Subir Sachdev

Department of Physics, Harvard University


The Luttinger theorem is a fundamental constraint on the structure of metallic phases of strongly correlated systems. Fermi surfaces which obey the Luttinger theorem are often referred to as “large”. However, it is possible to have “small” Fermi surfaces with electron-like quasiparticles, in certain metallic states (called FL*) with emergent gauge fields. I will review theories of small and large Fermi surfaces on the Anderson lattice (and the closely related Kondo lattice), and of the phase transitions between them, and describe their application to the heavy fermion compounds. Then I will describe a new approach to transferring these insights to the single-band Hubbard
model, and the physics of the cuprates. The key idea is to avoid fractionalizing the electron, and to instead fractionalize a paramagnon into a pair of ancilla qubits. This leads to a theory of a pseudogap metal as a FL* state, and of the quantum phase transition from FL* to an ordinary Fermi liquid, in a single band model.

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