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Spin and Charge Transport in Two-dimensional Electron Gases
Norwegian University of Science and Technology, Faculty of Natural Sciences and Technology, Department of Physics.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the continuing demand for miniaturisation from the electronics industry, it becomes increasingly important to understand the physical behaviour of nanometre sized conductors. State-of-the-art transistors have typical length scales of the order a few tens of nanometres. On these mesoscopic scales, quantum effects become important in certain semiconductor systems at low temperature, or in the case of the newly discovered graphene even at room temperature. The quantum effects can significantly alter the electronic behaviour of a device. Quantum transport on the mesoscopic scale is also interesting from a fundamental point of view, as it allows us to study the crossover between the quantum and classical regimes of physics.

The two-dimensional electron gases studied in this thesis can be created at the interface between a semiconductor and another semiconductor or an insulator. Graphene is another twodimensional conductor which holds great promises for use in electronics, due to high mobilities, gate controllable doping, and its intrinsic two-dimensionality.

In the first two papers of this thesis [1, 2] we investigate spin transport in graphene. The first paper [1] suggests a way of inducing a significant spin polarisation in the non-magnetic material, while the second paper [2] examines the role of the spin-orbit interaction in highly doped graphene.

The last two papers [3, 4] study the non-local electronic signal that can be induced between two normal contacts in the presence of a superconductor. The physical process responsible for this signal, crossed Andreev reflection, has been suggested as a candidate for generating entangled electrons in a solid state device. While experimentally such a signal has only been observed beyond linear response or in the presence of interactions, we show that it is possible to generate this type of signal also in linear response.

 

Place, publisher, year, edition, pages
NTNU, 2010.
Series
Doctoral theses at NTNU, ISSN 1503-8181 ; 2010:200
Identifiers
URN: urn:nbn:no:ntnu:diva-12673ISBN: 978-82-471-2379-9 (printed ver.)ISBN: 978-82-471-2380-5 (electronic ver.)OAI: oai:DiVA.org:ntnu-12673DiVA: diva2:420913
Public defence
2010-12-10, 00:00
Available from: 2011-06-07 Created: 2011-06-07 Last updated: 2011-06-07Bibliographically approved
List of papers
1. Spin transport in proximity-induced ferromagnetic graphene
Open this publication in new window or tab >>Spin transport in proximity-induced ferromagnetic graphene
2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 11, 115406- p.Article in journal (Refereed) Published
Abstract [en]

Ferromagnetic insulators deposited on graphene can induce ferromagnetic correlations in graphene. We estimate that induced exchange splittings Delta similar to 5 meV can be achieved by, e.g., using the magnetic insulator EuO. We study the effect of the induced spin splittings on the graphene transport properties. The exchange splittings in proximity-induced ferromagnetic graphene can be determined from the transmission resonances in the linear response conductance or, independently, by magnetoresistance measurements in a spin-valve device. The spin polarization of the current near the Dirac point increases with the length of the barrier, so that long systems are required to determine Delta experimentally.

Identifiers
urn:nbn:no:ntnu:diva-12538 (URN)10.1103/PhysRevB.77.115406 (DOI)000254542800158 ()
Note
©2008 The American Physical SocietyAvailable from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-06-07Bibliographically approved
2. Spin transport in doped graphene
Open this publication in new window or tab >>Spin transport in doped graphene
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:no:ntnu:diva-12537 (URN)
Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-06-07Bibliographically approved
3. Crossed Andreev reflection versus electron transfer in three-terminal graphene devices
Open this publication in new window or tab >>Crossed Andreev reflection versus electron transfer in three-terminal graphene devices
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 17, 174523- p.Article in journal (Refereed) Published
Abstract [en]

We investigate the transport properties of three-terminal graphene devices, where one terminal is superconducting and two are normal metals. The terminals are connected by nanoribbons. Electron transfer (ET) and crossed Andreev reflection (CAR) are identified via the nonlocal signal between the two normal terminals. Analytical expressions for ET and CAR in symmetric devices are found. We compute ET and CAR numerically for asymmetric devices. ET dominates CAR in symmetric devices, but CAR can dominate ET in asymmetric devices, where only the zero-energy modes of the zigzag nanoribbons contribute to the transport.

 

Identifiers
urn:nbn:no:ntnu:diva-12539 (URN)10.1103/PhysRevB.81.174523 (DOI)000278141600113 ()
Note
©2010 The American Physical SocietyAvailable from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-06-07Bibliographically approved
4. Focused crossed Andreev reflection
Open this publication in new window or tab >>Focused crossed Andreev reflection
2011 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 93, no 6, 67005- p.Article in journal (Refereed) Published
Abstract [en]

We consider non-local transport mediated by Andreev reflection in a two-dimensional electron gas (2DEG) connected to one superconducting and two normal metal terminals. A robust scheme is presented for observing crossed Andreev reflection (CAR) between the normal metal terminals based on electron focusing by weak perpendicular magnetic fields. At slightly elevated temperatures the CAR signature can be easily distinguished from a background of quantum interference fluctuations. The CAR-induced entanglement between electrons can be switched on and off over large distances by the magnetic field. Copyright (C) EPLA, 2011

Identifiers
urn:nbn:no:ntnu:diva-12540 (URN)10.1209/0295-5075/93/67005 (DOI)000288943800023 ()
Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-06-07Bibliographically approved

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