Understanding layered metals is of fundamental scientific interest. Their metallic properties cannot be understood in terms of text-book concepts such as the non-interacting electron model, energy bands, and Fermi liquid theory; concepts which work so well for common metals such as copper and bronze. The large interactions between electrons compared to their kinetic energy result in strong electronic correlations (i.e., the motion of any electron is correlated with the motion of others). The low effective dimensionality due to the layered crystal structure lead to large quantum fluctuations. Consequently, new quantum states of matter can be found in these materials. Fundamental questions arise about the nature of the low-lying quantum states of the system and the extent of the quantum coherence of the charge transport between the layers. In conventional metals, transport properties are well described by a picture in which the current is carried by weakly interacting charged fermions, leading to the concept of quasi-particles. However, in many advanced materials such concepts may no longer applicable.
One powerful experimental probe of the existence of quasi-particles is the frequency-dependent conductivity, usually in the infra-red to optical range. An important signature of the existence of quasi-particles is the Drude peak which occurs at zero frequency. It provides useful information about the effective mass and the lifetime of the quasi-particles. In many strongly correlated materials the Drude peak is only observed at relatively low temperatures and most of the spectral weight in the frequency dependent conductivity is not in the Drude peak but at higher frequencies and is characteristic of incoherent excitations.
Here is a talk I gave on this subject at Bristol University earlier this year.