Cryogenic static and magic-angle spinning nuclear magnetic resonance on superconductors

本课题研究的是静态和旋转固体中超导体的低温核磁共振。传统的导体允许电流通过，尽管当电流遇到电阻时，一些能量会消散为热。当温度低于临界温度（Tc）时，超导体对直流电无电阻。缺乏能量耗散使得超导体在科学和医学的广泛应用（核磁共振和核磁共振的磁铁，粒子加速器，发电机）中非常有趣，并且如果在有用的温度范围内无损且稳定，则可能用于大电流引线。新的高温超导体（HTS）材料的发现往往是偶然的，公认的超导理论并不能完全描述高温超导体。需要更多的信息来设计具有大临界场和Tc接近室温的稳定材料。因此，将精确的实验观测与局部结构和计算参数联系起来的详细研究，如在本建议中，是非常有价值的。核磁共振提供了局部结构和动力学的详细信息，具有原子分辨率，在很宽的温度范围内。静态样品的固体核磁共振波谱通常非常宽，因此可能会纠缠在一起，难以分离出特定的位点信息。对于某些原子核，核磁共振相互作用非常大，需要静态宽线谱。对于许多其他的核，提高分辨率和信噪比是通过围绕魔角旋转样本来实现的。魔角旋转（MAS）可以解析不相等的化学位点，由于技术限制，通常用于160k以上的研究。在世界上，只有少数MAS核磁共振探针工作在70 K以下（cryoMAS）。在英国，只有南安普顿有一个14t的cryoMAS系统。对于超导体来说，必须在较宽的温度范围内进行实验，尤其是在低温下，因为许多有趣的物理现象发生在150 K以下。在2 K和300 K之间的几个场（重要的是Tc取决于磁场）的静态和MAS核磁共振实验提供的实验数据将与从头计算的信息一起使用，以实现几何和动态约束方面的核磁共振特性的一致解释，从而提供对电子结构的洞察（费米能级的态密度，能隙）。迄今为止，还没有人进行过如此系统的结合低温、低温核磁共振和从头算方法的高温超导研究。我们的调查将在现场产生重大影响。高质量的数据将获得细粉末高温超导样品，这是最相关的技术应用，但更难以表征。与结构紊乱、晶界、氧化和掺杂作用相关的问题将是我们核磁共振研究的目标之一，并将补充使用其他技术获得的信息。结构模型的改进将有助于制定下一代HTS的指导方针。具体目标将是：(1)对表征良好的模型化合物进行初步研究，以调整实验条件、数据分析和计算。(2)在UoS中制备的Osmates具有表征良好的电子性能。核磁共振将解决有关碱离子的咔嗒运动的尚未解决的问题。(3)纯MgB2和掺杂MgB2，解决掺杂的结构作用和晶界的影响。(4)在富勒烯笼内含有氢分子的碱性富勒化物及其衍生物。从我之前对纯H2@C60的NMR工作中可以清楚地看出，H2具有显著的影响，改变了C60的动力学和相变。

This project is concerned with the cryogenic nuclear magnetic resonance (NMR) on superconductors in static and rotating solids. Conventional conductors allow electric current to flow through them, although some of the energy is dissipated into heat as the current encounters a resistance. For temperatures below the critical temperature (Tc) a superconductor exhibits no electrical resistance for DC currents. The lack of energy dissipation makes superconductors very interesting for a wide range of applications in science and medicine (magnets for NMR and MRI, particle accelerators, generators), and potentially useful for high current leads if lossless and stable in a useful temperature regime. The discovery of new high temperature superconductor (HTS) materials is often accidental, and the accepted theory for superconductivity does not fully describe HTS. More information is needed to enable the design of stable materials with large critical fields and with Tc closer to room temperature. Detailed studies relating precise experimental observations to the local structure and to the calculated parameters, as in this proposal, are therefore very valuable. NMR provides detailed information on the local structure and dynamics, with atomic resolution, in a wide temperature range. Solid state NMR spectra of static samples are often very broad, so that site specific information may be entangled and difficult to separate. For certain nuclei, the NMR interactions are so large that static wideline spectra are needed. For many other nuclei, improved resolution and signal to noise are achieved by rotating the sample about the magic-angle . Magic-angle spinning (MAS) can resolve inequivalent chemical sites, and is routinely applied mostly for studies above 160 K due to technical limitations. In the world, there are only a few MAS NMR probes operating below 70 K (cryoMAS). In the UK, a 14 T cryoMAS system is available uniquely in Southampton. For superconductors, it is essential to perform experiments in a wide temperature range, but expecially at cryogenic temperatures, because much of the interesting physics occurs below 150 K. Experimental data provided by static and MAS NMR experiments at several fields (important since Tc depends on magnetic fields), between 2 K and 300 K will be used, together with the information from ab initio calculations, to achieve a consistent interpretation of NMR properties in terms of geometrical and dynamical constraints, therefore providing insight into the electronic structure (density of states of the Fermi levels, energy gap). Nobody has ever undertaken such a systematic study of HTS combining cryostatic, cryoMAS NMR and ab initio methods. The impact of our investigation will be significant in the field. High quality data will be obtained on fine powder HTS samples, which are most relevant for technological applications, but more difficult to characterise. Problems related to structural disorder, grain boundaries, the role of oxidation and doping will be among the targets of our NMR studies, and will complement information obtained using other techniques. The improvement of structural models will help to draw guidelines for the next generation of HTS. Specific targets will be: (1) Preliminary studies on well characterised model compounds to tune up experimental conditions, data analysis and calculations. (2) Osmates prepared in UoS with well characterised electronic properties. NMR will address the unsolved questions about rattling motion of the alkali ions. (3) pure and doped MgB2, to address the structural role of the doping and the effects of the grain boundaries. (4) Alkali fullerides and derivatives with molecular hydrogen inside the fullerene cage. From my previous NMR work on pure H2@C60, it is clear that H2 has a significant effect and changes the dynamics and phase transition of C60.