Zhang Xinyi

Measurement of twist angle and surface torsional anchoring strength in a nematic liquid crystal cell

Abstract In this paper a novel method is developed for measuring the twist angle and surface torsional anchoring strength in nematic liquid crystal (NLC) cells. This method is based on a technique developed from the Jones optical propagation matrix. From the measurement, the actual twist angle in NLC cell and the deviation of the LC director on the boundary of the cell from the rubbing direction of the substrate are obtained. A theoretical discussion shows that the surface torsional anchoring strength has a great influence on the value of the deviation, and hence so it can be calculated.

Enhancement of fluorescence and Raman spectroscopies of undoped C60 film

The spectroscopic characteristics of C60 films deposited on different substrates have been investigated at room temperature. We find that the fluorescence of C60 film is enhanced by the presence of a rough silver surface and also by oxygen under excitation at 5145 A (2.41 eV) at room temperature. The enhancement of the fluorescence on the rough silver surface is mainly due to an appropriate work function of the silver surface and an external electromagnetic field induced by laser irradiation, which leads to a resonant coupling with the C60 molecular dipole. The fluorescence enhancement by oxygen is interpreted as resulting from the strong reduction in symmetry of C60 from Ih to C2v or lower due to photooxygenation of the C60 molecules. This also results in larger Jahn-Teller distortions. The kinetics of the photochemical reactions between oxygen and C60 have also been obtained with an observed rate constant of magnitude about 1.6 × 10−2s−1 by monitoring the changes of the fluorescence and Raman spectra with the irradiation time.

A novel sulfur-passivation method and magnetic overlayers on passivated III–V semiconductor surface

Abstract A sulfur passivation method for GaAs, CH3CSNH2 treatment has been developed. It is quite effective for removing the surface oxide layer and forming the sulfide passivation layer on GaAs surface, with sulfur atoms bound with Ga and As atoms. After being annealed, a stable sulfur passivation layer is formed. The enhancement of PL intensity reveals the reduction of surface non-radiative recombination and the density of surface states. Moreover, the investigation has been made for the role of S-passivation on interfacial interaction between magnetic overlayer and GaAs. The interdiffusion of As, Ga into overlayer is effectively inhibited, and the magnetization of Fe overlayers is enhanced. In addition, a relationship has been found between the surface chemical structure of the substrates and the magnetic property of overlayers.

Energy Transfer in Cubic PbF2:Gd Crystals

The emission and excitation spectra of cubic PbF2:Gd were measured. Compared with pure crystals, the emissions from Gd3+ were observed upon cation exciton excitation in lattices, while the emission from intrinsic self-trapped excitons was quenched. It indicates the energy transfer from exciton to the 4f state of Gd3+.

Electronic structure studies of Bi2Sr2CaCu2-xSnxO8+? system

Photoemission measurements have been carried out for Bi2Sr2CaCu2-xSnxO8+? system with conventional x-ray photoemission spectroscopy for core-level spectra and synchrotron radiation photoemission spectroscopy for valence band. With Sn doping, all core levels shift differently in binding energy, and the intensity near fermi energy becomes smaller in valence band. From the experiment, we can deduce that the shifts of all core levels and valence bands may involve some other mechanisms, such as electrostatic effects, in addition to binding energy referencing effects. We argue that the chemical environment plays a crucial role in the electronic structure of high-temperature superconductors.

A new method to enhance the magnetism of Fe overlayer on GaAs(100): sulfur passivation using CH3CSNH2

Ferromagnetic resonance (FMR) has been used to investigat the magnetism of Fe overlayer on S-passivated GaAs(100) pretreated by CH3CSNH2. Comparing with the magnetism of Fe overlayer on clean GaAs(100), we find that sulfur passivation can prevent As diffusion into Fe overlayer and weaken the interaction of As and Fe. It results in enhancing the magnetism of Fe overlayer on GaAs(100). We also investigate the effects of the pre-annealing of S- passivated GaAs(100) substrate on the magnetism of Fe overlayers. The results show that the maximum effective magnetization can be obtained at annealing temperature of 400 °C. According to the experimental results of synchrotron radiation photoemission, it can be explained by the change of chemical composition and surface structure of the passivation layer on GaAs(100) surface after the annealing.

Luminescence of SR irradiated fused quartz

Abstract The properity of amorphous SiO 2 is sensitive to the irradiation of energetic ions, electron beam, X- and γ-rays, even to VUV radiation. The luminescence of a-SiO 2 and irradiated a-SiO 2 has been investigated. Here, we report that the visible light can be emitted under the excitation of Ar + ion laser from the surface of bulk fused quartz which was irradiated by “white” SR in UHV at room temperature. The emission spectrum is an asymmetry band peaked at about 625 nm, with a FWHM of about 20 nm. The SR for irradiation is from the 800 MeV storage ring without any optical elements between the light source and the samples. The vacuum is kept below 5 × 10 −7 Pa, the average beam current is 100mA and the irradiation time is 3 hours. The intensity of the luminescence is rapidly decreased under the excitation of laser. It can stand for half an hour when the power of excited laser light is kept at 2W/cm 2 approximately.

Soft-x-ray photoemission study of Mn/GaP(100) interface

The interface formation and electronic structures of the Mn/GaP(100) interface are studied with synchrotron radiation photoemission. At the early stage of Mn deposition, Mn covers the whole GaP(100) surface. With the increase of coverage, Ga atoms can be exchanged by Mn atoms and diffuse into the Mn overlayer. However, P atoms remain always near the interfacial region. A significant difference of the electronic structures is observed between the ultra-thin and the thick Mn films. The explanations for this are given in the text.

Alkali Rb‐doped C60: The movement of the valence band (abstract)

Using synchrotron radiation light source, we have measured the photoemission spectra of undoped C60 and alkali Rb‐doped C60 to reveal their electron structure. We observed a complex movement of the top of the valence band through the process of alkali Rb doping. When Rb is doped into C60, at first the spectrum moves to the high binding energy side for about 0.5 eV. As doping continues, the spectrum moves to the low binding energy side again. The spectrum moves back toward the original site by a small amount until the stoichiometry reaches about 1. This phenomenon has not been reported in other alkali doped C60 systems. We suggest the backward movement is due to the instability of the RbC60, Rb3C60, and other phases.