Browsing by Author "Chu, CS"

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Andreev-level tunneling in a ballistic double superconductor normal-metal-superconductor junction
(1997-03-01)
Chang, VCY
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
The supercurrent in a mesoscopic, ballistic, and symmetrically stacked, double superconductor-normal-metal-superconductor (SNS) junction is studied. Our focus is to establish the features in the supercurrent that are associated with the Andreev-level tunneling between the two SNS junctions. Both the junction geometry dependence and the gap function dependence are studied and are analyzed in terms of the quasiparticle scattering processes. The quasiparticle scattering is worked out analytically by solving the Bogoliubov-de Gennes equation within the Andreev approximation. The current is calculated, following Beenakker and van Houten [Phys. Rev. Lett. 66, 3056 (1991)], and the result is found to differ from that of the transmission approach. Such discrepancy between the two approaches, however, is found to disappear for a single SNS junction. In our result, the currents in the two normal regions are checked to be the same.
Coherent quantum transport in narrow constrictions in the presence of a finite-range longitudinally polarized time-dependent field
(1999-07-15)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We have studied the quantum transport in a narrow constriction acted upon by a finite-range longitudinally polarized time-dependent electric field. The electric field induces coherent inelastic scatterings that involve both intrasubband and intersideband transitions. Subsequently, the dc conductance G is found to exhibit suppressed features. These features are recognized as the quasi-bound-state (QBS) features that are associated with electrons making transitions to the vicinity of a subband bottom, of which the density of states is singular. Having valleylike instead of diplike structures, these QBS features are different from the G characteristics for constrictions acted upon by a finite-range time-modulated potential. In addition, the subband bottoms in the time-dependent electric-field region are shifted upward by an energy proportional to the square of the electric field and inversely proportional to the square of the frequency. This effective potential barrier is originated from the square of the vector potential, and it leads to the interesting field-sensitive QBS features. An experimental setup is proposed for the observation of these features. [S0163-1829(99)01827-5].
Coherent quantum transport in the presence of a finite-range transversely polarized time-dependent field
(2000-10-01)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
This work investigates the quantum transport in a narrow constriction acted upon by a finite-range transversely polarized time-dependent field. A generalized scattering-matrix method is developed that has incorporated a time-dependent mode-matching scheme. The transverse field induces coherent inelastic scatterings that include both intersubband and intersideband transitions. These scatterings give rise to the DC conductance G a general suppressed feature that escalates with the chemical potential. In addition, particular suppressed features - the dip structures - are found in G. These features are recognized as the quasibound state (QBS) features that arise from electrons making intersubband transitions to the vicinity of a subband bottom. For the case of larger held intensities, the QBS features that involve more photons are more evident. These QBS features are closely associated with the singular density of states at the subband threshold. An experimental setup is proposed for the observation of these features. (C) 2000 Elsevier Science B.V. All rights reserved.
Connecting wave functions at a three-leg junction of one-dimensional channels
(2006-01-01)
Voo, KK
;
Chen, SC
;
Tang, CS
;
Chu, CS
;
電子物理學系
We propose a scheme to connect the wave functions on different one-dimensional branches of a three-leg junction (Y junction). Our scheme differs from that due to Griffith [Trans. Faraday Soc. 49, 345 (1953)] in the respect that ours can model the difference in the widths of the quasi-one-dimensional channels in different systems. We test our scheme by comparing results from a doubly connected one-dimensional system and a related quasi-one-dimensional system, and we find a good agreement. Therefore our scheme may be useful in the construction of one-dimensional effective models out of (multiply connected) quasi-one-dimensional systems.
Critical current of a ballistic asymmetric SNSNS junction
(1998)
Chang, VCY
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
The critical current through an asymmetrically stacked, double superconductor-normal-metal-superconduct (SNS) junction is studied in the mesoscopic and ballistic regime. The asymmetry in such SNSNS junctions excludes any intuitive choice for the phase phi(2) of the middle superconductor S-2. We propose to determine phi(2) from the condition that the currents in the two normal regions are the same. The Bogoliubov-de Gennes equation is solved analytically within the Andreev approximation. In the long S2 limit, we find that the current-phase relation (CPR) can be constructed directly from the CPRs of the two constituent SNS junctions, of which the longer SNS junction imposes a cutoff to the critical current. In short S-2 cases, the cutoff feature remains, but the cutoff value is affected by the Andreev-level tunneling. The dependencies of the SNSNS critical current on the length of S-2, and the asymmetry in the configuration are studied. (C) 1998 Elsevier Science Ltd. All rights reserved.
Current-phase relation of a ballistic asymmetric double superconductor-normal-metal-superconductor junction
(1998-09-01)
Chang, VCY
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
The zero-temperature current-phase (I-phi) relation (CPR) of a mesoscopic, ballistic, and asymmetrically stacked, double superconductor-normal-metal-superconductor (SNS) junction is studied. Here phi is the phase difference between the two superconducting end-electrodes. The lack of configuration symmetry in such asymmetric SNSNS junctions forbids a simple choice of values for the phase phi(2), of the middle superconductor. We propose to determine the values of phi(2), by equating the currents in the two normal regions. Two features in the CPR are found. First, the CPR of the asymmetric junction has a cutoff feature, whose origin is best demonstrated in the long middle superconductor (large L-2) cases, when the critical current of the double SNS junction is bounded by the SNS junction that has the longer normal region. This cutoff feature is more pronounced for longer L-2 and for higher degree of junction asymmetry. Second, in regions other than the cutoff region, the CPR of the asymmetric junction deviates only within a few percent from the CPR of the symmetric junction which has the same total length L-Total in the normal regions and the same L-2. This is in contrast with the greater sensitivity the CPR has to the changes in L-Total or L-2. (C) 1998 Elsevier Science B.V. All rights reserved.
dc spin current generation in a Rashba-type quantum channel
(2006-02-01)
Wang, LY
;
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We propose and demonstrate theoretically that resonant inelastic scattering (RIS) can play an important role in dc spin current generation. The RIS makes it possible to generate dc spin current via a simple gate configuration: a single finger gate that locates atop and orients transversely to a quantum channel in the presence of Rashba spin-orbit interaction. The ac-biased finger gate gives rise to a time variation in the Rashba coupling parameter, which causes spin-resolved RIS and, subsequently, contributes to the dc spin current. The spin current depends on both the static and the dynamic parts in the Rashba coupling parameter, alpha(0) and alpha(1), respectively, and is proportional to alpha(0)alpha(2)(1). The proposed gate configuration has the added advantage that no dc charge current is generated. Our study also shows that the spin current generation can be enhanced significantly in a double finger-gate configuration.
Differential conductance of a saddle-point constriction with a time-modulated gate-voltage
(1998-11-01)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
This work investigates how a time-modulated gate-voltage influences the differential conductance G of a saddle-point constriction. The constriction is modeled by a symmetric saddle-point potential and the time-modulated gate-voltage is represented by a potential of the form V(0)Theta(a/2 - x - x(c)) cos(omega t). For h omega less than half of the transverse subband energy level spacing, gate-voltage-assisted (suppressed) feature occurs when the chemical potential mu is less (greater) than but close to the threshold energy of a subband. Our results indicate that as mu increases, G exhibits, alternatively, the assisted and the suppressed feature. For a larger h omega, these two features may overlap. In addition, dip structures are found in the suppressed regime, and mini-steps are found in the assisted regime only when the gate-voltage covers a region sufficiently distant from the center of the constriction. (C) 1998 Elsevier Science B.V. All rights reserved.
Differential conductance of a saddle-point constriction with a time-modulated gate-voltage
(1998-11-01)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
This work investigates how a time-modulated gate-voltage influences the differential conductance G of a saddle-point constriction. The constriction is modeled by a symmetric saddle-point potential and the time-modulated gate-voltage is represented by a potential of the form V(0)Theta(a/2 - \x - x(c)\) cos(omega t). For h omega less than half of the transverse subband energy level spacing, gate-voltage-assisted (suppressed) feature occurs when the chemical potential mu is less (greater) than but close to the threshold energy of a subband. Our results indicate that as mu increases, G exhibits, alternatively, the assisted and the suppressed feature. For a larger h omega, these two features may overlap. In addition, dip structures are found in the suppressed regime, and mini-steps are found in the assisted regime only when the gate-voltage covers a region sufficiently distant from the center of the constriction. (C) 1998 Elsevier Science B.V. All rights reserved.
Dissipation in a partially coherent flux-driven ring
(2000-03-15)
Liu, MT
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We have studied a mesoscopic ring threaded by a magnetic flux that increases linearly with time. The ring is partially coherent such that conduction electrons in the ring will encounter incoherent scatterings. We have treated both the incoherent scatterings and the coherent inelastic processes on the same footing. A S-matrix model, as proposed by Buttiker(1,2) for incoherent scatterings, has been adopted for our situation. This allows us to solve exactly, and analytically, the coherent inelastic processes caused by the time-varying magnetic flux. Our results demonstrate unequivocally that, for the electrons emanating out of incoherent scatterings, the lower the energies of these electrons the greater will be their net contribution to the de component of the induced current. A physical explanation is presented.
Effects of a time-dependent transverse electric field on the quantum transport in narrow channels
(1996-01-01)
Chu, CS
;
Tang, CS
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
The quantum transport in a narrow channel (NC) is studied in the presence of a time-dependent delta-profile electric field. Thr electric field is taken to be tranversely polarized, with frequency omega, causing inter-subband and inter-sideband transitions. Suppression in the de conductance G is found, which escalates with the chemical potential. There are structures in G which are related to the quasi-bound state (QBS) features. Major dip, and dip-and-peak, structures occur when an incident electron makes transition to a subband edge by absorbing or emitting one, and two, h omega, respectively. Structures associated with 3h omega processes are recognized. The QBS are closely associated with the singular density of states (DOS) at subband bottoms. Our results demonstrate that, due to this singular features of the DOS, the interaction of the electron with the electric field has to be treated beyond finite order perturbation.
Effects of a time-dependent transverse electric field on the quantum transport in narrow channels
(1996-01-01)
Chu, CS
;
Tang, CS
;
電子物理學系
;
Department of Electrophysics
The quantum transport in a narrow channel (NC) is studied in the presence of a time-dependent delta-profile electric field. Thr electric field is taken to be tranversely polarized, with frequency omega, causing inter-subband and inter-sideband transitions. Suppression in the de conductance G is found, which escalates with the chemical potential. There are structures in G which are related to the quasi-bound state (QBS) features. Major dip, and dip-and-peak, structures occur when an incident electron makes transition to a subband edge by absorbing or emitting one, and two, h omega, respectively. Structures associated with 3h omega processes are recognized. The QBS are closely associated with the singular density of states (DOS) at subband bottoms. Our results demonstrate that, due to this singular features of the DOS, the interaction of the electron with the electric field has to be treated beyond finite order perturbation.
Effects of a time-dependent transverse electric field on the quantum transport in narrow channels
(1996-01-01)
Chu, CS
;
Tang, CS
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
The quantum transport in a narrow channel (NC) is studied in the presence of a time-dependent delta-profile electric field. The electric field is taken to be transversely polarized, with frequency omega, causing inter-subband and inter-sideband transitions Suppression in the de conductance G is found, which escalates with the chemical potential. There are structures in G which are related to the quasi-bound state (QBS) features. Major dip, and dip-and-peak, structures occur when an incident electron makes transition to a subband edge by absorbing or emitting one, and two, h omega, respectively. Structures associated with 3h omega processes are recognized. The QBS are closely associated with the singular density of states (DOS) at subband bottoms. Our results demonstrate that, due to this singular features of the DOS, the interaction of the electron with the electric field has to be treated beyond finite order perturbation.
Effects of an elastic scatterer on the DC spin current generation in a Rashba-type quantum channel
(2006-05-01)
Wang, LY
;
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
In this work, we consider a Rashba-type quantum channel (RQC) consisting of one AC-biased finger-gates (FG) that orient perpendicularly and located above the RQC. Such an AC-biased FG gives rise to a local time-modulation in the Rashba coupling parameter, and is shown recently to generate a DC spin current [L.Y. Wang, C.S. Tang, C.S. Chu, Cond-mat/0409291, 2004]. No charge current, however, is generated in this configuration. We explore the robustness of such DC spin current generation against elastic scattering in the RQC. The effect of backscattering is studied by introducing a static barrier that is uniform in the transverse dimension. The effects of both backscattering and subband mixing is studied by introducing a static partial-barrier that is spatially localized and non-uniform in the transverse dimension. In addition, we compare the cases of attractive and repulsive partial-barriers. It is found that attractive partial-barrier gives rise to additional DC spin current structures due to resonant inter-subband and inter-sideband transition to quasi-bound states formed just beneath subband thresholds. (c) 2006 Elsevier B.V. All rights reserved.
Effects of an impurity on the dissipation in a partially coherent flux-driven ring
(2000)
Liu, MT
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We have studied a mesoscopic ring threaded by a magnetic flux that increases linearly with time. The ring is partially coherent, such that conduction electrons in the ring will encounter incoherent scatterings. In addition, the electrons encounter elastic scatterings due to the presence of an impurity in the ring. We have adopted a S-matrix model, as proposed by Buttiker M. Buttiker, Phys. Rev. B 32 (1985) 1846; M. Buttiker, Phys. Rev. B 33 (1986) 3020], for the incoherent scatterings in this time-dependent situation. This allows us to treat the incoherent scatterings, the elastic scatterings and the coherent inelastic processes on the same footing. We have solved the problem exactly. Our results demonstrate that, in the case of a weak impurity, the lower the energies of the electrons that emanate out of incoherent scatterings, the greater will be their net contribution to the de component I-dc of the induced current. In the case of a strong impurity, however, I-dc alternates between regions of zero and nonzero values as the chemical potential mu increases. The peak value of I-dc in the nonzero region increases with mu. We find that these regions of zero, and nonzero, I-dc correspond closely with the gaps, and the bands, respectively, of a one-dimensional energy band. All these characteristics arise from the fact that the electrons traversing the ring have their energies shifted gradually until their energies fall upon a forbidden region, where they suffer total reflection. This total reflection at the forbidden region does not occur in a ring that has a constant flux. Rather, it results from the nonadiabatic effect of the changing flux. The evolution of the nonadiabatic effects in the intermediate impurity regime has also been investigated. (C) 2000 Elsevier Science Ltd. All rights reserved.
Effects of incoherent processes on the quantum transport through a finite-range time-modulated potential
(2001-02-01)
Liang, HC
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We study the effect of phase randomization on the quantum transport through a finite-range time-modulated potential. The time-modulated potential leads only to coherent inelastic scatterings, and the incoherent processes are described by an S-matrix model proposed originally by Buttiker for the case of time-independent transport. This S-matrix model provides us a systematic way to reinject into the system those electrons that have suffered with incoherent scatterings. The distribution of these reinjected electrons is determined by an effective chemical potential <()over tilde>. For comparison, we study the cases where the incoherent processes occur inside and outside the time-modulated region. The incoherent processes are found to broaden and to smear the structures in the conductance G that are associated with the coherent inelastic scatterings. However, the smearing is more effective when the incoherent processes occur inside the time-modulated potential region.
Fabrication and metrology of an electro-optic polymer light modulator based on waveguide-coupled surface plasmon resonance
(2005-03-01)
Chyou, JJ
;
Chu, CS
;
Shih, ZH
;
Lin, CY
;
Chen, SJ
This study presents a novel high-efficiency electro-optic (EO). light modulator whose operation is based on the attenuated total reflection effect, which the surface plasmon wave is excited by an incident waveguide light wave. The EO light modulator is fabricated using a nonlinear optical polymer material that is a side-chain EO polyimide with 2-[N-ethyl-4-(tricyanovinyl)anilino]ethanol chromophore. The modulator is characterized, and its performance tested, in terms of the thickness and dielectric constant of the polymer thin film, the EO coefficient, the insertion loss, and the modulation index. Additionally, the dynamic response of the EO light modulator is fully investigated and discussed. (c) 2005 Society of Photo-Optical Instrumentation Engineers.
Fano resonance in transport through a mesoscopic two-lead ring
(2005-10-01)
Voo, KK
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
The ballistic transport through a one-dimensional two-lead ring at zero magnetic field is studied. We have focused on the case in which the potential in the ring does not define closed cavities or dots. Even in the absence of well-defined quasibound states, we find Fano profiles in the transmission probability. Those Fano profiles appear at energies corresponding to the standing-wave states in the ring, but their occurrence depends sensitively on the commensurability of the system parameters. When the system parameters are commensurate, the widths of the profiles at some energies are infinitesimally small. These findings suggest that the conventional understanding of the Fano profiles as a result of the interference effect of the transition through resonant states and nonresonant continuum of states, might not account for all the Fano profiles seen in the transport measurements. Moreover, the sensitivity and tunability of the resonance with respect to the system parameters may be usable in the fabrication of electrical nanodevices.
Finger-gate array quantum pumps: Pumping characteristics and mechanisms
(2004-08-01)
Chung, SW
;
Tang, CS
;
Chu, CS
;
Chang, CY
;
電子物理學系
We study the pumping effects, in both the adiabatic and nonadiabatic regimes, of a pair of finite finger-gate array (FGA) on a narrow channel. Connection between the pumping characteristics and associated mechanisms is established. The pumping potential is generated by ac biasing the FGA pair. For a single pair (N=1) of finger gates (FG's), the pumping mechanism is due to the coherent inelastic scattering of the traversing electron to its subband threshold. For a pair of FGA with pair number N>2, the dominant pumping mechanism becomes that of the time-dependent Bragg reflection. The contribution of the time-dependent Bragg reflection to the pumping is enabled by breaking the symmetry in the electron transmission when the pumping potential is of a predominant propagating type. This propagating wave condition can be achieved both by an appropriate choice of the FGA pair configuration and by the monitoring of a phase difference phi between the ac biases in the FGA pair. The robustness of such a pumping mechanism is demonstrated by considering a FGA pair with only pair number N=4.
Low-lying exciton states in a type-II broken-gap quantum well structure: A variation-basis-set approach
(1997-04-01)
Chu, CS
;
Chu, KD
;
電子物理學系
;
Department of Electrophysics
The exciton binding energy and the low-lying exciton states in a type-II broken-gap quantum well structure are studied. The conduction electron and the hole that form the exciton are spatially separated and are each confined by a quantum well. A variation-basis-set approach is proposed in which a set of variation basis set wavefunctions are constructed from the eigenstates of a variation Hamiltonian. The calculation of the exciton states can be carried out systematically and the accuracy of the state energies can be estimated. Numerical examples are presented, showing the rapid convergence of the excitonic levels with respect to the size of the variation basis set in and beyond the small well width regime. The effective binding energies for the states are found to increase with the decreasing of the well width. The low-lying exciton states in and beyond the small well width regime are presented.