Book Description

Strongly correlated electron systems are one of the central topics of condensed matter physics. The myriad of combinations of diverse Fermiologies, phonon spectra and electron-electron, electron-phonon interactions, together with spin-orbit couplings, Kondo couplings, and effects of disorder and external magnetic fields, leads to a truly dazzling range of quantum many-body phenomena. Superconductivity (conventional and unconventional) and magnetism are among the most prominent examples of quantum phases of matter that occur in such systems. We know that powerful emergent principles such as symmetry and topology are required to explain these emergent phenomena. However, due to the inherent difficulty of studying systems with macroscopically large number of strongly interacting particles, there remains the challenge of connecting these somewhat abstract mathematical principles with the underlying microscopic interactions. In this thesis, we illustrate, through two examples of systems with electron-electron and electron-phonon interactions, how one can simplify intractable quantum chemistry problems by reducing them to effective model Hamiltonians that capture the essence of microscopic interactions important to low-energy excitations, which we can then study using a variety of tools, such as determinantal quantum Monte Carlo (DQMC), exact diagonalization, weak and strong coupling considerations and mean-field theory. In the first example we encounter a novel deconfined quantum critical point (DQCP) with emergent O(4) symmetry. In the second example we offer a phenomenological explanation of superconducting and insulating phases of twisted bilayer graphene. Lastly, we also visit the more field-theoretic problem of boson-fermion duality in two spatial dimensions, for which we provide an exact lattice construction. This duality is closely related to the half-filled Landau level problem in quantum Hall physics.