Departmental Oral Examination (Thesis Title: “Exotic Many-Body Behavior in Polaronic & Disordered Systems & Consequences at Finite Temperatures")

Speaker: 
JOHN SOUS
Event Date and Time: 
Fri, 2018-07-20 10:00 - 12:00
Location: 
AMPEL 311
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 
Public

Abstract:
In this thesis, we investigate the many-body “collective” behavior of particles dressed by interactions, baths excitations, and disorder.
First we consider two-carrier states in the Peierls model describing the modulation of the particle hopping due to lattice distortions. We compute the spectral response using the Momentum Average approximation. Combining accurate numerical techniques and analytical arguments, we provide a complete picture of the Peierls bipolarons. We find that polarons bind into strongly bound yet light bipolarons in the singlet sector, even at large values of the electron-phonon coupling strength. These bipolarons are expected to condense into a high-Tc superconductor. On the other hand, phonons mediate a repulsive interaction in the triplet sector, or equivalently, between two hard-core particles. In this case, the ground-state dimers bound by sufficiently attractive bare interactions exhibit two sharp transitions, one of which is a self-trapping transition, the first example at the two-carrier level. In both cases, phonons mediate unusual pair-hopping effective interactions between the carriers. We further study the excited spectrum for the two hard-core particles, a situation relevant to ultracold quantum simulators. We find that the repulsive phonon-mediated interaction binds a repulsive bipolaron embedded in the excited spectrum.
We then turn to the study of quenched randomness in an ultracold molecular plasma. We argue that the quenched ultracold plasma presents an experimental platform for studying quantum many-body physics of disordered systems in the long-time and finite energy-density limits. We analyze an experiment that quenches a plasma of nitric oxide to an ultracold system of Rydberg molecules, ions and electrons that exhibits a long-lived state of arrested relaxation. The qualitative features of this state fail to conform with classical models. We develop a microscopic quantum description for the arrested phase based on an effective many-body spin Hamiltonian that includes both dipole-dipole and van der Waals interactions. This effective model appears to offer a way to envision the essential quantum disordered non-equilibrium physics of this system.
This thesis thus examines the finite-temperature quantum many-body response in interacting systems coupled to baths and in disordered environments.
 

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