Second-harmonic generation in asymmetric quantum dots in the presence of a static magnetic field

The second-harmonic generation(SHG) coefficient in an asymmetric quantum dot(QD) with a static magnetic field is theoretically investigated.Within the framework of the effective-mass approximation,we obtain the confined wave functions and energies of electrons in the QD.We also obtain the SHG coefficient by the compact-density-matrix approach and the iterative method.The numerical results for the typical GaAs/AlGaAs QD show that the SHG coefficient depends strongly on the magnitude of magnetic field,parameters of the asymmetric potential and the radius of the QD.The resonant peak shifts with the magnetic field or the radius of the QD changing.     

Ultrasound attenuation of superfluid 3He in aerogel

We have performed longitudinal ultrasound (9.5 MHz) attenuation measurements in the B phase of superfluid 3He in 98% porosity aerogel down to the zero temperature limit for a wide range of pressures at zero magnetic field. The absolute attenuation was determined by direct transmission of sound pulses. Compared to the bulk fluid, our results revealed a drastically different behavior in attenuation, which is consistent with theoretical accounts with gapless excitations and a collision drag effect.     

Dielectric linear response of magnetized electrons: Drag force on ions

The drag force on ions moving in a magnetized electron plasma is calculated in dielectric linear response. Various representations of the dielectric function ε(k, ω) are investigated for their suitability to display the limits for an infinite and a vanishing magnetic field. While the influence of the magnetic field is negligible in certain regions of k-space, it introduces in other regions a strong oscillatory structure in the dielectric function. This requires a careful treatment of the multidimensional integrations necessary for the drag force. The contributions from oscillatory integrands are treated by the saddle point method. Explicit results are obtained for the dependence of the drag force on the magnetic field, the direction of motion of the ion relative to the magnetic field, the shielding in the electron plasma, its density and the anisotropy of the electron temperature. The importance of the collective response of the electrons is investigated for limiting cases of the magnetic field. The validity of the linearization of the dielectric theory is checked by comparison with results obtained by numerical simulation of the nonlinear Vlasov-Poisson equation. For strong magnetic fields and low ion velocities, the simulations rather agree with the complementary binary collision model than with linear response.     

A Two-Electron Quantum Ring Under Magnetic Fields

We calculate the energy levels of low-lying states of a two-electron quantum ring under the influence of perpendicular homogeneous magnetic field. Calculations are made by using the method of exact diagonalization within the effective-mass approximation. The ground-state electronic structures and angular momentum transitions as a function of the strength of a magnetic field have been revealed.     

Magnetic-field effect on the diamagnetic susceptibility of hydrogenic impurities in quantum well-wires

We have studied the magnetic field effects on the diamagnetic susceptibility and binding energy of a hydrogenic impurity in a quantum well-wire by taking into account spatially dependent screening. Using the effective-mass approximation within a variational scheme, binding energy and diamagnetic susceptibility of donor are obtained as a function of the magnetic field, length of the square quantum well-wire for different impurity positions. It is shown that the magnetic field effects on diamagnetic susceptibility can be more important for donors in quantum well-wires over a large range of wire dimensions.     

A Negative Donor D- in a Magnetic Field

Using the method of numerical matrix diagonalization within the effective-mass approximation, we investigated a D^--center quantum dot system subjected to a Gaussian potential confinement. We obtain the dependence of binding energies of the ground-states of the D^--center on the depth of Gaussian potential and the magnetic field strength. The result shows clearly that the binding energies of the ground-states of the D^--center are rather sensitive to the depth of potential and the strength of magnetic field.     

准高斯声束对超声造影剂的声辐射力研究*

超声造影剂的定向输运在超声医学成像领域有着极为重要的意义,而声辐射力作用是实现该过程的关键,相比于高斯声束,准高斯声束是无源亥姆霍兹方程的精确解,可以使用标准波分解法简化计算。因此,本文研究了准高斯声束对超声造影剂的声辐射力作用。文章首先分析了准高斯声束与高斯声束之间的相关性;随后通过数值计算求得了准高斯声束对超声造影剂模型的声辐射力函数与无量纲频率之间的关系;最后,本文研究了不同造影剂气泡情况下的声辐射力。研究结果表明:声辐射力函数随无量纲频率变化将在不同位置出现共振峰,不同的波束宽度值将改变辐射力强度,但不改变共振峰的位置。相关结果可为利用声辐射力定向输运超声造影剂至靶向位置提供理论参考。     

The Electron-Hole Pair in a Single Quantum Dot and That in a Vertically Coupled Quantum Dot

The energy spectra of low-lying states of an exciton in a single and a vertically coupled quantum dots arestudied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method ofnumerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the bindingenergy of the ground and the excited states of an exciton in a single quantum dot and that in a vertically coupledquantum dot as a function of the dot radius for different values of the distance and the magnetic field strength.     

Optical properties of semiconductor quantum dots in magnetic fields

Optical properties of semiconductor quantum dots in magnetic fields are reviewed. A theory is described based on a multi-band effective-mass approximation with a nonparabolic conduction electron dispersion, the direct Coulomb interaction, and the electron-hole exchange interaction taken into account. The transition from the quantum-confined Zeeman effect for a weak magnetic field to the quantum-confined Paschen-Back effect to a strong magnetic field is discussed in comparison with atomic spectra in magnetic fields. Experimental results of the optical properties of isolated CuCl, CdSSe, and Si quantum dots in magnetic fields are also discussed in conjunction with the theoretical results.     

Magnetic propulsion of intense lithium streams in a tokamak magnetic field

This paper describes the effect and gives the theory of magnetic propulsion which allows driving free surface plasma facing liquid lithium streams in tokamaks. In the approximation of a thin flowing layer the MHD equations are reduced to one integrodifferential equation which takes into account the propulsion effect, viscosity, and the drag force due to magnetic pumping and other interactions with the magnetic field. A stability criterion is obtained for stabilization of the "sausage" instability of the streams by centrifugal force.     

The Electron－Hole Pair in a Single Quantum Dot and That in a Vertically Coupled Quantum Dot

The energy spectra of low-lying states of an exciton in a single and a vertically coupled quantum dots are studied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method of numerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the binding energy of the ground and the excited states of an exciton in a single quantum dot and that in a vertically coupled quantum dot as a function of the dot radius for different vaJues of the distance and the magnetic field strength.     

Mean force on a small sphere in a sound field in a viscous fluid

A mean force exerted on a small rigid sphere by a sound wave in a viscous fluid is calculated. The force is expressed as a sum of drag force coming from the external steady flow existing in the absence of the sphere and contributions that are cross products of velocity and velocity derivatives of the incident field. Because of the drag force and an acoustic streaming generated near the sphere, the mean force does not coincide with the acoustic radiation pressure, i.e., the mean momentum flux carried by the sound field through any surface enclosing the sphere. If the sphere radius R is considerably smaller than the viscous wave penetration depth delta, the drag force can give the leading-order contribution (in powers of delta/R) to the mean force and the latter can then be directed against the radiation pressure. In another limit, delta< or =R, the drag force and acoustic streaming play a minor role, and the mean force reduces to the radiation pressure, which can be expressed through source strengths of the scattered sound field. The effect of viscosity can then be significant only if the incident wave is locally plane traveling.     

Superconductivity in the alkali metals

At ambient pressure there are 29 elemental superconductors in the periodic table, none of which is an alkali metal. The first alkali metal to become superconducting under high pressure is Cs followed years later by Li. Alkali metals are believed to be exemplary free-electron systems. The fact that an alkali metal becomes superconducting at all is surprising and is a result of the fact that under pressure it shows marked deviations from free-electron behaviour where, counterintuitively, bands narrow and gaps widen. For this reason the alkali metals are among the most interesting systems known to study in high-pressure experiments and superconductivity is one of their most fascinating properties.     

Infrared Magneto-Optical Absorption of Shallow Donors in GaAs/GaAsAl Quantum Dots

Infrared magneto-optical absorption spectra of shallow donor impurity states in GaAs/Ga-AsAl quantum dot are investigated theoretically using the effective-mass, approximation within the strong confinement regime. The numerical results show that for high magnetic fields, the transition energies for (0, 0) + (0, 1) increase linearly with enhancement of magnetic field strength, while the transition energies for (0, 0) → (0, -1) appear to diminish with increasing magnetic field strength.     

Singlet-Triplet Transitions of a Pöschl-Teller Quantum Dot

We study the energy spectra of a two-dimensional two-electron quantum dot (QD) with Pöschl-Teller confining potential under the influence of perpendicular homogeneous magnetic field. Calculations are made by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. A ground-state behavior (spin singlet-triplet transitions) as a function of the strength of a magnetic field is found. We find that the dot radius R of a Pöschl-Teller potential is important for the ground-state transition and the feature of ground-state for a Pöschl-Teller QD and a parabolic QD is similar when R is larger. The larger the well depth, the higher the magnetic field for the singlet-triplet transition of the ground-state of two interacting electrons in a Pöschl-Teller QD.     

Generation of Electrical Currents and Magnetic Fields by Grain Boundary Motion

The effect of an induced magnetic moment due to grain boundary motion in a magnetic field was studied theoretically in a microscopic and a mesoscopic approximation. It was found that the induced moment generates a drag force on the boundary, which depends on the orientation of the magnetic field with regard to the crystal axis, as observed experimentally. However, the magnitude of the theoretically predicted dependency is much smaller than experimentally observed and even opposite with regard to the orientation dependence. Therefore, the electromagnetic drag can be neglected in comparison with other driving forces for grain boundary motion, but the effect may play a role for fast moving dislocations in a magnetic field.     

Plasma motion in a filtered cathodic vacuum arc

A model is proposed for the flow of a plasma originating from a cathodic vacuum arc into a curvilinear magnetic field. The model gives good agreement with measurements obtained from a filtered cathodic-arc thin film deposition system. The important parameters involved in the motion of a vacuum arc plasma beam through a magnetic filter are examined. The analysis is based on the use of the guiding center approximation to describe the motion of the charged particles produced in the plasma where the thermal energy is negligible compared to the mass flow energy. Electron-ion collision effects are included within the framework of the drift model. It is shown that under the limiting condition of a collision frequency which is much higher than the cyclotron frequency of the electron, the motion of the plasma ions around the bend becomes independent of the magnetic field, with the number of ions traversing the filter significantly reduced. However, in the collisionless plasma case (cyclotron frequency higher than the collision frequency), the model predicts a square-law relationship between ion-saturation current and magnetic field , I_p ∞B²     

Two interacting electrons in a Gaussian confining potential quantum dot

Two interacting electrons in a Gaussian confining potential quantum dot are considered under the influence of a perpendicular homogeneous magnetic field. The energy levels of the low-lying states are calculated as a function of magnetic field. Calculations are made by using the method of few-body physics within the effective-mass approximation. A ground state behavior (singlet→triplet state transitions) as a function of the strength of a magnetic field has been found in the weak confinement case as a two-electron quantum dot with parabolic confining potential.     

Hydrostatic pressure and electric field effects on the normalized binding energy in asymmetrical quantum wells

We have investigated the simultaneous effects of the hydrostatic pressure and electric field on the ground subband level and on normalized binding energy of an on-center donor in asymmetrical GaAs/AlGaAs quantum wells within the effective-mass approximation and a variational approach. We found that the well size at which the impurity energy changes from positive to negative value (turning point) strongly depends on the asymmetry and hydrostatic pressure. As a key result, we suggest that the study of the normalized binding energy for various values of the electric field in direct and inverse polarization regimes can be used to feel the quantum well asymmetry and to unambiguously find out the effective pressure acting on a given heterostructure.