| Summary: | We discuss a new class of quantum phase transitions—deconfined Mott transition (DMT)—that describe a continuous transition between a Fermi liquid metal with a generic electronic Fermi surface and an electrical insulator without Fermi surfaces of emergent neutral excitations. We construct a unified U(2) gauge theory to describe a variety of metallic and insulating phases, which include Fermi liquids, fractionalized Fermi liquids (FL*), conventional insulators, and quantum spin liquids, as well as the quantum phase transitions between them. Using the DMT as a basic building block, we propose a distinct quantum phase transition—deconfined metal-metal transition (DM^{2}T)—that describes a continuous transition between two metallic phases, accompanied by a jump in the size of their electronic Fermi surfaces (also dubbed a ‘Fermi transition'). We study these new classes of deconfined metallic quantum critical points using a renormalization group framework at the leading nontrivial order in a controlled expansion and comment on the various interesting scenarios that can emerge going beyond this leading order calculation. We also study a U(1)×U(1) gauge theory that shares a number of similarities with the U(2) gauge theory and sheds important light on many phenomena related to DMT, DM^{2}T, and quantum spin liquids.
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