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Quantum transport and spin effects in lateral semiconductor nanostructures and graphene
Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology. (Mesoscopic Physics and Photonics)
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis studies electron spin phenomena in lateral semi-conductor quantum dots/anti-dots and electron conductance in graphene nanoribbons by numerical modelling. In paper I we have investigated spin-dependent transport through open quantum dots, i.e., dots strongly coupled to their leads, within the Hubbard model. Results in this model were found consistent with experimental data and suggest that spin-degeneracy is lifted inside the dot – even at zero magnetic field.

Similar systems were also studied with electron-electron effects incorporated via Density Functional Theory (DFT) in the Local Spin Density Approximation (LSDA) in paper II and III. In paper II we found a significant spin-polarisation in the dot at low electron densities. As the electron density increases the spin polarisation in the dot gradually diminishes. These findings are consistent with available experimental observations. Notably, the polarisation is qualitatively different from the one found in the Hubbard model.

Paper III investigates spin polarisation in a quantum wire with a realistic external potential due to split gates and a random distribution of charged donors. At low electron densities we recover spin polarisation and a metalinsulator transition when electrons are localised to electron lakes due to ragged potential profile from the donors. In paper IV we propose a spin-filter device based on resonant backscattering of edge states against a quantum anti-dot embedded in a quantum wire. A magnetic field is applied and the spin up/spin down states are separated through Zeeman splitting. Their respective resonant states may be tuned so that the device can be used to filter either spin in a controlled way.

Paper V analyses the details of low energy electron transport through a magnetic barrier in a quantum wire. At sufficiently large magnetisation of the barrier the conductance is pinched off completely. Furthermore, if the barrier is sharp we find a resonant reflection close to the pinch off point. This feature is due to interference between a propagating edge state and quasibond state inside the magnetic barrier.

Paper VI adapts an efficient numerical method for computing the surface Green’s function in photonic crystals to graphene nanoribbons (GNR). The method is used to investigate magnetic barriers in GNR. In contrast to quantum wires, magnetic barriers in GNRs cannot pinch-off the lowest propagating state. The method is further applied to study edge dislocation defects for realistically sized GNRs in paper VII. In this study we conclude that even modest edge dislocations are sufficient to explain both the energy gap in narrow GNRs, and the lack of dependance on the edge structure for electronic properties in the GNRs.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2008. , 66 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1202
Keyword [en]
Electronic transport, Spin related phenomena, Quantum dots, Quantum wires, Two-dimensional electron gas, 2DEG, Graphene
National Category
Other Physics Topics Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-12410ISBN: 978-91-7393-835-8 (print)OAI: oai:DiVA.org:liu-12410DiVA: diva2:2096
Public defence
2008-09-19, K3, Kåkenhus, Campus Norrköping, Linköpings universitet, Norrköping, 10:15 (English)
Opponent
Supervisors
Available from: 2008-09-24 Created: 2008-09-03 Last updated: 2009-03-10Bibliographically approved
List of papers
1. Spin splitting in open quantum dots
Open this publication in new window or tab >>Spin splitting in open quantum dots
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2004 (English)In: Europhysics letters, ISSN 0295-5075, Vol. 68, no 2, 261-267 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate that the magnetoconductance of small lateral quantum dots in the strongly coupled regime (i.e. when the leads can support one or more propagating modes) shows a pronounced splitting of the conductance peaks and dips which persists over a wide range of magnetic fields (from zero field to the edge-state regime) and is virtually independent of the magnetic field strength. Our numerical analysis of the conductance based on the Hubbard Hamiltonian demonstrates that this is essentially a many-body/spin effect that can be traced to a splitting of degenerate levels in the corresponding closed dot. The above effect in open dots can be regarded as a counterpart of the Coulomb-blockade effect in weakly coupled dots, with the difference, however, that the splitting of the peaks originates from interactions between electrons of opposite spin.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13470 (URN)10.1209/epl/i2004-10189-2 (DOI)
Available from: 2005-11-24 Created: 2005-11-24 Last updated: 2009-02-11Bibliographically approved
2. Spin polarization in open quantum dots
Open this publication in new window or tab >>Spin polarization in open quantum dots
2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, Vol. 73, no 3, 035319- p.Article in journal (Refereed) Published
Abstract [en]

We investigate coherent transport through open lateral quantum dots using recursive Green's function technique, incorporating exchange-correlation effects within the density functional theory (DFT) in the local spin-density approximation. At low electron densities the current is spin polarized and electron density in the dot shows a strong spin polarization. As the electron density increases the spin polarization in the dot gradually diminishes. These findings are consistent with available experimental observations. Results of our DFT-based modeling indicate that utilization of the simplified approaches that use phenomenological parameters and/or model Hamiltonians might not be always reliable for theoretical predictions as well as interpretations of the experiments.

Keyword
spin polarised transport, semiconductor quantum dots, Green's function methods, exchange interactions (electron), density functional theory, electronic density of states
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-13472 (URN)10.1103/PhysRevB.73.035319 (DOI)
Available from: 2005-11-24 Created: 2005-11-24 Last updated: 2009-02-19Bibliographically approved
3. Spin polarization in modulation-doped GaAs quantum wires
Open this publication in new window or tab >>Spin polarization in modulation-doped GaAs quantum wires
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, Vol. 77, no 16, 165306- p.Article in journal (Refereed) Published
Abstract [en]

We study spin polarization in a split-gate quantum wire focusing on the effect of a realistic smooth potential due to remote donors. Electron interaction and spin effects are included within the density functional theory in the local spin density approximation. We find that depending on the electron density, the spin polarization exhibits qualitatively different features. For the case of relatively high electron density, when the Fermi energy EF exceeds a characteristic strength of a long-range impurity potential Vdonors, the density spin polarization inside the wire is practically negligible and the wire conductance is spin-degenerate. When the density is decreased such that EF approaches Vdonors, the electron density and conductance quickly become spin polarized. With further decrease of the density the electrons are trapped inside the lakes (droplets) formed by the impurity potential and the wire conductance approaches the pinch-off regime. We discuss the limitations of the density functional theory in the local spin density approximation in this regime and compare the obtained results with available experimental data.

Place, publisher, year, edition, pages
Institutionen för teknik och naturvetenskap, 2008
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-11757 (URN)10.1103/PhysRevB.77.16530 (DOI)
Note
Original publication: M. Evaldsson, S. Ihnatsenka, and I. V. Zozoulenko, Spin polarization in modulation-doped GaAs quantum wires, 2008, Physical Review B, (77), 165306. http://dx.doi.org/10.1103/PhysRevB.77.165306. Copyright: The America Physical Society, http://prb.aps.org/Available from: 2008-05-08 Created: 2008-05-08 Last updated: 2015-02-13Bibliographically approved
4. Quantum antidot as a controllable spin injector and spin filter
Open this publication in new window or tab >>Quantum antidot as a controllable spin injector and spin filter
2004 (English)In: Applied Physics Letters, ISSN 0003-6951, Vol. 85, no 15, 3136-3138 p.Article in journal (Refereed) Published
Abstract [en]

We propose a device based on an antidot embedded in a narrow quantum wire in the edge-state regime, that can be used to inject and/or control spin-polarized current. The operational principle of the device is based on the effect of resonant backscattering from one edge state into another through localized quasibound states, combined with the effect of Zeeman splitting of the quasibound states in sufficiently high magnetic field. We outline the device geometry, present detailed quantum-mechanical transport calculations, and suggest a possible scheme to test the device performance and functionality.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-13471 (URN)10.1063/1.1804249 (DOI)
Available from: 2005-11-24 Created: 2005-11-24 Last updated: 2009-06-09Bibliographically approved
5. Resonant reflection at magnetic barriers in quantum wires
Open this publication in new window or tab >>Resonant reflection at magnetic barriers in quantum wires
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2007 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, Vol. 75, no 20, 205301- p.Article in journal (Refereed) Published
Abstract [en]

The conductance of a quantum wire containing a single magnetic barrier is studied numerically by means of the recursive Green's function technique. For sufficiently strong and localized barriers, Fano-type reflection resonances are observed close to the pinch-off regime. They are attributed to a magnetoelectric vortex-type quasibound state inside the magnetic barrier that interferes with an extended mode outside. We, furthermore, show that disorder can substantially modify the residual conductance around the pinch-off regime.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14775 (URN)10.1103/PhysRevB.75.205301 (DOI)
Available from: 2008-09-24 Created: 2008-09-24 Last updated: 2015-02-13
6. Magnetic barriers in graphene nanoribbons: Theoretical study of transport properties
Open this publication in new window or tab >>Magnetic barriers in graphene nanoribbons: Theoretical study of transport properties
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, Vol. 77, no 24, 245401- p.Article in journal (Refereed) Published
Abstract [en]

A theoretical study of the transport properties of zigzag and armchair graphene nanoribbons with a magnetic barrier on top is presented. The magnetic barrier modifies the energy spectrum of the nanoribbons locally, which results in an energy shift of the conductance steps toward higher energies. The magnetic barrier also induces Fabry–Pérot-type oscillations, provided the edges of the barrier are sufficiently sharp. The lowest propagating state present in zigzag and metallic armchair nanoribbons prevents confinement of the charge carriers by the magnetic barrier. Disordered edges in nanoribbons tend to localize the lowest propagating state, which get delocalized in the magnetic barrier region. Thus, in sharp contrast to the case of two-dimensional graphene, the charge carriers in graphene nanoribbons cannot be confined by magnetic barriers. We also present a method based on the Green's function technique for the calculation of the magnetosubband structure, Bloch states and magnetoconductance of the graphene nanoribbons in a perpendicular magnetic field. Utilization of this method greatly facilitates the conductance calculations, because, in contrast to existing methods, the present method does not require self-consistent calculations for the surface Green's function.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14777 (URN)10.1103/PhysRevB.77.245401 (DOI)
Available from: 2008-09-24 Created: 2008-09-24 Last updated: 2009-05-19
7. Edge disorder induced Anderson localization and conduction gap in graphene nanoribbons
Open this publication in new window or tab >>Edge disorder induced Anderson localization and conduction gap in graphene nanoribbons
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 16, 161407- p.Article in journal (Refereed) Published
Abstract [en]

We study the effect of the edge disorder on the conductance of the graphene nanoribbons (GNRs).We find that only very modest edge disorder is sufficient to induce the conduction energy gap inthe otherwise metallic GNRs and to lift any difference in the conductance between nanoribbonsof different edge geometry. We relate the formation of the conduction gap to the pronounced edgedisorder induced Anderson-type localization which leads to the strongly enhanced density of states atthe edges, formation of surface-like states and to blocking of conductive paths through the ribbons.

Place, publisher, year, edition, pages
American Physical Society, 2008
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14778 (URN)10.1103/PhysRevB.78.161407 (DOI)
Available from: 2008-09-24 Created: 2008-09-24 Last updated: 2013-06-12Bibliographically approved

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