Muon sites and couplings in magnetic and superconducting materials: towards high-throughput modelling

Abstract

This thesis consists of the theoretical study of the muon stopping sites and hyperfine interactions in magnetic compounds mostly using density functional theory (DFT) calculations, to aid in the interpretation of muon-spin spectroscopy (μSR) experimental measurements. A new high-throughput (HT) DFT-based calculation approach to automatically manage these calculations is introduced and benchmarked with a large set of magnetic materials already characterized by μSR experiments. It is also demonstrated in this thesis how the results of these calculations are utilized in characterizing material ground state properties.

First, the theoretical quantum mechanical approach for simulating the time dependent muon spin polarization is described together with its implementation in the open-source, UNDI software package. In particular, the computationally fast and efficient method by Celio is described. Also within this approach, the effects of the electric quadrupolar interactions relevant to accurately capture the physical properties for nuclei with large moment are incorporated. The approach is demonstrated for LiF, Cu, MnSi and utilized to study muon sites and magnetic properties in these compounds.

Another, the results of zero field μSR and nuclear magnetic resonance (NMR) measurements for iron-phosphide Fe2P are presented. This material is the parent compound of a large family of (Fe/Mn), (P/Si/B) alloys displaying first-order magnetic transitions, exploitable for magnetic energy-harvesting applications. Results of DFT calculations provide unique muon-stopping sites, muon hyperfine interactions and hyperfine at the P nuclei that allowed to further characterize and interprete μSR and NMR measurements, providing accurate description of the ferromagnetic ground state properties of Fe2P.

Next, efforts and approach towards the design and implementation of workflows for high-throughput DFT-based muon calculations are presented. The aim is to introduce a more user friendly calculation approach with less human intervention that allows to automatically manage, track and store muon calculation results. The work- flow is benchmarked on 16 selected magnetic compounds, that allowed to discuss the success and limitations of the approach at its current stage. Benchmark results show that further improvement on the workflow should include taking into account the muon charge states and proper treatment of electronic correlation effects.

Finally, results of the electronic and magnetic ground state properties together with the muon charge localization in Chromium Chalcogenide Cr2S3 are presented. Cr2S3 is one of the 16 magnetic compounds selected in the previous chapter for the benchmark of the workflow, where the poor treatment of electronic correlation within the conventional DFT and absence of muon charge state result in the encoun- tered failure of the workflow. Here, outside the workflow, calculation results show that adequate electronic correlation and muon charge treatment allow to accurately describe the ground state properties and the muon sites Cr2S3 in agreement with experiment. The results of the calculation show that the electronic properties are strongly dependent on the magnetic ordering.