Muon-Spin Studies in Organic Systems

This thesis reports muon-spin-relaxation ( muSR) studies in organic systems, including superconductors, ferromagnets, and antiferromagnetic spin -density-wave (SDW) systems. In the quasi two-dimensional superconductors kappa-(BEDT-TTF) _2-X, with X = Cu(NCS)_2 and Cu (N(CN)_2) Br, we found that the magnetic penetration depth displays linear -temperature variations below 0.5T_{c }, characteristic of anisotropic superconducting pairing with line nodes in the energy gap. In the (TMTSF)_2-X system, with X = PF_6, NO_3 and ClO_4, we observed a spontaneous internal magnetic field at ambient pressure, due to the onset of SDW order. Comparing the muSR results obtained in these three systems, we found a common SDW amplitude followed by the reduction of the SDW transition temperature with increasing effective dimensionality. These results are consistent with the mean-field theory for the Fermi-surface nesting model. In the PF_6 system, we observed a sharp phase transition followed by significant reduction of SDW amplitude at low temperatures, which is not expected from the mean-field theory. This indicates the importance of considering dynamic effects, such as the collective spinwave excitations. The large spinwave stiffness observed in the PF_6 compound is characteristic of an itinerant electron system. We have also performed muSR studies in an organic ferromagnet beta -phase p-NPNN and a molecular ferromagnet (DMeFc)(TCNE). In p-NPNN, our measurements provide direct observation of spontaneous magnetic order. The behaviour of the system is close to that of an isotropic three-dimensional Heisenberg spin-system. In (DMeFc)(TCNE), the relaxation rate in the paramagnetic phase displays a gradual variation with temperature, indicating that the slowing down of spin dynamics occurs over a wide temperature range. The temperature dependence of spin fluctuations between 1.5T_ {c} and 15T_{c} shows that thermal activation behavior expected for a spin chain with Ising character.