The destruction of particles has been long associated with high-energy physics and the construction of large-scale particle accelerators. It is perhaps less well-known that in solid state physics, the destruction of particles, or more precisely quasi-particles, takes place on a regular basis in a standard laboratory environment. Quasi-particles are the fundamental excitations of a metal. They are essentially electrons whose properties have been modified, typically through interactions with the atomic lattice and/or other electrons, leading to (amongst other things) larger effective masses. Simple changes in a sample's environment (eg through changes in its temperature, dimensionality or doping level) can alter the spectrum of quasi-particle excitations and thus lead to fundamentally new physics. In our research, we investigate ways in which the quasi-particle description breaks down in a host of exotic new materials known collectively as strongly correlated metals. This family of metals, that include the high-temperature superconductors and colossal magnetoresistance oxides, are not only interesting from a fundamental perspective; they also have huge technological potential.
Just as in conventional superconductors, where the scattering processes that dominate the electrical resistivity provide an important clue to the dominant pairing interaction (via the strength of the electron-phonon coupling), so an understanding of the normal state transport properties of high-temperature superconductors (HTS) is regarded as a key step towards the elucidation of the pairing mechanism for high temperature superconductivity. Whilst this remains the ultimate goal, normal state transport in HTS has emerged as a field in its own right and one of the most challenging (and controversial) topics in modern solid state physics. The ubiquitous linear-in-temperature resistivity at optimal doping, extending over a very wide temperature range, the strong temperature-dependence of the Hall coefficient, the violation of Kohler's rule and the divergence of the resistivity anisotropy are but some of the striking anomalies which have puzzled the community over the past two decades and inspired theorists to develop radical new concepts in many-body theory.
One of the goals of our research is to see how far the conventional picture of (quasi-particle) excitations above the Fermi surface, ie the velocity distribution of the most energetic electrons can explain these various anomalies. In order to do that however, we must first determine what that Fermi surface looks like. The schematic figure (which appeared in Nature in 2003) shows the first three-dimensional mapping of the Fermi surface of a high temperature superconductor. From this starting point, we have now been able to explain the temperature dependence of the resistivity and Hall effect for this compound by extracting further information on the anisotropy of the quasi-particle lifetime around the Fermi surface. The challenge now is to explain the evolution of the anomalous transport as we reduce the carrier concentration and increase the superconducting transition temperature. At some point, we expect the conventional picture to break down, but by studying in detail the way this occurs will, we believe, shed important light on the origin both of the anomalous transport and of the mechanism of high temperature superconductivity itself.
Landau's Fermi liquid theory has stood as the standard model for understanding metal physics for over 40 years. Indeed, it seemed for a long while that it was sufficient simply to ascribe a one-to-one correspondence between the low-energy excitations of a free Fermi gas and those of an interacting Fermi liquid, through a renormalisation process, to account for all the many-body interactions that exist inside a metallic system. This correspondence, however, is now thought to break down in spectacular fashion once the electrons inside the metal are confined to dimensions lower than three. Such materials, known collectively as low dimensional conductors, have exposed the limits of applicability of the Fermi liquid scenario and have opened up the possibility for an astonishing range of new exotic ground states, where magnetic, orbital, structural and electronic order all compete for the stable fixed point at zero temperature. We currently employ a range of magneto-transport techniques to study the charge dynamics of various low dimensional conductors, with the aim of following the evolution of their intrinsic behaviour as the dimensionality of the electronic ground state is changed through different control parameters, including disorder and the application of large magnetic fields.