Search for ferromagnetism in ultrathin epitaxial films: Cr/Au(100), Cr/Cu(100), and Fe/Cu(100)

The surface magneto-optic Kerr effect (SMOKE) technique was used to search for ferromagnetism in monolayer-range films of Cr and Fe grown on Au(100) and Cu(100). The growth modes were characterized using low energy electron diffraction (LEED) and Auger electron spectroscopy. The fcc structure of Cr could not be stabilized on Cu(100). Ferromagnetism was not observed for the Cr/Au(100) films at temperature above 100 K. Ferromagnetism also was not observed for fcc Fe/Cu(100) grown at room temperature; but for growth at >150°C, a ferromagnetic, metastable state was observed for the top layer of the Fe film, in the absence of bulk ferromagnetism. The ferromagnetic Fe/Au(100) system was used to establish the sensitivity of the approach.     

High-resolution photoemission from Ag(100), Ag(110), and Ag(111)

Energy distribution curves of photoelectrons emitted normal to (100), (110), and (111) faces of silver have been obtained at photon energies of 21.22, 16.85, and 11.83 eV. The results are compared with Christensen's relativistic band structure calculation of bulk silver yielding a close correspondence between experiment and theory. A surface state in the L gap immediately below the Fermi level is identified.     

Cu(100),(111),(110)表面晶格振动和力常数

采用嵌入原子方法得到了描述Cu基态相互作用性质的半经验函数;推导了表面面间力常数的计算公式,并计算了Cu(100),(111),(110)表面面间力常数,由此揭示了原子间相互作用在体内和在表面附近的区别,以及它们在不同表面结构中的差异。采用递推方法计算了相应的表面振动的投影态密度,与电子能量损失谱(EELS)所得到的实验结果符合得较好。 关键词：     

Langmuir evaporation from the (100), (111A), and (111B) faces of GaAs

We have measured the Langmuir evaporation of Ga and As from the (100), (111A), and (111B) faces of GaAs above and below the congruent evaporation temperature T_c. We have found that T_c is lowest for the (111B) face and highest for the (111A) face. These differences can be understood in terms of the different lifetimes of surface Ga on these faces. Furthermore, we have deduced that the evaporation processes are the rate limiting steps in the decomposition of GaAs. Below T_c, decomposition is controlled by the evaporation of Ga; above T_c it is controlled by the evaporation of As.     

Steady state shapes of periodic profiles on (110), (100), and (111) surfaces of nickel

Periodic surface profiles with amplitudes of ≦0.4 μm and periodicities of 4–20 μm were prepared on Ni(110), (100), and (111) single crystal surfaces. These crystals were annealed in ultra-high vacuum (UHV) at 1073–1327 K after they had been cleaned by Ar ion bombardment and investigated by Auger electron spectroscopy. The geometry of the profiles was studied in UHV by laser diffraction and outside the vacuum by interference microscopy. The profiles have sinusoidal shapes on Ni(110) but trapezoidal shapes on both the (100) and (111) surfaces. This type of faceting can be understood on the basis of the anisotropic surface energy of Ni, with cusps at the (100) and (111) orientations. Model calculations show in the case of anisotropic surface energy that periodic profiles develop facets which correspond to the low surface energy orientations (close-packed surfaces).     

The surface chemistry of ethylene adsorbed on Mo(100), oxygen-covered Mo(100) and MoO2

Ethylene adsorbed on Mo(100) and oxygen-covered Mo(100) can thermally decompose to yield hydrogen and adsorbed carbon, desorb molecularly, self-hydrogenate to produce ethane or dissociate to form adsorbed C₁ species which can hydrogenate to form methane. Complete thermal decomposition of the ethylene is proposed to take place on the four-fold sites on Mo(100) since the hydrogen yield decreases linearly with oxygen coverage. The ethylene desorption activation energy increases with increasing oxygen coverage suggesting that ethylene bonds to Mo(100) predominantly by donation of π electrons to the molybdenum surface. The ethylene hydrogenation activation increases as a function of oxygen coverage in accord with this effect. The yield of methane also varies with oxygen coverage so that no methane desorption is detected for clean Mo(100) but the yield increases with oxygen coverage reaching a maximum at a coverage of ≈0.6 ML and decreasing at higher coverages. Photoelectron spectroscopy results suggest that adsorbed oxygen increases the dissociative probability of ethylene. In addition, experiments in which carbenes are grafted onto the surface by decomposing methylene iodide show that carbenes are stabilized by the addition of oxygen to the surface. These effects both explain the increase in methane yield as a function of increasing oxygen coverage. The decrease at higher coverage is likely due, at least in part, to the lack of hydrogen. The ethane yield also decreases at higher coverages due to a similar effect.     

Angular distributions of H2 desorbed from the (100), (110), and (111) faces of copper crystals

The angular distributions of hydrogen molecules desorbed from copper have been measured for single-crystal surfaces of three different orientations: (100), (110), and (111). The surfaces were cleaned to the degree that the impurities were below the level of detection by Auger electron spectroscopy. The angular distributions were all significantly more peaked at the surface normal than the distribution corresponding to diffuse emission, i.e., where the molecular flux is proportional to cos θ, where θ is the angle measured from the surface normal. We have characterized the observed angular distributions by fitting the emperical expression cos^dθ to the data, where d is an adjustable parameter. The results are: d ≈ 5 for (100); d ≈ 2.5 for (110); d ≈ 6 for (111). The distributions are nearly symmetric about the azimuthal angle between the plane of detection and the principal axes of the surface lattice. Possible models are considered.     

The interacting states (020), (100), and (001) of H216O

A fit of 450 rotational levels of the 3 vibrational states (020), (100), and (001) of H₂¹⁶O has been performed using 57 effective constants. The Fermi-type interaction between (020) and (100) and the Coriolis type interaction between (100) and (001) as well as between (020) and (001) are taken into account. The part of the Hamiltonian which is diagonal in the vibrational quantum numbers is a Watson-type Hamiltonian. Most of the perturbed levels are well reproduced and the general agreement between experimental and calculated levels is satisfactory with 70% of the calculated ones falling within 15 × 10^?3 cm^?1 of the observed ones.     

C60 growth on Si(100), GaSe(0001) and GeS(001)

C₆₀ films have been grown in ultra high vacuum on various crystalline substrates and the structure of the films has been investigated by low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS). The C₆₀ films form randomly oriented nanocrystals on Si(100), mesoscopic polycrystals on GaSe(0001) and microscopic single crystals on GeS(001). The vibrational structure of the C₆₀/substrate interfaces is analyzed in detail by HREELS carried out in the dipole and impact scattering regimes. It is shown that the epitaxy of C₆₀ on GeS(001) is induced by the weak van der Waals bonding and the peculiar corrugation of the substrate surface.     

Chemisorption of CO on Cu(100), Ag(110) and Au(110)

The adsorption of CO on Cu, Ag and Au is studied using core and valence photoemission, X-ray absorption and autoionization of core excited states. The purpose is to investigate the nature of the adsorption bond starting out from the well-established chemisorption system CO/Cu(100)-c(2 × 2), and from the results we suggest that CO forms chemisorbed phases also on Ag(110) and Au(110). The photoemission spectra show strong shake-up satellites both for the valence levels and the core levels. The separation to the satellite appearing closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This is based on the widths of the XA resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state. The same trends regarding the shake-up intensities are observed both for the valence and core levels.     

Planar channeling of helium ions along (100), (110) and (111) MgO

Planar channelling of 1, 1.5 and 2 MeV ⁴He⁺ ions along (100), (110) and (111) MgO have been studied experimentally using Rutherford backscattering. Values of the half angle $\text{ψ}{}^{\mathrm{P}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, shoulder half angle $\text{ψ}{}^{\mathrm{s}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and surface minimum yield x^P_min have been determined for channelling with respect to the two sublattices. Agreements and discrepancies with existing theories are discussed.     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.     

100 keV 10-B + implantation into poly-(di-n-hexyl silane), (PDHSi)

100 keV¹⁰B⁺ ions were implanted into poly-(di-n-hexyl silane) in different directions at a fluence of 1×10¹⁴ cm^-2, and their depth distribution was determined by means of the neutron depth profiling technique. In no case were the projectile ions found to come to rest according to their predicted range profiles. Instead, they are always found to undergo considerable long-range migration. During the irradiation process this motion appears to be radiation-enhanced, and during the subsequent annealing steps one appears to deal with regular thermal diffusion. The implant redistribution is always found to be governed strongly by the self-created damage, insofar as both electronic and nuclear defects in the polymer act as trapping centers. Their population ratio is modified by thermal annealing.The as-implanted redistribution shows a pronounced directional dependence, which essentially is a consequence of the spatial distributions of the electronic and nuclear damage. The changes of the nuclear defect distribution during thermal annealing are studied by a specially developed tomographic method. It is found that boron is preferentially trapped along the irradiation direction, exhibiting quite pronounced prolate (i.e. cigar-like) distributions. This shape is almost unaffected by thermal annealing. PACS 61.80; 66.30; 66.30.L; 87.59.F     

Surface states calculation for the (100), (110) and (111) faces of Si

Using essentially the Heine and Jones model of the band structure for diamond-type semiconductors and a 3-dimensional surface state secular equation, we have calculated the bands of surface states for the (100), (110) and (111) orientations of the surface plane of ideal Si. Our bands for the (110) surface are very similar to those obtained by Jones. Based on these bands of surface states, we can explain reasonably both optical and electrical measurements on the (111) surface.