Renormalization group theory for fermions and order parameter fluctuations in interacting Fermi systems

TL;DR

This thesis develops a comprehensive renormalization group framework for analyzing quantum phase transitions and fluctuations in interacting Fermi systems, extending existing theories and computing critical exponents and fluctuation effects.

Contribution

It extends Hertz-Millis theory to include phases with symmetry-breaking and non-Gaussian fixed points, and introduces a coupled fermion-boson RG approach for quantum criticality.

Findings

Computed quantum critical exponents for Dirac fermions transitioning to superfluidity.

Demonstrated non-analytic behavior of propagators destroying Fermi liquid properties.

Quantified quantum fluctuation effects on fermionic gap and superfluid properties.

Abstract

In this thesis, we perform a comprehensive renormalization group analysis of two- and three-dimensional Fermi systems at low and zero temperature. We examine systems with spontaneous symmetry-breaking and quantum critical behavior by deriving and solving flow equations within the functional renormalization group framework. We extend the Hertz-Millis theory of quantum phase transitions in itinerant fermion systems to phases with discrete and continuous symmetry-breaking, and to quantum critical points where the zero temperature theory is associated with a non-Gaussian fixed point. We compute the finite temperature phase boundary near the quantum critical point explicitly including non-Gaussian fluctuations. We then set up a coupled fermion-boson renormalization group theory that captures the mutual interplay of gapless fermions with massless order parameter fluctuations when approaching a quantum critical point. As a first application, we compute the complete set of quantum critical exponents at the semimetal-to-superfluid quantum phase transition of attractively interacting Dirac fermions in two dimensions. Both, the order parameter propagator and the fermion propagator become non-analytic functions of momenta destroying the Fermi liquid behavior. We finally compute the effects of quantum fluctuations in the superfluid ground state of an attractively interacting Fermi system, employing the attractive Hubbard model as a prototype. The flow equations capture the influence of longitudinal and Goldstone order parameter fluctuations on non-universal quantities such as the fermionic gap and the fermion-boson vertex, as well as the exact universal infrared asymptotics present in every fermionic superfluid.