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

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.

This project has tremendous potential to impact on the development of stable high-temperature superconductors, which could lead to break-throughs in technological applications. Apart from academic beneficiaries, in the short term, our results will be of interest for the private sector among the companies interested in the production of superconductors. See for instance http://www.conectus.org for a list of some European companies with strong involvement in the commercial applications of superconductivity. The model systems to be examined under this feasibility study will not be immediately exploitable for commercial applications. However, they will be a useful tool to improve our understanding of HTS. Moreover, the new approach set by our work will provide guidelines useful for academic researchers as well as private companies with interest towards the development of the next generation of HTS materials. The benefits will come from: 1) new accurate experimental data (from cryogenic NMR) will provide a testbed for theories and calculations on superconductors. 2) the NMR data acquired on fine powders will provide structural information on materials with imperfections and short range order. It will help to draw guidelines on desirable properties of powder samples for practical applications. Just like academic beneficiaries, also the private and public sector will have full access to the results from our work, which will be disseminated through conferences, published in international scientific journals and advertised through the on line resources of our host institution, which are freely accessible to anybody with an internet connection. The applications may include 1) cheaper and/or more powerful superconducting magnets for NMR research and MRI (with clear impact for the society due to the medical applications, as MRI is a very important diagnostic tool). Several companies manufacture these instruments, and Bruker, Varian and Jeol are the main ones. 2) superconducting motors, generators, powder grids and so on. 3) high current leads if lossless and stable in a useful temperature regime 4) fast trains exploiting HTS (maglev) 5) ultra-fast computer hardware 6) the defence industry is among the users of superconductors