| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Gvozdic, Dejan M., et al.
(författare)
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Investigation of the super-efficient Rashba effect by simulation of Shubnikov-de Haas oscillations in a two-dimensional hole gas
- 2006
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : Elsevier BV. - 1386-9477 .- 1873-1759. ; 34:02-jan, s. 377-380
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Tidskriftsartikel (refereegranskat)abstract
- The Shubnikov-de Haas effect is frequently used for two-dimensional systems to determine individual subband populations, e.g. when the subbands are split by the Rashba effect. We have previously shown that the Rashba effect can give a wave vector splitting for holes that is up to three orders of magnitude larger than for electrons at the same electric field. To reach the optimum we have made a careful design of a modulation-doped quantum well with a top gate in which the negative differential Rashba effect is utilized. From the calculated hole Landau levels we determine the density of states at the Fermi energy and demonstrate a clear difference between the symmetric case without bias and the asymmetric case with a gate voltage of 100 meV, where the spin subband populations differ by a factor 3.
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| 7. |
- Gvozdić, Dejan M., et al.
(författare)
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Novel mechanism for order-of-magnitude enhancement of rashba effect in wide modulation-doped quantum wells
- 2007
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Ingår i: Physics of Semiconductors. - : American Institute of Physics (AIP). - 9780735403970 ; , s. 1371-1372
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Konferensbidrag (refereegranskat)abstract
- The Rashba effect leading to subband splitting in quantum wells is frequently taken to be proportional to some average electric field. We here show that taking the spatial variation of the electric field into account gives important effects. In particular we demonstrate that one can apply a moderate electric field to a wide modulation-doped quantum well and get an order-of-magnitude enhancement of the Rashba splitting characteristic of the built-in interface field. For small asymmetry the wave function localization and spin projection of a subband can be rapid functions of the in-plane wave vector.
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| 8. |
- Gvozdić, D. M., et al.
(författare)
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Novel mechanism for rapid spin flip with increasing in-plane wave vector in slightly asymmetric modulation-doped quantum wells
- 2008
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Ingår i: PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY. - : IOP Publishing.
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Konferensbidrag (refereegranskat)abstract
- We calculate the electric-field-induced spin splittings in wide slightly asymmetric modulation-doped quantum wells. When spin subbands are anticrossing we demonstrate twostep spin flips as the in-plane wave vector along the [11] direction is increased by 0.002 nm-1. At the beginning of this interval the y-component flips, at the end the x-component. Simultaneously the energy separation stays roughly constant below 1-eV and the wave functions are interchanged. A bias change of about 1 meV is sufficient to move the Fermi level from below to above the anticrossing region.
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| 9. |
- Gvozdic, Dejan M., et al.
(författare)
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Rapid spin flip in a spin subband at an anticrossing region in a slightly asymmetric modulation-doped quantum well
- 2008
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : Elsevier BV. - 1386-9477 .- 1873-1759. ; 40:6, s. 2081-2083
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Tidskriftsartikel (refereegranskat)abstract
- The spin splitting due to structure and bulk inversion asymmetry is calculated for electron subbands in wide slightly asymmetric InGaSb quantum wells. At anticrossings, rapid spin flips in two steps are found as the in-plane wave vector along the [1 1] direction is increased by 0.002 nm(-1). First the y-component and then the x-component is flipped. A change of bias of about 1 meV across the quantum well is sufficient move the Fermi level across the anticrossing region.
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| 10. |
- Gvozdic, D. M., et al.
(författare)
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Strong dependence of spin direction and wave function localization on In-plane wave vector in wide modulation-doped quantum wells
- 2007
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 61:1, s. 273-277
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Tidskriftsartikel (refereegranskat)abstract
- An important mechanism in spintronics is spin-splitting induced by structure and/or bulk inversion asymmetry. These effects are frequently assumed to depend on two parameters usually denoted by α and β respectively, and α is assumed to be proportional to some average electric field. We here demonstrate that the spatial dependence of the electric field gives very important effects absent in simpler models. These effects are particularly clear in wide modulation-doped quantum wells where there are two weakly interacting electron gases in the interface regions. Using an 8 × 8 k . p matrix approach we obtain anticrossings between interacting subbands at which the spin direction and/or wave function localization are found to be strong functions of the in-plane wave vector.
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| 11. |
- Gvozdic, D. M., et al.
(författare)
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Superefficient electric-field-induced spin-orbit splitting in strained p-type quantum wells
- 2006
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 73:6, s. 927-933
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Tidskriftsartikel (refereegranskat)abstract
- We investigate theoretically the efficiency of the Rashba effect, i.e. the spin-orbit splitting resulting from an electric field. In contrast to previous studies, where the carriers have usually been taken to be electrons, we focus on holes and are able to demonstrate remarkable improvements of the effect by several orders of magnitude. We also show that the frequently-neglected lattice-mismatch between GaAs and AlGaAs can be used to further enhance the efficiency of the wave vector splitting mechanism. The Rashba effect is the fundamental mechanism behind the Datta-Das spin transistor and we find that for a small electric field of 2 kV/cm the spin precession length becomes only 36nm.
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| 12. |
- Gvozdic, D. M., et al.
(författare)
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Superiority of p-type spin transistors
- 2006
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Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; T126, s. 21-26
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Tidskriftsartikel (refereegranskat)abstract
- The spintronic device that has probably stimulated the most 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 demonstrate that the relevant spin splitting in k-space can be made more than three orders of magnitude larger for holes than for electrons at the same electric field. This is partly achieved by utilizing the frequently neglected lattice-mismatch between GaAs and AlGaAs. We design heterostructures where this efficient Rashba effect should show up. Compared to present transistors, we conclude that electron-based spin transistors will have problems in becoming competitive but hole-based ones are much more promising.
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| 13. |
- Gvozdic, Dejan, et al.
(författare)
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Strong enhancement of Rashba effect in strained p-type quantum wells
- 2005
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Ingår i: Physics of Semiconductors, Pts A and B. - MELVILLE : AMER INST PHYSICS. - 0735402574 ; , s. 1423-1424
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Konferensbidrag (refereegranskat)abstract
- One of the most studied spintronic devices is the spin transistor proposed by Datta and Das. The mechanism behind this transistor is the Rashba effect: The inversion asymmetry caused by the gate voltage gives rise to a spin splitting. We show that the relevant spin splitting in k-space is typically two orders of magnitude larger in unstrained p-type quantum wells compared to n-type quantum wells. We also show that further order-of-magnitude improvement can be obtained by utilizing the frequently ignored lattice-mismatch between GaAs and AlGaAs.
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| 14. |
- Holmström, Petter, et al.
(författare)
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Efficient electroabsorption for mid-infrared wavelengths using intersubband transitions
- 2008
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Ingår i: PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY. - Bristol : IOP PUBLISHING LTD.
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Konferensbidrag (refereegranskat)abstract
- We have demonstrated efficient intersubband (IS) electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor phase epitaxy (MOVPE). An absorption modulation of 2300 cm(-1) at lambda=5.7 mu m due to Stark shift of the IS resonance was achieved with a low applied voltage swing of +/-0.5 V in a multipass waveguide structure. Two useful wavelength ranges of lambda approximate to 5.4-5.8 mu m and 6.3-6.6 mu m were obtained by considering the two flanks of the IS resonance. Based on the experimental results it is estimated that an electroabsorption modulator with a low peak-to-peak voltage of V-PP = 0.9 V can yield a modulation speed of f(3dB) = 120 GHz with the present material by using a strongly confining surface plasmon waveguide of 30 mu m length.
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| 15. |
- Holmström, Petter, et al.
(författare)
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Efficient infrared electroabsorption with 1 V applied voltage swing using intersubband transitions
- 2008
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 93:19
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Tidskriftsartikel (refereegranskat)abstract
- We have demonstrated efficient intersubband electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor-phase epitaxy. An absorption modulation of 6 dB (Delta alpha=2300 cm(-1)) at lambda similar to 5.7 mu m due to Stark shift of the intersubband resonance was achieved at a low applied voltage swing of +/- 0.5 V in a multipass waveguide structure. The interface intermixing was estimated by comparing experimental and theoretical Stark shifts. It is predicted that the present material in a strongly confining surface plasmon waveguide can yield an electroabsorption modulator with a peak-to-peak voltage of V-pp=0.9 V and modulation speed of f(3dB)approximate to 130 GHz.
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| 16. |
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| 17. |
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| 18. |
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| 19. |
- Liu, X. Y., et al.
(författare)
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Structural and optical properties of GaN/AlN multiple quantum wells for intersubband applications
- 2005
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 278:1-4, s. 397-401
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Tidskriftsartikel (refereegranskat)abstract
- GaN/AIN multiple quantum well structures of 1, 5 and 20 periods were grown by molecular beam epitaxy (MBE). To investigate structural parameters, symmetrical scan (0002) and reciprocal space mapping in the vicinity of the GaN (10 15) plane were made by X-ray diffraction (XRD). The layer thickness, composition and relaxation were determined and gave good agreement with simulated results. The 20 period multiple quantum well (MQW) sample exhibited an intersubband resonance at 360 meV, which corresponds well to the structure data determined by XRD.
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| 20. |
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