| 1. |
- Akabori, M., et al.
(författare)
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Realization of In0.75Ga0.25As two-dimensional electron gas bilayer system for spintronics devices based on Rashba spin-orbit interaction
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 112:11, s. 113711-
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Tidskriftsartikel (refereegranskat)abstract
- Narrow gap InGaAs two-dimensional electron gas (2DEG) bilayer samples are fabricated and confirmed to have good electronic qualities as well as strong Rashba-type spin-orbit interactions (SOIs). The 2DEG systems are realized by molecular beam epitaxy in the form of wide quantum wells (QWs) with thicknesses tQW∼40-120nm modulation doped in both the upper and lower InAlAs barriers. From the Hall measurements, the overall mobility values of μe ∼15 m2/V s are found for the total sheet electron density of ns ∼8 × 1011/cm2, although the ns is distributed asymmetrically as about 1:3 in the upper and lower 2DEGs, respectively. Careful low temperature magneto-resistance analysis gives large SO coupling constants of α ∼20 × 10 -12eV m as well as expected electron effective masses of m*/m0 ∼0.033-0.042 for each bilayer 2DEG spin sub-band. Moreover, the enhancement of α with decrease of tQW is found. The corresponding self-consistent calculation, which suggests the interaction between the bilayer 2DEGs, is carried out and the origin of α enhancement is discussed.
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| 2. |
- Almbladh, Carl-Olof, et al.
(författare)
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Accuracy of the Hartree-Fock and Local Density Approximations for Electron Densities: A study for Light Atoms
- 1983
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Ingår i: Physica Scripta. - 0031-8949. ; 28:3, s. 193-389
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Tidskriftsartikel (refereegranskat)abstract
- We compare the electron densities and Hartree potentials in the local density and the Hartree-Fock approximations to the corresponding quantities obtained from more accurate correlated wavefunctions. The comparison is made for a number of two-electron atoms, Li, and for Be. The Hartree-Fock approximation is more accurate than the local density approximation within the 1s shell and for the spin polarization in Li, while the local density approximation is slightly better than the Hartree-Fock approximation for charge densities in the 2s shell. The inaccuracy of the Hartree-Fock and local density approximations to the Hartree potential is substantially smaller than the inaccuracy of the local density approximation to the ground-state exchange-correlation potential.
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| 3. |
- Ekenberg, Ulf, et al.
(författare)
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Analysis of electric-field-induced spin splitting in wide modulation-doped quantum wells
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:20, s. 205317-
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation. We argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than the commonly used Rashba term dependent on k and electric field. It is pointed out that the expectation value of the electric field in a subband is sometimes not unique because the expectation values can even have opposite signs for the spin-split subband components. Symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting are also discussed. We apply a well established multiband approach to wide modulation-doped InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.
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| 4. |
- Ekenberg, Ulf, et al.
(författare)
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Control of spontaneous spin splitting in an asymmetric quantum well with the use of strain and/or magnetic field
- 2001
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - 1386-9477 .- 1873-1759. ; 10:03-jan, s. 81-85
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Tidskriftsartikel (refereegranskat)abstract
- Spin-orbit coupling combined with inversion asymmetry gives rise to spin splitting even in the absence of an applied magnetic field. The size of this spin splitting can be controlled by changing the degree of asymmetry using a gate voltage. We present here other less obvious ways of controlling the spontaneous spin splitting in a two-dimensional hole gas, where these effects are particularly large. Applying moderate stress can easily decrease the spin splitting by an order of magnitude. The mechanism is the strain-induced energy shift of the heavy-hole and light-hole subbands, which diminishes the degree of band mixing, which is found to be strongly correlated to the spin splitting. An applied magnetic field causes an additional Zeeman splitting, but we find that a magnetic field of 1T can be sufficient to practically erase the difference between a symmetric quantum well (without subband splitting) and an asymmetric quantum well. We have simulated Shubnikov-de Haas oscillations and found that two periodicities in 1/B can occur even for one filled spin-degenerate hole subband.
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| 5. |
- Ekenberg, Ulf, et al.
(författare)
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Spin splitting in modulation-doped semiconductor quantum wells
- 2009
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Ingår i: Quantum Wells. - : Nova Science Publishers, Inc.. - 9781606925577 ; , s. 385-425
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Bokkapitel (refereegranskat)abstract
- We review different ways to achieve a spin splitting of two-dimensional electron and hole subbands with the combination of inversion asymmetry and spin-orbit interaction. In particular we focus on novel mechanisms to achieve a substantial spin splitting with a small applied bias across the sample. We discuss the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation and argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than commonly used terms dependent on k and electric field. One of our findings is that the expectation values of the electric field can differ substantially and even have opposite signs for the spin-split components of a subband. Thus the frequent assignment of one expectation value to a subband is sometimes not appropriate. We also discuss symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting. Our approach is applied to wide modulation-doped n-type InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. For a slightly asymmetric quantum well we demonstrate a reversal of the spin direction in a spin subband in two steps as the in-plane wave vector is increased a little. Our most significant results pertain to the superefficient Rashba effect for holes. With a careful design of doping profile and strain we find that the wave vector splitting for hole subbands can be made several thousand times stronger than for electrons at the same electric field. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.
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| 6. |
- Gustafsson, Oscar, et al.
(författare)
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Long-wavelength infrared quantum-dot based interband photodetectors
- 2011
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Ingår i: Infrared physics & technology. - : Elsevier BV. - 1350-4495 .- 1879-0275. ; 54:3, s. 287-291
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Tidskriftsartikel (refereegranskat)abstract
- We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley-Read-Hall generation-recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal-organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 x 10(11) cm(-2), 1 x 10(12) cm(-2) and 3 x 10(10) cm(-2) were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6-12 mu m. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II inter-band transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.
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| 7. |
- Gustafsson, Oscar, et al.
(författare)
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Photoluminescence photoresponse from InSb/InAs-based quantum dot structures
- 2012
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Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 20:19, s. 21264-71
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Tidskriftsartikel (refereegranskat)abstract
- InSb-based quantum dots grown by metal-organic vapor-phase epitaxy (MOVPE) on InAs substrates are studied for use as the active material in interband photon detectors. Long-wavelength infrared (LWIR) photoluminescence is demonstrated with peak emission at 8.5 μm and photoresponse, interpreted to originate from type-II interband transitions in a p-i-n photodiode, was measured up to 6 μm, both at 80 K. The possibilities and benefits of operation in the LWIR range (8-12 μm) are discussed and the results suggest that InSb-based quantum dot structures can be suitable candidates for photon detection in the LWIR regime.
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| 8. |
- Gvozdic, Dejan M., et al.
(författare)
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Beyond the Rashba model
- 2006
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : Elsevier BV. - 1386-9477 .- 1873-1759. ; 32:02-jan, s. 458-461
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Tidskriftsartikel (refereegranskat)abstract
- We analyze some common approximations made in connection with the Rashba effect, where a macroscopic electric field gives rise to a spin splitting. We demonstrate that the size of the Rashba splitting is not given by the expectation value of the electric field or some other average electric field, as is commonly assumed. Instead we find that the local electric field near an interface of a wide asymmetric modulation-doped quantum well can give rise to a Rashba splitting that is an order of magnitude larger than expected from the average electric field. The localization of the wave functions of the spin subbands can be quite sensitive to the parallel wave vector. Clear deviations from an energy-independent wave vector splitting occur when nonparabolicity is taken into account.
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| 9. |
- Gvozdic, D. M., et al.
(författare)
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Comparison of performance of n- and p-type spin transistors with conventional transistors
- 2005
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Ingår i: Journal of Superconductivity. - : Springer Science and Business Media LLC. - 0896-1107 .- 1572-9605. ; 18:3, s. 349-356
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Tidskriftsartikel (refereegranskat)abstract
- A spintronic device that has stimulated much research interest is the Datta-Das spin transistor. The mechanism behind it called the Rashba effect is that an applied voltage gives rise to a spin splitting. We propose ways to optimize this effect. The relevant spin splitting in k-space is predicted to increase with electric field at a rate that is more than two orders of magnitude larger for holes than for electrons. Furthermore, the almost negligible lattice-mismatch between GaAs and AlGaAs can be used to further enhance the advantage of hole-based spin transistors. Compared to present transistors we conclude that electron-based spin transistors will have problems to become competitive but hole-based ones are much more promising.
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| 10. |
- Gvozdic, D. M., et al.
(författare)
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Efficient switching of Rashba spin splitting in wide modulation-doped quantum wells
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:5
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Tidskriftsartikel (refereegranskat)abstract
- The authors demonstrate that the size of the electric-field-induced Rashba spin splitting in an 80 nm wide modulation-doped InGaSb quantum well can depend strongly on the spatial variation of the electric field. In a slightly asymmetric quantum well it can be an order of magnitude stronger than for the average uniform electric field. For even smaller asymmetry spin subbands can have wave functions and/or expectation values of the spin direction that are completely changed as the in-plane wave vector varies. The Dresselhaus effect [Phys. Rev. 100, 580 (1955)] can give an anticrossing at which the spin rapidly flips.
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