Introduction

The quantum many-body problem is one of the most profound challenges in modern physics. Many of the most intriguing and topical phenomena in contemporary condensed matter physics, such as high temperature superconductivity (high-Tc), colossal magneto resistance, metal-insulator transitions and quantum criticality are intimately related to strong electron-electron interactions, or “correlations”, between relatively local electrons in atomic d or f shells.
We currently focus on the study of ruthenium and rhodium based 4d transition metal oxides. These materials exist near the critical value of U/W and show a wide range of non-trivial phases including low-dimensional Fermi liquids, spin-triplet super-conductivity, Mott insulating and orbitally ordered states.

The extremely high purity of Ruthenate and rhodate single crystals (some of the best samples are being grown in St. Andrews by J. Farrell and R. Perry) opens a unique window to the correlated electron problem: It allows one to study an interacting fluid in a nearly perfect low-dimensional lattice, free of the complications arising from doping induced disorder. A recent example of the remarkably simple Fermi liquid state observed in the new correlated metal Sr2RhO4 is shown in the figure on the right. More detailed informations can be found in recent publications (PRL 96, 246402 (2006), New J. Phys. 8, 175 (2006)).

Angle-resolved photoemission (ARPES)

Much of modern condensed matter physics is concerned with the response of complex materials to external stimuli, such as applied electric and magnetic fields, heat, pressure or electromagnetic radiation. 
This response depends on the low-energy excitations of a material, such as phonons, different magnetic excitations, and most importantly the electron like quasiparticles on or near the Fermi surface.

Angular resolved photoemission (ARPES) is a uniquely powerful tool for probing such electronic quasiparticles. It is virtually the only spectroscopy with direct momentum resolution, a key advantage for the study of anisotropic and low-dimensional materials. ARPES data contain the band structure of a material, and, because the signal is proportional to the single particle spectral function, give detailed information on the propagation of electrons in the full many-body system. The former is demonstrated in the figure on the left, showing the well-known Fermi surface of Cu measured by angular resolved photoemission.