Fermiology of High Temperature Superconductors

Despite having been discovered more than twenty years ago, the unusual properties of high temperature cuprate superconductors (HTSC) are still far from being well understood. The most important question is of course, why is the superconducting transition temperature so high and how might it be raised even further? A strongly linked question to this is, what is the nature of the normal (non-superconducting) state of these materials and how does this evolve as the number density of conduction electrons is varied by chemical doping? Clearly, a complete understanding of the former requires knowledge of the latter. Until recently our main experimental probes of the normal state were macroscopic transport measurements (electrical resistivity, Hall effect etc.) and angle resolved photoemission spectroscopy (ARPES). The recent discovery that quantum oscillations can also be observed in some cuprate superconductors at very high magnetic field (> 40 Tesla) has made a very big impact on the subject. Most importantly, it reveals that the true nature of the Fermi surface of the slightly doped cuprates is significantly different to that suggested previously by ARPES measurements. The main features of the quantum oscillation technique are first that the measurements probe the electronic structure right at the Fermi level with much higher energy and momentum resolution than ARPES, and second that they probe the bulk of the sample and so are not influenced by surface defects or reconstructions. A second related technique for investigating the Fermi surface is angle dependent magnetoresistance (ADMR). This has recently proven a very successful technique particularly for investigating more heavily doped HTSC and has revealed in great detail the full three-dimensional Fermi surface topology on one particular cuprate family. Such three-dimensional information is currently inaccessible to ARPES. ADMR has an additional advantage over conventional quantum oscillation techniques in that it can also reveal the temperature and momentum dependence of the transport scattering rate, a key parameter that largely determines the evolution of the anomalous transport properties of HTSC across the phase diagram.Both the quantum oscillations and ADMR have now become powerful techniques in the study of HTSC because of the recent development of facilities which are able to produce very high magnetic fields (up to 100 Tesla) and recent improvements in the signal-to-noise ratios at these facilities. The goal of this proposal is to make best use of these timely developments and to study quantum oscillations and ADMR in a variety of different HTSC materials at very high magnetic fields. In so doing we aim to gain a deeper understanding of how the normal state electronic structure of high temperature cuprate superconductors evolves as a function of the electron density. We will also attempt to couple our Fermi surface studies to measurements of their resistivity and Hall effect in the limiting low temperature (high magnetic field) regime.