The temperature dependence of evaporation field for Gomer-type field-evaporation mechanisms

Theoretical formulae are developed for the temperature dependence of the onset evaporation field F°, assuming a parabolic surface-atom bonding well and a Gomer-type mechanism for the escape process. Recent experimental results for tungsten and molybdenum, when replotted in the form $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ versus $\text{1}\text{F}\text{°}$, exhibit linear behaviour in the range from about 60 to 150 K. Deviations occur below this (at 50 K for W, 35 K for Mo); the deviation temperatures are compatible with theoretical estimates of the critical temperature at which ion tunnelling effects become important. Values of the zero-Q evaporation field extrapolated from the linear regime (74 V/nm for W, 60 V/nm for Mo) are significantly higher than observed values of F° near 78 K or values of F^e derived from the Müller-Schottky formula; the difference can be explained by taking into account repulsive ion-surface and F² potential-energy terms. Our theoretical picture seems generally self-consistent at the classical level, and it is concluded that the low-temperature field-evaporation process for the more field-desorption-resistant metals is, at this level, now basically understood. Field evaporation may now be used to investigate atomic behaviour at charged surfaces, but greater care is needed over the standardization of evaporation conditions and the consistent choice of field calibration.     

Field emission and field ion microscope study of Ga,In and Sn on W: Structure,work function,diffusion and binding energy

Gallium, indium and tin were deposited on a tungsten tip by making a contact between the tip and these metals in the liquid state. The activation energies of diffusion of the adsorbates on tungsten were found to be 0.29 eV for Ga, 0.35 eV for In and 0.71 eV for Sn. The adsorbates were field-evaporated by gradually increasing a positive tip voltage by a small increment each time and the variation of the work function with the decreasing coverage was examined for each evaporation stage. The result indicates that the adatoms assume one of two different adsorption states. The adatoms bound as strongly as in a bulk crystal were field-evaporated at a low evaporation field. The remaining adatoms form a more strongly bound covering layer which maximizes the average work function of the covered surfaces, 4.75 eV for Ga, 4.63 eV for In and 5.10 eV for Sn, and are field-evaporated at a significantly higher field. The covering layer of the strongly bound adatoms were observed on the areas from the {001} to {114} planes and were hardly noticed on the {011} and 112 areas. The arrangement of the strongly bound adatoms, particularly on the {114} planes, is found to be a precise replica of the substrate arrangement. Thus, the surface density of the adatoms is exactly the surface density of the substrate atoms. The observed results suggest that an adatom occupying a tungsten lattice site and contacting four substrate tungsten atoms can establish unusually strong bonding with the substrate.     

Field ion image qualities of Ga,In and Sn on W and work functions

It has been found previously that Ga, In and Sn form a pseudomorphic monolayer on W which maximizes the work function of the covered surface. In the present work the bright and sharp field ion images of the pseudomorphic monolayer are shown comparing the images of the substrate tungsten. The interesting finding is that the images of the covering layer are most distinct for Sn which gives rise to the largest increment in the work function, and are least for In which has the smallest work function increment. A simple calculation on the penetration probaility of an electron into the potential barrier between an imaging gas atom and a metal surface indicates that the probability increases significantly with the work function and consequently the covered area would be images brightly. The calculation also suggests that the work function, that is the surface charge distribution, is one of the key factors controlling the quality of the field ion images.     

New interpretation of surface states in W and Mo

The linear combination of muffin tin orbirtals (LCMTO) technique has been used to calculate surface states in the s, dz² band gap of W and Mo. These states are found to be very sensitive to surface reconstruction effects in that they become more localized and their energies shift by ～ 0.08 Ry. Furthermore, reconstruction reduces the dangling bond charge density in the vacuum region.     

Surface dependence of field ion energy distributions,He on W

Kinetic energy distributions of helium ions produced by field ionization above the (100), (110), and (111) planes of tungsten field emitters are reported. The energy resolution of these measurements if 0.3 eV fwhm. The main peak of these distributions is found to shift by as much as 0.7 eV at $\text{F = 5.3 V/}\text{A}\text{?}$, T = 21 K, when the origin of the ions collected is changed slightly. Several possible explanations for this shift are discussed. The most plausible of these involves electron tunneling through a field adsorbed helium atom. These results have important implications for the use of field ion energy distributions as a probe of the density of electronic states of the emitter.     

On the temperature dependence of the level density parameter and its effect upon neutron evaporation spectra

Effects associated with the temperature dependence of the level density parameter,a(T), are investigated in connection with neutron evaporation processes. Different approximations, for this temperature dependence, are compared for the case of neutron emission from the compound nucleus²⁰⁹Pb.     

Electron-paramagnetic-resonance of ZnTe:Ga and ZnTe:Sn

Photosensitive EPR measurements ofS-state centers of gallium and tin in ZnTe single crystals are reported. In addition to large hyperfine interaction with the impurity nuclei the spectra show partially resolved superhyperfine structures arising from three shells of tellurium ligands. Both HF and SHF splittings decrease with increasing temperature. The samples exhibitp-type photoconductivity with the same spectral sensitivity observed with the generation and depletion of the EPR-spectrum.     

Pressure dependence of the Spin-Density Wave states in chromium doped with Si,Ge, and Sn

Abstract

The pressure dependence of the N&l temperature (T_N) has been studied in Cr-Si, Cr-Ge, and Cr-Sn alloys. In all alloys we have found a transition to the commensurate anti-ferromagnetic Spin-Density-Wave (SDW) state above a certain threshold concentration. Such a behavior was also found for transition metal additions and explained on the basis of a two-band nesting model, in which the transition metal additions act as electron donors to the d-band of Cr. The applicability of the two-band nesting model for Cr-Si, Cr-Ge, and Cr-Sn will be discussed.     

The electric field gradient and its temperature dependence for indium in beryllium

The electric field gradient (EFG) for¹¹⁷InBe has been determined by a TDPAC measurement to be eq_tot=1.83(1) 10¹⁷V/cm² at room temperature. The sources were prepared by ion — implantation of¹¹⁷Cd in a Be single crystal. The EFG was found to decrease considerably with increasing temperature. The results are compared to those of other impurities in Be.     

The ultrasonic cleaning process: its dependence on the energy density,time of action,temperature, and modulation of the sonic field

Objective evaluation of the efficiency of ultrasonic cleaning machines has hitherto been frustrated by the lack of suitable measuring processes for the quantitative determination of the degree of cleaning, and the acoustic energy existing in the bath. A method for obtaining reproducible measurements of the degree of cleaning and the effects of various parameters on the ultrasonic cleaning process are described. In particular, the relations between the energy density, the electrical power supplied to the ultrasonically excited tanks, and the degree of cleaning are discussed. Experiments carried out using different types of oscillation modulation show that optimal cleaning efficiency is achieved with a simple half-wave modulation form.     

Hydrogen on W(100): temperature dependence of LEED and angle-resolved ESD

For hydrogen adsorption on W(100), the evolution of the c(2 × 2) LEED intensities and of the H⁺ ESD signal with H coverage have been investigated for various adsorption and annealing temperatures. Striking changes have been found for the half-order LEED intensities in the temperature range 140–360 K, in agreement with other workers, where the H⁺ signal showed only minor differences. The maxima of the LEED and the ESD intensities, however, occurred at the same exposure throughout this range (≈25% of saturation coverage). A temperature dependent variation of the height of the H⁺ maximum was observed which was reversible up to the desorption temperature of the β₂ hydrogen phase. The H⁺ ESDIAD lobe was found to have a polar FWHM of about 21°, independent of temperature between 140 and 450 K, and without any azimuthal dependence. These results provide evidence for the assumption that the observable H⁺ ions desorb from reconstructed sites. The number of these sites depends on temperature and hydrogen coverage, as shown by the change of the H⁺ current with these parameters. The transition from H on reconstructed to H on unreconstructed sites is of the order-order type; the energy difference between the two different adsorbate situations is about 135 meV/site at the quarter coverage. The consistency of the results and conclusions with a bridge-site model for H adsorption is shown. Elastic interactions lead to agglomeration of adsorbed H. The azimuthal isotropy of the ESDIAD lobes is interpreted by a superposition of emission from various types of bridge-sites which smear out the anisotropy expected for individual bridge-sites.     

The electric field gradient and its temperature dependence for ruthenium in scandium and yttrium

The electric field gradients (EFG) for ruthenium in scandium and yttrium metal were determined by TDPAC measurements to be 19(4) × 10¹⁷ V/cm² and 5.5(12) × 10¹⁷ V/cm² respectively at room temperature. The EFG for Ru in Sc was found to vary considerably in the temperature range from 14 to 700 K, whereas for Ru in Y only an extremely small temperature dependence of the EFG was observed.     

Effect of the emitter temperature and emitter atom ionization potential on field evaporation

The field evaporation of nickel, nichrome alloy, and tungsten carbide at different temperatures is studied with a time-of-flight atomic probe and a field emission microscope. The charge of evaporating ions does not depend on the emitter temperature: it decreases with decreasing evaporating field F _ev. If F _ev does not vary with temperature, so does the charge of the ions. In the case of multicomponent emitters with different ionization potentials of the components, the components evaporate at the same values of F _ev in the form of atoms and ionized clusters. The reason for such behavior is that the initial evaporation of the easily ionizable component decreases the binding energy of harder-to-ionize ones to the point where they can evaporate at the same field.     

Width and temperature dependence of lithography-induced magnetic anisotropy in (Ga,Mn)As wires

In-plane magnetic anisotropy of 40-μm-long (Ga,Mn)As wires with different widths (0.4, 1.0, and 20 μm) has been investigated between 5 and 75 K by measuring anisotropic magneto-resistance (AMR). The wires show in-plane 〈1 0 0〉 cubic and [−1 1 0] uniaxial anisotropies, and an additional lithography-induced anisotropy along the wire direction in narrow wires with width of 0.4 and 1.0 μm. We derive the temperature dependence of the cubic, uniaxial, and lithography-induced anisotropy constants from the results of AMR, and find that a sizable anisotropy can be provided by lithographic means, which allows us to control and detect the magnetization reversal process by choosing the direction of the external magnetic fields.     

聚酰亚胺电导率随温度和电场强度的变化规律

介质深层充电对航天器安全运行构成了重大威胁.以聚酰亚胺为代表的此类聚合物绝缘介质的电导率受温度影响显著,又因为充电过程中局部产生强电场(10~7V/m量级),因此,其电导率模型需要综合考虑温度和强电场的影响,这对介质深层充电的仿真评估意义重大.已有的两类模型,不是低温区间不适用,就是没有充分考虑强电场的影响.基于跳跃电导理论,本文分析对比了现有电导率模型,提出了适用于较宽温度范围且合理考虑强电场增强效应的电导率新模型,并采用某型聚酰亚胺电导率测试数据做出验证.此外,为了提高新模型在强电场下的低温适用范围,尝试对强电场因子中的温度做变换,取得了满意的效果.参数敏感度分析表明新模型在电导率拟合与外推方面具有参数少、适用性强的优势.     

FIM observation of A Ga(3 × 3) superstructure on W and Mo(011)

FIM observation has revealed that the atomic arrangement of a monoatomic Ga layer on the W and Mo(011) plane is the (3 × 3) superstructure formed by zigzag chains of Ga atoms. Observed images indicate that the Ga atoms of the zigzag chains are displaced in the [01&#x0304;1] or [011&#x0304;] direction from an open W lattice site to the position where the displaced atom is stabilized by contacting three substrate W atoms.