{"id":{"repo_id":"aachen","oai_identifier":"oai:publications.rwth-aachen.de:50132"},"canonical_url":"https://search.dev.ndltd.org/etd/aachen/oai:publications.rwth-aachen.de:50132","repository":{"repo_id":"aachen","name":"RWTH Aachen University","base_url":"https://publications.rwth-aachen.de/oai2d"},"display":{"title":"Transport spectroscopy and control of molecular quantum dots","abstract":"A quantum dot is a small confined region in space, e.g, an area in a two-dimensional electron gas confined by metallic gates, where both charge and orbital degrees of freedom become quantized. Recently, also quantum dots consisting of single molecules and carbon nanotubes (macro-molecules) have been contacted and measured in transistor-like setups. Such systems are interesting from an application point of view due to their small size and possibility of bottom-up fabrication by chemical synthesis. They may also display new interesting physics, such as quantized mechanical degrees of freedom and magnetic anisotropy. One of the first major challenges is to understand the properties of such molecular quantum dots inside the transport junction, which can deviate significantly from those of the isolated dot due to the close proximity of the large electrodes. Here the transport current through the system can act as a tool to investigate its properties, i.e., a transistor setup is a used as a spectroscopic tool. The main objective of this thesis is to advance this field by developing a general, explicit theory for transport spectroscopy and applying this to analyze generic models, predicting new effects, and in experimental collaborations, trying to interpret actual transport spectra and compare with model calculations using the developed method. A more longterm goal is to use quantum dots as active electronic components. Here molecular dots offer exciting new possibilities: chemical meta-stability and long life-times of excitations open the door to switching applications, mechanical degrees of freedom enable them to be used as the ultimate limit of NEMS (nano electro-mechanical systems) and long spin decoherence times have even lead to suggestions of molecule-based quantum computers. Such control of quantum dots is also an important theme of this thesis. Transport through molecular quantum dots presents a challenging theoretical problem: the coupling to the electrodes introduces a huge number of degrees of freedom, while a finite bias voltage prevents methods of equilibrium statistical physics from being used. Additionally, local interactions on the dot are typically large, preventing them from being treated perturbatively. In this thesis a generalized master equation approach is used, which treats interactions on the dot as well as the non-equilibrium condition exactly, while the electrode couplings are treated perturbatively. The main theoretical advance is the derivation of explicit expressions for the transport rates, valid for very general quantum dot models, up to next-to-leading order in the electrode couplings. Based on the developed formalism a previously unnoticed resonance, associated with coherent tunneling of electron pairs, is predicted. This occurs even in the well-studied non-equilibrium Anderson model. An important application studied in this thesis is coupling between charge tunneling and quantized vibrational modes of the molecular quantum dot. Several models are studied, showing interesting physics such as transport-based measurements of the vibrational Q-factor, coherent coupling between mechanical and electronic degrees of freedom and vibration-induced spin-blockade. Additionally, experimental transport data on a suspended carbon nanotube are shown to indicate that an electronic state can pump the vibrational mode out of equilibrium. Also experiments on carbon nanotubes filled with fullerene molecules (\"peapods\") are studied. The observed spectroscopic effects are reproduced by a model including a coherent coupling of nanotube and fullerene states. Such a coupling is essential if one wants to exploit the fullerene degrees of freedom in applications. Finally, experimental data on transport through single-molecule magnets are analyzed and compared with model calculations. The transport signature of the zero-field splitting, i.e., the energy cost of a quantized rotation of the spin-vector away from the easy axis, allows the magnetic anisotropy to be extracted. Due to the presence of a gate-electrode this could be done for the first time in multiple redox states of the molecule, which was seen to exhibit an enhanced magnetic anisotropy upon both reduction and oxidization.","abstract_html":"A quantum dot is a small confined region in space, e.g, an area in a two-dimensional electron gas confined by metallic gates, where both charge and orbital degrees of freedom become quantized. Recently, also quantum dots consisting of single molecules and carbon nanotubes (macro-molecules) have been contacted and measured in transistor-like setups. Such systems are interesting from an application point of view due to their small size and possibility of bottom-up fabrication by chemical synthesis. They may also display new interesting physics, such as quantized mechanical degrees of freedom and magnetic anisotropy. One of the first major challenges is to understand the properties of such molecular quantum dots inside the transport junction, which can deviate significantly from those of the isolated dot due to the close proximity of the large electrodes. Here the transport current through the system can act as a tool to investigate its properties, i.e., a transistor setup is a used as a spectroscopic tool. The main objective of this thesis is to advance this field by developing a general, explicit theory for transport spectroscopy and applying this to analyze generic models, predicting new effects, and in experimental collaborations, trying to interpret actual transport spectra and compare with model calculations using the developed method. A more longterm goal is to use quantum dots as active electronic components. Here molecular dots offer exciting new possibilities: chemical meta-stability and long life-times of excitations open the door to switching applications, mechanical degrees of freedom enable them to be used as the ultimate limit of NEMS (nano electro-mechanical systems) and long spin decoherence times have even lead to suggestions of molecule-based quantum computers. Such control of quantum dots is also an important theme of this thesis. Transport through molecular quantum dots presents a challenging theoretical problem: the coupling to the electrodes introduces a huge number of degrees of freedom, while a finite bias voltage prevents methods of equilibrium statistical physics from being used. Additionally, local interactions on the dot are typically large, preventing them from being treated perturbatively. In this thesis a generalized master equation approach is used, which treats interactions on the dot as well as the non-equilibrium condition exactly, while the electrode couplings are treated perturbatively. The main theoretical advance is the derivation of explicit expressions for the transport rates, valid for very general quantum dot models, up to next-to-leading order in the electrode couplings. Based on the developed formalism a previously unnoticed resonance, associated with coherent tunneling of electron pairs, is predicted. This occurs even in the well-studied non-equilibrium Anderson model. An important application studied in this thesis is coupling between charge tunneling and quantized vibrational modes of the molecular quantum dot. Several models are studied, showing interesting physics such as transport-based measurements of the vibrational Q-factor, coherent coupling between mechanical and electronic degrees of freedom and vibration-induced spin-blockade. Additionally, experimental transport data on a suspended carbon nanotube are shown to indicate that an electronic state can pump the vibrational mode out of equilibrium. Also experiments on carbon nanotubes filled with fullerene molecules ("peapods") are studied. The observed spectroscopic effects are reproduced by a model including a coherent coupling of nanotube and fullerene states. Such a coupling is essential if one wants to exploit the fullerene degrees of freedom in applications. Finally, experimental data on transport through single-molecule magnets are analyzed and compared with model calculations. The transport signature of the zero-field splitting, i.e., the energy cost of a quantized rotation of the spin-vector away from the easy axis, allows the magnetic anisotropy to be extracted. Due to the presence of a gate-electrode this could be done for the first time in multiple redox states of the molecule, which was seen to exhibit an enhanced magnetic anisotropy upon both reduction and oxidization.","abstract_has_math":false,"creators":["Leijnse, Martin"],"institution":"Publikationsserver der RWTH Aachen University","degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":["Wegewijs, Maarten Rolf"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2009,"date_issued":"2009","date_published":"2009","updated_at":"2026-07-30T19:40:16Z","subjects":["info:eu-repo/classification/ddc/530","Quantenpunkte","Transport","Spektroskopie","Physik","quantum dots","transport spectroscopy"],"languages":["eng"],"rights":["info:eu-repo/semantics/openAccess"],"rights_urls":[],"identifier_entries":[{"key":"dc:identifier","label":"Identifier","values":["https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112688%22"],"render_values":[{"text":"https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112688%22","href":"https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112688%22","code":true}]}]},"links":{"outbound_url":"https://publications.rwth-aachen.de/record/50132","outbound_label":"Repository record","outbound_source":"dc:identifier"},"source_record":{"url":"https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A50132","prefix":"oai_dc"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Wegewijs, Maarten Rolf"]},{"key":"dc:creator","label":"Author","values":["Leijnse, Martin"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:coverage","label":"Dc Coverage","values":["DE"]},{"key":"dc:date","label":"Dc Date","values":["2009"]},{"key":"dc:publisher","label":"Institution","values":["Publikationsserver der RWTH Aachen University"]},{"key":"dc:relation","label":"Dc Relation","values":["info:eu-repo/semantics/altIdentifier/urn/urn:nbn:de:hbz:82-opus-31669"]},{"key":"dc:type","label":"Dc Type","values":["info:eu-repo/semantics/doctoralThesis","info:eu-repo/semantics/publishedVersion"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["info:eu-repo/classification/ddc/530","Quantenpunkte","Transport","Spektroskopie","Physik","quantum dots","transport spectroscopy"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["eng"]},{"key":"dc:rights","label":"Dc Rights","values":["info:eu-repo/semantics/openAccess"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["https://publications.rwth-aachen.de/record/50132","https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112688%22"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["A quantum dot is a small confined region in space, e.g, an area in a two-dimensional electron gas confined by metallic gates, where both charge and orbital degrees of freedom become quantized. Recently, also quantum dots consisting of single molecules and carbon nanotubes (macro-molecules) have been contacted and measured in transistor-like setups. Such systems are interesting from an application point of view due to their small size and possibility of bottom-up fabrication by chemical synthesis. They may also display new interesting physics, such as quantized mechanical degrees of freedom and magnetic anisotropy. One of the first major challenges is to understand the properties of such molecular quantum dots inside the transport junction, which can deviate significantly from those of the isolated dot due to the close proximity of the large electrodes. Here the transport current through the system can act as a tool to investigate its properties, i.e., a transistor setup is a used as a spectroscopic tool. The main objective of this thesis is to advance this field by developing a general, explicit theory for transport spectroscopy and applying this to analyze generic models, predicting new effects, and in experimental collaborations, trying to interpret actual transport spectra and compare with model calculations using the developed method. A more longterm goal is to use quantum dots as active electronic components. Here molecular dots offer exciting new possibilities: chemical meta-stability and long life-times of excitations open the door to switching applications, mechanical degrees of freedom enable them to be used as the ultimate limit of NEMS (nano electro-mechanical systems) and long spin decoherence times have even lead to suggestions of molecule-based quantum computers. Such control of quantum dots is also an important theme of this thesis. Transport through molecular quantum dots presents a challenging theoretical problem: the coupling to the electrodes introduces a huge number of degrees of freedom, while a finite bias voltage prevents methods of equilibrium statistical physics from being used. Additionally, local interactions on the dot are typically large, preventing them from being treated perturbatively. In this thesis a generalized master equation approach is used, which treats interactions on the dot as well as the non-equilibrium condition exactly, while the electrode couplings are treated perturbatively. The main theoretical advance is the derivation of explicit expressions for the transport rates, valid for very general quantum dot models, up to next-to-leading order in the electrode couplings. Based on the developed formalism a previously unnoticed resonance, associated with coherent tunneling of electron pairs, is predicted. This occurs even in the well-studied non-equilibrium Anderson model. An important application studied in this thesis is coupling between charge tunneling and quantized vibrational modes of the molecular quantum dot. Several models are studied, showing interesting physics such as transport-based measurements of the vibrational Q-factor, coherent coupling between mechanical and electronic degrees of freedom and vibration-induced spin-blockade. Additionally, experimental transport data on a suspended carbon nanotube are shown to indicate that an electronic state can pump the vibrational mode out of equilibrium. Also experiments on carbon nanotubes filled with fullerene molecules (\"peapods\") are studied. The observed spectroscopic effects are reproduced by a model including a coherent coupling of nanotube and fullerene states. Such a coupling is essential if one wants to exploit the fullerene degrees of freedom in applications. Finally, experimental data on transport through single-molecule magnets are analyzed and compared with model calculations. The transport signature of the zero-field splitting, i.e., the energy cost of a quantized rotation of the spin-vector away from the easy axis, allows the magnetic anisotropy to be extracted. Due to the presence of a gate-electrode this could be done for the first time in multiple redox states of the molecule, which was seen to exhibit an enhanced magnetic anisotropy upon both reduction and oxidization."]},{"key":"dc:source","label":"Dc Source","values":["Aachen : Publikationsserver der RWTH Aachen University 142 S. : Ill., graph. Darst. (2009). = Aachen, Techn. 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One of the first major challenges is to understand the properties of such molecular quantum dots inside the transport junction, which can deviate significantly from those of the isolated dot due to the close proximity of the large electrodes. Here the transport current through the system can act as a tool to investigate its properties, i.e., a transistor setup is a used as a spectroscopic tool. The main objective of this thesis is to advance this field by developing a general, explicit theory for transport spectroscopy and applying this to analyze generic models, predicting new effects, and in experimental collaborations, trying to interpret actual transport spectra and compare with model calculations using the developed method. A more longterm goal is to use quantum dots as active electronic components. Here molecular dots offer exciting new possibilities: chemical meta-stability and long life-times of excitations open the door to switching applications, mechanical degrees of freedom enable them to be used as the ultimate limit of NEMS (nano electro-mechanical systems) and long spin decoherence times have even lead to suggestions of molecule-based quantum computers. Such control of quantum dots is also an important theme of this thesis. Transport through molecular quantum dots presents a challenging theoretical problem: the coupling to the electrodes introduces a huge number of degrees of freedom, while a finite bias voltage prevents methods of equilibrium statistical physics from being used. Additionally, local interactions on the dot are typically large, preventing them from being treated perturbatively. In this thesis a generalized master equation approach is used, which treats interactions on the dot as well as the non-equilibrium condition exactly, while the electrode couplings are treated perturbatively. The main theoretical advance is the derivation of explicit expressions for the transport rates, valid for very general quantum dot models, up to next-to-leading order in the electrode couplings. Based on the developed formalism a previously unnoticed resonance, associated with coherent tunneling of electron pairs, is predicted. This occurs even in the well-studied non-equilibrium Anderson model. An important application studied in this thesis is coupling between charge tunneling and quantized vibrational modes of the molecular quantum dot. Several models are studied, showing interesting physics such as transport-based measurements of the vibrational Q-factor, coherent coupling between mechanical and electronic degrees of freedom and vibration-induced spin-blockade. Additionally, experimental transport data on a suspended carbon nanotube are shown to indicate that an electronic state can pump the vibrational mode out of equilibrium. Also experiments on carbon nanotubes filled with fullerene molecules (\"peapods\") are studied. The observed spectroscopic effects are reproduced by a model including a coherent coupling of nanotube and fullerene states. Such a coupling is essential if one wants to exploit the fullerene degrees of freedom in applications. Finally, experimental data on transport through single-molecule magnets are analyzed and compared with model calculations. The transport signature of the zero-field splitting, i.e., the energy cost of a quantized rotation of the spin-vector away from the easy axis, allows the magnetic anisotropy to be extracted. Due to the presence of a gate-electrode this could be done for the first time in multiple redox states of the molecule, which was seen to exhibit an enhanced magnetic anisotropy upon both reduction and oxidization."],"dc:identifier":["https://publications.rwth-aachen.de/record/50132","https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112688%22"],"dc:language":["eng"],"dc:publisher":["Publikationsserver der RWTH Aachen University"],"dc:relation":["info:eu-repo/semantics/altIdentifier/urn/urn:nbn:de:hbz:82-opus-31669"],"dc:rights":["info:eu-repo/semantics/openAccess"],"dc:source":["Aachen : Publikationsserver der RWTH Aachen University 142 S. : Ill., graph. Darst. (2009). = Aachen, Techn. 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