V.E. de Carvalho

XPS AND EELS STUDY OF THE BISMUTH SELENIDE

Abstract A Bi 2 Se 3 crystal was studied by XPS (X-Ray Photoelectron Spectroscopy) and EELS (Electron Energy Loss Spectroscopy). Al Kα radiation and an electron beam energy in the range of 0.1 to 2 keV were used, respectively, to probe a Se-terminated (0001) surface. Samples of the constituent elements (Bi and Se) have also been measured with the same setup. The core level chemical shifts obtained show that a charge transfer occurs in Bi 2 Se 3 . The spectra in the valence band region suggest that the density of states in the compound may be obtained by combining the spectra of the constituent elements, that the electronic states in the vicinity of the gap region consist of a mixture of the metal and calcogen p -orbitals and that the band arising from the valence s -orbitals occurs about 12 eV below the valence band maximum. The EELS spectra allow to identify the bulk plasmons for the three materials and the Bi5 d 3 and Bi5 d 5 interband transitions. Considerations of the energies of the Bi5 d transitions as measured by XPS and EELS indicate that the bottom of the conduction band of the compound is 1.2 eV above the Fermi level. The EELS results also shows evidence that the losses occurring at 6.4 eV in the compound and at 5.4(5.5) eV in Bi(Se) have their origins in some surface process. We suggest that they may be associated to a surface plasmon.

Unconstrained optimisation in surface crystallography by leed: Preliminary results of its application to CdTe(110)

This paper describes the application to LEED of a form of the well-known method of unconstrained optimisation, originally developed by Hooke and Jeeves. It has been found to be of great use in the iterative searching that must be performed as a part of the process of discovering the atomic geometry of a surface using LEED. By using such an optimisation scheme, the speed with which a crystal surface structure may be found is greatly increased. The codification of the search strategy used in LEED in this way should also result in greater confidence being placed in LEED as a surface structure analysis technique. The CdTe(110) surface is used as the first application of this new scheme using newly collected, high quality experimental data. Nine structural and non-structural parameters were investigated to give a preliminary result for this surface.

Global search in photoelectron diffraction structure determination using genetic algorithms

Photoelectron diffraction (PED) is an experimental technique widely used to perform structural determinations of solid surfaces. Similarly to low-energy electron diffraction (LEED), structural determination by PED requires a fitting procedure between the experimental intensities and theoretical results obtained through simulations. Multiple scattering has been shown to be an effective approach for making such simulations. The quality of the fit can be quantified through the so-called R-factor. Therefore, the fitting procedure is, indeed, an R-factor minimization problem. However, the topography of the R-factor as a function of the structural and non-structural surface parameters to be determined is complex, and the task of finding the global minimum becomes tough, particularly for complex structures in which many parameters have to be adjusted. In this work we investigate the applicability of the genetic algorithm (GA) global optimization method to this problem. The GA is based on the evolution of species, and makes use of concepts such as crossover, elitism and mutation to perform the search. We show results of its application in the structural determination of three different systems: the Cu(111) surface through the use of energy-scanned experimental curves; the Ag(110)–c(2 × 2)-Sb system, in which a theory–theory fit was performed; and the Ag(111) surface for which angle-scanned experimental curves were used. We conclude that the GA is a highly efficient method to search for global minima in the optimization of the parameters that best fit the experimental photoelectron diffraction intensities to the theoretical ones.

The Simulated Annealing Global Search Algorithm Applied To The Crystallography Of Surfaces By Leed

Surface structure determination by Low Energy Electron Diffraction (LEED) is based on a comparison between experimentally measured and theoretically calculated intensity versus energy I(V) curves for the diffracted beams. The level of agreement between these, for different structural models, is quantified using a correlation function, the so-called R factor. Minimizing this factor allows one to choose the best structure for which the theoretical simulations are computed. Surface structure determination thus requires an exhaustive search of structural parameter space in order to minimize the R factor. This minimization is usually performed by the use of directed search methods, although they have serious limitations, most notably their inability to distinguish between false and real structures corresponding to local and global R factor minima. In this work we present the implementation of a global search method based on the simulated annealing algorithm, as suggested earlier by Rous, using the Van Hove and Tong standard LEED code and the results of its application to the determination of the structure of the Ag(111) and CdTe(110) surfaces. Two different R factors, RP and R1, have been employed in the structural searches, and the statistical topographies of these two factors were studied. We have also implemented a variation of the simulated annealing algorithm (Fast Simulated Annealing) and applied it to these same two systems. Some preliminary results obtained with this algorithm were used to compare its performance with the original algorithm proposed by Rous.

A layer-by-layer study of CdTe(110) surface Debye temperature and thermal vibrations by low energy electron diffraction

The Debye temperature and the thermal vibrations of the first two surface layers of the CdTe(110) have been studied by low energy electron diffraction (LEED). The full dynamic LEED calculations were performed using the LEEDFIT code that allows one to carry out optimizations of the Debye temperature and vibrational amplitudes of each atom in each surface layer.

Quantitative low-energy electron diffraction analysis of MnO(100) films grown on Ag(100)

We report a high-pressure investigation of the relaxor ferroelectric lead zinc niobate PbZn{sub 1/3}Nb{sub 2/3}O{sub 3} (PZN) up to 46 GPa, which is the highest pressure yet attained in the study of relaxors. The evolution of both Raman and x-ray scattering with pressure gives evidence for important pressure instabilities, which find its expression in three successive phase transitions. The observed pressure-induced suppression of diffuse scattering above 5 GPa is similar to recent reports and supports the hypothesis that this is a general feature in relaxors at high pressures.Stable pairing states of superfluid {sup 3}He in aerogel are examined in the case with a global uniaxial anisotropy which may be created by applying a uniaxial stress to the aerogel. Due to such a global anisotropy, the stability region of an Anderson-Brinkman-Morel (ABM) pairing state becomes wider. In a uniaxially stretched aerogel, the pure polar pairing state with a horizontal line node is predicted to occur, as a three-dimensional superfluid phase, over a measurable width just below the superfluid transition at T{sub c}(P). A possible relevance of the present results to the case with no global anisotropy is also discussed.A series of Ca{sub x}CoO{sub 2} (0.15{<=}x{<=}0.40) materials have been prepared by means of an ion exchange reaction from Na{sub x}CoO{sub 2}. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion, and twinning. Systematic structural analysis, in combination with the experimental data of Na{sub x}CoO{sub 2} (0.15{<=}x{<=}0.8) and Sr{sub x}CoO{sub 2} (1.5{<=}x{<=}0.4) systems, suggests that there are two common well-defined cation ordered states corresponding, respectively, to the orthorhombic superstructure at around x=1/2 and the 3{sup 1/2}ax3{sup 1/2}a superstructure at around x=1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33<x<0.5. The TEM observations also reveal an additional periodic structural distortion with q{sub 2}=a{sup *}/2 in materials for x{<=}0.35. This structural modulation also appears in the remarkable superconducting phase Na{sub 0.33}CoO{sub 2}{center_dot}1.3H{sub 2}O.Electrical resistance, thermoelectric power, dc magnetization, ac susceptibility, and electron spin resonance (ESR) are investigated for the polycrystalline Nd{sub 1-x}Sr{sub 1+x}CoO{sub 4} (x=0.25, 0.33, and 0.60). Powder x-ray diffraction (XRD) confirms that these compounds crystallize in K{sub 2}NiF{sub 4}-type structure with space group I4/mmm. The specimens exhibit ferromagnetic and semiconducting behaviors. With Sr doping, the lattice parameter c increases, the cusp intensity related to spin-glass states weakens, and the ferromagnetic property intensifies. The transport mechanisms in high temperature range obey Arrhenius law and might be understood by small polaron models. The magnetic properties present spin-glass states at {approx}18 K and Griffiths singularity at {approx}210 K.In this work we report on a low-energy electron diffraction (LEED) study of MnO(100) thick films grown on Ag(100) in order to determine their surface geometry. The LEED results indicate a topmost layer rumple of (4.8{+-}2.0)% with the oxygen ions moving towards the vacuum side. These results are in line with other surface structure determinations carried out on the (100) surface of different oxides having rock-salt structure but are in disagreement with MEIS results reported in the literature for the MnO(100) using a MnO single crystal.We report the observation of Co{sup 3+}/Co{sup 4+} short-range charge ordering in 10% Ho-doped SrCoO{sub 3-x} by means of high resolution neutron powder diffraction. The associated one-dimensional commensurate modulation, which can be described with the propagation vector q{sub CO}=(0 0 1/2) with respect to the cubic perovskite cell Pm3m, occurs for compositions close to x=0.20, corresponding to a 1:1 Co{sup 3+}/Co{sup 4+} ratio and extends over clusters of finite size (D{approx}250 A). The bond valence sums for the Co{sup 3+} and Co{sup 4+} sites are +3.07(7) and +3.95(11) (x=0.19), very close to their nominal values +3 and +4. We attribute this astonishing observation to the one-dimensional (1D) character of the associated distortion pattern, whose elastic stabilization is eased with respect to the 3D arrays reported for other charge-ordered perovskite oxides.The compounds RNi{sub 2}Mn (R=Tb, Dy, Ho, and Er) with a MgCu{sub 2}-type structure have been synthesized. The R to transition metal atom ratio is confirmed to be 1:3 using the energy dispersive spectroscopy. The structural and magnetic properties have been investigated by various experimental methods. The x-ray diffraction patterns (XRD) can be well indexed with a cubic Laves cell and space group Fd3m. The refinement results of the XRD patterns show the presence of vacancies in the crystallographic structure. The ordering temperatures T{sub C} have been derived to be 131, 94, 75, and 50 K for R=Tb, Dy, Ho and Er, respectively, which are much higher than those of their corresponding RNi{sub 2} and RMn{sub 2} compounds. A large difference of M-T curves between zero-field-cooling and field-cooling magnetization for all samples at a certain temperature range is observed in a low field, which can be understood in the terms of narrow-domain-wall pinning and a sensitive temperature dependence of coercivity.The structure of liquid CdTe was investigated at pressures up to 23.5 GPa using synchrotron x-ray diffraction. The structure factor, S(Q), and the pair distribution function, g(r), drastically change in two pressure regions, 1.8-3.0 and 7.0-9.0 GPa, accompanied with marked increase in the average coordination number. These findings suggest that there exists at least three stable liquid forms below 23.5 GPa. The pressure interval of the structural change is much smaller compared to other liquids of tetrahedrally bonded materials. Comparing the shapes of S(Q) and g(r) and other structural parameters with the respective data for the reference materials reveals that the lowest- and intermediate-pressure forms have the same local structures as the crystalline counterpart (zinc-blende-like local structure and a NaCl-like local structure), while the highest-pressure form has a different local structure from that in the crystalline form.The charge distributions of slow atomic particles that are singly scattered, multiply scattered, recoiled, and sputtered from metal surfaces are analyzed in terms of both nonadiabatic particle-substrate electron transfer and electron transfer from electronically excited substrates. The results are compared to experimental data for 50 eV Na{sup +} ions scattered from Cu(001), and Al atoms sputtered and recoiled from Al(100). The comparison allows for a quantitative determination of the role of substrate excitations in surface charge exchange. In addition, an analysis of kinetic electron emission (KEE) is carried out using similar low-energy metal projectile-metal substrate systems. Contributions to KEE from various nonadiabatic processes are quantitatively evaluated, including the same process that is responsible for charge formation in single-scattering experiments. The results are compared to experimental KEE data induced by Na{sup +} impinging on Ru(0001). The contributions of nonadiabatic one-electron processes are shown to be small when realistic particle-substrate parameters are used. Many-electron interactions are assumed to play an important role in explaining KEE and, as an illustration, a simplified hot-spot model is outlined.Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo{sub 2}O{sub 5.48} compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at T{sub OD}=776 K. Across the transition the perovskite structure loses its vacancy ordering, and the crystal symmetry changes from orthorhombic Pmmm to tetragonal P4/mmm. The resistivity measurements for temperatures above {approx}350 K yield high values of {rho}, indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole (polaronic) hopping is proposed.Granular films composed of well defined nanometric Co particles embedded in an insulating ZrO{sub 2} matrix were prepared by pulsed laser deposition in a wide range of Co volume concentrations (0.15<x<0.43). High-resolution transmission electron microscopy (TEM) showed very sharp interfaces between the crystalline particles and the amorphous matrix. Narrow particle size distributions were determined from TEM and by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. The magnetic particle size varies little for Co volume concentrations x<0.32 and increases as the percolation limit is approached. The tunneling magnetoresistance (TMR) was successfully reproduced using the Inoue-Maekawa model. The maximum value of TMR was temperature-independent within 50-300 K, and largely increased at low T, suggesting the occurrence of higher-order tunneling processes. Consequently, the tunneling conductance and TMR in clean granular metals are dominated by the Coulomb gap and the inherent particle size distribution.The five independent elastic moduli of single-crystalline hexagonal boron nitride (h-BN) are determined using inelastic x-ray scattering. At room temperature the elastic moduli are in units of GPa C{sub 11}=811, C{sub 12}=169, C{sub 13}=0, C{sub 33}=27.0, and C{sub 44}=7.7. Our experimental results are compared with predictions of ab initio calculations and previously reported incomplete datasets. These results provide solid background for further theoretical advances and quantitative input to model elasticity in boron nitride (BN) nanotubes.I argue that certain bosonic insulator-superfluid phase transitions as an interaction constant varies are driven by emergent geometric properties of insulating states. I examine the renormalized chemical potential and population of disordered bosons at different energy levels. These quantities define the geometric aspect of an effective low energy Hamiltonian which I employ to investigate various resonating states and quantum phase transitions. In a mean field approximation, I also demonstrate that the quantum phase transitions are in the universality class of a percolation problem.The electronic structure and physical properties of {gamma}-Sn{sub 3}N{sub 4} in the spinel structure are investigated by first-principles calculations. The calculated band structure, electronic bonding, and optical properties are compared with two well-studied spinel nitrides {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4}. {gamma}-Sn{sub 3}N{sub 4} is a semiconductor with a direct band gap of 1.40 eV and an attractive small electron effective mass of 0.17. Its optical properties are different from that of {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4} because of the difference in the conduction band minimum. The Sn K, Sn L{sub 3}, Sn M{sub 5}, and N K edges of the x-ray-absorption near-edge structure spectra in {gamma}-Sn{sub 3}N{sub 4} are calculated using a supercell approach and are found to be rich in structures. These spectra are discussed in the context of the electronic structure of the unoccupied conduction band in the presence of the electron core-hole interaction. These calculated spectra can be used for the characterization of this novel compound.The structure of the incommensurate phase of Rb{sub 2}ZnCl{sub 4} has been determined at 194 K (2 K above the lock-in transition) within the soliton regime using satellites up to fifth order. The rather anharmonic modulation functions agree with the expected steplike functions supported by theoretical arguments. In addition, the constancy of the ratio between the amplitudes of the fifth-order and first-order harmonics, a relation predicted by theory, indicate the correctness of the model and imply a value of 0.4 for the soliton density n{sub s}. A symmetry mode analysis shows that the incommensurate structure is consistent with the one of the lock-in phase in the sense that the displacement pattern of every symmetry mode remains unaltered in the transition except for a global change in the amplitudes.X-ray diffraction of SnO{sub 2} (cassiterite) at high pressures and temperatures demonstrates the existence of four phase transitions to 117 GPa. The observed sequence of phases for SnO{sub 2} is rutile-type (P4{sub 2}/mnm){yields}CaCl{sub 2}-type(Pnnm){yields}pyrite-type(Pa3){yields}ZrO{sub 2} orthorhombic phase I (Pbca){yields}cotunnite-type (Pnam). Our observations of the first three phases are generally in agreement with earlier studies. The orthorhombic phase I and cotunnite-type structure (orthorhombic phase II) were observed in SnO{sub 2} for the first time. The Pbca phase is found at 50-74 GPa during room-temperature compression. The cotunnite-type structure was synthesized when SnO{sub 2} was compressed to 74 GPa and heated at 1200 K. The cotunnite-type form was observed during compression between 54-117 GPa with additional laser heating carried out at 91 and 111 GPa. Fitting the pressure-volume data for the high-pressure phases to the second-order Birch-Murnaghan equation of state yields a bulk modulus of 259(26) GPa for the Pbca phase and 417(7) GPa for the cotunnite-type phase.We report x-ray photoelectron spectroscopy (XPS) study of Na and K adlayers on icosahedral Al{sub 70.5}Pd{sub 21}Mn{sub 8.5} (i-Al-Pd-Mn) quasicrystal. The Na 1s core-level exhibits a continuous linear shift of 0.8 eV towards lower binding energies (BE) with increasing coverage up to one monolayer (ML) saturation coverage. In the case of K/i-Al-Pd-Mn, a similar linear shift in the K 2p spectra towards lower BE is observed. In both cases, the plasmon related loss features are observed only above 1 ML. The substrate core-level peaks, such as Al 2p, do not exhibit any shift with the adlayer deposition up to the highest coverage. Based on these experimental observations and previous studies of alkali metal growth on metals, we conclude that below 1 ML, both Na and K form a dispersed phase on i-Al-Pd-Mn and there is hardly any charge transfer to the substrate. The variation of the adlayer and substrate core-level intensities with coverage indicates layer by layer growth.We report the magnetic properties of the ZnL{sub 2}S{sub 4} (L=Er,Tm,Yb) olivines, in which the magnetic lanthanide ions are in a potentially frustrated geometry consisting of sawtooth chains of corner-sharing triangles. Fits to the high-temperature magnetic susceptibility yielded Curie-Weiss temperatures of {theta}{sub W}{approx_equal}-4, -13, and -75 K for the Er, Tm, and Yb compounds, respectively. None of the compounds displayed magnetic long-range order above T=1.8 K. The lack of ordering at temperatures near {theta}{sub W} may be attributed to either the low dimensionality of the structure or the frustrating effect of the triangular geometry.

STRUCTURAL DETERMINATION OF THE InSb(110) SURFACE BY THE AUTOMATED TENSOR LEED PROGRAM

The InSb(110) surface structure has been re-examined using the tensor LEED approach. A refinement of the structure as well as the influence of the Debye temperature on the structure determination is presented.

Study of the InSb(110) surface Debye temperature by the tensor LEED

The recently developed automated tensor LEED Program (ATLP) has been used to study the InSb(110) surface structure. The ATLP was installed on a Sun SPARCstation SLC, where all the calculations were carried out. A refinement of the structure, obtained by theory-experiment comparison, is presented, as well as the influence of the Debye temperature on the structure determination.

Atomistic modeling of Au-Ag nanoparticle formation

Atomistic modeling of the formation process of Au-Ag nanoparticles is performed using the Bozzolo-Ferrante-Smith method for alloys for the energetic. Silver segregation to the surface, surface structure and orientation, and weak Au-Ag interactions are identified as the main features driving the formation process.

Difração de elétrons de baixa energia (LEED) e a determinação da estrutura atômica de superfícies ordenadas

We present a brief summary of the steps that lead to the demonstration that electron scattering can result in a diffraction pattern. We include a description of the intrinsic processes in low energy electron diffraction, a technique that, at present, is routinely used for surface structural determination in an atomic scale. We also present some illustrative examples.