Quantum Materials Studied with muSR and beta-NMR

在两种材料之间的边界附近，所有的电子、磁性和结构性质都会发生变化。因此，薄膜或晶体的近表面区域通常具有不同于本体的特性。在某些情况下，可以用电场或磁场来控制这些特性。利用这种效应的设备对磁存储信息产生了巨大的影响。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有趣，因为这是我们期望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（如超导性）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子/磁性能。这将有助于创建一个更统一的图像，电子和磁性离子如何在低温下组织成物质的相干状态，这些属性如何在界面附近改变，以及如何用外场控制它们。潜在的影响来自于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期望。

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.