| 1. |
- Brusheim, Patrik, et al.
(author)
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Field-driven geometrical phases in a time-periodic quantum system
- 2009
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 79:20
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Journal article (peer-reviewed)abstract
- We apply Floquet theory to explore the geometry of the Hilbert space under the influence of a time-periodic field. The geometrical phase is found to be induced by field-driven hybridizations when the photon energy of the driving field is close to the transition energies of the states of a quantum system. The phases of two hybridized states are phase locked to each other. We show that the geometrical phase is in general related to the Rabi frequency of the hybrid states. We also show that when the photon energy is equal to the transition energy of two states the geometrical phase acquired by each state is given exactly by an integer multiple of pi, independent of the strength of the driving field. We illustrate the derived generic properties of the geometric phase with an experimentally realizable quantum-wire system. It is shown that the interference between conductance channels in the wire presents a way to identify the geometrical phase.
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| 2. |
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| 3. |
- Brusheim, Patrik, et al.
(author)
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Multiterminal multimode spin-dependent scattering matrix formalism: Electron and hole quantum spin transport in multiterminal junctions
- 2008
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:8
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Journal article (peer-reviewed)abstract
- We present a derivation of a scattering matrix method providing an exact multimode solution to spin-dependent quantum transport in multiterminal structures. The method is formulated in a general language such that it can readily be applied to any spin-S system with spin interactions. We apply the formalism to spin-1/2 electron and spin-3/2 hole transport in three- and four-terminal structures. It is shown that the existence of a third lead lifts constraints on the flux polarization of two-terminal electron transport. A spin-rectification property in a three-terminal system with Rashba spin-orbit interaction is demonstrated. We furthermore find that a four-terminal structure can partition a fully spin-polarized electron flux into two oppositely polarized fluxes. For holes, we calculate the polarization vector of both the injected states as well as the outgoing states in a three-terminal structure. Close to the onset of propagating channels, the hole polarization exhibits peak-dip structures attributed to the angular-momentum dependent Fano resonances in the three-terminal junction. We rigorously show that when the outgoing state is restricted to a single channel, the polarization is uniquely determined by the outgoing lead state, independent of the scattering details of the structure.
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| 4. |
- Brusheim, Patrik, et al.
(author)
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Spin filtering devices based on ferromagnetic stripe modulated double quantum-dot structures
- 2006
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 73:4
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Journal article (peer-reviewed)abstract
- We report on a theoretical study of spin-dependent electron transport in double quantum dot structures, made from a two-dimensional electron gas, with a local magnetic field modulation. Spin-dependent conductance and probability density of electrons in the structures are calculated and the underlying physics of the results is discussed. We include in the study not only the magnetic field component perpendicular to the two-dimensional electron gas plane, but also a consideration of the in-plane component of the magnetic field. It is shown that giant spin polarization (similar to 100%) of the conductance, with tunable spin polarity, can be achieved with the double-dot structures. It is also shown that the structures can be used as efficient spin filtering devices at temperatures well above that defined by the spin splitting energy.
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| 5. |
- Brusheim, Patrik, et al.
(author)
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Spin Hall effect and zitterbewegung in an electron waveguide
- 2006
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 74:20
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Journal article (peer-reviewed)abstract
- We study spin-resolved probability distributions for electrons in a multichannel waveguide in the presence of a spin-orbit interaction. For a spin-polarized electron injection, a zitterbewegung pattern is predicted in the probability distribution of electrons in the waveguide. For a spin-unpolarized injection, the spin-resolved electron probability in the waveguide shows spin accumulations. In addition to the spin Hall phenomenon-namely, accumulations of opposite spins at the lateral edges of the waveguide-we predict the existence of a regular stripe pattern of spin accumulations in the internal region of the waveguide. We show that the predicted zitterbewegung and spin Hall effect stem from the same mechanism and are formed from coherent states of electrons in the waveguide.
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| 6. |
- Brusheim, Patrik, et al.
(author)
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Spin transport and spin Hall effect in an electron waveguide in the presence of an in-plane magnetic field and spin-orbit interaction
- 2007
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 75:19
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Journal article (peer-reviewed)abstract
- We study electron spin transport and the spin Hall effect in a quantum waveguide in the presence of Rashba spin-orbit interaction (SOI) and a homogeneous in-plane magnetic field, as well as the energy dispersion relation and spin polarization properties of the electron states in the corresponding infinite system. A general expression for the spin polarization of the outgoing flux, resulting from an arbitrarily polarized injection source, is derived for the quantum waveguide without the assumption of spin-degenerate leads. We further derive constraints on the spin-resolved transmission coefficients by analyzing the symmetries of the system. These constraints will impose restrictions on the spin-dependent conductances as well as the spin polarization of the outgoing flux. For an applied in-plane magnetic field parallel to the waveguide, conductance oscillations as a function of the SOI strength are diminished when the Zeeman energy exceeds the Rashba energy, due to spin alignment with the applied magnetic field. Furthermore, the calculated spin and charge probability distributions in the SOI waveguide region show spin Hall patterns and zitterbewegung patterns, respectively. For an applied transverse in-plane magnetic field, the SOI induced effective magnetic field is parallel or antiparallel with the applied field. The conductance of the system is then dominantly determined by spin-conserved transport processes, and spin-flipped transport processes can occur only through interchannel scattering and are therefore greatly suppressed. Furthermore, no zitterbewegung pattern in the charge density distribution can be found in the SOI region when the electrons are injected from a lead with spin polarization along the applied in-plane transverse field direction. However, the spin polarization probability distribution can still show a spin Hall pattern in the SOI region and the sign of the spin polarization in the spin Hall pattern can be reversed by tuning the applied magnetic field strength. For the corresponding infinite waveguide systems with uniform SOI and in-plane magnetic field, we show that the interplay between the applied in-plane magnetic field and the SOI induced effective magnetic field, together with subband hybridizations, will create a wave-vector-dependent spin polarization of the Bloch states with rich features and that these features cannot be captured by employing a simple one-dimensional model.
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| 7. |
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| 8. |
- Brusheim, Patrik, et al.
(author)
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Symmetry of hole spin transport in a two-terminal quantum system
- 2006
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 74:23
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Journal article (peer-reviewed)abstract
- We study two-terminal hole spin transport in a p-type, light-hole-heavy-hole coupled, quantum-well system under the influence of Rashba spin-orbit interaction. An expression for the spin polarization of the outgoing hole flux is derived. It is shown that the in-plane spin polarization vanishes identically whenever the outgoing lead supports only heavy-hole channels. This result is independent of the details of the system. By analyzing the symmetries of the system, constraints on the spin-dependent transmission amplitudes are derived. It is shown that under time-reversal symmetry, the total hole spin polarization vanishes whenever the outgoing lead supports only one spin-degenerate hole channel, which is also independent of the details of the system. It is further shown that generally it is difficult to achieve spin polarized currents from a laterally defined Rashba hole device without breaking its transverse reflection symmetry.
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| 9. |
- Brusheim, Patrik
(author)
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Theoretical Studies of Spin-Dependent Quantum Phenomena in Semiconductor Nanostructures
- 2008
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Doctoral thesis (other academic/artistic)abstract
- The study of spin transport has emerged at the forefront of condensed matter physics during the last decade. Much of the interest has undoubtedly been sparked by the advent of the spintronics paradigm and the idea of creating novel electronic devices based on the spin degree of freedom of the carriers. However, a great deal of interest is due to the rich physics that emerges from the study of spin phenomena. In this thesis we will study quantum spin-dependent phenomena as it appears in nano-scale structures formed in semiconducting materials. We will set up a formalism for quantum spin transport and apply it to investigate novel coherent spin phenomena such as the spin Hall effect and Zitterbewegung, as well as spin conductance and polarisation in mesoscopic structures. Symmetry relations are derived which sets rigorous constraints on the spin transport properties. We will furthermore consider many-body effects in spin-orbit coupled systems and coherent geometrical phase properties due to interaction with a time-periodic field. Computational techniques based on the scattering matrix formalism have been developed for spin-dependent transport in quantum structures with spin-orbit interaction and local magnetic field modulations. The techniques have been applied to the studies of the spin Hall effect and Zitterbewegung in both electron and the less studied hole systems, multi-terminal spin-dependent transport with spin-orbit interaction, and the problem of creating a source of spin-polarised carriers. In the presence of spin-orbit interactions the spin Hall effect arises as a spin accumulation along the transverse edges of a waveguide, and as a spin-stripe pattern in the internal part of the waveguide. We show that the Zitterbewegung is directly linked to the spin Hall effect and only differ in the injection conditions. We further find that flux polarisation in a multi-terminal structure is strongly influenced by the existence of multiple lead channels and give rise to spin-rectification and spin-partitioning. A source of spin-polarised carriers is proposed by demonstrating that a current of large spin polarisation, at a temperature above the spin-splitting temperature, can be achieved in a double-dot structure influenced by a non-uniform magnetic field. Symmetry considerations are powerful tools to derive rigorous constraints on spin transport properties. By a symmetry analysis we show that the in-plane polarisation of hole transport necessarily vanishes whenever the outgoing lead only supports heavy hole channels. It is also shown that the complete spin-polarisation vanishes identically under time-reversal symmetry whenever the outgoing lead supports one, doubly degenerate, hole channel independent of the number of incoming channels or the details of the scatterer. On the topic of interactions, recent measurements of Coulomb blockade of holes in nanowire quantum dots are analysed by formulating a pseudo-spin Hartree-Fock language to extract a hole exchange interaction energy. We furthermore study the geometrical response of quantum confined particles to a time-periodic electric field and show that field-induced angular momentum and spin transitions are associated with a geometrical phase indicating a non-trivial topology of the projective Hilbert space.
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| 10. |
- Csontos, D., et al.
(author)
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Lande-like formula for the g factors of hole-nanowire subband edges
- 2008
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:3
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Journal article (peer-reviewed)abstract
- We have analyzed theoretically the Zeeman splitting of hole-quantum-wire subband edges. As is typical for any bound state, their g factor depends on both an intrinsic g factor of the material and an additional contribution arising from a finite bound-state orbital angular momentum. We discuss the quantum-confinement-induced interplay between bulk-material and orbital effects, which is nontrivial due to the presence of strong spin-orbit coupling. A compact analytical formula is provided that elucidates this interplay and can be useful for predicting Zeeman splitting in generic hole-wire geometries.
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| 11. |
- Csontos, D., et al.
(author)
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Spin-3/2 physics of semiconductor hole nanowires: Valence-band mixing and tunable interplay between bulk-material and orbital bound-state spin splittings
- 2009
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 79:15
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Journal article (peer-reviewed)abstract
- We present a detailed theoretical study of the electronic spectrum and Zeeman splitting in hole quantum wires. The spin-3/2 character of the topmost bulk-valence-band states results in a strong variation in subband-edge g factors between different subbands. We elucidate the interplay between quantum confinement and heavy-hole-light-hole mixing and identify a certain robustness displayed by low-lying hole-wire subband edges with respect to changes in the shape or strength of the wire potential. The ability to address individual subband edges in, e.g., transport or optical experiments enables the study of hole states with nonstandard spin polarization, which do not exist in spin-3/2 systems. Changing the aspect ratio of hole wires with rectangular cross section turns out to strongly affect the g factor of subband edges, providing an opportunity for versatile in situ tuning of hole-spin properties with possible application in spintronics. The relative importance of cubic crystal symmetry is discussed, as well as the spin splitting away from zone-center subband edges.
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| 12. |
- Fuhrer, Andreas, et al.
(author)
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Fano effect in a quantum-ring-quantum-dot system with tunable coupling
- 2006
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 73:20
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Journal article (peer-reviewed)abstract
- Transport measurements are presented on a quantum ring that is tunnel-coupled to a quantum dot. When the dot is in the Coulomb blockade regime, but strongly coupled to the open ring, Fano line shapes are observed in the current through the ring, when the electron number in the dot changes by 1. The symmetry of the Fano resonances is found to depend on the magnetic flux penetrating the area of the ring and on the strength of the ring-dot coupling. At temperatures above T=0.65 K the Fano effect disappears, while the Aharonov-Bohm interference in the ring persists up to T=4.2 K. Good agreement is found between these experimental observations and a single-channel scattering matrix model including decoherence in the dot.
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| 13. |
- Roddaro, Stefano, et al.
(author)
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Spin States of holes in ge/si nanowire quantum dots.
- 2008
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In: Physical Review Letters. - 1079-7114. ; 101:18
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Journal article (peer-reviewed)abstract
- We investigate tunable hole quantum dots defined by surface gating Ge/Si core-shell nanowire heterostructures. In single level Coulomb-blockade transport measurements at low temperatures spin doublets are found, which become sequentially filled by holes. Magnetotransport measurements allow us to extract a g factor g;{*} approximately 2 close to the value of a free spin-1/2 particle in the case of the smallest dot. In less confined quantum dots smaller g factor values are observed. This indicates a lifting of the expected strong spin-orbit interaction effects in the valence band for holes confined in small enough quantum dots. By comparing the excitation spectrum with the addition spectrum we tentatively identify a hole exchange interaction strength chi approximately 130 mueV.
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| 14. |
- Sun, Jie, et al.
(author)
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Frequency mixing and phase detection functionalities of three-terminal ballistic junctions
- 2007
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In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 18:19
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Journal article (peer-reviewed)abstract
- Three-terminal ballistic junctions (TBJs) are fabricated from a high-mobility InP/In0.75Ga0.25As heterostructure by electron-beam lithography. The voltage output from the central branch is measured as a function of the voltages applied to the left and right branches of the TBJs. The measurements show that the TBJs possess an intrinsic nonlinearity. Based on this nonlinearity, a novel room-temperature functional frequency mixer and phase detector are realized. The TBJ frequency mixer and phase detector are expected to have advantages over traditional circuits in terms of simple structure, small size and high speed, and can be used as a new type of building block in nanoelectronics.
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| 15. |
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| 16. |
- Sun, Jie, et al.
(author)
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Novel nanoelectronic device applications based on the nonlinearity of three-terminal ballistic junctions
- 2007
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In: Physics of Semiconductors, Pts A and B. - : AIP. - 0094-243X .- 1551-7616. ; 893, s. 1471-1472
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Conference paper (peer-reviewed)abstract
- Nanometer-scale electron devices containing three-terminal ballistic junctions are fabricated by electron-beam lithography on InP/InGaAs two-dimensional electron gas materials. Based on the intrinsic nonlinearity of the devices, frequency mixer, phase detector and RS flip-flop memory functioning at room temperature are successfully achieved. The devices have simple structure layout and small size, and are expected to function at high speed.
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| 17. |
- Sun, Jie, et al.
(author)
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Novel room-temperature functional analogue and digital nanoelectronic circuits based on three-terminal ballistic junctions and planar quantum-wire transistors
- 2008
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In: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6596 .- 1742-6588. ; 100, s. 052073-052073
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Conference paper (peer-reviewed)abstract
- Three-Terminal ballistic junctions (TBJs) and planar quantum-wire transistors (QWTs) are emerging nanoelectronic devices with various novel electrical properties. In this work, we realize novel nanoelectronic analogue and digital circuits with TBJs and planar QWTs made on In0.75Ga0.25As/InP two-dimensional electron gas (2DEG) material. First we show that a single TBJ can work as a frequency mixer or a phase detector. Second, we fabricate an integrated nanostructure containing two planar QWTs, which can be used as an RS flip-flop element. Third, we make a nanoelectronic circuit by the integration of two TBJs and two planar QWTs. This circuit shows the RS flip-flop functionalities with much larger noise margins in both high and low level inputs. All measurements in this work are done at room temperature.
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| 18. |
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| 19. |
- Zhang, Lebo, et al.
(author)
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Multimode electron transport through quantum waveguides with spin-orbit interaction modulation: Applications of the scattering matrix formalism
- 2005
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 72:4
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Journal article (peer-reviewed)abstract
- We present a formulation of the scattering matrix method for spin-dependent electron transport in a quantum waveguide with spin-orbit interaction (SOI). All the required Hamiltonian matrices needed in the implementation of the formulation are represented in a basis of the transverse spatial eigenstates and the spin eigenstates of the leads. Thus the method has great flexibility and can be easily applied to systems with complex geometrical structure, potential distribution, and SOI strength profile. Also, the method is numerically stable and can be used to treat spin-dependent multisubband scattering processes accurately. We have applied the method to the spin-dependent electron transport in quasi-one-dimensional (Q1D) conductors, with a region of the Rashba SOI of uniform strength and with a region containing a Rashba SOI superlattice, made from a semiconductor heterostructure. The total conductance, spin-dependent conductances, and spin polarization of the system are calculated for a fully spin-polarized electron beam injected from a lead into the SOI region. For the Q1D conductor with a single region of the Rashba SOI, it is found that when the Fermi energy is set at a value, for which the total conductance is at a plateau, the spin-dependent conductances show regular oscillations with increasing SOI strength. This is approximately true even when the total conductance is at a high plateau and thus multiple subbands in the waveguide are open for conduction. However, when the Fermi energy is set at a value close to the onset of a subband (with the subband index n >= 2), the spin-polarized conductances plotted against the SOI strength and the SOI region length show sharp resonance features or complex fluctuations. These irregular conductance characteristics arise from SOI-induced strong coupling between subbands. For the Q1D conductor modulated by an array of strong Rashba SOI stripes, the total conductance shows regular superlattice behavior, while the spin-dependent conductances show complex behavior with regions of slow oscillations and regions of rapid oscillations. As in the Q1D conductor with a single SOI region, the slow oscillations are found in the energy regions where the total conductance is at plateaus. However, the rapid oscillations appear at energies close to the onsets of subbands with the subband index n >= 2. These oscillations originate from strong spin scattering by localized states formed in the SOI-modulated superlattice region.
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