{"id":{"repo_id":"mit","oai_identifier":"oai:dspace.mit.edu:1721.1/130218"},"canonical_url":"https://search.dev.ndltd.org/etd/mit/oai:dspace.mit.edu:1721.1/130218","repository":{"repo_id":"mit","name":"MIT","base_url":"https://dspace.mit.edu/oai/request"},"display":{"title":"Dynamic polarizability and collective modes in narrow-band electron systems","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_html":"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_has_math":false,"creators":["Lewandowski, Cyprian(Cyprian Krzysztof)"],"institution":"Massachusetts Institute of Technology","degree_name":"Doctoral","degree_level":null,"degree_discipline":null,"degree_department":"Massachusetts Institute of Technology. Department of Physics","school":null,"contributors":[],"advisors":["Leonid S. Levitov."],"committee_chairs":[],"committee_members":[],"year":2020,"date_issued":"2020","date_published":"2020","updated_at":"2026-07-22T22:22:25Z","subjects":["Physics."],"languages":["eng"],"rights":["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."],"rights_urls":["http://dspace.mit.edu/handle/1721.1/7582"],"identifier_entries":[]},"links":{"outbound_url":"https://hdl.handle.net/1721.1/130218","outbound_label":"Handle","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Leonid S. Levitov."]},{"key":"dc:contributor.department","label":"Department","values":["Massachusetts Institute of Technology. Department of Physics","Phys"]},{"key":"dc:contributor.other","label":"Dc Contributor Other","values":["Massachusetts Institute of Technology. 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Please reuse MIT thesis content according to the MIT Libraries Permissions Policy, which is available through the URL provided."]},{"key":"dc:rights.uri","label":"Rights URI","values":["http://dspace.mit.edu/handle/1721.1/7582"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://hdl.handle.net/1721.1/130218"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["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)."]},{"key":"dc:description.abstract","label":"Abstract","values":["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."]},{"key":"dc:description.degree","label":"Dc Description Degree","values":["Ph. D."]},{"key":"dc:title","label":"Title","values":["Dynamic polarizability and collective modes in narrow-band electron systems"]}]}],"canonical_facts":{"dc:contributor.advisor":["Leonid S. Levitov."],"dc:contributor.department":["Massachusetts Institute of Technology. Department of Physics","Phys"],"dc:contributor.other":["Massachusetts Institute of Technology. Department of Physics."],"dc:creator":["Lewandowski, Cyprian(Cyprian Krzysztof)"],"dc:date.accessioned":["2021-03-22T17:39:58Z"],"dc:date.available":["2021-03-22T17:39:58Z"],"dc:date.issued":["2020"],"dc:description":["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)."],"dc:description.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."],"dc:description.degree":["Ph. D."],"dc:identifier.uri":["https://hdl.handle.net/1721.1/130218"],"dc:language.iso":["eng"],"dc:publisher":["Massachusetts Institute of Technology"],"dc:rights":["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."],"dc:rights.uri":["http://dspace.mit.edu/handle/1721.1/7582"],"dc:subject":["Physics."],"dc:title":["Dynamic polarizability and collective modes in narrow-band electron systems"],"dc:type":["Thesis"],"thesis:degree_name":["Doctoral"]},"updated_at":"2026-07-22T22:22:25Z"}