Renormalization group and Ward identities in quantum liquid phases and in unconventional critical phenomena

TL;DR

This paper reviews how the renormalization group and Ward identities, which encode symmetry constraints, help analyze quantum liquid phases and critical phenomena, including non-Fermi liquids, superfluids, and metal-insulator transitions.

Contribution

It demonstrates the use of Ward identities to reduce variables and derive exact descriptions in various quantum many-body systems and phase transitions.

Findings

Ward identities reveal non-Fermi-liquid behavior in 1D fermions

They provide an exact description of superfluid excitations

They relate disorder effects to Landau parameter renormalizations

Abstract

By reviewing the application of the renormalization group to different theoretical problems, we emphasize the role played by the general symmetry properties in identifying the relevant running variables describing the behavior of a given physical system. In particular, we show how the constraints due to the Ward identities, which implement the conservation laws associated with the various symmetries, help to minimize the number of independent running variables. This use of the Ward identities is examined both in the case of a stable phase and of a critical phenomenon. In the first case we consider the problems of interacting fermions and bosons. In one dimension general and specific Ward identities are sufficient to show the non-Fermi-liquid character of the interacting fermion system, and also allow to describe the crossover to a Fermi liquid above one dimension. This crossover is examined both in the absence and presence of singular interaction. On the other hand, in the case of interacting bosons in the superfluid phase, the implementation of the Ward identities provides the asymptotically exact description of the acoustic low-energy excitation spectrum, and clarifies the subtle mechanism of how this is realized below and above three dimensions. As a critical phenomenon, we discuss the disorder-driven metal-insulator transition in a disordered interacting Fermi system. In this case, through the use of Ward identities, one is able to associate all the disorder effects to renormalizations of the Landau parameters. As a consequence, the occurrence of a metal-insulator transition is described as a critical breakdown of a Fermi liquid.