Novel Properties of Electrons Near Intefaces Studied with muSR and beta-NMR

在两种材料（或表面）之间的界面附近，由于电子的平移对称性和离域性质的破坏，所有的电子、磁性和结构性质都发生了变化，其中电子波函数的特征随距离边界的距离而变化。在某些情况下，这些变化很小，局限于几个原子层，而在其他情况下，集体行为在许多纳米甚至微米上被改变。磁性多层膜中的巨磁电阻是一个显著的例子，它对磁存储产生了巨大的影响。只有少数实验方法能够以深度分辨的方式探测局部磁和电子特性。我们已经在TRIUMF开发了其中一种称为低能β探测核磁共振（β - nmr）。另一种密切相关的方法是低能μ子自旋/弛豫，这是由瑞士保罗·谢勒研究所（PSI）开发的。这两种方法都将用于探索界面附近和异质结构内电子的相干特性，以解决凝聚态物理中的重要问题。这包括SrTiO3/LaAlO3界面的磁/电子性质，拓扑绝缘体的电磁响应以及高度相关系统中超导和磁序参数的深度依赖关系。我们的主要目标是探索/控制量子材料界面和表面附近的电子/磁性能。研究结果将用于检验理论预测和寻找新现象。这将有助于建立一个更统一的图像，了解电子如何在低温下组织成物质的相干状态，以及它们如何在界面附近和异质结构内对电场和磁场做出反应。潜在的影响来自于对未来设备将越来越多地利用界面和异质结构内电子特性的期望。我们将使用新开发的低能muSR和低能β - nmr技术，它们具有相似的原理，但提供互补的信息。μ子（或放射性核）的磁矩充当局部磁/电子环境的探针。传统的介子自旋和核磁共振是在1957年随着弱相互作用宇称破坏的发现而被发明的。然而，这里使用的特定变体相对较新，仍在发展中。与传统核磁共振不同的是，β -核磁共振和muSR中的信号与样本量无关，可以在nm长度尺度上作为函数深度进行监测。因此，这两种方法都非常适合研究薄膜和异质结构。它们都是磁共振的形式，但提供了互补的信息。μ子是静态或准静态磁场的敏感探针，而8Li对慢电子弛豫过程（例如金属中的korringa弛豫）更敏感，因为它的寿命（1.2s）要长得多，而μ子的寿命为2.2微秒。

All electronic, magnetic and structural properties are altered near an interface between two materials (or a surface) due to the broken translational symmetry and delocalized nature of electrons in which the character of the electronic wavefunction varies as a function of distance from the boundary. In some cases these changes are small and confined to a few atomic layers while in other situations, the collective behaviour is modified over many nanometers or even microns. Giant magneto-resistance in magnetic multilayers is one notable example which had a dramatic impact on magnetic storage. Only a few experimental methods are capable of probing local magnetic and electronic properties in a depth resolved manner. We have developed one of them at TRIUMF called low energy beta-detected nuclear magnetic resonance (beta-NMR). Another closely related method is low energy muon spin rotation/relaxation, which has been developed at the Paul Scherrer Institute (PSI) in Switzerland. Both methods will be used to explore the coherent properties of electrons near interfaces and within heterostructures in order to address important problems in condensed matter physics. This includes the magnetic/electronic properties of the SrTiO3/LaAlO3 interface, the electromagnetic response of topological insulators and the depth dependence of the superconducting and magnetic order parameter in highly correlated systems. Our main objective is to explore/control the electronic/magnetic properties near the interfaces and surfaces of quantum materials. The results will be used to test theoretical predictions and search for new phenomena. This is turn will help create a more unified picture of how electrons organize themselves into coherent states of matter at low temperatures and how they respond to electric and magnetic fields near interfaces and within heterostructures. The potential impact comes from the expectation that future devices will increasingly utilize on the properties of electrons at interfaces and within heterostructures. We will use newly developed techniques of low energy muSR and low energy beta-NMR, which have similar principles but provide complementary information. The magnetic moment of the muon (or radioactive nucleus) acts as a probe of the local magnetic/electronic environment. Conventional muon spin rotation and beta-NMR were invented in 1957 along with the discovery of parity violation in weak interactions. However, the particular variants used here are relatively new and still developing. The key points are that unlike conventional NMR, the signals in beta-NMR and muSR are independent of sample size and can be monitored as a function depth on a nm length scale. Consequently both methods are ideally suited to studies of thin films and heterostructures. They are both forms of magnetic resonance but provide complementary information. The muon is a sensitive probe of static or quasi static magnetic fields whereas 8Li is much more sensitive to slow electronic relaxation processes (e.g.Korringa relaxation in a metal) due to the much longer lifetime (1.2s), compared to 2.2 microseconds for the muon.