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Leonid S. Levitov. Lewandowski, Cyprian(Cyprian Krzysztof) Massachusetts Institute of Technology. Department of Physics. Massachusetts Institute of Technology. Department of Physics 2021-03-22T17:39:58Z 2021-03-22T17:39:58Z 2020 2020 https://hdl.handle.net/1721.1/130218 1241733506 Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, May, 2020 Cataloged from student-submitted PDF version of thesis. Includes bibliographical references (pages 114-123). The family of moiré materials, in particular the magic angle twisted bilayer graphene, has emerged recently as a platform to study strongly interacting physics. This thesis analyzes the impact of the ultranarrow Bloch bands and strong electron-electron interactions on the dynamical polarization response of these systems. Strong interactions alter the collective charge dynamics in a number of interesting ways, in particular by stiffening the frequency-momentum dispersion of surface plasmons and making it much stronger than that of the underlying narrow-band carriers. Strongly dispersing plasmons pierce through the particle-hole continuum and extend in the forbidden energy band above it. This behavior enables decoupling of plasmons from particle-hole excitations. Such over-the-band plasmons are unable to decay into particle-hole pairs and thus are not subject to Landau damping. As a result, plasmons acquire longer lifetimes as well as an enhanced spatial optical coherence. The optical coherence manifests itself in spatial interference patterns that provide telltale signatures of over-the-band plasmons that are readily accessible in near-field imaging experiments. We further show that the over-the-band plasmon dispersion remains robust in the presence of ordering of the narrow-band carriers. The specific examples of a Wigner crystal and a Mott-Hubbard order, worked out in detail, show that interaction-driven gap opening has no impact on the over-the-band plasmon dispersion. Lastly, we consider the implications of the mechanisms behind the over-the-band behavior for achieving of unidirectional collective modes. We present a new mechanism for plasmon nonreciprocity the magnitude of which is controllable through the strength of electron-electron interactions, which makes it particularly pronounced in the moiré materials. by Cyprian Lewandowski. Ph.D. Ph.D. Massachusetts Institute of Technology, Department of Physics 123 pages eng Massachusetts Institute of Technology MIT theses may be protected by copyright. Please reuse MIT thesis content according to the MIT Libraries Permissions Policy, which is available through the URL provided. http://dspace.mit.edu/handle/1721.1/7582 Physics. Dynamic polarizability and collective modes in narrow-band electron systems Thesis application/pdf 2021-03-22T17:39:17Z Publication Doctoral Phys unknown unknown <Publication xmlns="https://www.openaire.eu/cerif-profile/1.1/" id="eefb6f73-36af-4561-8b29-b722b3b06faf"> <Type xmlns="https://www.openaire.eu/cerif-profile/vocab/COAR_Publication_Types">http://purl.org/coar/resource_type/c_1843</Type> <Language>eng</Language> <Title>Dynamic polarizability and collective modes in narrow-band electron systems</Title> <PublishedIn> <Publication> </Publication> </PublishedIn> <PublicationDate>2020</PublicationDate> <Authors> <Author> <DisplayName>Lewandowski, Cyprian(Cyprian Krzysztof)</DisplayName> <Affiliation> <OrgUnit> </OrgUnit> </Affiliation> </Author> </Authors> <Editors> </Editors> <Publishers> <Publisher> <DisplayName>Massachusetts Institute of Technology</DisplayName> <OrgUnit /> </Publisher> </Publishers> <License>http://dspace.mit.edu/handle/1721.1/7582</License> <Keyword>Physics.</Keyword> <Abstract>The family of moiré materials, in particular the magic angle twisted bilayer graphene, has emerged recently as a platform to study strongly interacting physics. This thesis analyzes the impact of the ultranarrow Bloch bands and strong electron-electron interactions on the dynamical polarization response of these systems. Strong interactions alter the collective charge dynamics in a number of interesting ways, in particular by stiffening the frequency-momentum dispersion of surface plasmons and making it much stronger than that of the underlying narrow-band carriers. Strongly dispersing plasmons pierce through the particle-hole continuum and extend in the forbidden energy band above it. This behavior enables decoupling of plasmons from particle-hole excitations. Such over-the-band plasmons are unable to decay into particle-hole pairs and thus are not subject to Landau damping. As a result, plasmons acquire longer lifetimes as well as an enhanced spatial optical coherence. The optical coherence manifests itself in spatial interference patterns that provide telltale signatures of over-the-band plasmons that are readily accessible in near-field imaging experiments. We further show that the over-the-band plasmon dispersion remains robust in the presence of ordering of the narrow-band carriers. The specific examples of a Wigner crystal and a Mott-Hubbard order, worked out in detail, show that interaction-driven gap opening has no impact on the over-the-band plasmon dispersion. Lastly, we consider the implications of the mechanisms behind the over-the-band behavior for achieving of unidirectional collective modes. We present a new mechanism for plasmon nonreciprocity the magnitude of which is controllable through the strength of electron-electron interactions, which makes it particularly pronounced in the moiré materials.</Abstract> <Access xmlns="http://purl.org/coar/access_right" > </Access> </Publication>