Science https://doi.org/gfrhdz (2018)

The transport properties of conventional metals can be understood in terms of Landau–Fermi liquid theory. The theory holds that conductivity arises when a quasiparticle carrying the same amount of charge and spin as a single electron responds to an external field. Despite its remarkable success, the theory fails to explain the behaviour of a broad class of correlated materials, referred to as ‘bad metals’. Now, Peter Brown and co-workers have used a strongly interacting Fermi gas to study the underlying mechanism.

The minimum model for studying the physics of strongly correlated electrons is the Fermi–Hubbard model, which can be realized using ultracold atoms trapped in an optical lattice. Based on the measurements of the diffusion constant and the compressibility of the system, Brown and colleagues derived the conductivity through the Nernst–Einstein relation. The inverse of the conductivity exhibited linear temperature dependence, violating the Mott–Ioffe–Regel limit — a hallmark of bad metals.