Stranger than Metals

TL;DR

Strange metals defy traditional resistivity behavior, showing continuous transport properties across temperature ranges, challenging particle-based models, and possibly requiring new physical principles to explain their non-local transport phenomena.

Contribution

This review synthesizes transport and spectroscopic data on strange metals, exploring quantum criticality, Planckian dissipation, and gauge theories to understand their unconventional behavior.

Findings

Resistivity behavior in strange metals is continuous across temperature ranges.

Strange metallicity correlates with superfluid density in cuprates.

Non-local transport suggests the need for new theoretical frameworks.

Abstract

Although the resistivity in traditional metals increases with temperature, its $T$ dependence vanishes at low or high temperature, albeit for different reasons. Here, we review a class of materials, known as \lq strange' metals, that can violate both principles. In materials exhibiting such behavior, the change in slope of the resistivity as the mean free path drops below the lattice constant, or as $T \rightarrow 0$, can be imperceptible, suggesting complete continuity between the charge carriers at low and high $T$. Since particles cannot scatter at length scales shorter than the interatomic spacing, strange metallicity calls into question the relevance of locality and a particle picture of the underlying current. This review focuses on transport and spectroscopic data on candidate strange metals with an eye to isolate and identify a unifying physical principle. Special attention is paid to quantum criticality, Planckian dissipation, Mottness, and whether a new gauge principle, which has a clear experimental signature, is needed to account for the non-local transport seen in strange metals. For the cuprates, strange metallicity is shown to track the superfluid density, thereby making a theory of this state the primary hurdle in solving the riddle of high-temperature superconductivity.