{"id":{"repo_id":"uiuc","oai_identifier":"oai:www.ideals.illinois.edu:2142/113898"},"canonical_url":"https://search.dev.ndltd.org/etd/uiuc/oai:www.ideals.illinois.edu:2142/113898","repository":{"repo_id":"uiuc","name":"University of Illinois - Urbana-Champaign","base_url":"https://www.ideals.illinois.edu/oai-pmh"},"display":{"title":"Electronic collective modes in two dilute conductors with momentum-resolved electron energy-loss spectroscopy","abstract":"Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2022-04-06 without embargo terms","abstract_html":"Submission original under an indefinite embargo labeled &#x27;Open Access&#x27;. The submission was exported from vireo on 2022-04-06 without embargo terms","abstract_has_math":false,"creators":["Rubeck, Samantha Ilene"],"institution":"University of Illinois at Urbana-Champaign","degree_name":"Ph.D.","degree_level":"Dissertation","degree_discipline":"Physics","degree_department":null,"school":null,"contributors":["Abbamonte, Peter","Eckstein, Jim","Wagner, Lucas","Gadway, Bryce"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2022,"date_issued":"2022-04-29T21:34:47Z","date_published":"2022-04-29T21:34:47Z","updated_at":"2026-07-22T22:24:53Z","subjects":["Physics"],"languages":["en","eng"],"rights":["Copyright 2021 Samantha Rubeck"],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"http://hdl.handle.net/2142/113898","outbound_label":"Handle","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Abbamonte, Peter","Eckstein, Jim","Wagner, Lucas","Gadway, Bryce"]},{"key":"dc:creator","label":"Author","values":["Rubeck, Samantha Ilene"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2022-04-29T21:34:47Z","2021-12","2021-12-03"]},{"key":"dc:type","label":"Dc Type","values":["text","Thesis"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Physics"]},{"key":"thesis:degree_level","label":"Degree Level","values":["Dissertation"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Ph.D."]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["University of Illinois at Urbana-Champaign"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Physics"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en","eng"]},{"key":"dc:rights","label":"Dc Rights","values":["Copyright 2021 Samantha Rubeck"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["http://hdl.handle.net/2142/113898"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2022-04-06 without embargo terms","The student, Samantha Rubeck, accepted the attached license on 2021-12-01 at 19:21.","The student, Samantha Rubeck, submitted this Dissertation for approval on 2021-12-01 at 19:40.","This Dissertation was approved for publication on 2021-12-03 at 07:59.","DSpace SAF Submission Ingestion Package generated from Vireo submission #17350 on 2022-04-06 at 17:10:37","Made available in DSpace on 2022-04-29T21:34:47Z (GMT). No. of bitstreams: 2 RUBECK-DISSERTATION-2021.pdf: 32658673 bytes, checksum: 3df1a369301db46955ba453e3f186fe7 (MD5) LICENSE.txt: 4212 bytes, checksum: 9f0feacc3e5a8e8985c9c3d337d3a922 (MD5) Previous issue date: 2021-12-03","Much of modern condensed matter research tries to understand strongly correlated electron systems. These materials exhibit a variety of interesting quantum phenomena that stem from strong electron interactions and the emergent quasiparticles that define the system. Understanding the collective mode dynamics of these particles is key to understanding the macroscopic behavior of these materials. One quantity that contains fundamental information about the about these boson collective modes is the charge susceptibility χ(q,ω) which contains information about the propagation of density fluctuations that are mediated by bosonic excitations. Until recently though, it was not possible to measure χ(q, ω) at low energy (< 100 meV) and with the momentum resolution and accuracy needed to see interesting phenomena. With the development of momentum-resolved electron energy-loss spectroscopy (M-EELS) in the Abbamonte Group at UIUC, χ(q, ω) can now be probed at low energy scales of interest. Here we use M-EELS to study the collective modes of several strongly correlated materials with interesting low energy physics. Bose condensed phases of excitons have the potential to realize macroscopic quantum phenomena at unprecedented high temperatures [1,2]. When excitons Bose condense in a real material, however, some rearrangement of the charge density inevitably results, leading to a distortion of the crystal lattice [3–6]. This raises the question of whether there can ever be a distinction between a Bose condensate of excitons and a conventional, structural phase transition that breaks the same symmetry. Here, we use inelastic electron scattering (M-EELS) to study copper-intercalated TiSe2, in which exciton condensation can be directly observed as a soft electronic mode at the exciton condensation temperature, T_XC [7]. While the lattice distortion in CuxTiSe2 persists to x > 0.10 [8], we find the exciton condensate is fully suppressed by x = 0.014, which is short of the semimetal-metal transition we identify at x = 0.025. Our observations indicate that the excitonic and lattice instabilities split as x is increased, showing that structural and excitonic subsystems can exhibit separate transitions and may be distinct subsystems that break different symmetries. In addition we found that although exciton condensation is suppressed, exciton fluctuations remain for most of the doping phase diagram. These fluctuations have been theorized to aid in superconducting pairing in this system [9,10]. SrTi(1−x)NbxO3, is an electron doped ionic semiconductor that exhibits aborted ferroelectricity due to quantum fluctuations [11], dilute (unconventional) superconductivity that survives even when the plasmon energy (ω_p) is lower than the Fermi energy (E_F ) [12], and charge transport properties that suggests bad metal behavior [13]. In general, the polar nature of ionic semiconductors leads to coupling of the longitudinal optical (LO) phonons to collective charge modes, such as plasmons, due to long range polarization fields. Understanding the dynamics of these collective modes, which have been implicated in superconducting pairing [14] [15] [16], can provide insight into the nature of these unusual properties. Here, we use inelastic electron scattering (M-EELS) with a fracturing surface preparation technique to study the doping, temperature, and momentum dependence of the charge collective modes in SrTi(1−x)NbxO3. We measure propagating and diffusive acoustic phonons, Fuchs-Kliewer optical phonons and overtones that get suppressed with doping due to metallic screening, a 93 meV phonon mode that becomes highly damped and asymmetric at x=0.002, and a very broad plasmon that blue-shifts with decreasing temperature. We find that the plasmon in SrTi(1−x)NbxO3 remains dispersion-less at all temperatures and dopings even with other propagating collective modes (acoustic phonon), contrary to RPA predictions and dispersions in other polar doped semiconductors [17]. The width of the plasmon deviates from RPA predictions by an order of magnitude. In addition, the energy of the plasmon measured in M-EELS is less than that of other techniques like infrared spectroscopy for samples of comparable carrier densities."]},{"key":"dc:format","label":"Dc Format","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["Electronic collective modes in two dilute conductors with momentum-resolved electron energy-loss spectroscopy"]}]}],"canonical_facts":{"dc:contributor":["Abbamonte, Peter","Eckstein, Jim","Wagner, Lucas","Gadway, Bryce"],"dc:creator":["Rubeck, Samantha Ilene"],"dc:date":["2022-04-29T21:34:47Z","2021-12","2021-12-03"],"dc:description":["Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2022-04-06 without embargo terms","The student, Samantha Rubeck, accepted the attached license on 2021-12-01 at 19:21.","The student, Samantha Rubeck, submitted this Dissertation for approval on 2021-12-01 at 19:40.","This Dissertation was approved for publication on 2021-12-03 at 07:59.","DSpace SAF Submission Ingestion Package generated from Vireo submission #17350 on 2022-04-06 at 17:10:37","Made available in DSpace on 2022-04-29T21:34:47Z (GMT). No. of bitstreams: 2 RUBECK-DISSERTATION-2021.pdf: 32658673 bytes, checksum: 3df1a369301db46955ba453e3f186fe7 (MD5) LICENSE.txt: 4212 bytes, checksum: 9f0feacc3e5a8e8985c9c3d337d3a922 (MD5) Previous issue date: 2021-12-03","Much of modern condensed matter research tries to understand strongly correlated electron systems. These materials exhibit a variety of interesting quantum phenomena that stem from strong electron interactions and the emergent quasiparticles that define the system. Understanding the collective mode dynamics of these particles is key to understanding the macroscopic behavior of these materials. One quantity that contains fundamental information about the about these boson collective modes is the charge susceptibility χ(q,ω) which contains information about the propagation of density fluctuations that are mediated by bosonic excitations. Until recently though, it was not possible to measure χ(q, ω) at low energy (< 100 meV) and with the momentum resolution and accuracy needed to see interesting phenomena. With the development of momentum-resolved electron energy-loss spectroscopy (M-EELS) in the Abbamonte Group at UIUC, χ(q, ω) can now be probed at low energy scales of interest. Here we use M-EELS to study the collective modes of several strongly correlated materials with interesting low energy physics. Bose condensed phases of excitons have the potential to realize macroscopic quantum phenomena at unprecedented high temperatures [1,2]. When excitons Bose condense in a real material, however, some rearrangement of the charge density inevitably results, leading to a distortion of the crystal lattice [3–6]. This raises the question of whether there can ever be a distinction between a Bose condensate of excitons and a conventional, structural phase transition that breaks the same symmetry. Here, we use inelastic electron scattering (M-EELS) to study copper-intercalated TiSe2, in which exciton condensation can be directly observed as a soft electronic mode at the exciton condensation temperature, T_XC [7]. While the lattice distortion in CuxTiSe2 persists to x > 0.10 [8], we find the exciton condensate is fully suppressed by x = 0.014, which is short of the semimetal-metal transition we identify at x = 0.025. Our observations indicate that the excitonic and lattice instabilities split as x is increased, showing that structural and excitonic subsystems can exhibit separate transitions and may be distinct subsystems that break different symmetries. In addition we found that although exciton condensation is suppressed, exciton fluctuations remain for most of the doping phase diagram. These fluctuations have been theorized to aid in superconducting pairing in this system [9,10]. SrTi(1−x)NbxO3, is an electron doped ionic semiconductor that exhibits aborted ferroelectricity due to quantum fluctuations [11], dilute (unconventional) superconductivity that survives even when the plasmon energy (ω_p) is lower than the Fermi energy (E_F ) [12], and charge transport properties that suggests bad metal behavior [13]. In general, the polar nature of ionic semiconductors leads to coupling of the longitudinal optical (LO) phonons to collective charge modes, such as plasmons, due to long range polarization fields. Understanding the dynamics of these collective modes, which have been implicated in superconducting pairing [14] [15] [16], can provide insight into the nature of these unusual properties. Here, we use inelastic electron scattering (M-EELS) with a fracturing surface preparation technique to study the doping, temperature, and momentum dependence of the charge collective modes in SrTi(1−x)NbxO3. We measure propagating and diffusive acoustic phonons, Fuchs-Kliewer optical phonons and overtones that get suppressed with doping due to metallic screening, a 93 meV phonon mode that becomes highly damped and asymmetric at x=0.002, and a very broad plasmon that blue-shifts with decreasing temperature. We find that the plasmon in SrTi(1−x)NbxO3 remains dispersion-less at all temperatures and dopings even with other propagating collective modes (acoustic phonon), contrary to RPA predictions and dispersions in other polar doped semiconductors [17]. The width of the plasmon deviates from RPA predictions by an order of magnitude. In addition, the energy of the plasmon measured in M-EELS is less than that of other techniques like infrared spectroscopy for samples of comparable carrier densities."],"dc:format":["application/pdf"],"dc:identifier":["http://hdl.handle.net/2142/113898"],"dc:language":["en","eng"],"dc:rights":["Copyright 2021 Samantha Rubeck"],"dc:subject":["Physics"],"dc:title":["Electronic collective modes in two dilute conductors with momentum-resolved electron energy-loss spectroscopy"],"dc:type":["text","Thesis"],"thesis:degree_discipline":["Physics"],"thesis:degree_level":["Dissertation"],"thesis:degree_name":["Ph.D."],"thesis:institution_name":["University of Illinois at Urbana-Champaign"]},"updated_at":"2026-07-22T22:24:53Z"}