Quantum Materials Studied with muSR and beta-NMR

所有的电子、磁性和结构性质都会在两种材料的边界附近发生变化。因此，薄膜或晶体的近表面区通常具有与块体不同的性质。在某些情况下，可以利用电场或磁场来控制这些属性。使用这种效应的设备对信息的磁性存储产生了戏剧性的影响。我研究的第二个主题是量子材料的相干特性。材料的低温特性特别令人感兴趣，因为这是我们预计相干效应最大的地方。在低温下表现出有趣的量子现象的材料(如超导)有时被称为“量子材料”。我们的目标是探索并最终控制量子材料界面和表面附近的电/磁性质。这将有助于创建一幅更统一的图景，了解电子和磁离子在低温下如何将自己组织成物质的相干状态，这些性质在界面附近如何变化，以及如何通过外场控制它们。潜在的影响来自对未来设备将越来越多地利用量子材料的界面和薄膜特性的预期。 我们将使用强大的新方法在纳米尺度上探索局部磁性作为深度的函数。其中一种被称为低能贝塔检测核磁共振(贝塔核磁共振)的技术已经在加拿大开发出来。瑞士的保罗·谢勒研究所(PSI)有可能使用一种密切相关的方法，即低能Muon自旋旋转/弛豫(MuSR)。这两种方法具有相似的原理，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。这两种方法与通常所说的核磁共振成像密切相关。所有形式的核磁共振都包括产生非平衡自旋极化，然后观察随时间变化的极化。MuSR和β-核磁共振的不同之处在于，极化是通过Muon或原子核的各向异性衰减特性来检测的。因此，可以在比常规核磁共振或核磁共振所需的小10个数量级的结构中观察到信号。特别是，这些方法使人们能够探测非常薄的薄膜的内部磁性和电学性质，这些薄膜比人类头发的厚度薄一百万倍。

All electronic, magnetic and structural properties are altered near a boundary between two materials. Consequently a thin film or the near surface region of a crystal has properties that in general are different than in the bulk. In some cases it may be possible to control such properties with an electric or magnetic field. Devices using such effects have had a dramatic impact on magnetic storage of information. A second theme for my research is coherent properties of quantum materials. The low temperature properties of materials are particularly interesting since that is where we expect effects due to coherence to be largest. Materials which exhibit interesting quantum phenomena at low temperatures (e.g. superconductivity) are sometimes called “quantum materials”. Our objectives are to explore and eventually control the electronic/magnetic properties near the interfaces and surfaces of quantum materials. This will help create a more unified picture of how electrons and magnetic ions organize themselves into coherent states of matter at low temperatures, how these properties change near interfaces and how they can be controlled with external fields. The potential impact comes from the expectation that future devices will increasingly utilize the properties of interfaces and thin films of quantum materials. We will use powerful new methods to explore local magnetic properties as a function of depth on a nanometer length scale. One of them, called low energy beta-detected nuclear magnetic resonance (beta-NMR), has been developed in Canada. A closely related method, low energy muon spin rotation/relaxation (muSR) , is possible at the Paul Scherrer Institute (PSI) in Switzerland. The two methods 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. The two methods are closely related to nuclear magnetic resonance imaging commonly known as MRI. All forms of nuclear magnetic resonance involve generation of a non-equilibrium spin polarization followed by observation of the time dependent polarization. muSR and beta-NMR are distinct in that the polarization is detected through the anisotropic decay properties of the muon or nucleus. Consequently a signal can be observed in structures that are 10 orders of magnitude smaller than is required in conventional NMR or MRI. In particular these methods allow one to probe the internal magnetic and electronic properties in very thin films which are a million times thinner than of a human hair.