Structural Characteristics of Cerium Oxide Nanocrystals Prepared by the Microemulsion Method

The aim of this work is to investigate the microstructure development of erium oxide nanocrystal,prepared by the microemulsion process,as a function of annealing temperature in air.Combined with the HRTEM and the thermogravimetric-differential thermal analysis in air.Combined withthe HRTEm and the thremogravimetric-differential thermal analysis(TG-DTA),the XRD patterns reveal that the sample annealed at 623 K is amorphous,and the formation of cerium oxide nanocrystal occurs above 773 K.The local structural and electronic properties in the nanocrystallization process are probed by X-ray absorption spectra (XAS) at the Ce L3 edge.It is found that the phase structure changes from triclinic to cubic (CeO2),and the electroic structure changes from Ce^3 to Ce^4 upon increasing the annealing temperature.     

TIME—RESOLVED X—RAY SPECTRA AND ATOMIC PROCESSES

In this article we will discuss time-resolved He-like X-ray spectra of titanium ions from a TFTR tokamak plasma[1] and of iron ions from solar flares[2] in particular attention to a presence of high energy electrons which affect the spectra and ionization balances.We consider a model that a hot component interacts with a bulk plasma.With this model,the time evolution of the spectra and ionization balaces derived therefrom can be described consistently.     

Electronic relaxation of deep bulk trap and interface state in ZnO ceramics

This paper investigates the electronic relaxation of deep bulk trap and interface state in ZnO ceramics based on dielectric spectra measured in a wide range of temperature, frequency and bias, in addition to the steady state response. It discusses the nature of net current flowing over the barrier affected by interface state, and then obtains temperature-dependent barrier height by approximate calculation from steady I--V (current--voltage) characteristics. Additional conductance and capacitance arising from deep bulk trap relaxation are calculated based on the displacement of the cross point between deep bulk trap and Fermi level under small AC signal. From the resonances due to deep bulk trap relaxation on dielectric spectra, the activation energies are obtained as 0.22 eV and 0.35 eV, which are consistent with the electronic levels of the main defect interstitial Zn and vacancy oxygen in the depletion layer. Under moderate bias, another resonance due to interface relaxation is shown on the dielectric spectra. The DC-like conductance is also observed in high temperature region on dielectric spectra, and the activation energy is much smaller than the barrier height in steady state condition, which is attributed to the displacement current coming from the shallow bulk trap relaxation or other factors.     

UV–visible spectral characterization and density functional theory simulation analysis on laser-induced crystallization of amorphous silicon thin films

The effect of laser energy density on the crystallization of hydrogenated intrinsic amorphous silicon （a-Si：H） thin films was studied both theoretically and experimentally. The thin films were irritated by a frequency-doubled （λ= 532 nm） Nd：YAG pulsed nanosecond laser. An effective density functional theory model was built to reveal the variation of bandgap energy influenced by thermal stress after laser irradiation. Experimental results establish correlation between the thermal stress and the shift of transverse optical peak in Raman spectroscopy and suggest that the relatively greater shift of the transverse optical （TO） peak can produce higher stress. The highest crystalline fraction （84.5%） is obtained in the optimized laser energy density （1000 mJ/cm2） with a considerable stress release. The absorption edge energy measured by the UV- visible spectra is in fairly good agreement with the bandgap energy in the density functional theory （DFT） simulation.     

Micromagnetic simulation on the dynamic susceptibility spectra of cobalt nanowires arrays: the effect of magnetostatic interaction

<正>Micromagnetic simulations have been performed to obtain the dynamic susceptibility spectra of 4×4 cobalt nanowire arrays with different spatial configurations and geometries.The susceptibility spectra of isolated wires have also been simulated for comparison purposes.It is found that the susceptibility spectrum of nanowire array bears a lot of similarities to that of an isolated wire,such as the occurrences of the edge mode and the bulk resonance mode. The simulation results also reveal that the susceptibility spectrum of nanowire array behaves like that of single isolated wire as the interwire distance grows to an extent,which is believed due to the decrease of magnetostatic interaction among nanowires,and can be further confirmed by the static magnetic hysteresis simulations.In comparison with single nanowire,magnetostatic interaction may increase or decrease the resonance frequencies of nanowire arrays assuming a certain interwire distance when the length of array increases.Our simulation results are also analysed by employing the Kittel equation and recent theoretical studies.     

Experimental Observations Indicating the Topological Nature of the Edge States on HfTe_5

The topological edge states of two-dimensional topological insulators with large energy gaps furnish ideal conduction channels for dissipationless current transport. Transition metal tellurides X_(Te5)X=Zr, Hf) are theoretically predicted to be large-gap two-dimensional topological insulators, and the experimental observations of their bulk insulating gap and in-gap edge states have been reported, but the topological nature of these edge states still remains to be further elucidated. Here, we report our low-temperature scanning tunneling microscopy/spectroscopy study on single crystals of HfTe5. We demonstrate a full energy gap of ～80 meV near the Fermi level on the surface monolayer of HfTe5 and that such an insulating energy gap gets filled with finite energy states when measured at the monolayer step edges. Remarkably, such states are absent at the edges of a narrow monolayer strip of one-unit-cell in width but persist at both step edges of a unit-cell wide monolayer groove. These experimental observations strongly indicate that the edge states of HfTe5 monolayers are not trivially caused by translational symmetry breaking, instead they are topological in nature protected by the 2 D nontrivial bulk properties.     

Detection of spin current through a quantum dot with Majorana bound states

The spin transport properties are theoretically investigated when a quantum dot(QD) is side-coupled to Majorana bound states(MBSs) driven by a symmetric dipolar spin battery. It is found that MBSs have a great effect on spin transport properties. The peak-to-valley ratio of the spin current decreases as the coupling strength between the MBS and the QD increases. Moreover, a non-zero charge current with two resonance peaks appears in the system. In the extreme case where the dot–MBS coupling strength is strong enough, the spin current and the charge current are both constants in the non-resonance peak range. When considering the effect of the Zeeman energy, it is interesting that the resonance peak at the higher energy appears one shoulder. And the shoulder turns into a peak when the Zeeman energy is big enough. In addition, the coupling strength between the two MBSs weakens their effects on the currents of the system. These results are helpful for understanding the MBSs signature in the transport spectra.     

Finite size effects on the helical edge states on the Lieb lattice

For a two-dimensional Lieb lattice,that is,a line-centered square lattice,the inclusion of the intrinsic spin–orbit(ISO)coupling opens a topologically nontrivial gap,and gives rise to the quantum spin Hall(QSH) effect characterized by two pairs of gapless helical edge states within the bulk gap.Generally,due to the finite size effect in QSH systems,the edge states on the two sides of a strip of finite width can couple together to open a gap in the spectrum.In this paper,we investigate the finite size effect of helical edge states on the Lieb lattice with ISO coupling under three different kinds of boundary conditions,i.e.,the straight,bearded and asymmetry edges.The spectrum and wave function of edge modes are derived analytically for a tight-binding model on the Lieb lattice.For a strip Lieb lattice with two straight edges,the ISO coupling induces the Dirac-like bulk states to localize at the edges to become the helical edge states with the same Dirac-like spectrum.Moreover,it is found that in the case with two straight edges the gapless Dirac-like spectrum remains unchanged with decreasing the width of the strip Lieb lattice,and no gap is opened in the edge band.It is concluded that the finite size effect of QSH states is absent in the case with the straight edges.However,in the other two cases with the bearded and asymmetry edges,the energy gap induced by the finite size effect is still opened with decreasing the width of the strip.It is also proposed that the edge band dispersion can be controlled by applying an on-site potential energy on the outermost atoms.     

Photoluminescence of ZnSe—ZnS Single Quantum Wells Grown by Vapour Phase Epitaxy

We study the photoluminescence (PL) of ultra thin layer ZnSe quantum Wells in ZnS barriers.Samples with different well widths are grown by vapour phase epitaxy and the PL spectra of these samples are measured by the excitation of a 500W Hg lamp.The peak positions of the bands coming from the excitonic luminescence show a larger blueschift with respect to the energy of free excitons in the ZnSe bulk material.The observed variation of the full width at half maximum and peak position of the bands in the spectra with the well width are interpreted to the formation of the ZnSxSe1-x alloy layer due to the interdiffusion in the interfaces between ZnSe and ZnS.According to the behaviour of the excitons in the smaller conduction band offset,the exciton binding energy is estimated from the dependence of the PL intensity on the temperature.from this result,excitons seem to show nearly three-dimensional characteristics.     

Modulation of energy spectrum and control of coherent microwave transmission at single-photon level by longitudinal field in a superconducting quantum circuit

We study the effect of longitudinally applied field modulation on a two-level system using superconducting quantum circuits. The presence of the modulation results in additional transitions and changes the magnitude of the resonance peak in the energy spectrum of the qubit. In particular, when the amplitude λ_z and the frequency ω_l of the modulation field meet certain conditions, the resonance peak of the qubit disappears. Using this effect, we further demonstrate that the longitudinal field modulation of the Xmon qubit coupled to a one-dimensional transmission line could be used to dynamically control the transmission of single-photon level coherent resonance microwave.     

The QE numerical simulation of PEA semiconductor photocathode

Several kinds of models have already been proposed to explain the photoemission process. The exact photoemission theory of the semiconductor photocathode was not well established after decades of research. In this paper an integral equation of quantum efficiency (QE) is constructed to describe the photoemission of positive electron affinity (PEA) of the semiconductor photocathode based on the three-step photoemission model. Various factors (e.g., forbidden band gap, electron affinity, photon energy, incident angle, degree of polarization, refractive index, extinction coefficient, initial and final electron energy, relaxation time, external electric field and so on) have an impact on the QE of the PEA semiconductor photocathode, which are entirely expressed in the QE equation. In addition, a simulation code is also programmed to calculate the QE of the K₂CsSb photocathode theoretically at 532 nm wavelength. By and large, the result is in line with the expected experimental value. The reasons leading to the distinction between the experimental and theoretical QE are discussed.     

Synthesis and characterization of Ca2Sn1-xCexO4 with blue luminescence originating from Ce^4＋ charge transfer transition

Ce^4＋-doped Ca2SnO4 with a one-dimensional structure, which emits bright blue light, is prepared by using a solid-state reaction method. The x-ray diffraction results show that the Ce^4＋ ions doped in Ca2SnO4 occupy the Sn^4＋ sites. The excitation and emission spectra of Ca2Sn1-xCexO4 appear to have broad bands with peaks at - 268nm and -442nm, respectively. A long excited-state lifetime （-83μs） for the emission from Ca2Sn1-xCexO4 suggests that the luminescence originates from a ligand-to-metal Ce^4＋ charge transfer （CT）. The luminescent properties of Ca2Snl_xCexO4 have been compared with those of Sr2CeO4, which is the only material reported so far to show Ce^4＋ CT luminescence. More interestingly, it is observed that the emission intensity of Ca2Sn1-xCexO4 with a small doping concentration （x - 0.03） is comparable to that of Sr2CeO4 in which the concentration of active centre is 100%.     

Microstructure and optical absorption of Au-MgF2 nanoparticle cermet films

The microstructure and optical absorption of Au-MgF2 nanoparticle cermet films with different Au contents are studied. The microstructural analysis shows that the films are mainly composed of the amorphous MgF2 matrix with embedded fcc Au nanoparticles with a mean size of 9.8-21.4nm. Spectral analysis suggests that the surface plasma resonance （SPR） absorption peak of Au particles appears at λ=492-537nm. With increasing Au content, absorption peak intensity increases, profile narrows and location redshifts. Theoretical absorption spectra are calculated based on Maxwell-Garnett theory and compared with experimental spectra.     

X-ray-boosted photoionization for the measurement of an intense laser pulse

Investigations show that X-ray-boosted photoionization(XBP) has the following advantages for in-situ measurements of ultrahigh laser intensity I and field envelope F(t)(time t,pulse duration τL,carrier-envelope-phase Φ):accuracy,dynamic range,and rapidness.The calculated XBP spectra resemble inversely proportional functions of the photoelectron momentum shift.The maximum momentum p and the observable value Q(defined as a double integration of a normalized photoelectron energy spectrum,PES) linearly depend on I1/2 and τL,respectively.Φ and F(t) can be determined from the PES cut-off energy and peak positions.The measurable laser intensity can be up to and over 1018 W/cm2 by using high energy X-rays and highly charged inert gases.     

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.     

Study of neutron spectra in a water bath from a Pb target irradiated by 250 MeV protons

Spallation neutrons were produced by the irradiation of Pb with 250 MeV protons. The Pb target was surrounded by water which was used to slow down the emitted neutrons. The moderated neutrons in the water bath were measured by using the resonance detectors of Au, Mn and In with a cadmium (Cd) cover. According to the measured activities of the foils, the neutron flux at different resonance energies were deduced and the epithermal neutron spectra were proposed. Corresponding results calculated with the Monte Carlo code MCNPX were compared with the experimental data to check the validity of the code. The comparison showed that the simulation could give a good prediction for the neutron spectra above 50 eV, while the finite thickness of the foils greatly effected the experimental data in low energy. It was also found that the resonance detectors themselves had great impact on the simulated energy spectra.     

Off-resonant double-resonance optical-pumping spectra and their application in a multiphoton cesium magneto-optical trap

We present an investigation of double-resonance optical pumping(DROP) spectra under the condition of singlephoton frequency detuning based on a cesium 6S1/2–6P3/2–8S1/2ladder-type system with a room-temperature vapor cell.Two DROP peaks are found, and their origins are explored. One peak has a narrow linewidth due to the atomic coherence for a counterpropagating configuration; the other peak has a broad linewidth, owing to the spontaneous decay for a copropagating configuration. This kind of off-resonant DROP spectrum can be used to control and offset-lock a laser frequency to a transition between excited states. We apply this technique to a multiphoton cesium magneto-optical trap, which can efficiently trap atoms on both red and blue sides of the two-photon resonance.     

Antiferromagnetic ferromagnetic transition in zigzag graphene nanoribbons induced by substitutional doping

Using first-principles calculations based on density functional theory, we show that the ground state of zigzag-edged graphene nanoribbons(ZGNRs) can be transformed from antiferromagnetic(AFM) order to ferromagnetic(FM) order by changing the substitutional sites of N or B dopants. This AFM–FM transition induced by substitutional sites is found to be a consequence of the competition between the edge and bulk states. The energy sequence of the edge and bulk states near the Fermi level is reversed in the AFM and FM configurations. When the dopant is substituted near the edge of the ribbon, the extra charge from the dopant is energetically favorable to occupy the edge states in AFM configuration. When the dopant is substituted near the center, the extra charge is energetically favorable to occupy the bulk states in FM configuration. Proper substrate with weak interaction is necessary to maintain the magnetic properties of the doped ZGNRs. Our study can serve as a guide to synthesize graphene nanostructures with stable FM order for future applications to spintronic devices.     

In situ electronic structural study of VO_2 thin film across the metal–insulator transition

The in situ valence band photoemission spectrum （PES） and X-ray absorption spectrum （XAS） at V LⅡ-LⅢ edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal–insulator transition （MIT） temperature （TMIT=67 ℃）. The spectra show evidence for changes in the electronic structure depending on temperature. Across the TMIT, pure V 3d characteristic d‖ and O 2p-V 3d hybridization characteristic πpd, σpd bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V LⅢ-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator–semiconductor, semiconductor–metal processes, and vice versa. The conventional MIT at around the TMIT=67 ℃ is actually a semiconductor–insulator transformation point.     

Temperature－Induced Switching－Over of the Luminescence Transitions in GaInNAs／GaAs Quantum Wells

Photoluminescence (PL) spectra of the GaInNAs/GaAs single quantum well (SQW) with different N compositions are carefully studied in a range of temperatures and excitation power densities. The anomalous S-shape temperature dependence of the PL peak is analysed based on the competition and switching-over between the peaks related to N-induced localized states and the peak related to interband excitonic recombination. It is found that with increasing N composition, the localized energy increases and the turning point of the S-shape temperature dependence occurs at higher temperature, where the localized carriers in the handrail states obtain enough thermal activation energy to be dissociated and delocalized. The rapid thermal annealing (RTA)effectively reduces the localized energy and causes a decrease of the switching-over temperature.