Multi-frequency high-pressure electron spin resonance study of low dimensonal organic conductors

Electronic and magnetic properties of quasi-one dimensional ormore widely low-dimensional systems aswell as their related correlated electron phenomena have been at the very frontier of condensed matter physics for quite some time. The reduced dimensionality in these systems offers a unique possibility of direct comparison with model calculations. Furthermore the competing ordered ground states in strongly correlated low-dimensional systems lead to rich phase diagrams comprising many electronic phases, such as superconductivity, charge/spin densitywaves, different electric charge distributions, or a novel formof metalicity. These phases are generically sensitive to a variety of parameters, such as temperature, magnetic field, pressure, a certain degree of irregularity, etc. In this thesis, we employ Electron Spin Resonance (ESR) spectroscopy using strong magnetic fields, high microwave frequencies and high hydrostatic pressure to study a recently discovered family of quasi-one-dimensional organic charge transfer salts, the δ-(EDT-TTF-CONMe2)2X; family to gain insight into the complex physics of strongly correlated low-dimensional compounds. ESR is a highly efficient technique to study organic conductors. This can be easily understood if one realizes that ESR is observable inmost organic conductorswhile only very few "ordinary" metals show detectable ESR signals, even at low temperatures. Indeed, this feature results principally from the electronic low-dimensionality of these systems which controls the spin relaxation process. This tendency is even more reinforced by the fact that most organic molecules only contain light chemical elements for which the effect of the spin-orbit coupling on the ESR linewidth remains small. For this reason, ESR spectroscopy is certainly amajor tool to investigate the paramagnetic state of organic conductors. Moreover, in samples where a magnetic order is present, the large frequency bandwidth of the spectrometers used in the experimental setup (4 GHz–420 GHz) makes the resonance signal accessible at low temperatures in the ordered state, where the resonance frequencywould be too high for conventional ESRspectrometers. Our device hence represents an efficient tool to probe different magnetic ground states and, in particular, the antiferromagnetic state, which is most commonly found in organic salts. Organic conductors exhibit a high degree of sensitivity with respect to magnetic fields and applied pressure. To combine the high sensitivity of the ESR method with the sensitivity and thus fine tuneability of these organic systems by magnetic field and pressure, we build a unique high-field, high-frequency and high-pressure ESR spectrometerwhich is well suited to the study of organic materials. Compared to other devices of its kind, the ESR spectrometer built in our laboratory as a part of this thesis enables a wide parameter space to be explored: magnetic fields from 0 – 16 T, microwave frequencies in the 105 – 420 GHz