Browsing by Author "Yang, TJ"

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A method of measuring the contribution of the image potential energy of the shallow impurity ground state in a quantum well
(2001-10-01)
Yang, TJ
;
Hsu, SY
;
電子物理學系
;
Department of Electrophysics
We show that the binding energy of the shallow impurity (at the center of the quantum well) ground state is strongly reduced by the presence of a metallic mirror at a few effective Bohr radii from the quantum well. For a given depth of the quantum well, we find that the absolute value of the image potential energy without the metallic mirror is equal to that with a fixed metallic mirror while a certain width of the quantum well is met. Hence. it is proposed that the binding energy of the shallow impurity ground state in the quantum well with and without the metallic mirror can be separately measured by the variation of the width of quantum well. The contribution of the image potential energy to the binding energy of the shallow impurity ground state may then be deduced. (C) 2001 Elsevier Science B.V. All rights reserved.
A Raman study of ZnSe at high pressure
(1997-06-01)
Chou, WC
;
Lin, CM
;
Ro, RS
;
Ho, CS
;
Hong, DY
The ZnSe was studied by Raman scattering spectroscopy at pressures up to 36 GPa at first time. The present researchers have found splittings of transverse optical (TO) phonon at 4.65 and 7.61 GPa. In addition, the Raman signals of the longitudinal optical (LO) phonon disappeared at 14.35 GPa. The disappearance of the LO phonon is attributed to the semiconductor-metal phase transition. However, the TO phonon peaks were still visible above the metallization pressure.
A Raman study of ZnSe at high pressure
(1997-06-01)
Chou, WC
;
Lin, CM
;
Ro, RS
;
Ho, CS
;
Hong, DY
The ZnSe was studied by Raman scattering spectroscopy at pressures up to 36 GPa at first time. The present researchers have found splittings of transverse optical (TO) phonon at 4.65 and 7.61 GPa. In addition, the Raman signals of the longitudinal optical (LO) phonon disappeared at 14.35 GPa. The disappearance of the LO phonon is attributed to the semiconductor-metal phase transition. However, the TO phonon peaks were still visible above the metallization pressure.
A silicon nanowire with a Coulomb blockade effect at room temperature
(2002-05-17)
Hu, SF
;
Wong, WZ
;
Liu, SS
;
Wu, YC
;
Sung, CL
An extremely narrow and thin silicon wire has been fabricated on a silicon-on-insulator wafer (see Figure). The room-temperature Coulomb blockade effects as well as the influence of a capacitively coupled gate on the transport properties of this conducting silicon quantum wire are studied. The results obtained are encouraging for the application of such wires in single-electron transistors.
Analysis of average transport critical current density of oxide superconductors deposited on a silver base
(1996-01-01)
Lee, WD
;
Yang, TJ
;
Chiou, BS
;
電子物理學系
;
電控工程研究所
Considering the resistivity of a silver base and the contact resistance of a silver base with a bulk superconductor, the dependence of average critical current density J(c)(D,H-a) of a high-T-c, oxide superconductor deposited oil a silver base on the sample dimension D and the applied magnetic field H-a is calculated using a generalized critical-state model, which was recently extended to low field by Chen and Yang [Physica C 224 (1994) 345]. The self-field effect of the average critical current was also taken into account in the model. Based on the generalized critical state model, new behaviors of J(c)(D, H-a) in the low-field range of H-c1 less than or equal to H-a much less than H-c2 are obtained. In addition, a constant J(c)(D, H-a) for 0 less than or equal to H-a less than or equal to H-c1 is obtained, which is different from the results derived from a model that does not include the self-field effect. However, these theoretical results can be fitted to the experimental data to obtain some important macroscopic material parameters and may be used to interpret the dimension effect of transport J(c) of a high-T-c oxide superconductor deposited on a silver base.
Andreev scattering in semiconductor-superconductor junctions containing a finite width semiconductor region applied by magnetic fields
(2004-05-01)
Hsue, YC
;
Yang, TJ
;
Gu, BY
;
Wang, J
;
電子物理學系
We present numerical studies of conductance of quasi-particles (QPs) in junctions consisting of semiconductor (Sm) two-dimensional electronic gas (2DEG) and superconductor (S) containing a finite width 2DEG region (referred to magnetic barrier) applied by a magnetic field perpendicular to the 2DEG plane. Total conductance as a function of beta = mu(N)/homega(c), (where mu(N) is the chemical potential of semiconductor and omega(c) = eB/cm(N)(*) is the cyclotron frequency) strongly depends on the normal or superconducting state of the superconductor. It demonstrates that the Andreev reflection (AR) plays a dominant role. The total conductance of G((NBN)) or G((NBS)) as a function of beta for the NBN or NBS junction displays monotonically increasing behavior with different rising slopes. We observe the existence of a critical magnetic field for switching off the conductance. This critical field of G((NBS)) is a half of that of G((NBN)) in the matching case of the Fermi level and effective mass of QPs in materials forming the junctions. G((NBS)) is usually larger than G((NBN)) for a given magnetic field. The 3D plot of the conductance G((NBN)) and G((NBS)) as functions of beta and the incident angle theta of the QPs displays a flat plateau over a finite domain region of [beta,theta]. However, in the mismatching case of the Fermi energy and effective mass of QPs, G((NBS)) is lower than G((NBN)) because the AR of the holelike QPs now becomes incomplete and normal reflection (NR) of the electronlike QPs is considerably enhanced. The corresponding 3D plot of conductance against beta and theta exhibits heavy spike oscillations around its occupied boundary. We present explicit analytic expressions of the conductance of QPs for this model junction on the basis of classical cyclotron orbit of the QPs in magnetic fields and the quantum mechanical calculation. The characteristics of conductance can be well interpreted by this phenomenological physical picture.
Anion-terminated zinc-blende CdS(100) surface
(1997-10-15)
Lin, CM
;
Tsai, MH
;
Yang, TJ
;
Chuu, DS
;
電子物理學系
Using the local-orbital density-functional molecular-dynamics method, we have studied the surface structure of the strain-stabilized zinc-blende monolayer anion-terrainated CdS(100) surface. By analogy with the GaAs(100) surface, we have considered four structural models and found that the structure with a (2 x 4) unit cell, in which there are a two-dimer unit and a two-shifted-dimer unit, is the most favorable. These results can be understood in terms of physical properties associated with ionicity.
Anisotropic pinning effect on a Nb thin film with triangular arrays of pinning sites
(2004-06-01)
Wu, TC
;
Kang, PC
;
Horng, L
;
Wu, JC
;
Yang, TJ
A superconducting Nb thin film with triangular arrays of pinning sites has been fabricated to investigate the commensurate vortex lattices. Vortex-vortex interactions in this pattern film make the observed anisotropy in the pinning properties. Periodic minima appear in the magnetoresistance curves at equal field intervals that can be associated with matching effect. The transport properties at near critical temperatures show anisotropic pinning properties related to the configuration of pinning centers. The critical current is depressed when the driving force is applied along the short diagonal in the rhombic unit cell of pinning array. It implies that the pinning potential along the short diagonal creates a moving channel of vortices in triangular arrays of defects. The pinned and interstitial vortices can be movable in an incommensurate vortex row and a straight unhindered one-dimensional path. (C) 2004 American Institute of Physics.
Applying a modified plane-wave expansion method to the calculations of transmittivity and reflectivity of a semi-infinite photonic crystal
(2004-07-01)
Hsue, YC
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
We propose a modified plane-wave expansion method to calculate transmittivity and reflectivity of a semi-infinite photonic crystal (PC) with interface. This method is based on an expanded completeness basis, including both the propagation and evanescence modes. We use this approach to deal with two kinds of problems: one is to determine the normal direction of the largest attenuation strength for a semi-infinite PC in the gap frequencies; the other is to calculate the transmittivity and reflectivity of a PC slab. To demonstrate the extensive utilization of our approach, we revisit the same system as studied by K. Sakoda [Phys. Rev. B 52, 8992 (1995)] and find that our results are in good agreement with ones obtained by Sakoda's paper.
Calculation of numerical microcanonical ensemble method on 2D statistical models
(1998-06-01)
Wang, JZ
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
In our earlier work, we proposed a new microcanonical ensemble method and showed that the numerical results of this method are correct in 1D models. In this paper, the numerical data in 2D models are presented. One of our calculations is concentrated on fermionic models, because fermionic models behave quite differently from bosonic models when the dimension is larger than two (there may be boson-fermion duality in 1D). Our results come out to be consistent with exact values. To our knowledge, there is no other numerical methods which can present reliable calculation on fermionic models with large lattice, especially when the temperature is low. In addition to the test on fermionic models, we also show our results on quantum 2D XY model. We calculate longitudinal spin-spin correlation, specific heat, vortex density and vortex pair density. These results agree with the calculations by other methods. The data of vortex density and vortex pair density seem to. display directly the unbinding of vortices and antivortices.
Characteristics of transmission of electrons in a quantum wire tangentially attached to a superconductor ring threaded by magnetic flux
(2002-04-01)
Gu, BY
;
Lu, Y
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
We present numerical investigations of the transmission properties of electrons in a normal quantum wire tangentially attached to a superconductor ring threaded by magnetic flux. A point scatterer with a delta-function potential is placed at node to model scattering effect. We find that the transmission characteristics of electrons in this structure strongly depend on the normal or superconducting state of the ring. The transmission probability as a function of the energy of incident electrons, in the case of a superconductor ring threaded by one quantum magnetic flux, emerges one deep dip, imposed upon the first broad bump in spectrum. This intrinsic conductance dip originates from the superconductor state of the ring. When increasing the magnetic flux from one quantum magnetic flux to two, the spectrum shifts toward higher energy region in the whole. This conductance dip accordingly shifts and appears in the second bump. In the presence of a point-scatterer at the node, the spectrum is substantially modified. Based on the condition of I he formation of the standing wave functions in the ring and the broken of the time-reserve symmetry of Schrodinger equation after switching magnetic flux, the characteristics of transmission of electrons in this structure can be well understood.
Comparison of irreversibility lines of silver-sheathed Bi-2223, Tl-1223 and Tl-1234 superconducting tapes
(1996-01-01)
Wei, JC
;
Yang, TJ
;
Liu, RS
;
光電工程學系
;
Department of Photonics
The irreversibility lines of silver-sheathed (Bi,Pb)(2)Sr2Ca2Cu3O10 (Bi-2223), (Tl,Pb,Bi)(Sr,Ba)(2)Ca2Cu2O9 (T1-1223) and (Tl,Pb)Sr2Ca3Cu4O11 (T1-1234) superconducting tapes were investigated and compared with those of other superconducting systems. We have demonstrated that the shape of the irreversibility lines of T1-1223 and T1-1234 superconducting tapes are similar in behavior to those of YBa2Cu3O7 (Y-123) and (Tl,Pb)Sr-2(Ca,Y)Cu2O7 (T1-1212) systems and better than those of Tl2Ba2Ca2Cu3O10 (T1-2223) and (Bi,Pb)(2)Sr2Ca2Cu5O10 (Bi-2223) superconductors. This means that the flux pinning ability of T1-1223 and T1-1234 superconducting tapes was stronger than that of Bi-2223 and T1-2223 superconductors. The possible reason for controlling flux pinning can be attributed to the intrinsic crystal structure difference in the materials.
Comparison of irreversibility lines of silver-sheathed Bi-2223, Tl-1223 and Tl-1234 superconducting tapes (vol 43, pg 83, 1996)
(1996-10-01)
Wei, JC
;
Yang, TJ
;
Liu, RS
;
光電工程學系
;
Department of Photonics
Contour of the attenuated length of an evanescent wave at constant frequency within a band gap of photonic crystal
(2004-02-01)
Hsue, YC
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
The plane wave method is normally applied to determine the eigenfrequency of a two-dimensional (2D) photonic crystal. A slight change to this eigenvalue equation makes the wave number its eigenvalue providing a direct means to determine the attenuated length of the evanescent modes at the frequency within the photonic band gap. The contour of the length of attenuation of the evanescent modes in a square lattice can be determined using the proposed wave number eigenvalue equation. The wave number eigenvalue equation for the two-dimensional (3D) photonic crystal can also be obtained using a derivation similar to that for the 2D photonic crystal. Possible applications of the proposed calculation-method are presented. (C) 2003 Elsevier Ltd. All rights reserved.
Coupling technique for efficient interfacing between silica waveguides and planar photonic crystal circuits
(2004-12-20)
Chau, YF
;
Yang, TJ
;
Lee, WD
;
電子物理學系
;
Department of Electrophysics
We describe a two-step-size tapered structure with one defect pair that can markedly enhance the coupling efficiency at the entrance and exit terminals of a planar photonic crystal (PPC) waveguide. PPC waveguides are composed of circular dielectric rods set in two-dimensional square lattices. On the basis of our simulations, we found that the optimized scheme maximizes the power transmission above 90% at a wavelength of 1.55 mum. Besides, one can control the central frequency for optical communications by determining this defect configuration in an optimization procedure. Moreover, by properly adjusting the defect radii in PPC tapers, one can use the PPC circuit as a good reflector. (C) 2004 Optical Society of America.
Current distribution and vortex-vortex interaction in a superconducting film of finite thickness
(1996-11-01)
Wei, JC
;
Yang, TJ
;
電子物理學系
;
光電工程學系
;
Department of Electrophysics
The current distribution due to a single vortex in a superconducting film of finite thickness is calculated, based on the London model. At large distances from the vortex core, the current density on the surface of the superconductor was found to follow the 1/r(2) law which has a simple expression, J(s)(r, 0) approximate to phi(0)[coth(d/lambda) + csch (d/lambda)]/2 pi mu(0) lambda r(2). We also find that the interaction energy between two vortices has the form, U-int(r/(12)) phi(0)(2)[H-0(r(12)/lambda') - Y-0(r(12)/lambda l)], when the film is not too thick and at large distances between two vortices. This potential differs from Pearl's result by the new effective penetration depth lambda' = 2 lambda coth (d/lambda).
Effects of the quantum size and an off-center donor atom on energy levels in a quantum dot
(1998-08-01)
Wang, JZ
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
Gorecki and Brown [J. Phys. B 22, 2659 (1989)] proposed a variational theory for the ground state. They assumed the trial wave function in confined systems as f phi, where f acted as a variable contour function to minimize the energy. In practical calculations, it is only possible to calculate electronic energy levels at some shapes of f to decide which f gives the minimal energy. Due to this constraint in the variational calculation of f, their method gives unsatisfactory results in asymmetrical quantum problems, and therefore is rarely used. We generalize their method by assuming the trial wave function as f psi, where psi is a linear combination of basis functions. In our improvement, the use of the linear combination of basis functions acts not only to simulate efficiently the asymmetrical state but also to the give energy levels of the excited state. For the same reason, our method can be applied in open systems and in the problem where a new potential exists. Therefore, the goal of our improvement is to propose a general variational method. As applications, the changes of binding energies and donor states in spherical and ellipsoidal semiconductor quantum dots are studied when the position of the donor is shifted and when the size of the dot is changed.
Efficient mode coupling technique between photonic crystal heterostructure waveguide and silica waveguides
(2005-09-15)
Chau, YF
;
Yang, TJ
;
Gu, BY
;
Lee, WD
;
電子物理學系
We report an efficient mode coupling technique between silica waveguides (SWGs) and a planar photonic crystal heterostructure waveguide (PPCHWG) composed of two semi-infinite two-dimensional photonic crystals (PCs) with different filling fractions. By calculating the transmittance at output end of SWG, we find that the designed structures possess the merits of high coupling efficiency and relaxing fabricating tolerance. The maximal transmittance exceeds 90% at a wavelength near 1.65 mu m. The high transmission efficiency of PPCHWGs can be achieved without introducing any defect structures for optimal mode matching. Besides, the width of PPCHWGs can be flexibly adjusted via changing the separation of two component PCs in PC heterostructures. (c) 2005 Elsevier B.V. All rights reserved.
Exact solution of the Ginzburg-Landau equation for the upper critical field of a d(x2-y2) superconductor
(1999-04-01)
Dai, MC
;
Yang, TJ
;
Ting, CS
;
電子物理學系
;
Department of Electrophysics
A d(x2-y2) superconductor is modeled as the superconducting layers in the a-b plane, whose coupling in the c direction is approximated by the effective mass, within the Ginzburg-Landau theory. In this work, this model is applied to the system where the, coherence length along the c direction is greater than the layer spacing. Based on our model, we calculate the upper critical field in a magnetic field lying in a-e plane and tilted by an angle from the c axis. According to our results, the curvature of H-c2(T) is upward, and the slope - dH(c2)(T)/dT depends on the angle between the c axis and the external field. The most interesting feature is that the ratio of the c-direction parameter related to the effective mass to the n-b plane parameter connected with the effective mass can influence H-c2. As the rafio is decreased, H-c2. becomes increased. We also find that there is no admixture of s-wave component in the critical regime and believe that the upward curvature of the H-c2(T) is the characteristic property of a d-wave superconductor. [S0163-1829(99)01914-1].
Formation of multiple-quanta vortices at circular defects in a superconducting thin film
(1997-08-01)
Wei, JC
;
Horng, L
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
The possibility of the formation of multiple-quanta vortices in a superconducting thin film with circular defects is discussed. It is shown that the condition for creating two-quanta vortices on a circular defect depends on the radius of defect, the separation between defects and the configuration of defects.