| 1. |
- Andresen, SE, et al.
(författare)
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Spin-polarized electron tunneling across a Si delta-doped GaMnAs/n-GaAs interface
- 2003
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Ingår i: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 94:6, s. 3990-3994
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Tidskriftsartikel (refereegranskat)abstract
- We study the spin-polarized tunneling of electrons from the valence band of GaMnAs into the conduction band of n-type GaAs with Si delta-doping at the interface. The injection of spin-polarized electrons is detected as circular polarized emission from a GaInAs/GaAs quantum well light emitting diode, corresponding to magneto-optical Kerr effect loops. The angular momentum selection rules are simplified by the strain-induced heavy-hole/light-hole splitting, allowing a direct relation between circular polarization and spin-polarization. Comparison with the influence of Zeeman splitting allow us to conclude a spin-injection from the majority spin-band.
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| 2. |
- Linke, H, et al.
(författare)
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Quantum ratchets and quantum heat pumps
- 2002
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Ingår i: Applied Physics A: Materials Science & Processing. - : Springer Science and Business Media LLC. - 1432-0630. ; 75:2, s. 237-246
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Tidskriftsartikel (refereegranskat)abstract
- Quantum ratchets are Brownian motors in which the quantum dynamics of particles induces qualitatively new behavior. We review a series of experiments in which asymmetric semiconductor devices of sub-micron dimensions are used to study quantum ratchets for electrons. In rocked quantum-dot ratchets electron-wave interference is used to create a non-linear voltage response, leading to a ratchet effect. The direction of the net ratchet current in this type of device can be sensitively controlled by changing one of the following experimental variables: a small external magnetic field, the amplitude of the rocking force, or the Fermi energy. We also describe a tunneling ratchet in which the current direction depends on temperature. In our discussion of the tunneling ratchet we distinguish between three contributions to the non-linear current-voltage characteristics that lead to the ratchet effect: thermal excitation over energy barriers, tunneling through barriers, and wave reflection from barriers. Finally, we discuss the operation of adiabatically rocked tunneling ratchets as heat pumps.
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| 3. |
- Löfgren, Anneli, et al.
(författare)
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Symmetry of magnetoconductance fluctuations of quantum dots in the nonlinear response regime
- 2006
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 73:23
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the symmetry of magnetoconductance fluctuations of phase-coherent, two-terminal quantum dots in the nonlinear regime of transport. Specifically, we consider open, ballistic quantum dots (electron billiards) with and without symmetry axes parallel and perpendicular to the current direction and formulate a set of novel symmetry relations not observed in devices with lower symmetry. We experimentally confirm these relations, demonstrating that high-quality materials and modern semiconductor technology allow the fabrication of devices with almost perfect symmetry. Small deviations from the intended symmetry, presumably due to impurities and fabrication limitations, do exist and can be detected. We also take into account circuit-induced asymmetries of the measured conductance due to bias-dependent depletion and demonstrate that this effect can be experimentally distinguished from rectification effects that are due to a lack of device symmetry. Some open questions regarding the role of a magnetic field in the nonlinear regime of transport are highlighted.
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| 4. |
- Mathieu, R, et al.
(författare)
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Magnetization of ultrathin (Ga,Mn)As layers
- 2003
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 68:18: 184421
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Tidskriftsartikel (refereegranskat)abstract
- Kerr rotation and superconducting quantum interference device magnetometry measurements were performed on ultrathin (Ga0.95Mn0.05)As layers. The thinner layers (below 250 A) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 A) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary ion mass spectrometry experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers.
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| 5. |
- Sorensen, BS, et al.
(författare)
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Dependence of Curie temperature on the thickness of epitaxial (Ga,Mn)As film
- 2002
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 66:23
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Tidskriftsartikel (refereegranskat)abstract
- We present the magnetotransport properties of very thin (5-15 nm) single (Ga,Mn)As layers grown by low-temperature molecular beam epitaxy. A lower (Ga,Mn)As thickness limit of 5 nm for the ferromagnetic phase and the dependence of the Curie temperature on (Ga,Mn)As thickness are determined from electrical-transport measurements. The Curie temperature is determined to be 97 K for the thinnest ferromagnetic sample and is found to decrease for increasing layer thickness. A carrier density of 7.1x10(20) cm(-3) for the 5 nm thick (Ga,Mn)As layer is determined from Hall measurements. Differences between magnetotransport properties of thick and thin (Ga,Mn)As layers are observed and discussed.
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| 6. |
- Sorensen, BS, et al.
(författare)
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Effect of annealing on carrier density and Curie temperature in epitaxial (Ga,Mn)As thin films
- 2003
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:14, s. 2287-2289
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Tidskriftsartikel (refereegranskat)abstract
- We report a clear correspondence between changes in the Curie temperature and carrier density upon annealing in epitaxially grown (Ga,Mn)As layers with thicknesses in the range between 5 and 20 nm. The changes are dependent on the layer thickness, indicating that the (Ga,Mn)As-GaAs interface has importance for the physical properties of the (Ga,Mn)As layer. The magnetoresistance shows additional features when compared to thick (Ga,Mn)As layers, that are at present of unknown origin. (C) 2003 American Institute of Physics.
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