--- _id: '10759' abstract: - lang: eng text: In this Thesis, I study composite quantum impurities with variational techniques, both inspired by machine learning as well as fully analytic. I supplement this with exploration of other applications of machine learning, in particular artificial neural networks, in many-body physics. In Chapters 3 and 4, I study quasiparticle systems with variational approach. I derive a Hamiltonian describing the angulon quasiparticle in the presence of a magnetic field. I apply analytic variational treatment to this Hamiltonian. Then, I introduce a variational approach for non-additive systems, based on artificial neural networks. I exemplify this approach on the example of the polaron quasiparticle (Fröhlich Hamiltonian). In Chapter 5, I continue using artificial neural networks, albeit in a different setting. I apply artificial neural networks to detect phases from snapshots of two types physical systems. Namely, I study Monte Carlo snapshots of multilayer classical spin models as well as molecular dynamics maps of colloidal systems. The main type of networks that I use here are convolutional neural networks, known for their applicability to image data. alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Wojciech full_name: Rzadkowski, Wojciech id: 48C55298-F248-11E8-B48F-1D18A9856A87 last_name: Rzadkowski orcid: 0000-0002-1106-4419 citation: ama: Rzadkowski W. Analytic and machine learning approaches to composite quantum impurities. 2022. doi:10.15479/at:ista:10759 apa: Rzadkowski, W. (2022). Analytic and machine learning approaches to composite quantum impurities. Institute of Science and Technology Austria. https://doi.org/10.15479/at:ista:10759 chicago: Rzadkowski, Wojciech. “Analytic and Machine Learning Approaches to Composite Quantum Impurities.” Institute of Science and Technology Austria, 2022. https://doi.org/10.15479/at:ista:10759. ieee: W. Rzadkowski, “Analytic and machine learning approaches to composite quantum impurities,” Institute of Science and Technology Austria, 2022. ista: Rzadkowski W. 2022. Analytic and machine learning approaches to composite quantum impurities. Institute of Science and Technology Austria. mla: Rzadkowski, Wojciech. Analytic and Machine Learning Approaches to Composite Quantum Impurities. Institute of Science and Technology Austria, 2022, doi:10.15479/at:ista:10759. short: W. Rzadkowski, Analytic and Machine Learning Approaches to Composite Quantum Impurities, Institute of Science and Technology Austria, 2022. date_created: 2022-02-16T13:27:37Z date_published: 2022-02-21T00:00:00Z date_updated: 2024-02-28T13:01:59Z day: '21' ddc: - '530' degree_awarded: PhD department: - _id: GradSch - _id: MiLe doi: 10.15479/at:ista:10759 ec_funded: 1 file: - access_level: closed checksum: 0fc54ad1eaede879c665ac9b53c93e22 content_type: application/zip creator: wrzadkow date_created: 2022-02-21T13:58:16Z date_updated: 2022-02-22T07:20:12Z file_id: '10785' file_name: Rzadkowski_thesis_final_source.zip file_size: 17668233 relation: source_file - access_level: open_access checksum: 22d2d7af37ca31f6b1730c26cac7bced content_type: application/pdf creator: wrzadkow date_created: 2022-02-21T14:02:54Z date_updated: 2022-02-21T14:02:54Z file_id: '10786' file_name: Rzadkowski_thesis_final.pdf file_size: 13307331 relation: main_file success: 1 file_date_updated: 2022-02-22T07:20:12Z has_accepted_license: '1' language: - iso: eng month: '02' oa: 1 oa_version: Published Version page: '120' project: - _id: 2564DBCA-B435-11E9-9278-68D0E5697425 call_identifier: H2020 grant_number: '665385' name: International IST Doctoral Program publication_identifier: issn: - 2663-337X publication_status: published publisher: Institute of Science and Technology Austria related_material: record: - id: '10762' relation: part_of_dissertation status: public - id: '8644' relation: part_of_dissertation status: public - id: '7956' relation: part_of_dissertation status: public - id: '415' relation: part_of_dissertation status: public status: public supervisor: - first_name: Mikhail full_name: Lemeshko, Mikhail id: 37CB05FA-F248-11E8-B48F-1D18A9856A87 last_name: Lemeshko orcid: 0000-0002-6990-7802 title: Analytic and machine learning approaches to composite quantum impurities type: dissertation user_id: c635000d-4b10-11ee-a964-aac5a93f6ac1 year: '2022' ... --- _id: '8958' abstract: - lang: eng text: "The oft-quoted dictum by Arthur Schawlow: ``A diatomic molecule has one atom too many'' has been disavowed. Inspired by the possibility to experimentally manipulate and enhance chemical reactivity in helium nanodroplets, we investigate the rotation of coupled cold molecules in the presence of a many-body environment.\r\nIn this thesis, we introduce new variational approaches to quantum impurities and apply them to the Fröhlich polaron - a quasiparticle formed out of an electron (or other point-like impurity) in a polar medium, and to the angulon - a quasiparticle formed out of a rotating molecule in a bosonic bath.\r\nWith this theoretical toolbox, we reveal the self-localization transition for the angulon quasiparticle. We show that, unlike for polarons, self-localization of angulons occurs at finite impurity-bath coupling already at the mean-field level. The transition is accompanied by the spherical-symmetry breaking of the angulon ground state and a discontinuity in the first derivative of the ground-state energy. Moreover, the type of symmetry breaking is dictated by the symmetry of the microscopic impurity-bath interaction, which leads to a number of distinct self-localized states. \r\nFor the system containing multiple impurities, by analogy with the bipolaron, we introduce the biangulon quasiparticle describing two rotating molecules that align with respect to each other due to the effective attractive interaction mediated by the excitations of the bath. We study this system from the strong-coupling regime to the weak molecule-bath interaction regime. We show that the molecules tend to have a strong alignment in the ground state, the biangulon shows shifted angulon instabilities and an additional spectral instability, where resonant angular momentum transfer between the molecules and the bath takes place. Finally, we introduce a diagonalization scheme that allows us to describe the transition from two separated angulons to a biangulon as a function of the distance between the two molecules." alternative_title: - ISTA Thesis article_processing_charge: No author: - first_name: Xiang full_name: Li, Xiang id: 4B7E523C-F248-11E8-B48F-1D18A9856A87 last_name: Li citation: ama: Li X. Rotation of coupled cold molecules in the presence of a many-body environment. 2020. doi:10.15479/AT:ISTA:8958 apa: Li, X. (2020). Rotation of coupled cold molecules in the presence of a many-body environment. Institute of Science and Technology Austria. https://doi.org/10.15479/AT:ISTA:8958 chicago: Li, Xiang. “Rotation of Coupled Cold Molecules in the Presence of a Many-Body Environment.” Institute of Science and Technology Austria, 2020. https://doi.org/10.15479/AT:ISTA:8958. ieee: X. Li, “Rotation of coupled cold molecules in the presence of a many-body environment,” Institute of Science and Technology Austria, 2020. ista: Li X. 2020. Rotation of coupled cold molecules in the presence of a many-body environment. Institute of Science and Technology Austria. mla: Li, Xiang. Rotation of Coupled Cold Molecules in the Presence of a Many-Body Environment. Institute of Science and Technology Austria, 2020, doi:10.15479/AT:ISTA:8958. short: X. Li, Rotation of Coupled Cold Molecules in the Presence of a Many-Body Environment, Institute of Science and Technology Austria, 2020. date_created: 2020-12-21T09:44:30Z date_published: 2020-12-21T00:00:00Z date_updated: 2023-09-20T11:30:58Z day: '21' ddc: - '539' degree_awarded: PhD department: - _id: MiLe doi: 10.15479/AT:ISTA:8958 ec_funded: 1 file: - access_level: open_access checksum: 3994c54a1241451d561db1d4f43bad30 content_type: application/pdf creator: xli date_created: 2020-12-22T10:55:56Z date_updated: 2020-12-22T10:55:56Z file_id: '8967' file_name: THESIS_Xiang_Li.pdf file_size: 3622305 relation: main_file success: 1 - access_level: closed checksum: 0954ecfc5554c05615c14de803341f00 content_type: application/x-zip-compressed creator: xli date_created: 2020-12-22T10:56:03Z date_updated: 2020-12-30T07:18:03Z file_id: '8968' file_name: THESIS_Xiang_Li.zip file_size: 4018859 relation: source_file file_date_updated: 2020-12-30T07:18:03Z has_accepted_license: '1' language: - iso: eng month: '12' oa: 1 oa_version: Published Version page: '125' project: - _id: 26031614-B435-11E9-9278-68D0E5697425 call_identifier: FWF grant_number: P29902 name: Quantum rotations in the presence of a many-body environment - _id: 2688CF98-B435-11E9-9278-68D0E5697425 call_identifier: H2020 grant_number: '801770' name: 'Angulon: physics and applications of a new quasiparticle' publication_identifier: issn: - 2663-337X publication_status: published publisher: Institute of Science and Technology Austria related_material: record: - id: '5886' relation: part_of_dissertation status: public - id: '8587' relation: part_of_dissertation status: public - id: '1120' relation: part_of_dissertation status: public status: public supervisor: - first_name: Mikhail full_name: Lemeshko, Mikhail id: 37CB05FA-F248-11E8-B48F-1D18A9856A87 last_name: Lemeshko orcid: 0000-0002-6990-7802 title: Rotation of coupled cold molecules in the presence of a many-body environment type: dissertation user_id: c635000d-4b10-11ee-a964-aac5a93f6ac1 year: '2020' ... --- _id: '1189' abstract: - lang: eng text: "Within the scope of this thesis, we show that a driven-dissipative system with\r\nfew ultracold atoms can exhibit dissipatively bound states, even if the atom-atom\r\ninteraction is purely repulsive. This bond arises due to the dipole-dipole inter-\r\naction, which is restricted to one of the lower electronic energy states, resulting\r\nin the distance-dependent coherent population trapping. The quality of this al-\r\nready established method of dissipative binding is improved and the application\r\nis extended to higher dimensions and a larger number of atoms. \ Here, we simu-\r\nlate two- and three-atom systems using an adapted approach to the Monte Carlo\r\nwave-function method and analyse the results. Finally, \ we examine the possi-\r\nbility of finding a setting allowing trimer \ states but prohibiting dimer states.\r\nIn the context of open quantum systems, such a three-body bound states corre-\r\nsponds to the driven-dissipative analogue of a Borromean state. These states can\r\nbe detected in modern experiments with dipolar and Rydberg-dressed ultracold\r\natomic gases.\r\n" article_processing_charge: No author: - first_name: Clemens full_name: Jochum, Clemens last_name: Jochum citation: ama: Jochum C. Dissipative Few-Body Quantum Systems. 2016. apa: Jochum, C. (2016). Dissipative Few-Body Quantum Systems. Technical University Vienna. chicago: Jochum, Clemens. “Dissipative Few-Body Quantum Systems.” Technical University Vienna, 2016. ieee: C. Jochum, “Dissipative Few-Body Quantum Systems,” Technical University Vienna, 2016. ista: Jochum C. 2016. Dissipative Few-Body Quantum Systems. Technical University Vienna. mla: Jochum, Clemens. Dissipative Few-Body Quantum Systems. Technical University Vienna, 2016. short: C. Jochum, Dissipative Few-Body Quantum Systems, Technical University Vienna, 2016. date_created: 2018-12-11T11:50:37Z date_published: 2016-11-28T00:00:00Z date_updated: 2021-01-12T06:48:57Z day: '28' extern: '1' language: - iso: eng main_file_link: - open_access: '1' url: http://repositum.tuwien.ac.at/obvutwhs/content/titleinfo/1517088 month: '11' oa: 1 oa_version: Published Version page: '94' publication_status: published publisher: Technical University Vienna publist_id: '6164' status: public supervisor: - first_name: Mikhail full_name: Lemeshko, Mikhail id: 37CB05FA-F248-11E8-B48F-1D18A9856A87 last_name: Lemeshko orcid: 0000-0002-6990-7802 - first_name: Peter full_name: Rabl, Peter last_name: Rabl title: Dissipative Few-Body Quantum Systems type: dissertation user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87 year: '2016' ...