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Confinement Sensitivity in Quantum Dot Spin Relaxation
Stockholm University, Faculty of Science, Department of Physics. (Atomic Physics)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum dots, also known as artificial atoms, are created by tightly confining electrons, and thereby quantizing their energies. They are important components in the emerging fields of nanotechnology where their potential uses vary from dyes to quantum computing qubits. Interesting properties to investigate are e.g. the existence of atom-like shell structures and lifetimes of prepared states.

Stability and controllability are important properties in finding applications to quantum dots. The ability to prepare a state and change it in a controlled manner without it loosing coherence is very useful, and in some semiconductor quantum dots, lifetimes of up to several milliseconds have been realized. Here we focus on dots in semiconductor materials and investigate how the confined electrons are effected by their experienced potential.

The shape of the dot will effect its properties, and is important when considering a suitable model. Structures elongated in one dimension, often called nanowires, or shaped as rings have more one-dimensional characteristics than completely round or square dots. The two-dimensional dots investigated here are usually modeled as harmonic oscillators, however we will also consider circular well models.

The effective potential confining the electrons is investigated both in regard to how elliptical it is, as well as how results differ when using a harmonic oscillator or a circular well potential. By mixing spin states through spin-orbit interaction transitioning between singlet and triplet states becomes possible with spin independent processes such as phonon relaxation. We solve the spin-mixing two-electron problem numerically for some confinement, and calculate the phonon transition rate between the lowest energy singlet and triplet states using Fermi's golden rule.

The strength of the spin-orbit interaction is varied both by changing the coupling constants, and by applying an external, tilted, magnetic field. The relation between magnetic field parameters and dot parameters are used to maximize state lifetimes, and to model experimental results.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2017. , 64 p.
National Category
Nano Technology Other Physics Topics Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-142133ISBN: 978-91-7649-809-5 (print)ISBN: 978-91-7649-810-1 (electronic)OAI: oai:DiVA.org:su-142133DiVA: diva2:1091193
Public defence
2017-06-08, sal FB55, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.

Available from: 2017-05-16 Created: 2017-04-26 Last updated: 2017-05-16Bibliographically approved
List of papers
1. Performance of the coupled-cluster singles and doubles method applied to two-dimensional quantum dots
Open this publication in new window or tab >>Performance of the coupled-cluster singles and doubles method applied to two-dimensional quantum dots
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 3, 035112Article in journal (Refereed) Published
Abstract [en]

An implementation of the coupled-cluster single and double excitations (CCSD) method on two-dimensional quantum dots is presented. Advantages and limitations are studied through comparison with other high accuracy approaches, including another CCSD implementation, for up to twelve confined electrons. The possibility to effectively use a very large basis set is found to be an important advantage compared to full configuration interaction implementations. The error in the ground-state energy introduced by truncating at triple excitations is shown to be comparable to the difference between the results from the variation and diffusion Monte Carlo methods. Convergence of the iterative solution of the coupled cluster equations is found for surprisingly weak confinement strengths even when the full electron-electron interaction is treated as a perturbation. The relevance of the omitted excitations is investigated through comparison with full configuration interaction results. DOI: 10.1103/PhysRevB.87.035112

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-87693 (URN)10.1103/PhysRevB.87.035112 (DOI)000313331900002 ()
Funder
Swedish Research Council
Note

AuthorCount:3;

Available from: 2013-02-15 Created: 2013-02-14 Last updated: 2017-04-28Bibliographically approved
2. Two-electron quantum dot in tilted magnetic fields: Sensitivity to the confinement model
Open this publication in new window or tab >>Two-electron quantum dot in tilted magnetic fields: Sensitivity to the confinement model
Show others...
2013 (English)In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 86, no 10, 430Article in journal (Refereed) Published
Abstract [en]

Semiconductor quantum dots are conventionally treated within the effective-mass approximation and a harmonic model potential in the two-dimensional plane for the electron confinement. The validity of this approach depends on the type of the quantum-dot device as well as on the number of electrons confined in the system. Accurate modeling is particularly demanding in the few-particle regime, where screening effects are diminished and thus the system boundaries may have a considerable effect on the confining potential. Here we solve the numerically exact two-electron states in both harmonic and hard-wall model quantum dots subjected to tilted magnetic fields. Our numerical results enable direct comparison against experimental singlet-triplet energy splittings. Our analysis shows that hard and soft wall models produce qualitatively different results for quantum dots exposed to tilted magnetic fields. Hence, we are able to address the sensitivity of the two-body phenomena to the modeling, which is of high importance in realistic spin-qubit design.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-96095 (URN)10.1140/epjb/e2013-40677-x (DOI)000325608100003 ()
Note

AuthorCount:5;

Available from: 2013-11-14 Created: 2013-11-11 Last updated: 2017-04-28Bibliographically approved
3. Confinement sensitivity in quantum dot singlet-triplet relaxation
Open this publication in new window or tab >>Confinement sensitivity in quantum dot singlet-triplet relaxation
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-142132 (URN)
Available from: 2017-04-26 Created: 2017-04-26 Last updated: 2017-04-28Bibliographically approved

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