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Based on the Green's function method and tight-binding model, we have investigated tunneling magnetoresistance (TMR) in a two-dimensional quantum anomalous Hall insulator (QAHI)/topological superconductor (TSC)/QAHI hybrid structure with its Chern number N = 1 and N = 0 in the central TSC region. It is found that the TMR effect can emerge in our proposed structure. In the presence of the chiral gapless Majorana edge state, the 100% TMR ratio is robust to disorder and is independent of the length of TSC. Further calculations show that the TMR effect could be adjusted by the relative angle between magnetizations of left and right QAHI electrodes and magnetization strength. In addition, we also calculate the TMR effect in the QAHI/TSC/QAHI hybrid structure with its Chern number N = 2 in the central TSC region. We found that our conclusions are still applicable.

One of the important challenges that graphene must overcome before it can legitimately declare its irreplaceable position among the fields of plasmonic materials is to achieve efficiently coupling to external light. In this paper, we demonstrate an effective solution to this problem by exciting localized surface plasmon polaritons (SPPs) on graphene-coated nanowire arrays (GCNAs). These SPPs are analyzed by introducing a universal scaling law that considerably simplifies the understanding of these modes. Meanwhile, numerical experiments are carried out to demonstrate the theoretical analysis of plasmon excitations. The excited SPPs permit the control through both geometrical and physical properties. The proposed structure can be used as a tunable optical filter, a highly sensitive refractive index sensor, and other plasmonic modulation devices.

By using the Green's function method, we have investigated spin Hall conductance (SHC) in a four terminal quantum spin Hall insulator. The results show that the intrinsic spin orbit coupling in a HgTe/CdTe hetero-structure interface naturally leads to separate probability distributions for the two spins in coordinate space, which leads to the spin Hall effect in our proposed device. We also find that the SHC of bulk states exhibits an oscillatory behavior as a function of the device width and persists at a broad device width. In addition, we calculate the effects of disorder on the SHC of the bulk states of non-trivial and trivial topology. The results indicate that the spin up and spin down conductances show different degrees of suppression by disorder; thus the SHC could be significantly enhanced by the disorder. This kind of property has a great value to practical applications.

In measurement of the microwave surface resistance, Rs(T), of a high-Tc film with a host-cavity method, in which the cavity material is usually copper, the systematic error in Rs at low temperature can be significantly reduced through calibration with the data obtained by a niobium host cavity. Using a cavity excited in the TE011 mode at 13.6 GHz, the procedure is illustrated for a c-axis oriented YBa2Cu3O7−δ film fabricated on 36 mm diameter copper disk with yttria-stabilized-zirconia and chromium buffer layers. The temperature dependence in Rs(T) was consistent with that of the penetration depth; both quantities behavior could be fit well by a modified two-fluid model, in which the fraction of the pairing normal carriers obeyed (T/Tc)2 rather than (T/Tc)4 with some 20% of the charge carriers remaining normal.

The energy of a biexciton in a GaAs–AlxGa1−xAs quantum well is calculated variationally by use of a two-parameter trial wave function. The calculated binding energy, relative to two well-separated excitons, is greater than that given previously by Kleinman [Phys. Rev. B 28, 871 (1983)]. Our binding energy takes its greatest value when the well width is approximately 10 A. The ratio of the binding energy of a biexciton to that of an exciton for a range of well thickness (5–300 A) is found to be from 0.127 to 0.284, which agrees fairly well with previous experimental results. The results of our approach are compared with those of earlier theories.

The microwave surface reactance of copper was measured over the temperature range from 300 to 14 K using the TE011 mode of a cylindrical cavity. The result was that with decreasing temperature the ratio of the reactive-to-resistive component remained at around 1 from 300 to 75 K, and increased to 1.32 at 14 K, in contrast to the theoretical extreme anomalous limit, equal to or higher than ∛. The discrepancy between the measured and theoretical values may be attributed to the quality of the material and the surface finishing. In this experiment an accurate measurement of the resonant frequency is important. Associated with the frequency, we discuss two thermal-expansion measurement techniques: cavity dilatometry and capacitance dilatometry. Further, the usefulness of the present data for studying high-Tc films will be mentioned.

We present the analysis of experimental results on second‐harmonic generation in a novel ferroelectric liquid crystal (FLC) material, o‐ nitroalkoxyphenyl biphenylcarboxylate 1, synthesized specifically for large nonlinear coefficients. All nonzero dij coefficients have been measured, with the largest being d22 = 0.6 ± 0.3 pm/V. Relative values of refractive indices are also given suggesting uniaxial optical properties with large birefringence (nz − nx≊ 0.20 at λ=1.064 μm). This material demonstrates the flexibility of FLC molecular synthesis to form thermodynamically stable macroscopically aligned materials for second‐order nonlinear optics applications.

We have carried out an extensive simulation study for the spin autocorrelation function at T=∞ of the one‐dimensional classical Heisenberg model with four different types of isotropic bilinear nearest‐neighbor coupling: uniform exchange, alternating exchange, and two kinds of random exchange. For the long‐time tails of all but one case, the simulation data seem incompatible with the simple ∼t−1/2 leading term predicted by spin diffusion phenomenology.

Considering the difference of energy bands in graphene and silicene, we put forward a new model of the graphene-silicene-graphene (GSG) heterojunction. In the GSG, we study the valley polarization properties in a zigzag nanoribbon in the presence of an external electric field. We find the energy range associated with the bulk gap of silicene has a valley polarization more than 95%. Under the protection of the topological edge states of the silicene, the valley polarization remains even the small non-magnetic disorder is introduced. These results have certain practical significance in applications for future valley valve.

A relative permeability model for transient two-phase flow in fractal porous media is derived based on the fractal characteristics of pore size distribution and the assumption that porous media consists of capillary bundles. The functions in this model are tortuosity fractal dimension, pore fractal dimension, and maximum and minimum pore diameters. Every parameter has clear physical meaning without the use of empirical constants. Good agreement between model predictions and experimental data is obtained, the sensitive parameters that influence the relative permeability are specified and their effects on relative permeability are discussed.

This computer simulation study provides further evidence that spin diffusion in the one‐dimensional classical Heisenberg model at T=∞ is anomalous: 〈Sj(t)⋅Sj〉 ∼t−α1 withα1 ≳1/2. However, the exponential instability of the numerically integrated phase‐space trajectories transforms the deterministic transport of spin fluctuations into a computationally generated stochastic process in which the global conservation laws are still satisfied to high precision. This may cause a crossover in 〈Sj(t)⋅Sj〉 from anomalous spin diffusion (α1 ≳ 1/2) to normal spin diffusion (α1 = 1/2) at some characteristic time lag that depends on the precision of the numerical integration.

The dynamics of the classical XXZ model with uniform interaction is nonlinear for N≥2 spins and nonintegrable for N≥3. However, the nonlinearities disappear in the thermodynamic limit N→∞, and the spin autocorrelation functions can be determined exactly for infinite temperature. The function 〈Szi(t)Szi〉 exhibits a Gaussian decay to a nonzero constant, and the function 〈Sxi(t)Sxi〉 decays algebraically to zero or like a Gaussian, depending on the type (easy axis or easy plane) and amount of uniaxial anisotropy.

We propose a model to study the spin transport properties of a regular polygonal quantum ring with two leads, which is subjected to Rashba spin-orbit coupling (SOC). The transmission coefficient has been derived analytically using a standard quantum network method. It is found that in the absence of Rashba SOC, there are two conditions for which zero conductances arise when the leads are asymmetrically located, and in this case, the transmitted electron is unpolarized. In the presence of SOC, however, one kind of conductance zero can be lifted by the Rashba SOC, while the other persists. We also study the conductance of spin-polarized carriers. It is found that Rashba SOC can be used as an effective mean for inducing spin flip. In addition, based on our calculations, the device considered may demonstrate the properties of a spin filter.

This paper describes using the finite difference method to study the screening effect on the binding energy of excitons in quantum wires (QWRs). We have derived an analytical formula for the effective screened interaction potential between an electron and a hole in an exciton. The exciton binding energy as functions of the screening length, carrier density, and QWR width is calculated and is found to be sensitive to the screen length when the length is less than 1 × 103 A. Furthermore, when the screen length is less than 3 × 102 A, the exciton binding energy decreases rapidly with decreasing screen length. It is also found that the wider the QWR is, the more pronounced the screening effect will be. The transition from a dilute exciton gas to a dense electron-hole plasma in a QWR is clearly shown on a phase diagram for the carrier density-temperature plane.

Using the one-dimensional effective potential method, we theoretically study the properties of an exciton in vertically coupled quantum dots (VCQDs) under the influence of an applied magnetic field. The ground state transition energies of the heavy-hole exciton are calculated in a single quantum dot and in VCQDs, respectively. Our results are in good agreement with those of experiments. In symmetrical VCQDs, the entanglement of the exciton state monotonously increases with increasing barrier width, and can even attain the value of the maximally entangled state. When the symmetry of the VCQDs is broken, the entanglement of the exciton state almost entirely breaks down under the combined influences of tunneling and the Coulomb interactions.

We have investigated the effect of the coupling of localized surface plasmon (LSP) on the transmission properties of a compound structure with metallic particle and hole arrays. It is found that the variation in longitudinal interval G between particle and hole arrays leads to the shift in transmission peak and the new transmission peak, which are due to the change in coupling strength of LSP modes and the formation of coaxial guide mode, respectively. The lateral displacement Lx (parallel to the polarization direction of incident light) results in the splitting of transmission peak, originating from the variation in coupling manner of LSP modes, while the lateral displacement Ly (perpendicular to the polarization direction of incident light) causes the reduction and redshift in transmission peak.

Using first-principles band structure methods, we investigate the interactions between different donors in In2O3. Through the formation energy and transition energy level calculations, we find that an oxygen-vacancy creates a deep donor level, while an indium-interstitial or a tin-dopant induces a shallow donor level. The coupling between these donor levels gives rise to even shallower donor levels and leads to a significant reduction in their formation energies. Based on the analysis of the PBE0-corrected band structure and the molecular-orbital bonding diagram, we demonstrate these effects of donor–donor binding. In addition, total energy calculations show that these defect pairs tend to be more stable with respect to the isolated defects due to their negative binding energies. Thus, we may design shallow donor levels to enhance the electrical conductivity via the donor–donor binding.

Based on the finite-difference time-domain method, we demonstrate multiple enhanced transmission bands through subwavelength compound periodic array of rectangular holes by adjusting the cutoff wavelength of each hole. The results show that the transmission spectrum of this structure is almost a compound of that of the corresponding periodic hole arrays with simple cell, when the holes are arranged appropriately. It is also found that the surface plasmon resonance peak and the localized resonance peak exhibit different behaviors as the symmetry of compound periodic structure varies. The proposed method provides an effective way to achieve enhanced transmission simultaneously at different selected wavelengths in the visible and infrared regions.

The properties of an exciton bound to a neutral donor (D0,X) in a parabolic GaAs quantum-well wire in the presence of a parallel magnetic field are investigated using the finite-difference method within the quasi-one-dimensional effective potential model. The magnetic field effects on the binding energy of the D0,X is studied for various parabolic potentials. The results show that high magnetic fields significantly increase the binding energy of the D0,X in the case of weak spatial confinement. In addition, we also found that with weak spatial confinement, the magnetic field has much stronger effects on the D0,X than it does on the biexciton, but with strong spatial confinement it has less effect.

The properties of ionized-donor-bound excitons (D+,X) in a parabolic quantum-well wire in the presence of a magnetic field are studied numerically in the framework of effective-mass envelope function theory. The binding energy of (D+,X) structures is calculated as a function of the oscillator length for different values of the magnetic field by using the one-dimensional effective potential model and the finite-difference method. The results show that the binding energy increases as the oscillator length decreases, and that it is also increased by enhancing the magnetic field. The binding energies of the complex for the two different dissociation processes are taken into account, and their behaviors are discussed in detail. The Haynes factor is found to increase rapidly with decreasing oscillator length in the case of strong parabolic potential confinement. In addition, the average interparticle distances and the probability density distributions are investigated for a given set of values of the oscillator...

The problem of a biexciton confined in parabolic GaAs quantum-well wires in the presence of a perpendicular magnetic field is discussed. We apply an analytical one-dimensional formula for the effective interaction potential between the confined charge carriers to study the binding energies of a biexciton, which results in a numerical “exactly” solvable model. The ratio of the binding energy of a biexciton to that of an exciton for a range of confinement lengths (l

The ground state of a negatively charged donor (D−) in a parabolic GaAs quantum-well wire in the presence of a magnetic field is investigated using the finite difference method within the quasi-one-dimensional effective potential model. The magnetic effects on the binding energies of the ground state of a D− center are calculated for various parabolic potentials. The distance between the electrons and the donor ion and the distance between the two electrons are also calculated, respectively, as a function of the strength of the parabolic potential and the magnetic field. We find that the interplay of the spatial confinement and the magnetic confinement of electrons in quantum-well wires leads to complicated behavior of the binding energies of the D− center and that the increase of the electron-donor ion attraction dominates the increase of the electron-electron repulsion as the spatial and magnetic confinement increases for the ground state of a D− center in a parabolic quantum-well wire.

The spin-dependent Andreev reflection (AR) tunneling through a ferromagnet double-quantum-dot superconductor (F-DQD-S) is studied using the nonequilibrium Green’s function method. The F-DQD-S is a T-shaped hybrid structure where one of the dots is coupled to the F and S electrodes and the other dot is coupled to the first dot only. On each dot the intradot spin-flip process is present. The behavior of the F-DQD-S tunneling conductance is determined by the competition among the interdot coupling, the intradot spin-flip process, and the coupling of the first dot to the electrodes. When the interdot coupling is present, the quantum interference effects between the two dots cause secondary peaks in the tunneling conductance besides the primary one, indicating a structure in the AR spectrum. On the other hand, the spin-flip scattering process suppresses the AR peaks. With the spin-flip scattering strength increasing, the resonance peaks are split.

The effects of compressive stress on the binding energy of shallow-donor impurity states in symmetrical GaAs-Ga1−xAlxAs double quantum dots are calculated variationally using a parameterized wave function within the effective-mass approximation. Results are obtained for different dot and barrier widths, donor ion positions, and compressive stresses along the growth direction of the structure. In the direct-gap regime (for stress values up to 13.5 kbar) the binding energy increases linearly with the stress. In the indirect-gap regime (for stress values greater than 13.5 kbar) and with the donor ion at the center of the dot, the binding energy increases up to a maximum and then decreases. For all donor ion positions, the binding energy shows nonlinear behavior in the indirect-gap regime due to the Γ-X crossing effect. In the limit of single quantum wells, the results we obtain are in good agreement with those previously obtained for the case in which the donor ion is at the center of the well.

The properties of a hydrogenic impurity in a parabolic GaAs quantum-well wire in the presence of the magnetic field are investigated using the finite-difference method within the quasi-one-dimensional effective potential model. The magnetic effects on the energies and binding energies of the ground and lowest excited states of a hydrogenic impurity in a parabolic GaAs quantum-well wire are studied for various parabolic potentials. The calculated results indicate that the interplay of the spatial confinement and the magnetic confinement of electrons in the quantum-well wires leads to complicated binding energies of the hydrogenic impurity, and high magnetic fields significantly increase the binding energies of the hydrogenic impurity in the case of weak spatial confinement.

In studies of dynamic correlation functions the focus is, in general, on their long‐time asymptotic behavior and the singularity structure of the associated spectral densities. It turns out that the analysis of the same dynamic correlation functions from a quite different perspective is equally useful and revealing. The focus of our study is on the properties of spectral densities at high frequencies, specifically their decay law, expressible asΦ0■ω■ ∼exp■ − ω2■λ■, in terms of a characteristic exponent λ. That decay law governs the growth rate of the sequence of recurrents which determine the relaxation function in the continued‐fraction representation. The value of λ contains valuable information on the underlying dynamical processes taking place in the system, which, in some sense, is complementary to that inferred from the singularity structure of spectral densities. A detailed study of the dynamics of various quantum and classical spin models has prompted us to introduce the concept of ‘‘universality ...

An extremely small hole of approximately 10 μm in diameter was made on a glass plate by a special technique. Using this small hole, together with a turbomolecular pump, a pressure difference of seven orders of magnitude was achieved. Using this system, a direct analysis of the gas components in CO2 lasers of 300‐mm‐long waveguide type and 800‐mm‐long common type during their short‐term lifetime has been performed. The following results were obtained: (1) In the case of filling with pure CO2, a direct mass‐spectrometric analysis of CO2 decomposition process has been performed showing a decomposition of 80% in partial pressure. (2) In the case of filling with pure CO, the continuous increase of CO2 was observed, and an equilibrium CO2 peak height of about 8% of the total peak height was reached. (3) In the case of filling with a laser gas mixture, the variations of all components were observed, and an analysis of the curve shapes showed that there exists a cataphoresis effect. (4) When a procedure of ‘‘disc...

Quantum spin chains of higher spin are interesting in the case of both ferromagnets and antiferromagnets. They comprise a wide range of model types—Bethe–Ansatz integrable, nonintegrable, and with limited integrability. It is not well appreciated that the two‐magnon (two‐body) problems is always solvable, by Bethe–Ansatz and Green’s‐function techniques, regardless of the integrability character of the entire model, with interesting implications in the context of solitons and bound solitons. Chubukov and Kveschenko have suggested using the two‐magnon problem in a test for integrability and claim to find additional classes of integrable spin chains for s=1 and s= (3)/(2) . Here we investigate this situation.