An upgraded external beam PIXE setup for multielemental analysis of atmospheric aerosol samples

The external beam particle‐induced X‐ray emission (PIXE) setup has been upgraded by introducing a new silicon drift detector with the aim of obtaining better minimum detection limits (MDLs) at the 2 × 1.7 MV Tandetron accelerator of the Beijing Normal University in China. The upgraded external beam PIXE setup is equipped with two silicon drift detectors for PIXE analysis of low and high Z elements. A surface barrier detector for Rutherford backscattering spectrometry monitors beam and helium flow. Two kinds of aerosol filter samples (quartz fiber filters and Teflon filters) were studied. A 200‐μm thick Mylar absorber in front of the medium‐high energy X‐rays detector was adopted, and it got the best MDLs for atmospheric aerosol samples analysis. Multielemental analysis of quartz fiber filter aerosol samples was achieved. For more accurate and better MDLs of low Z elements in PIXE analysis, it is necessary to keep sufficient helium flow behind the thin samples.     

Modification of bonding properties and microstructure of resin-cement interface by coupling agents

To solve the problem of organic-inorganic light conductive composite interface features, transparent resin and cement matrix were used as carriers to further study features of the organic-inorganic interface formed by transparent resin and cement matrix and the interface modification of the coupling agent. The bond strength, micro-hardness, microstructure and surface morphology of a resin light conductive cementitious materials (RLCCM) interface were evaluated by tests of tensile and oblique shear, micro-hardness, environmental scanning electron microscopy, and atomic force microscope. The results show that the silane A-151 and aluminate coupling agent could significantly improve interface bond properties of RLCCM. At 7 d, the interface tensile bond strength of the silane A-151 and aluminate coupling agent increased by 117 and 105%, respectively. At 28 d, strength on average increased by 73%. At 7 d, interface shear strength the silane A-151 and aluminate coupling agent of 45° increased by 43 and 53%, respectively, At 28 d, strength on average increased by 40%. The transparent resin performance weakened region thickness up to 100 μm; the hardness of the transparent resin in the transition region was increased by 19.6 and 39.9%, respectively. Silane coupling agent A-151 and cement hydration products formed flat spherical particles with diameter of approximately 78 nm, and these particles mosaicked and fused in the surface of the hydrate; therefore, the surface became denser and smoother.     

Frequency and intensity readouts of micro-wave electric field using Rydberg atoms with Doppler effects

A scheme of electric-field measurement of micro-waves is proposed in Rydberg atoms with Doppler effects. A cascade-type electromagnetically-induced-transparency (EIT) system is disturbed by a perturbative field coupling a metastable transition. The original dark state splits and two EIT window appear with a central absorption peak. When a micro-wave (MW) field couples the Rydberg transition, the central absorption peak is divided into two. The frequency splitting of two central peaks is proportional to the MW field intensity, which can be used to probe MW electric field strength. This frequency-readout method based on a double-dark state system increases the probe sensitivity nearly by a factor of 7, compared with that of the single-dark state case. At room temperature, Doppler effects can enhance the absorption spectrum, moreover, its peak value varies linearly with the MW field strength. This can also serve to measure MW electric field strength. Numerical results show that the latter intensity-readout method after Doppler averaging improves the probe sensitivity by a factor of 10 with respect to the case without Doppler effects.     

Observation of Spin and Valley Splitting of Landau Levels under Magnetic Tunneling in Graphene/Boron Nitride/Graphene Structures

Resonance magnetic tunneling in heterostructures formed by graphene single sheets separated by a hexagonal boron nitride barrier and bounded by two gates has been investigated in a strong magnetic field, which has allowed observing transitions between spin- and valley-split Landau levels with various indices belonging to different graphene sheets. An unexpected increase with the temperature in the interlayer tunneling conductance owing to transitions between the Landau levels in strong magnetic fields cannot be explained by existing theories.     

Unidirectionally Emitting Quantum Cascade Microcavity Lasers with Patterned Contacts

We investigate unidirectionally emitting quantum cascade microcavity lasers with semicircle-patterned top contacts. We employ novel patterned top contacts while fabricating notched ellipse-shaped cavity lasers. We study experimentally the microcavity-structure-based quantum cascade (QC) laser material with a long infrared wavelength of ~10 μm. Then we characterize microcavity lasers with patterned contacts and compare them with nonpatterned ones and observe a lower operating injection threshold current as a consequence of this relatively straightforward technique, with the unidirectional emission feature being kept. We obtain a maximum light output peak power of 16 mW with unidirectional emission at a far-field divergence angle of ~7° at a full width of half maximum, while the patterned device shows low threshold even in the microcavity laser with a size of 150 μm. Furthermore, we also carry out a reliability test of the QC microcavity lasers with semicircle-patterned top contacts, and the testing results show no sudden failure or severe light-output-power drop during an operating time of thousand hours.     

Stability of the DX- center in GaAs quantum dots

Using a first-principles band structure method, we study how the size of quantum dots affects the stability and transition energy levels of defects in GaAs. We show that, although a negatively charged DX- center is unstable in bulk GaAs:Si with respect to the tetrahedral coordinated Si(-)(Ga), it becomes stable when the dot size is small enough. The critical size of the dot is about 14.5 nm in diameter. The reason for the stabilization is the strong quantum-confinement effect, which increases the formation energy of Si(-)(Ga) more than that of the DX- defect center. Our studies show that defect properties in quantum dots could be significantly different from those in bulk semiconductors.     

Electronic band structure of a type-II ’W’ quantum well calculated by an eight-band k·p model

In this paper, we present an investigation of type-II 'W' quantum wells for the InAs/Ga_1-xIn_xSb/AlSb family, where 'W' denotes the conduction profile of the material. We focus our attention on using the eight-band k?p model to calculate the band structures within the framework of finite element method. For the sake of clarity, the simulation in this paper is simplified and based on only one period---AlSb/InAs/Ga_1-xIn_xSb/InAs/AlSb. The obtained numerical results include the energy levels and wavefunctions of carriers. We discuss the variations of the electronic properties by changing several important parameters, such as the thickness of either InAs or Ga_1-xIn_xSb layer and the alloy composition in Ga_1-xIn_xSb separately. In the last part, in order to compare the eight-band k?p model, we recalculate the conduction bands of the 'W' structure using the one-band k?p model and then discuss the difference between the two results, showing that conduction bands are strongly coupled with valence bands in the narrow band gap structure. The in-plane energy dispersions, which illustrate the suppression of the Auger recombination process, are also obtained.     

Pseudo analytical solution to time periodic stiffness systems

An analytical form of state transition matrix for a system of equations with time periodic stiffness is derived in order to solve the free response and also allow for the determination of system stability and bifurcation. A pseudoclosed form complete solution for parametrically excited systems subjected to inhomogeneous generalized forcing is developed,based on the Fourier expansion of periodic matrices and the substitution of matrix exponential terms via Lagrange-Sylvester theorem. A Mathieu type of equation with large amplitude is presented to demonstrate the method of formulating state transition matrix and Floquet multipliers. A two-degree-of-freedom system with irregular time periodic stiffness characterized by spiral bevel gear mesh vibration is presented to find forced response in stability and instability. The obtained results are presented and discussed.     

Solution synthesis of ZnO nanotubes via a template-free hydrothermal route

ZnO nanotubes were successfully synthesized by a simple template-free hydrothermal method. X-ray powder diffraction and transmission electron microscopy were used to characterize the as-prepared ZnO nanotubes. The average size of the nanotubes is 200-500 nm in length and 20-30 nm in diameter. In addition, a further investigation of the optimized synthetic conditions has been carried out.