The electronic properties of the Cs/GaAs(100) interface and the formation of metastable Cs clusters

The method of threshold photoemission spectroscopy is used to investigate the electronic properties of the ultrafine gallium-enriched Cs/GaAs(100) interface. The rearrangement of the spectrum of surface photo-emission as a function of Cs coating, as well as the temperature dependence of the spectrum, enable one to identify two phases of adsorption with strong (Cs-Ga) and weak (Cs-Cs) bonds. In the first phase of adsorption with the coating of approximately 0.3 monolayers, two surface bands are detected which are due to the local interaction of cesium adatoms with gallium dimers. It is found that the transition from the first to the second phase of adsorption occurs with the Cs coating of approximately 0.7 monolayers, which corresponds to the saturation of all dangling bonds of gallium on the gallium-enriched GaAs(100) surface. In the second phase of adsorption with the coating of more than 0.7 monolayers, a number of additional photoemission singularities are observed in the spectra, whose emergence is associated with the formation of metastable Cs formations. Photoemission peaks at 1.9 and 2.17 eV may be associated with the excitation of quasi-two-and/or quasi-three-dimensional Cs clusters, and the peaks at 2.05, 2.4, and 2.78 eV may be associated with the excitation of an interface plasmon and of surface and bulk Cs plasmons, respectively.     

Prediction of Pressure-Induced Structural Transition and Mechanical Properties of Mg Y from First-Principles Calculations

Using the particle swarm optimization algorithm on crystal structure prediction,we first predict that Mg Y alloy undergoes a first-order phase transition from Cs Cl phase to P4/NMM phase at about 55 GPa with a small volume collapse of 2.63%.The dynamical stability of P4/NMM phase at 55 GPa is evaluated by the phonon spectrum calculation and the electronic structure is discussed.The elastic constants are calculated,after which the bulk moduli,shear moduli,Young's modui,and Debye temperature are derived.The brittleness/ductile behavior,and anisotropy of two phases under pressure are discussed in details.Our results show that external pressure can change the brittle behavior to ductile at10 GPa for Cs Cl phase and improve the ductility of Mg Y alloy.As pressure increases,the elastic anisotropy in shear of Cs Cl phase decreases,while that of P4/NMM phase remains nearly constant.The elastic anisotropic constructions of the directional dependences of reciprocals of bulk modulus and Young's modulus are also calculated and discussed.     

Composite behavior and multidegeneracy in high-pressure phases of Cs and Rb

The pressure-driven phases Cs III and Rb III having large unit cells are shown to be peculiar examples of commensurate modulated composites with two monatomic subsystems of striking simplicity. The two subsystems are obverse-reverse layers, symmetry related but misfitted. Modulations are smooth and describable by a few parameters within a well-defined superspace symmetry. Ab initio density-functional theory calculations show that the composite character is reflected in their physical behavior. Cs III has a low-energy mode with phason character corresponding to the relative sliding of the neighboring misfitted layers, the energy barrier being lower than 0.01 meV/atom, which is most favorable for transforming to other configurations. These phases possess a quasidegenerate energy landscape, close to the signature of incommensurate systems and quasicrystals.     

Initial stages of the interaction of sodium and cesium with gold

The desorption of Cs and Na atoms from the corresponding layers applied to a gold film deposited on textured tungsten ribbon with a preferred orientation of the (100) surface is studied by thermal desorption spectroscopy with the products of thermal desorption scanned on a pulsed time-of-flight mass spectrometer. The Cs atoms evaporated at T = 300 K are desorbed by two phases, one of which can be identified with the filling of a monolayer and the other can be attributed to the formation of the CsAu compound. The Na atoms evaporated at T = 300 K are desorbed by three phases associated with the formation of a monolayer coating, a sodium compound of with gold, and a multilayer sodium film.     

铯氧比对砷化镓光电阴极激活结果的影响

实验和理论分析结果表明，激活成功的砷化镓光电阴极的铯氧比存在一个最佳值.砷化镓光电阴极铯氧比的控制可通过调节激活过程中铯源和氧源的加热电流大小来实现.激活实验结果表明，铯氧电流比适中的样品，首次进氧时，光电流上升速度最快，激活后的阴极量子效率最高，稳定性好.当偏离这个比例，过大或过小时，光电流上升速度都会减慢，激活结果也比前者差.随着铯氧电流比的增大，铯氧交替的总次数随之减少.最佳铯氧电流比的调节应以首次进氧时光电流的上升速度最快为准，一旦确定后在整个铯氧交替过程中保持不变.     

Phase transitions and127I,133Cs quadrupole couplings in polycrystalline Cesium metaperiodate

The¹²⁷I and¹³³Cs NMR spectra of polycrystalline CsIO₄, a material of potential interest for transformation of laser irradiation, have been measured in the temperature range 170–440 K and the line shape has been analyzed taking into account first and second order quadrupole interactions and chemical shift anisotropy. Quadrupole coupling constants and asymmetry parameters have been determined as a function of temperature.¹²⁷I and¹³³Cs data consistently show two phase transitions in the ranges 243–300 K and 420–440 K with a temperature hysteresis for the first transition and a range for coexistence of two phases. The time evolution of the NMR signal suggests the existence of piezomagnetism, which was believed to exist only in crystals with a magnetic structure.     

Properties of a new ferroelectric crystal family: MCd(NO2)3 [M=NH4, K,Rb, Cs,Tl]

Single crystals of MCd(NO₂)₃, with M=NH₄, K, Rb, Cs, and Tl, respectively, were grown and investigated by means of optical and dielectric methods. The crystals show rhombohedral symmetry (possible space group R 3) and multi-domain patterns at ambient temperature. The compounds exhibit phase transitions above room-temperature of first (M=K) and second order (M=Rb, Cs, Tl), respectively, into cubic high-temperature phases. Observed dielectric anomalies and hysteresis loops point to ferroelectric properties and suggest a relaxational order-disorder mechanism of the nitrite ions to be responsible for the transitions. The MCd-(NO₂)₃-compounds are seen to represent a new family of ferroelectrics with a cubic prototypic phase closely related to the structural type of the perovskites. The transions are classified by symmetry considerations, and a possible structure of the cubic high-temperature phase with space group Pm3 is proposed.     

The atomic geometries of the ordered (2 × 2) and (√3 × √3)R30° phases of K, Rb, and Cs on Rh(111) in comparison with Ru(0001)

The local adsorption geometries of K, Rb and Cs in the (√3 × √3)R30° and (2 × 2) phases on a Rh(111) surface at coverages of 0.33 and 0.25 ML, respectively, are determined by analyzing LEED intensity data. For all (√3 × √3)R30° phases investigated, the three-fold hcp site is found. For the (2 × 2) overlayer, K remains in the hcp position, while Cs favors the on-top position. For the case of Rb-(2 × 2), LEED analysis suggests occupation of the unusual two-fold bridge site. Since LEED analysis of the Rb-(2 × 2) phase is not completely conclusive, additional experimental evidence is necessary to firmly establish this adsorption geometry.     

Hypersonic wave anomalies in Cs3H(SeO4)2 at the transitions above room temperature

The elastic properties of Cs3H ( SeO4 ) ₂ are investigated by Brillouin spectroscopy in the temperature range 20-220 covering the two transitions III II and II I occurring at and , respectively. Phase I is known to be a protonic conductive one. Discontinuities of elastic constants are generally observed at both transitions, implying first orderness. In phases II and I, a slight broadening of the Brillouin lines is detected. The results are discussed in comparison with compounds of the families XHSeO4 (X = NH4, Rb and Cs) and CsH2BO4 (B = As and P) which also undergo a transition to a superionic phase. In the conductive phase, it appears that the lattice anharmonicity is weaker in Cs3H ( SeO4 ) ₂ than in these other compounds. Received 16 October 1998     

NEA光电阴极的(Cs,O)激活工艺研究

在自行研制的负电子亲和势光电阴极性能评估实验系统上,首次利用动态光谱响应技术和变角X射线光电子能谱(XPS)表面分析技术研究了GaAs光电阴极的(Cs,O)激活工艺.获得了首次导Cs、(Cs,O)导入以及(Cs,O)循环的优化激活条件.XPS分析给出GaAs(Cs,O)的最佳激活层厚度为0.82 nm,首次导Cs达到峰值光电发射时的Cs覆盖率为0.71个单层.在优化激活条件下,可以在国产反射式GaAs上获得1025 μA/lm的积分灵敏度.     

Variation of the electronic and adsorption properties of GaAs(100) in the transition from an As-to Ga-rich surface

This paper reports on a threshold photoemission study of the variation of electronic properties occurring as Cs is adsorbed on GaAs(100) and the surface transfers gradually from the As-to Ga-rich state. The ionization energy and integrated photoemission current are studied as functions of the Cs coverage. The minimum of ionization energy and the corresponding Cs dose are established to differ substantially for the As-and Ga-rich GaAs(100) surfaces. The first observation is reported of anomalous curves with two ionization-energy minima, which are characteristics of surfaces in an intermediate state, with Ga and As dimers present. The sticking coefficient of Cs to the surface enriched in As is found to be several times smaller than that for the Ga-rich surface.     

Bond contraction in monolayers: 2d-hexagonal Cs

The first ab-initio all-electron theoretical/computational study of the energetics of a hexagonal Cs monolayer reveals that the CsCs nearest neighbor distance in the monolayer is reduced by about 11% compared with the corresponding distance in bulk Cs. This finding is in excellent agreement with measurements of the packing density of a hexagonal Cs overlayer on a W(110) surface. Model calculations show that this contraction is related to the formation of bands and the polarizability of the Cs 5p semi-core states.     

Frequency-selective laser optical pumping and spin exchange of cesium with129Xe and131Xe in a high magnetic field

We report the experimental results of frequency-selective laser optical pumping and spin exchange of Cs with¹²⁹Xe and¹³¹Xe in a high magnetic field of 11.74 T. Our results show that hyperpolarized¹²⁹Xe and¹³¹Xe nuclear magnetic resonance (NMR) signals exhibit alternating phases when the laser frequency for pumping the cesium atoms is changed, which is explained on the basis of the high-field optical pumping of Cs. We obtain about 3% polarization of the¹²⁹Xe. The electron-spin polarization of the Cs atoms has been measured to be about 22% with a simple NMR method.     

Evolution of Photocurrent during Coadsorption of Cs and O on GaAs （100）

The photocurrent and spectral response characteristics of gallium arsenide （GaAs） are obtained by a multiinformation measurement system, and the evolution of the photocurrent versus the Cs：O flux ratio is investigated. The experimental results show that the photocurrent increases approximately exponentially after the first exposure to Cs until a maximum sensitivity is reached, the detailed evolution process and the ultimate photocurrent are different for different samples. These differences are analysed, and according to the process of coadsorption of Cs and oxygen on GaAs, an equation is presented to explain the increase of photocurrent.     

LEED-Auger characterization of GaAs during activation to negative electron affinity by the adsorption of Cs and O

A combination of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) has been used to study the formation of the negative electron affinity (NEA) condition on surfaces of p-type, degenerate, (100) and (111) GaAs. Activation to NEA is achieved by adsorbing Cs and O onto atomically clean GaAs in repetitive cycles of first Cs and then O. Before activation, the clean GaAs surfaces exhibit their characteristic LEED patterns. However, once obtained, there is no significant correlation between the quality of these LEED patterns and the final activation. The adsorption of both Cs and O during activation to NEA is amorphous. Auger measurements have shown that the first photoemission maximum occurs after the adsorption of about a half monolayer of Cs. The initial O adsorption occurs on the GaAs surface between the Cs atoms. The adsorbed O interacts strongly with Cs at any stage during the activation. Peak photosensitivities, after completion of the Cs and O adsorptions, were in the range 400 to 1100 $\text{μA}\text{lumen}$. The final activation does not correlate with the quantity of Cs and O on the surface. The temperature dependence of the photosensitivity of NEA GaAs (100) activated at ?170°C has a broad maximum at about ?50°C and a subsidiary maximum at about 160°C. In addition, the photoemission at ?170°C can be either increased or decreased by having heated the sample up to 200°C, even though no Cs or O desorption has taken place. These results can be traced to changes in work function rather than to changes in bulk properties. While the LEED patterns from clean GaAs show no structural changes with temperature, such changes are observed when Cs is on the surface. It is suggested that changes both in photoemission and in LEED patterns are due to the temperature-induced mobility of Cs on GaAs. An atomic model for the NEA surface is discussed in terms of a layer of Cs and O atoms about 10 Å thick on the GaAs.     

X-ray diffraction study of liquid Cs up to 9.8 GPa

We describe an x-ray diffraction study of liquid Cs at high pressure and temperature conducted in order to characterize the structural changes associated with the complex melting curve and phase transitions observed in the solid phases. At 3.9 GPa we observe a discontinuity in the density of the liquid accompanied by a decrease in the coordination number from about 12 to 8, which marks a change to a nonsimple liquid. The specific volume of liquid Cs, combined with structural analysis of the diffraction data, strongly suggest the existence of dsp(3) electronic hybridization above 3.9 GPa, similar to that reported on compression in the crystalline phase.     

A simple model for melting curve maxima

A “shouldered” repulsive inter-atomic potential, in conjunction with a statistical mechanics perturbation theory which treats the fluid and solid phases in a consistent way, leads to a maximum in the (P, T) coexistence curve, in qualitative agreement with the melting curve of Cs.     

Rare gas coadsorption with Cs or K on Ag(111)

Rare gas (RG) coadsorption with submonolayer amounts of Cs or K on Ag(111) was studied using low-energy electron diffraction (LEED). A crossover in the alkali-RG interaction from repulsive to attractive was observed as a function of alkali species and of alkali coverage. The K---RG interaction was observed to be repulsive at all coverages, while the Cs---RG interaction was observed to be attractive at low Cs coverages and apparently repulsive at high Cs coverages. For the K + RG adsorption system, desorption data were analyzed to determine the spreading pressure in the alkali layer, thus showing that RG can be used as a 2D manometer in some coadsorption systems. From the spreading pressure it is possible to obtain some information about the properties of the adsorbed alkali such as the energy differences between commensurate and incommensurate phases. We also demonstrate that work function measurements from such coadsorption systems do not necessarily have a simple interpretation.     

Similarities and contrasts between critical point behavior of heavy fluid alkalis and d-dimensional Ising model

A brief summary is first given of recent progress in establishing the near-critical point behavior of the fluid alkalis Rb and Cs. Departure from the law of Rectilinear Diameters is emphasized, along with its consequences for theories emphasizing homogeneity and scaling. The behavior as the critical points of Rb and Cs are approached is compared and contrasted with the d-dimensional Ising model.