Ephraim BERNHARDT

PhD Student

Resarch activities at CPHT: Condensed Matter

Thesis:  "Gauge fields, Interacting topological phases and localization"

Advisor: Karyn Le Hur

Research interests:  Condensed Matter, Gauge fields, Topological phases, Localization, Interactions

Abstract

 The objective of this PhD thesis is to study a new interface between quantum phases of matter, localization aspects from impurities and topology. An on-going work, started from the Master internships is to study the interplay between topological phenomena induced by U(1) gauge fields from a magnetic field and many-body localization effects coming from the interaction with impurities in various geometries, such as a quantum ladder. Here, impurities are described microscopically through the introduction of another particle species described by a Z2 gauge theory. Therefore, one can address various regimes of localization, from static to mobile impurities related to Mott physics in the strong-coupling regime. The new physics questions lead to the development of new tools, going from analytical and quantum field theory methods to numerical techniques, to characterize the dynamics and topological response linked to many-body localization physics. This PhD thesis also introduces probes coming from quantum information, such as entanglement entropy and bipartite fluctuations. This PhD thesis is directly related to new developments in cold atoms, mesoscopic physics and light-matter systems allowing to implement gauge fields and study time-dependent dynamics. These gauge theories also bridge with models on a lattice describing topological insulators and high-Tc superconductors.

Publication

Bernhardt E, Elouard C, Le Hur K.
Topologically protected quantum dynamo effect in a driven spin-boson model.
Physical Review A. 2023;107(2):022219.
DOI: 10.1103/PhysRevA.107.022219

Bernhardt E, Yang F, Le Hur K.
Localization dynamics from static and mobile impurities.
Physical Review B. 2021;104(11):115113.
DOI: 10.1103/PhysRevB.104.115113