Quantum critical metals and loss of quasiparticles

Strange metals develop near quantum critical points in a variety of strongly correlated systems. Some of the issues that are central to the field include how the quantum critical state loses quasiparticles, how it drives superconductivity and to what extent the strange-metal physics in different classes of correlated systems are interconnected. In this Review, we survey some of these issues from the vantage point of heavy-fermion metals. We describe the notion of Kondo destruction and how it leads to several crucial effects. These include a transformation of the Fermi surface from large to small when the system is tuned across the quantum critical point, a loss of quasiparticles everywhere on the Fermi surface when it is perched at the quantum critical point and a dynamical Planckian scaling in various physical properties including charge responses. We close with a discussion about the connections between the strange-metal physics in heavy-fermion metals and its counterparts in the cuprates and other correlated materials. The strange-metal state that develops close to a quantum critical point in strongly correlated electron systems is not well understood. This Review summarizes how the notion of Kondo destruction can describe much of the experimental phenomenology.