Muon stopping sites in magnetic systems from density functional theory

This thesis concerns the use of density functional theory (DFT) to determine muon stopping sites in crystalline solids. New tools for carrying out these calculations are introduced and these techniques are demonstrated through the results of calculations on the skyrmion-hosting semiconductors GaVS and GaVS and the heavy-fermion metals URuSi and CeRuSi. The results of three studies on significantly different magnetic systems are presented, where in each case the interpretation of the results of muon-spin spectroscopy (SR) experiments is aided by knowledge of the muon site. The results of SR measurements on the iron-pnictide compound FeCrAs are presented and indicate a magnetically ordered phase throughout the material below =105(5) K. There are signs of fluctuating magnetism in a narrow range of temperatures above involving low-energy excitations, while at temperatures well below a characteristic freezing of dynamics is observed. Using DFT, a distinct muon stopping site is proposed for this system. The results of transverse-field (TF) SR measurements on the molecular spin ladder compound (Hpip)CuBr, [Hpip=(CHN)] are reported. Characteristic behaviour in each of the regions of the phase diagram is identified in the TF SR spectra. Analysis of the muon stopping sites, calculated using DFT, suggests that the muon plus its local distortion can lead to a local probe unit with good sensitivity to the magnetic state. Finally, the results of SR measurements on the charge density wave system 1T-TaS are presented, which show three distinct phases versus temperature. The critical exponents for each of these phases are compared with the predictions of quantum spin liquid models. Using DFT, a quantum delocalised state for the muon between the TaS layers is proposed, which is used in conjunction with its associated hyperfine interactions to determine the coupling of the muon to the diffusing spinons.