报告题目：Development of Nuclear Magnetic Resonance Techniques For the Study of Topological Band Structures

报 告 人：Louis Bouchard，UCLA

报告时间：2015年10月29日13：30

报告地点：理科楼B406

报告摘要：In recent years, the emergence of gapless topologically protected edge states in the solid state without the need to apply an external field has led to searches for new phases of condensed matter in new and existing materials. For example, some thermoelectrics and Kondo insulators have been shown to be topological insulators (TIs). The edge states give rise to exotic phenomena include the quantum anomalous Hall effect, fractional quantum anomalous Hall effect, topological superconductor, fractional time-reversal invariance, topological crystalline insulator and the topological magneto-electric effect. Because the interesting properties of TIs are found at edges and interfaces, they are challenging to study experimentally. In this talk, I will present new experimental approaches to study the electronic and magnetic properties of such topological materials based on nuclear spin interactions (namely, using nuclear magnetic resonance, NMR). Among the techniques, we shall discuss a type of radioactive ion beam spectroscopy to resolve properties as function of depth, and with nanoscale resolution. Such studies not only reveal substantial modulations of the material properties at these length scales, but also reveal new parameters such as s-d exchange integrals which cannot be obtained by other means. Because they do not rely on transport, NMR techniques may offer new and less ambiguous ways to separate bulk from surface contributions. Unlike ARPES, the method is not limited to n-type materials and one can easily probe p-type materials. NMR is also useful in the case of materials with high defect content even up to room temperature. I will discuss the distinct responses of nuclear spins based on dipolar and quadrupolar moments. The new methods could have implications in the design of devices, in the search for novel physics and in the optimization of material properties.