| Among all the superconductors discovered so far, high temperature cuprate superconductors (HTCS) are unique not only in their high critical temperatures but also in their diverse physical properties depicted by the well known temperature-doping phase diagram. Depending on the doping levels, they can be antiferromagnetic insulator, spin glass, superconductor, normal metal or even some other strange phase; in other words, there exist multiple quantum critical points along the doping axis. A common way to control the doping level is either oxygen annealing or cation substitution. When these two schemes are combined, the available doping range can be extended far beyond what can be reached with only one. By applying this to MBE grown BSCCO films, we show how and where the quantum phase transitions occur as a function of doping, using in-plane resistivity as a measure of this. Another and completely different approach to control the doping level is to use electric field effect. One of the long sought questions in the electric field effect study on cuprates is whether it is possible to drive a non-superconducting cuprate system into a superconductor simply by inducing high enough density of carriers. We answer this question based on our results from epitaxially fabricated field effect transistor-like cuprate structures. A third approach related to the doping control is search for a microfabrication technique for in-plane S-AFI(or US)-S cuprate structures, where 'AFI' stands for antiferromagnetic insulator and 'US' for underdoped superconductor. Such a structure has recently attracted considerable attention due to its relationship to the so called SO(5) theory for HTCS mechanism. We show how such spatial doping control can be achieved by combining a simple microfabrication technique with thermal treatments. |
