Correlated magnets and superconductors explored using muon-spin rotation

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Implanted muons give a unique local perspective on quantum materials. By measuring the rotation of the implanted muon spin in the local magnetic field, which in turn results from the dipolar field arising from nearby magnetic moments, one can follow the magnetic order parameter as a function of temperature. If there are a range of muon sites the technique can provide information about the internal magnetic field distribution. Even above the magnetic transition temperature, the measured relaxation of the muon spin can provide information about magnetic fluctuations and spin dynamics. In superconductors the technique is particularly valuable because the measured internal field distribution can allow us to infer the pairing mechanism. However, there is a fundamental limitation of this technique which comes from the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. We have found that this problem can be addressed using electronic-structure calculations using a technique we have developed which we call "DFT+mu", density functional theory with an included muon. This D.Phil. project will involve experiments using muons on new magnets and superconductors, as well as investigations motivated by the DFT+mu method. The aim is to further our understanding of the magnetic and superconducting materials, but also to extend the muon-spin rotation technique by gaining new insights into the nature of muon sites.