic materials do not form single domains with all the magnetic moments of the electrons aligned in the same direction. Instead, multiple domains with different orientations occur separated by domain walls where the magnetic moments twist from one orientation to the other. If the 2 electrons entering a superconductor are taken from different domains, it may be easier to pair the electrons up - a process known as cross Andreev reflection. To do this the two domains need to be placed within the superconducting coherence length - the distance over which the pairing of electrons happens. This distance is typically within 10s to 100s of nm, which is also the length scale of a domain wall width in many materials.These effects are only just within the range of experimental investigation. However, such effects have already been shown, theoretically at least, to form the basis for devices to manipulate and control the spin orientations of electrons. The use of an electron's spin, in addition to its charge, to carry information is the subject of much current research and the development of so called 'spintronics' is widely held to be key to opening up a new era in electronics as fundamental as the development of the transistor. This project both depends on and complements that research. The combination of spintronic elements in superconducting devices will give insight into the optimization of the materials and design of devices in spintronics research as well as elucidating the fundamental physics of the interaction of superconductivity and magnetic materials. In essence, this would be the equivalent in superconducting devices of the transition from passive magnetic devices to spin active devices.