Theory of Strange Metals from Hot Fermions.

We study a metal where a large Fermi surface coexists with a set of `hot spots' with a high density of states. The hot electrons occupy a small fraction of the Brillouin zone, yet qualitatively modify the properties of the entire system. We emphasize the importance of scattering processes in which two electrons from the large, `cold' Fermi surface scatter into one hot and one cold electron. These lead to a `strange metallic' state with anomalous, non-Fermi liquid thermodynamic and transport properties. Scattering into hot electrons that are effectively classical (non-degenerate) in a finite portion of the Brillouin zone leads to a marginal Fermi liquid. This explains, in detail, the phenomenology of Sr$_3$Ru$_2$O$_7$ in field, including $T$-linear resistivity and a $T\log(1/T)$ electronic specific heat. Hot electrons that are instead localized near a point in the Brillouin zone, such as a two-dimensional van Hove singularity, lead to different power laws. We show that the transport behavior of strained Sr$_2$RuO$_4$ is recovered from this picture.