Synthesis,Characterization of 9, 9＇-Bianthracene and Fabrication of 9, 9＇-Bianthracene Field-Effect Transistors

We synthesize and purify 9, 9＇-bianthracene with the purity up to 96.4%. The electronic and crystallographic structures of 9, 9＇-bianthracene are studied. The results of a joint experimental investigation based on a combination of x-ray diffraction （XRD） spectra, hydrogen nuclear magnetic （HNMR） spectra, infrared absorption （FT-IR） spectra, and mass spectra （MS） of 9, 9＇-bianthracene are obtained. The uniform compact film is observed by an atomic-force microscope （AFM）. Organic field effect transistors （OFETs） with an active layer based on the synthesized 9, 9＇-bianthracene are fabricated for the first time. Its field-effect mobility is as large as 0.067 cm^2 /（V·s） and the on/off ratio is above 5 ×10^4. The result demonstrates that the oligomerization of a small semiconductor molecule is an effective method to develop high-mobility organic semiconductors.     

An Artificially Garnet Crystal Materials Using In Terahertz Waveguide

A hypothesis is brought forward that the materials with low propagation loss in both optical and microwave band may exhibit good performance in terahertz （THz） band because THz wave band interspaces those two wave bands. For the purpose-of exploring a kind of low-loss material for THz waveguide, Lu2.1Bi0.9Fe5O12（LuBiIG） garnet films are prepared by liquid phase epitaxy （LPE） method on a gadolinium gallium garnet （GGG） substrate from lead-free flux because of the good properties in both optical and microwave bands. In microwave band, the ferromagnetic resonance （FMR） linewidth of the film 2△H = 2.8-5.1Oe; in optical band, the optical absorption coefficient is 600cm^-1 at visible range and about 100-170cm^-1 when the wavelength is longer than 800nm. In THz range, our hypothesis is well confirmed by a THz-TDS measurement which shows that the absorbance of the film for THz wave is 0.05-0.3 cm 1 and the minimum value appears at 2.3 THz. This artificial ferromagnetic material holds a great promise for magnetic field tunable THz devices such as waveguide, modulator or switch.     

High-resolution diode-laser spectroscopy of calcium

Saturated-absorption signals on the calcium 657 nm transition are observed by direct absorption using diode lasers and a high flux atomic-beam cell. Line-widths as narrow as 65 kHz are observed with a high signal-to-noise ratio. Prospects for using this system as a compact wavelength/frequency reference are considered.     

First-Principles Study of Li Doping in a Double-Wall Carbon Nanotube

By performing first-principles calculations, we study Li doping in a double-wall carbon nanotube where a （5,0） tube is confined inside a （14,0） tube. There are three possible sites for Li doping and two of them are energetically favorable. The change of energy band structure is closely related to the doping sites and the charge transfer is investigated. Bader charge analysis indicates that Li prefers to donate its electron to the inner （5,0） tube. Moreover, the Li capacity of the system can reach LIC4.75 which makes it a promising candidate for Li-ion battery materials.     

Analysis of X-ray spectral data with genetic algorithms

An algorithmic method for the analysis of X-ray line spectra using genetic algorithms is presented. This technique permits the extraction of diagnostic information on the emitting medium from the spectral data. As an example of the method, plasma electron number density and temperature are extracted from the analysis of X-ray spectral data recorded in an Ar-doped inertial-confinement-fusion core. For the study of a sequence of gradually changing spectra, a combination of genetic algorithms and case-based reasoning that learns from experience is used to accelerate the analysis. The technique is general and can be applied to other plasma spectroscopy studies including analysis of spatially and temporally resolved line absorption or emission data.     

Electronic Structure and Optical Properties of La-Doped SrTiO3 and Sr2TiO4 by Density Function Theory

The effect of La doping on the electronic structure and optical properties of SrTiO₃ and Sr₂TiO₄ is investigated by the first-principles calculation of plane wave ultrasoft pseudopotential based on the density function theory (DFT). The calculated results reveal that the electron doping in the case of Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ can be described within the rigid band model. The La³⁺ ions fully acts as electron donors in Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ systems and the Fermi level shifts further into the conduction bands (CBs) for Sr_1.875La_0.125TiO₄ compared to Sr_0.875La_0.125TiO₃. The two systems exhibit n-type degenerate semiconductor features. At the same time, the density of states (DOS) of the two systems shift towards low energies and the optical band gaps are broadened. The Sr_1.875La_0.125TiO₄ is highly transparent with the transmittance about 90% in the visible range, which is larger than that of Sr_0.875La_0.125TiO₃(85%). The wide band gap, small transition probability and weak absorption due to the low partial density of states (PDOS) of impurity in the Fermi level result in the optical transparency of the films...     

Absorption cross-section fluctuations driven by continuous and discrete laser frequency variations

Though the operational definition of a photon’s absorption cross-section is straightforward, the definition has a fundamental and well-known subtlety. In order to make contact with theory, the incident photons must be monoenergetic and should impinge on the target atom or molecule at a constant rate. To some extent, however, these two constraints are always violated: no field is perfectly monochromatic and laser photons bunch (for both technical and fundamental reasons). Thus, the textbook definition of cross-section must be understood in terms of a mean, in which case the quantum nature of a field and the practicability of its realization must be recognized as sources of cross-section variance. Here, we consider the role that continuous and discrete random frequency fluctuations play in determining the variance of an absorption cross-section. In particular, we investigate the differing roles of phase diffusion and mode-partition noise, finding that discontinuous mode variations have a significant and not easily discerned effect on an absorption cross-section’s variance. As a by-product of our studies, agreement between theory and experiment highlights the utility of a hybrid absorption cross-section for use in the quasi-static approximation, and also suggests an exponential relationship between the rate of mode hops and injection current in cleaved-facet diode lasers.     

Effect of Coma Aberration on Orbital Angular Momentum Spectrum of Vortex Beams

Spiral spectra of vortex beams with coma aberration are studied. It is shown that the orbital angular momentum （OAM） states of vortex beams with coma aberration are different from those aberration-free vortex beams. Spiral spectra of beams with coma aberration are spreading. It is found that in the presence of coma aberration, the vortex beams contain not only the original OAM component but also other components. A larger coma aberration coefficient and/or a larger beam waist will lead to a wider spreading of the spiral spectrum. The results may have potential applications in information encoding and transmittance.     

Preparation and Photocurrent Performance of Highly Ordered Titania Nanotube Implanted with Ag/Cu Metal Ions

Ag/Cu-doped titania nanotubes （Ag/TiNT, Cu/TiNT） are prepared by a metal vapor vacuum arc implanter. A scanning electronic microscope is employed for microstruetural characterization. The photo-current performance of doped titania nanotubes under UV and visible light is tested by an electrochemical workstation CS300UA, the results show that the absorption edge of both Ag/TiNT and Cu/TiNT samples shifts to the visible light region and the band gap becomes narrower. Ag/TiNT possesses better photo-current ability than Cu/TiNT under UV and visible light. Titania doped with Ag and Cu metal ions is also studied based on the linearized augmented planewave method implemented by WIEN2k package, the result becomes better with the experimental performance.     

High resolution measurements of the hyperfine structure of atomic Lanthanum for energetically low lying levels of odd parity

Doppler-reduced saturation absorption spectroscopy is applied to study the hyperfine structure of excited levels of Lanthanum. 16 transitions in the near infrared wavelength range are investigated. Precise values for the magnetic dipole hyperfine structure constants A as well as for the electric quadrupole hyperfine structure constants B of the isotope ¹³⁹La are determined for 14 levels of odd parity and nine levels of even parity. For levels of even parity a good agreement is found with values from previous measurements using sub-Doppler methods. For levels of odd parity previously determined values are improved and for two levels new values of the hyperfine structure constants are reported.     

Experimental Research of Spontaneous Evolution from Ultracold Rydberg Atoms to Plasma

The spontaneous evolution from ultracold Rydberg atoms to plasma is investigated in a caesium MOT by using the method of field ionization. The plasma transferred from atoms in different Rydberg states （n = 22-32） are obtained experimentally. Dependence of the threshold time of evolving to plasma and the threshold number of initial Rydberg atoms on the principal quantum number of initial Rydberg states is studied. The experimental results are in agreement with hot-cold Rydberg-Rydberg atom collision ionization theory.     

Experimental Study on Liquid Free Surface in Buoyant-Thermocapillary Convection

We investigate the surface deformations of buoyant-thermocapillary convection in a rectangular cavity due to gravity and temperature gradient between the two sidewalls. The cavity is 52mm×42 mm in horizontal cross section, the thickness of liquid layer h is changed from 2.5 mm to 6.5 mm. Surface deformations of h = 3.5 mm and 6.0mm are discussed and compared. Temperature difference is increased gradually, and the flow in the liquid layer will change from stable convection to unstable convection. Two kinds of optical diagnostic system with image processor are developed for study of the kinetics of buoyant-thermocapillary convection, they give out the information of liquid free surface. The quantitative results are calculated by Fourier transform and correlation analysis, respectively. With the increasing temperature gradient, surface deformations calculated are more declining. It is interesting phenomenon that the inclining directions of the convections in thin and thick liquid layers are different. For a thin layer, the convection is mainly controlled by thermocapillary effect. However, for a thick layer, the convection is mainly controlled by buoyancy effect. The surface deformation theoretically analysed is consistent with our experimental results. The present experiment proves that surface deformation is related to temperature gradient and thickness of the liquid layer. In other words, surface deformation lies on capillary convection and buoyancy convection.     

A Closed-Form Model for Position-Dependent Potential across the Channel in DG-MOSFETs

A closed-form model for electrostatic potential distribution in the direction normal to the channel for double-gate （DG） MOSFETs is presented. The effects of doping （NA for nMOS） and minority carriers both are taken into account for the first time, in solving Poisson＇s equation analytically. Excellent agreement between model-predicted results and numerical device simulation is achieved for a wide range of body thickness, light or high channeldoping, under various bias conditions. This complete closed form for position-dependent potential distribution has wide applications for MOS compact modelling and device design.     

Absorption measurements in dense cesium vapor using a UV–violet light-emitting diode

We present results of absorption measurements in a dense superheated cesium vapor generated in an all-sapphire cell using a UV–violet light-emitting diode as a continuum source. Due to the very effective thermal destruction of Cs₂ molecules, a number of diffuse and satellite bands appear around higher members of cesium principal series lines, which are not easily visible in saturated cesium vapor. From the temperature dependence of the diffuse features we can distinguish short-range singlet transitions from temperature-independent spectral features that stem either from a shallow lowest triplet state or from the long-range photo association spectra of triplet or singlet Cs₂ molecules. The limits of the present interpretation of the observed bands in the UV–violet spectral region are discussed. Received: 25 May 2000 / Revised version: 20 September 2000 / Published online: 13 December 2000     

Reversibility of Temperature-Induced Liquid Transition in Pb26Sn42Bi32 Melt: Experimental Evidence with Electrical Property

Exploring nature of liquid structures and properties is becoming more interesting in various fundamental and applied fields. With different resorts including the resistivity method, temperature-induced liquid-liquid structure transitions （TI-LLST） have been suggested and verified to occur in some liquid alloys, while the reversibility of TI- LLST has rarely been examined as yet. Unlike some other investigated liquid alloys, here we show that electrical resistivity of Pb26Sn42Bi32 melt exhibit an anomalous change in the first heating run and a reversible change in the following cooling and heating cycles. Taking account of the structural sensitivity of electrical resistivity, the abnormal patterns suggest two sorts of TI-LLST that are irreversible and reversible in the liquid ternary alloy, respectively. This interesting phenomenon together with other growing evidence imply that liquid structures and their change characteristics are multiform and complex.     

Element Substitution Effect in Transition Metal Oxypnictide Re(O1-xFx)TAs (Re=rare earth, T=transition metal)

Different element substitution effects in transition metal oxypnictide Re(O_1-xF_x)TAs, with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, are studied. Similarto the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145K, which is ascribed to the spin-density-wave instability. Electron doping by F increases T_c to about 50K. While in the case of Gd, Tc is reduced below 10K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO_1-xF_xRuAs and CeO_1-xF_xRuAs compounds. The metallic behaviour is observed down to 4K.     

Spectroscopy and dynamics of neutral atoms in superfluid helium

Optical spectra and radiative lifetimes of neutral atoms in superfluid helium have been studied. The absorption and emission spectra of Ag, Mg, Yb, Al, Ga, and In were found to exhibit shifts and broadening typical of atoms residing in microscopic He bubbles, showing that this type of trapping is fairly general. The radiative lifetimes measured for these atoms are close to the free space values, indicating that the surrounding bubble hardly perturbs the electronic orbitals during photo emission. One exception observed in Sr is discussed, where competing autoionization substantially decreases the lifetime of a high excited triplet state. A transient non-bubble state with sharp, free atom like spectra is seen during the first few s after dispersion for many atomic species. The dynamics of this state are unusual, with for example very short radiative lifetimes measured for light alkali atoms.     

Properties of Field Aligned Current in Plasma Sheet Boundary Layers in Magnetotail: Cluster Observation

Field aligned current （FAC） distribution in the plasma sheet boundary layers （PSBLs） in the magnetotail is studied statistically by analysing magnetic field data from the Cluster 4-point measurements. The results show that the FAC distribution on the dusk side is not the same as that on the dawn side in the magnetotail. On the each side earthward and tailward, FA C occurrences are different; occurrence and average current density of FA Cs in the northern hemisphere are different from those in the southern hemisphere. This implies that the FACs have dusk-dawn side asymmetry, polarity asymmetry and inter hemisphere difference in the magnetotail. The present results give a good observation evidence for study on the FAC mechanism.     

Improvement of High Temperature Characteristics for SiGeC p-i-n Diodes with Carbon Incorporation

Temperature-dependent characteristics of SiGeC p-i-n diodes axe analysed and discussed. Based on the ISE data, the temperature-dependent physical models applicable for SiGeC/Si diodes are presented. Due to the addition of carbon into the SiGe system, the thermal stability of SiGeC diodes are improved remarkably. Compared to SiGe diodes, the reverse leakage current of SiGeC diodes is decreased by 97.1% at 400 K and its threshold voltage shift is reduced over 65.3% with an increasing temperature from 300 K to 400 K. Furthermore, the fast and soft reverse recovery characteristics are also obtained at 400 K for SiGeC diodes. As a result, the most remarkable feature of SiGeC diodes is the better high-temperature characteristics and this can be applied to high temperature up to 400 K.