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This paper theoretically studies the high-electron-mobility transistor (HEMT) driven by the terahertz radiation. It calculates the gate-to-source/drain admittance and the detection responsivity of the HEMT as a function of the signal frequency. It finds that the peaks of the admittances and the responsivity show redshift, and the heights of the peaks decrease with increasing the lengths of the source-to-gate and gate-to-drain spacing. Such phenomena may be useful in designing different HEMTs by utilizing the effects associated with the plasma oscillations excitation.

The effects of bias on the dynamical localization of two interacting electrons in a pair of coupled quantum dots driven by external AC fields have been numerically investigated. With an effective two-site model and Floquet formalism, the time-dependent Schrodinger equation is numerically solved and the Pmin, the minimum of the population evolution of the initial state within a certain time period, is used to quantify the degree of the dynamical localization. Results indicate that the bias can change the energy of the initial state and break the dynamical symmetry of the system with a pure AC field. And the amplitude of the AC field with dynamical localization phenomenon changes with bias. All the numerical results are explained by the perturbation theory and two-level approximation.

The combined interaction of Coulomb interaction and ac fields with two electrons in a quantum dot molecule is studied respectively with numerical simulation, perturbation theory and the approximation of driven two-level model. The dynamical localization occurs with the ac field whose ratio of the amplitude to the angular frequency is a root of n-order Bessel functions, where n is determined by the Coulomb interaction energy. Such results are explained with either the driven two-level approximation or the degenerated three-level model and verified by the numerical simulations.

Within the framework of effective mass approximation, we have calculated the electronic structure of the two laterally coupled quantum dots with a donor by the finite element method. The calculated results show that the bond states of quantum-dot molecules are quite sensitive to the donor positions. By varying the donor position, the transition from covalent to ionic bond state is realized for some electronic states. Some extreme cases are also discussed for comparison.

We present a perturbation investigation of dynamic localization condition of two electrons in a strong dc–ac biased quantum dot molecule. By reducing the system to an Hubbard-type effective two-site model and by applying Floquet theory, we find that the dynamical localization phenomenon occurs under certain values of the large strength of the dc and ac field. This demonstrates the possibility of using appropriate dc–ac fields to manipulate dynamical localized states in mesoscopic devices, which is an essential component of practical schemes for quantum information processing. Our conclusion is instructive to the field of quantum function devices.

We study the dynamics of two interacting electrons in a coupled-quantum-dot system with a time-dependent external electric field. The numerical results of the two-particle states reveal that the dynamical localization still exists under appropriate dc and ac voltage amplitudes. Such localization is different from the stationary localization phenomenon. Our conclusion is instructive for the field of quantum function devices.

Within the effective-mass approximation, we investigate the electronic structure of hexagonal quantum-disc clusters using the finite element method. With an increasing amount of quantum dots in the cluster, the electronic energy levels quickly expand into mini-bands, each consisting of discrete, unevenly distributed energy levels. The corresponding electronic eigenfunctions are linear combinations of the electron orbits in each quantum dot. The spatial symmetry of the combination is the same as the electronic eigenfunction of a single quantum dot.

We investigate the dynamics of two interaction electrons confined to one-dimensional quantum dot array in an ac electric field. We find that initially localized electrons will remain localized in the absence of Coulomb interaction if the ratio of the ac field magnitude to the frequency is a root of the ordinary zero-order Bessel function. In contrast to the case without Coulomb interaction, no matter what the value is, the electrons are delocalized and the delocalization effect depends on the ratio U/omega and eaE/omega, where U is the strength of Coulomb interaction, a is the lattice constant, and E and omega are the ac held amplitude and frequency, respectively.

The electronic structure of the quantum-dot molecules in an electric field is investigated by the finite element method with the effective mass approximation. The numerical calculation results show that the valence bond of the quantum-dot molecule alternates between covalent bonds and ionic bonds as the electric field increases. The valence-bond property can be reflected by the oscillator strength of the intraband transition. The bound state with the highest energy level in the quantum-dot molecule gradually changes into a quasibound state when the electric field increases.

The effects of the magnetic field on the valence bond property of the double-quantum-dot molecule are numerically studied by the finite element method and perturbation approach because of the absence of cylindrical symmetry in the horizontally coupled dots. The calculation results show that the energy value of the ground state changes differently from that of the first excited state with increasing magnetic field strength, and they cross under a certain magnetic field. The increasing magnetic field makes the covalent bond state change into an ionic bond state, which agrees qualitatively with experimental results and makes ionic bond states remain. The oscillator strength of transition between covalent bond states decreases distinctly with the increasing magnetic field strength, when the molecule is irradiated by polarized light. Such a phenomenon is possibly useful for actual applications.

The electronic energy spectrum and wavefunction of a quantum-dot molecule are studied by means of the finite-element solution of the single electron Schrodinger equation. We find that the nature of the coupling can be covalent, ionic, or ``intermediate" new mixed states, depending on various parameters such as the separation distance between two dots, the height of potential barrier, matching of the energies and parities of the orbital localized on each dot. The bond property can be used to explain the experimental result obtained by Oosterkamp et al. (1998 Nature 395 873).

The glass is in a non-equilibrium state in nature, and relaxation might occur towards the equilibrium state at a certain temperature. When heating a quenched glass, relaxation can be resolved as temperature approaches to the glass transition, and further heating leads to enthalpy recovery as the system turns into an equilibrium supercooled liquid. The released energy involving the relaxation relative to the original quenched state is, in magnitude, identical to the gained energy in enthalpy recovery, showing a memory effect. In this paper, we discuss the enthalpy behaviors involved in a cooling and reheating cycle around the glass transition in various glass forming systems such as oxides, metal alloys, and small molecular systems. The cooling and heating rates are fixed to be -/+ 20 K/min with the related cooling and heating heat capacity curves being determined. It is found that the relaxation enthalpy involved in the cooling/heating cycles is closely related to the enthalpy of fusion for the glass forming materials, and the basically linear correlation implies the similarity between the glass transition and melting behaviors with regard to the atomic rearrangements involved in the relaxation and solidification processes. The determining of the cooling and heating heat capacity curves also helps establish the enthalpy relaxation/recovery spectra of various glasses, and the symmetry of the spectrum is associated with the fragility of glass-forming material. For the material of low or medium fragilities, the symmetry of the enthalpy relaxation spectrum is observed to be somehow dependent on the fragility, while for the high fragility glass, the symmetry keeps almost constant. The dependence of fragility on the glass transition thermodynamics is also discussed, and low melting entropy and high fragility are shown to reduce effectively the liquid-crystal Gibbs free energy difference. Using the correlation between the relaxation enthalpy and kinetic fragility reported in our previous studies, the glass transition thermodynamics for the case of the most fragile glass with m= 175 is evaluated, especially compared with the second phase transition of thermodynamics. The results provide a new understanding of the thermodynamics of the relaxation in glassy material and the glass transition.

The compression of Zr44.4Nb7Cu13.5Ni10.8Be24.3 bulk metallic glass (BMG) is investigated at room temperature up to 39 GPa using {in situ} high-pressure energy dispersive x-ray diffraction with a synchrotron radiation source. The equation of state of the BMG is obtained by the calculation of the radial distribution function. Pressure-induced structural relaxation is exhibited. It is found that below about 6 GPa, the existence of excess free volume contributes to the rapid structural relaxation, which gives rise to rapid volumetric change, and the structural relaxation results in structural stiffness under higher pressure.

The Pd39Ni10Cu30P21 bulk metallic glass is isothermally relaxed under various pressures. The degree of the structural relaxation is evaluated in terms of the enthalpy recovery behaviours involved in the irreversible glass transition processes by using a temperature-modulated differential scanning calorimetry technique. A roughly Linear increase of the recovery enthalpy is observed within the experimental pressure range fi-om 2.67 to 4.45 GPa, which reflects the release of the frozen-in enthalpy in the as-quenched glass with increasing relaxation pressure. The pressure dependence of the timescale of the enthalpy recovery processes is also exhibited.

Anisotropic localization of Dirac fermions in graphene along both the x and y axes was studied using the transfer-matrix method. The two-parameter scaled behavior around the Dirac points was observed along the x axis with off-diagonal disorder. In contrast, the electronic state along the y axis with armchair edges was delocalized, which can be described well by single parameter scaling theory. This implies that the breakdown of the single-parameter scaling is related to the zigzag edge along the x axis. Furthermore, dimerization induced by the substrate suppresses the two-parameter scaling behavior along the x axis and preserves the delocalized state along the y axis. Our results also demonstrate anisotropic localization in graphene with diagonal disorder that can be tuned by dimerization.

The structural, elastic, electronic, and thermodynamic properties of ZrxNb1−xC alloys are investigated using the first principles method based on the density functional theory. The results show that the structural properties of ZrxNb1−xC alloys vary continuously with the increase of Zr composition. The alloy possesses both the highest shear modulus (215 GPa) and a higher bulk modulus (294 GPa), with a Zr composition of 0.21. Meanwhile, the Zr0.21 Nb0.79C alloy shows metallic conductivity based on the analysis of the density of states. In addition, the thermodynamic stability of the designed alloys is estimated using the calculated enthalpy of mixing.

This paper studies the constraint conditions for coherence destruction in tunneling by using perturbation theory and numerical simulation for an AC-field with bias and Coulomb interaction between electrons in a quantum dot molecule. Such conditions can be described by using the roots of a Bessel function Jn(x), where n is determined by both the bias and the Coulomb interactions, and x is the ratio of the amplitude to the frequency of the AC-field. Under such conditions, a coherent suppression of tunneling occurs between localized electronic states, which results from the dynamical localization phenomenon. All the conditions are verified with numerical simulations.

A cataphoresis discharge tube of 7mm inner diameter and 38cm active length was made for the He-Sr+ laser. The recombination laser at 430.5nm and the R-M transition laser at 1.03μm were obtained with the modified Blumlein circuit by high-frequency longitudinal pulsed discharge. The laser components are concentrated on the 430.5nm wavelength. Dependence of working parameters, such as the pulse frequency, the supply voltage and the helium pressure on laser output characteristics were measured and discussed. The maximum laser output power of 819mW and specific power of 56mW/cm3 were obtained.

According to Holstein's theory about radiation trapping effect, the influence of gas temperature and ground-state population density on the effective lifetime of the resonance level is discussed. The results show that the ground-state population density is the dominating factor in the radiation trapping effect. The threshold parameters of radiatoin trapping have been calculated for seven resonance metastable transition laser lines in Ca+, Sr+, Ba+ and Hg+ in different ionization ratios. The threshold temperature of radiation trapping was found to be consistent with the initial lasing temperature for the ionization ratio of 5%, which matches the ionization ratio of 3% to 5% observed in experiment. This consistency reveals that the threshold condition of radiation trapping effect is an important factor for population inversion in lastrs based on the ionic resonance to metastable transition.

Under consideration of the basic factors such as the level hyperfine structure and isotope split etc., the spectral structures of spontaneous emission of copper atom 578/511nm lines are calculated. A sealed operation CuBr laser with periodical refreshment of the buffer gas are designed and constructed. The 578/511nm spectral structures, produced by CuBr laser at lower Ne buffer gas pressure were measured under different working temperatures and exciting voltages. The spectral structure of spontaneous emission of copper atom 578/511nm lines have multi-peaks similar to their laser spectral structure, but the 578nm laser line isstrongly dependent on the working temperature and the exciting voltage. Some possible reasons are suggested to qualitatively explain the experimental results.

For a type of Fredholm integral equation with weak singularity integral kernel of logarithm function, the exact solutions is deduced through a calculus procedure when the free term is a triangle function. The singular character of solution is also analyzed when the variable is approaching the extreme points.

With a two-site Hubbard model and Floquet theorem, we investigate the dynamic behaviour of an exciton confined in a coupled double-quantum-dot system driven by an alternating current(AC) electrical field. The calculation shows that the quasi-energy levels versus the amplitude of the external field undergo a series o f exact-crossing and avoid-crossing. Generally, the initially localized state can remain forever at the first exact-crossing point of two ground quasi-energy levels. Increasing the interaction of the electron and hole, the degree of dynamical localization is enhanced. At the second exact-crossing point of ground quasi-energy levels, this kind of dynamical localization is decayed. At the point of avoid-crossing and offset from the exact-crossing, the dynamical localiz ation ceases to maintain. This demonstrates the possibility to control the excit on states on extremely short time scales using the appropriate AC field with the adjustable amplitude and frequency, which may be useful for quantum information processing.

We calculated the electronic structure of double quantum-dot molecule with a generalized LCAO approach and the finite element method.Compared with real molecule,the quantum dot molecule can be in the form of covalent bonding or ionic bonding between the two same (or different) quantum dots under different conditions.The condition is related to the distance between the two quantum dots and the energy of each quantum dot.The effects of the potential and radius of the quantum dots on the electronic structure have also been studied with the finite element method.

The electronic spectra for the Vicsek fractal at any stage have been discussed using the tight-binding method with the nearest-neighbor interaction approximation. The calculated results have shown the relationship between the persistent modes from one stage to the next. The particularity of the spectra for the fifth stage of Vicsek fractal obtained by numerical calculation was analyzed.