Microwave spectroscopy of highly underdoped yttrium barium cuprate

The manner in which the Mott insulating state gives way to superconductivity when the number of charge carriers is changed is one of the fundamental questions about high-Tc superconductors that remains unanswered. This is because of both the complexity of the high-Tc superconductors and the lack of experimental data on very clean underdoped samples. We have developed a novel technique for continuously varying the doping in a single sample of Y13a2CusOs+, without changing the oxygen content. This allows us to access different dopings with transition temperatures between 0 I i and 17 K. This thesis presents our data on the temperature and doping dependence of the superfluid density, which shows clear linear temperature dependence, typical of a good d-wave superconductor. An apparent phase transition as the doping is varied through a transition temperature of 11 I< is discussed and evidence is shown for a small spectroscopic energy gap that is proportional to Tc.