Published online by Cambridge University Press: 01 February 2011
The microscopic origin of unconventional superconductivity1 is one of the most fascinating questions2–3 in condensed matter physics. In recent years, important questions have been raised about the nature of the normal (metallic, non-superconducting) state above the superconducting critical transition temperature (Tc). In fact, it is now generally agreed4,5 that a better understanding of the nature of the normal state is crucial to our understanding of the microscopic origin of the superconducting state.