Browsing by Author "Chu, C. S."

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All-electric spin pumping in quantum channels with a single finger-gate capacitor
(2017-02-07)
Wang, L. Y.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
In this paper, we show that a single finger-gate capacitor (FGC) can generate pure spin pumping in a quantum channel (QC). Two dynamic fields, ac spin-orbit interaction and ac potential energy, both induced by the FGC onto the QC, are the agents driving the spin pumping. Smooth spatial profiles of the two ac fields are taken into account both perturbatively and full numerically for the nonadiabatic spin pumping. Our perturbative approach reveals that the spin-pumping mechanism is resonant sideband processes associated with simultaneous coupling of the two ac fields with traversing carriers. Full sideband-process treatment is carried out numerically by a time-dependent scattering matrix method. The same spin-pumping mechanism holds also for the case of a single finger-gated QC, albeit with smaller pumping amplitudes.
Asymmetric valley-resolved beam splitting and incident modes in slanted graphene junctions
(2016-01-18)
Hsieh, S. H.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
Electron injection into a graphene sheet through a slanted armchair graphene nanoribbon (AGNR) is investigated. An incident mode, or subband, in the AGNR is valley-unpolarized. Our attention is on the valley-resolved nature of the injected electron beams and its connection to the incident mode. It is known for a normal injection that an incident mode will split symmetrically into two valley-resolved beams of equal intensity. We show, in contrast, that slanted injections result in asymmetric valley-resolved beam splitting. The most asymmetric beam splitting cases, when one of the valley-resolved beams has basically disappeared, are found and the condition derived. This is shown not due to trigonal warping because it holds even in the low incident energy regime, as long as collimation allows. These most asymmetric beam splitting cases occur at energies within an energy interval near and include the subband edge of an incident mode. The physical picture is best illustrated by a projection of the slanted AGNR subband states onto that of the 2D graphene sheet. It follows that the disappearing of a valley-resolved beam coincides with the situation that the group velocities of the projected states in the corresponding valley are in backward directions. (C) 2016 AIP Publishing LLC.
Asymmetries in intrinsic spin-Hall effect in low in-plane magnetic field
(2008-10-01)
Wang, L. Y.
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Chu, C. S.
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Mal'shukov, A. G.
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電子物理學系
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Department of Electrophysics
Effects of low in-plane magnetic field on bulk spin densities and edge spin accumulations of a diffusive two-dimensional semiconductor stripe are studied. Focusing upon the Dresselhaus-type intrinsic spin-orbit interaction (SOI), we look for the symmetry, or asymmetry, characteristics in two magnetic-field orientations: along and transverse to the stripe. For longitudinal field, the out-of-plane spin density S(z) exhibits odd parity across the stripe and even parity in the magnetic field and is an edge accumulation. For transverse field, the out-of-plane S(z) becomes asymmetric in both spatial and field dependencies and has finite bulk values for finite magnetic fields. Our results support utilizing low in-plane magnetic fields for the probing of the underlying SOI.
Competing interplay between Rashba and cubic-k Dresselhaus spin-orbit interactions in spin-Hall effect
(2009-05-01)
Chang, R. S.
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Chu, C. S.
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Mal'shukov, A. G.
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電子物理學系
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Department of Electrophysics
Focusing on the interplay between the Rashba and cubic-k Dresselhaus spin-orbit interactions (SOI), we calculate the spin accumulation S-z and the spin polarizations S-i(B) at, respectively, the lateral edges and in the bulk of the two-dimensional electron gas. Their dependences on both the ratio between the Rashba and the Dresselhaus SOI coupling constants and the electron densities are studied systematically. Strong competition features in S-z are found. In the Dresselhaus-dominated regime S-z changes sign when the electron density is large enough. In the Rashba-dominated regime S-z is essentially suppressed. Most surprising is our finding that the Rashba-dominated regime occurs when alpha approximate to 2 beta, where alpha and beta are the Rashba and the effective linear-k Dresselhaus SOI coupling constants, respectively. For the spin polarizations S-i(B), the Rashba-dominated regime occurs when alpha >=beta. Our results point out that decreasing parallel to alpha parallel to leads to the restoration of the spin accumulation S-z.
Gate-induced localized states in graphene: Topological nature in their formation
(2018-12-26)
Wang, L. Y.
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Chang, Che-Yuan
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
In this work, we show that an N-C-chain gate potential along the armchair chain (direction (y) over cap) of a graphene sheet gives rise to topological localized states (LSs): one branch for N-C = 1 and two branches for N-C >= 2. These LSs are shown to form whenever the gate-induced potential V-0 is nonzero. The topological nature behind the formation of these LSs is revealed (for N-C = 1, 2) by showing, for V-0 not equal 0, that the LS-secular equation can be cast into a pseudospin-rotation form on which rotation upon a valley-associated pseudospin is to equate with another valley-associated pseudospin. Both pseudospins are on the same side of the gate potential. That the rotation angle of the pseudospin-rotation operator falls within the range of variation of the relative angle Delta theta(p) between the two pseudospins, as the energy E varies across the entire energy gap for a given k(y), demonstrates the topological nature and the inevitability of the LS branch formation. These topological LS branches exhibit Dirac-point characteristics, with dispersion relations leading out from the Dirac point (at k(y) = 0). For general multiple (N-C > 1) carbon chain gate-potential cases, the number N-LS of LS branches are found to increase with V-0, up to a maximum of N-LS,(max) = N-C. Yet LS branches carrying the Dirac-point characteristics are found to be fixed at two.
Large RKKY coupling from multiple scattering in armchair graphene nanoribbons
(2020-05-12)
Van Minh Nguyen
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We consider the Ruderman-Kittel-Kasuya-Yosida (RKKY) type coupling between two magnetic impurities on metallic armchair graphene nanoribbons (AGNR). Our key findings are that the lowest-order RKKY (LO-RKKY) coupling does not describe the coupling physics, and a full implementation of electron multiple scattering in RKKY coupling leads to orders of magnitude enhancement in the coupling magnitude. Key physics of our findings are revealed in our analysis of the LO-RKKY coupling. Within the Born approximation for the electron-magnetic-impurity scattering, the LO-RKKY coupling has inherited the singular density of states feature so that incident electrons at AGNR-subband band edges give finite contributions. Our other finding, that contribution from a gapped subband to LO-RKKY coupling is very small, implies then that severe cancellations must have occurred between states at its subband band edges and states not at its subband band edges. Magnitudes of the cancellation terms are each of the same order as the LO-RKKY coupling. This cancellation no longer occurs when full multiple scattering is reinstated. Contributions to the RKKY coupling from states at subband band edges are entirely suppressed, due to the restoring of the finiteness in the wave functions of these states. Contributions from states not from the subband band edges, though modified, retain their orders of magnitude. Analytic expressions have been obtained to further illustrate the physics.
Localized states in continuum in low-dimensional systems
(2006-10-01)
Voo, Khee-Kyun
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
It is shown in this paper that for open systems, states that are localized in space, discrete in energy, and embedded in the continuum of extended states can be sustained by low-dimensional and channeled leads. These states have an origin different from that of analogous states discussed by von Neumann and Wigner [Phys. Z. 30, 465 (1929)]. A few representative systems are discussed. These states cause, for example, infinitely sharp Fano resonance in transport when they are marginally destroyed.
Nodal adsorbate bound states in armchair graphene nanoribbons: Fano resonances and adsorbate recognition in weak disorder
(2014-11-21)
Chiu, C. H.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We consider adsorbates on metallic armchair-graphene nanoribbons (AGNRs). An adsorbed atom on a nodal site of the gapless subband in the AGNR is found to induce a bound state. The nodal-adsorbate bound state alone does not cause reflection in the AGNR quantum transport in the one propagating-channel regime. Yet its manifestation, as a Fano resonance in G, is brought forth by the presence of a non-nodal adsorbate (or a scatterer) located on a longitudinally separated site. Both the adsorbate-dependent nature and the significantly enhanced energy resolution of the Fano structure in G invite adsorbate recognition. The Fano peak is shown to be robust against weak disorder such as from edge vacancies, due to the localized nature of the Fano resonant state. Adsorbates F and OH, described within an extended Huckel model, are considered as examples.
Nonuniversality of the intrinsic inverse spin-Hall effect in diffusive systems
(2012-04-02)
Wang, L. Y.
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Mal'shukov, A. G.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We studied the electric current induced in a two-dimensional electron gas by the spin current, in the presence of Rashba and cubic Dresselhaus spin-orbit interactions. We found out that the factor relating the electric and spin currents is not universal, but rather depends on the origin of the spin current. Drastic distinction has been found between two cases: the spin current created by diffusion of an inhomogeneous spin density, and the pure homogeneous spin current. We found that in the former case the inverse spin-Hall effect electric current is finite, while it turns to zero in the latter case, if the spin-orbit coupling is represented by Rashba interaction.
Rashba-type spin accumulation near a void at a system edge
(2011-09-30)
Wang, L. Y.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We consider the spatial distribution of spin accumulation S(z) near a void (edge void) located at a system edge. The edge void is semicircular in shape with a radius R(0) approximate to l(so), the spin-relaxation length, and it is formed out of a Rashba-type two-dimensional electron system in the diffusive regime. The nonuniform driving field provides the essential condition, and diffusive contributions to the spin currents from spin polarizations provide the primary impetus for the spin accumulation. The edge void reveals an underlying asymmetry of the bulk-void counterpart, namely, a finite bulklike spin flow at the system edge. This bulklike spin flow gives rise to spin accumulation when open boundary occurs. The bulklike spin flow is found to exhibit a similar spatial profile as that of Sz at the system edge. This bulklike spin flow is expected to be a primary impetus for local spin injection at the sample edge near the edge void when a local protrusion occurs. Spin orientation of the bulklike spin flow varies with the locations on the edge-void boundary.
Resonant spin dipole induced by an in-plane potential gradient spin-orbit interaction
(2007-10-01)
Chen, K. Y.
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Chu, C. S.
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Mal'shukov, A. G.
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電子物理學系
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Department of Electrophysics
Spin-orbit interaction (SOI) arising from in-plane potential gradient is invoked for the generation of spin accumulation in a driven electric field. The SOI and a local in-plane potential pattern together bring about resonant spin dependent scatterings to electrons in a nonequilibrium distribution. In the vicinity of a ring-shaped potential barrier pattern, a spin dipole distribution with a resonant dipole strength characteristic is obtained. As the chemical potential mu is increased across one such resonant energy, the dipole strength manifests both sign reversal and large amplitude enhancement. The scattering resonance, thus, provides an additional knob for the manipulation of the spin accumulation.
Robust level coincidences in the subband structure of quasi-2D systems
(2012-12-01)
Winkler, R.
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Wang, L. Y.
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Lin, Y. H.
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Chu, C. S.
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電子物理學系
Recently, level crossings in the energy bands of crystals have been identified as a key signature for topological phase transitions. Using realistic models we show that the parameter space controlling the occurrence of level coincidences in energy bands has a much richer structure than anticipated previously. In particular, we identify robust level coincidences that cannot be removed by a small perturbation of the Hamiltonian compatible with the crystal symmetry. Different topological phases that are insulating in the bulk are then separated by a gapless (metallic) phase. We consider HgTe/CdTe quantum wells as a specific example. (c) 2012 Elsevier Ltd. All rights reserved.
Spin cloud induced around an elastic scatterer by the intrinsic spin Hall effect
(2006-08-18)
Mal'shukov, A. G.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
Similar to the Landauer electric dipole created around an impurity by the electric current, a spin polarized cloud of electrons can be induced by the intrinsic spin Hall effect near a spin independent elastic scatterer. It is shown that in the ballistic range around the impurity, such a cloud appears in the case of Rashba spin-orbit interaction, even though the bulk spin Hall current is absent.
Spin generation in a Rashba-type diffusive electron system by nonuniform driving field
(2010-03-01)
Wang, L. Y.
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Chu, C. S.
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Mal'shukov, A. G.
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電子物理學系
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Department of Electrophysics
We show that the Rashba spin-orbit interaction contributes to edge spin accumulation S(z) in a diffusive regime when the driving field is nonuniform. Specifically, we solve the case of nonuniform driving field in the vicinity of a circular void locating in a two-dimensional electron system and we identify the key physical process leading to the edge spin accumulation. The void has radius R(0) in the range of spin-relaxation length l(so) and is far from both source and drain electrodes. The key physical process we find is originated from the nonuniform in-plane spin polarizations. Their subsequent diffusive contribution to spin current provides the impetus for the edge spin accumulation S(z) at the void boundary. The edge spin accumulation is proportional to the Rashba coupling constant alpha and is in a spin-dipole form oriented transversely to the driving field. We expect similar spin accumulation to occur if the void is at the sample edge.
Spin Hall effect in a Josephson contact
(2008-09-01)
Mal'shukov, A. G.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
The spin Hall effect on the Josephson tunneling through a two-dimensional normal contact with a spin-orbit split conduction band has been studied in the diffusive regime and at the zero electric bias. Linearized Usadel equations for triplet components of the pairing function in the presence of intrinsic spin-orbit interaction have been derived. These equations have been employed for analysis of the spin Hall effect induced by the super-current. We predict that a nondissipative out-of-plane spin Hall polarization accumulates at lateral edges and an in-plane polarization is induced throughout the entire normal region. At the same time, in contrast to the spin Hall effect in normal systems, the spin current is absent in the considered case of the stationary Josephson effect.
Spin orientation and spin-Hall effect induced by tunneling electrons
(2007-12-01)
Mal'shukov, A. G.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
It is shown that a flux of unpolarized electrons across a symmetric double barrier quantum well induces a spin polarization inside the well. Besides, the transmitted current acquires a spin polarized component and the spin-Hall current flows in the planar direction. These phenomena are due to a combined effect of Dresselhaus interaction and the spin-orbit interaction induced by gradients of heterostructure material parameters. In contrast to previous studies of the spin filtering effect, we predict that it can be observed in the case of an isotropic distribution of incident electrons.
Spin-Hall current and spin polarization in an electrically biased SNS Josephson junction
(2011-08-10)
Mal'shukov, A. G.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
Periodic in time spin-Hall current and spin polarization induced by a dc electric bias have been calculated in a superconductor-normal two-dimensional electron gas-superconductor (SNS) Josephson junction. We assumed that the band energies of electrons in the normal system are split due to Rashba spin-orbit coupling. The transport parameters have been calculated within the diffusion approximation and by using perturbation expansion over a small SN contact transparency. We found out that, in contrast to the stationary Josephson effect, the spin-Hall current does not turn to zero. Besides a direct proximity effect caused by a Cooper pair's transition into a triplet state, the spin current and polarization are also driven by a periodic electric field associated with the charge imbalance.
Spin-Hall interface resistance in terms of Landauer-type spin dipoles
(2007-02-01)
Mal'shukov, A. G.
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Wang, L. Y.
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Chu, C. S.
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電子物理學系
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Department of Electrophysics
We considered the nonequilibrium spin dipoles induced around spin-independent elastic scatterers by the intrinsic spin-Hall effect associated with the Rashba spin-orbit coupling. The spin polarization normal to the two-dimensional electron gas (2DEG) has been calculated in the diffusion range around the scatterer. Although around each impurity this polarization is finite, we found that the corresponding macroscopic spin density obtained via averaging of individual spin dipole distributions over impurity positions is zero in the bulk. At the same time, the spin density is finite near the boundary of 2DEG, except for a special case of a hard wall boundary, that is, when it turns to 0. The boundary value of the spin polarization can be associated with the interface spin-Hall resistance determining the additional energy dissipation due to spin accumulation.
Temporal universal conductance fluctuations in RuO(2) nanowires due to mobile defects
(2011-10-17)
Lien, An-Shao
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Wang, L. Y.
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Chu, C. S.
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Lin, Juhn-Jong
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電子物理學系
Temporal universal conductance fluctuations (TUCF's) are observed in RuO(2) nanowires at cryogenic temperatures. The fluctuations persist up to very high T similar to 10 K. Their root-mean-square magnitudes increase with decreasing T, reaching similar to 0.2e(2)/h at T less than or similar to 2 K. These fluctuations are shown to originate from scattering of conduction electrons with rich amounts of mobile defects in artificially synthesized metal oxide nanowires. TUCF characteristics in both one-dimensional saturated and unsaturated regimes are identified and explained in terms of current theories. Furthermore, the TUCF's as a probe for the characteristic time scales of the mobile defects (two-level systems) are discussed.
Temporal universal conductance fluctuations in RuO2 nanowires due to mobile defects
(2011-10-17)
Lien, An-Shao
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Wang, L. Y.
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Chu, C. S.
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Lin, Juhn-Jong
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電子物理學系
Temporal universal conductance fluctuations (TUCF's) are observed in RuO2 nanowires at cryogenic temperatures. The fluctuations persist up to very high T similar to 10 K. Their root-mean-square magnitudes increase with decreasing T, reaching similar to 0.2e(2)/h at T less than or similar to 2 K. These fluctuations are shown to originate from scattering of conduction electrons with rich amounts of mobile defects in artificially synthesized metal oxide nanowires. TUCF characteristics in both one-dimensional saturated and unsaturated regimes are identified and explained in terms of current theories. Furthermore, the TUCF's as a probe for the characteristic time scales of the mobile defects (two-level systems) are discussed.