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

由于电子的平移对称性和离域性质的破坏，在两种材料(或表面)之间的界面附近，所有的电子、磁性和结构性质都会改变，其中电子波函数的性质随着与边界的距离而变化。在某些情况下，这些变化很小，仅限于几个原子层，而在另一些情况下，集体行为在许多纳米甚至微米范围内被改变。磁性多层膜的巨磁电阻就是一个显著的例子，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域的磁性和电性。我们在TRIUMF上开发了其中之一，称为低能β-检测核磁共振(β-核磁共振)。另一种密切相关的方法是瑞士保罗·谢勒研究所(PSI)开发的低能Muon自旋旋转/弛豫。这两种方法都将被用来探索界面附近和异质结构内电子的相干性质，以解决凝聚态物理中的重要问题。这包括了SrTiO_3/LaAl_3界面的磁/电性质，拓扑绝缘体的电磁响应，以及高度关联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/磁性质。结果将被用来检验理论预测和寻找新的现象。这将有助于创建一幅更统一的图景，了解电子在低温下如何将自己组织成物质的相干状态，以及它们如何对界面附近和异质结构内的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中的电子性质的预期。我们将使用新开发的低能量MuSR和低能量β-核磁共振技术，它们具有相似的原理，但提供了互补的信息。Muon(或放射性原子核)的磁矩充当了局部磁/电子环境的探测器。1957年，随着弱相互作用中宇称破坏的发现，人们发明了常规的µ子自旋旋转和β-核磁共振。然而，这里使用的特定变体是相对较新的，仍在开发中。关键是，与传统的核磁共振不同，β-核磁共振和muSR中的信号与样本大小无关，可以作为纳米尺度上的函数深度进行监测。因此，这两种方法都非常适合于薄膜和异质结构的研究。它们都是磁共振的形式，但提供了互补的信息。Muon是静态或准静态磁场的敏感探测器，而8Li对慢电子弛豫过程(例如金属中的Korringa弛豫)更敏感，因为它的寿命要长得多(1.2秒)，而Muon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.