Low-energy Electron Diffraction Intensity Analysis Research Articles

Structure-property relationship of reversible magnetic chirality tuning

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction mediates collinear magnetic interactions via the conduction electrons of a nonmagnetic spacer, resulting in a ferro- or antiferromagnetic magnetization in magnetic multilayers. Recently it has been discovered that heavy nonmagnetic spacers are able to mediate an indirect magnetic coupling that is noncollinear and chiral. This Dzyaloshinskii-Moriya-enhanced RKKY interaction causes the emergence of a variety of interesting magnetic structures, such as skyrmions and spin spirals. Here, we show by spin-polarized scanning tunneling microscopy that the interchain coupling between manganese oxide chains on Ir(001) can reproducibly be switched from chiral to collinear antiferromagnetic by increasing the oxidation state of ${\mathrm{MnO}}_{2}$, while the reverse process can be induced by thermal reduction. The underlying structure--property relationship is revealed by low-energy electron diffraction intensity analysis. Density functional theory calculations suggest that the magnetic transition may be caused by a significant increase of the Heisenberg exchange which overrides the Dzyaloshinskii-Moriya interaction upon oxidation.

Quantitative Multilayer Cu(410) Structure and Relaxation Determined by QLEED

Industrially relevant catalytically active surfaces exhibit defects. These defects serve as active sites; expose incoming adsorbates to both high and low coordinated surface atoms; determine morphology, reactivity, energetics, and surface relaxation. These, in turn, affect crystal growth, oxidation, catalysis, and corrosion. Systematic experimental analyses of such surface defects pose challenges, esp., when they do not exhibit order. High Miller index surfaces can provide access to these features and information, albeit indirectly. Here, we show that with quantitative low-energy electron diffraction (QLEED) intensity analyses and density functional theory (DFT) calculations, we can visualize the local atomic configuration, the corresponding electron distribution, and local reactivity. The QLEED-determined Cu(410) structure (Pendry reliability factor RP ≃ 0.0797) exhibits alternating sequences of expansion (+) and contraction (−) (of the first 16 atomic interlayers) relative to the bulk-truncated interlayer spacing of ca. 0.437 Å. The corresponding electron distribution shows smoothening relative to the bulk-determined structure. These results should aid us to further gain an atomic-scale understanding of the nature of defects in materials.

Surface structure and stacking of the commensurate (13×13)R13.9∘ charge density wave phase of 1T−TaS2(0001)

By quantitative low-energy electron diffraction (LEED) we investigate the extensively studied commensurate charge density wave (CDW) phase of trigonal tantalum disulphide (1T-TaS$_2$), which develops at low temperatures with a $\left(\sqrt{13}\times\sqrt{13}\right)$R13.9{\deg} periodicity. A full-dynamical analysis of the energy dependence of diffraction spot intensities reveals the entire crystallographic surface structure, i.e. the detailed atomic positions within the outermost two trilayers consisting of 78 atoms as well as the CDW stacking. The analysis is based on an unusually large data set consisting of spectra for 128 inequivalent beams taken in the energy range 20-250 eV and an excellent fit quality expressed by a bestfit Pendry R-factor of R=0.110. The LEED intensity analysis reveals that the well-accepted model of star-of-David-shaped clusters of Ta atoms for the bulk structure also holds for the outermost two TaS$_2$ trilayers. Specifically, in both layers the clusters of Ta atoms contract laterally by up to 0.25 $\r{A}$ and also slightly rotate within the superstructure cell, causing respective distortions as well as heavy bucklings (up to 0.23 $\r{A}$) in the adjacent sulphur layers. Most importantly, our analysis finds that the CDWs of the 1$^{\text{st}}$ and 2$^{\text{nd}}$ trilayer are vertically aligned, while there is a lateral shift of two units of the basic hexagonal lattice (6.71 $\r{A}$) between the 2$^{\text{nd}}$ and 3$^{\text{rd}}$ trilayer. The results may contribute to a better understanding of the intricate electronic structure of the reference compound 1T-TaS$_2$ and guide the way to the analysis of complex structures in similar quantum materials.

Crystallographic structure and energetics of the Rh(1 0 0)-(3 × 1)-2O phase

In this study we investigate the crystallographic structure of the Rh(1 0 0)-()-2O phase by quantitative low energy electron diffraction (LEED) and scanning tunnelling microscopy as well as the energetics of the system applying density functional theory calculations (DFT). The () structure forms upon exposing the clean Rh(1 0 0) surface to 1200 L of oxygen at 520 K. A full-dynamical LEED intensity analysis (Pendry R-factor ) reveals an oxygen-induced shifted row-reconstruction of the rhodium top layer where every third Rh-row is displaced by half a surface lattice parameter along the [0 1 1]-direction. There are two oxygen atoms within the unit cell which assume threefold coordinated sites on both sides of the shifted Rh-row with one bond to the shifted and two bonds to the unshifted rows. DFT calculations yield a total energy gain of 0.27 eV per oxygen atom compared to adsorption on the unreconstructed surface. This by far overcompensates the energetic penalty of 0.10 eV per oxygen atom for shifting the Rh-row and thus drives the substrate reconstruction. A coadsorption of oxygen at remaining regular sites of the substrate is not observed in experiment and is found to be energetically unfavorable.

Structure and ordering of oxygen on unreconstructed Ir(100)

The adsorption of oxygen on the unreconstructed Ir(100) surface is investigated by a combination of experimental and theoretical methods comprising low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density-functional theory (DFT). Apart from the well-known $(2\ifmmode\times\else\texttimes\fi{}1)$-O phase, we find a new $(3\ifmmode\times\else\texttimes\fi{}1)$-O phase for temperatures below 180 K. Our DFT calculations predict these two phases to be the only fundamental ground states of the system in the coverage range up to 0.5 monolayers. An analysis of the phase transitions as a function of coverage reveals extended coexistence ranges between the clean surface and the $3\ifmmode\times\else\texttimes\fi{}1$ phase, or between the $3\ifmmode\times\else\texttimes\fi{}1$ and $2\ifmmode\times\else\texttimes\fi{}1$ phases, respectively. As a function of temperature, both phases undergo order-disorder transitions at about 650 K for the $2\ifmmode\times\else\texttimes\fi{}1$ phase and 180 K for the $3\ifmmode\times\else\texttimes\fi{}1$ phase, the latter being only partially reversible. The complete ordering behavior can be consistently explained by the energetics of model defect structures calculated by DFT. The crystallographic structure of the phases is determined by full-dynamical LEED intensity analyses, yielding excellent agreement between experimental and calculated data sets (Pendry $R$-factors: ${R}_{P}\ensuremath{\approx}0.1$). Oxygen was found to assume bridge sites always inducing significant relaxations within the substrate. The derived structural parameters coincide with the respective predictions from DFT on the picometer scale. It is also shown that remnants and precursor stages of the clean surface's reconstruction can only be detected through the application of real-space methods such as STM. The overarching objective of the present study is to demonstrate how precisely and accurately such an adsorption system can be investigated nowadays by using a concerted experimental and theoretical approach.

Structure and oscillatory multilayer relaxation of the bismuth (100) surface

We present a combined experimental and theoretical study of the surface structure of single crystal Bi(100) via scanning tunneling microscopy (STM), low-energy electron diffraction intensity versus energy (LEED-IV) analysis and density functional theory (DFT). We find that the surface is unreconstructed and shows an unusually large oscillatory multilayer relaxation down to the sixth layer. This unexpected behavior will be explained by a novel mechanism related to the deeply penetrating electronic surface states. STM reveals wide (100) terraces, which are separated by two-layer high steps in which the shorter of the two interlayer spacings is terminating this surface, consistent with the LEED structural analysis and DFT.

Structure of ultrathin Pd films determined by low-energy electron microscopy and diffraction

Palladium (Pd) films have been grown and characterized in situ by low-energy electron diffraction (LEED) and microscopy in two different regimes: ultrathin films 2–6 monolayers (ML) thick on Ru(0001), and ∼20 ML thick films on both Ru(0001) and W(110). The thinner films are grown at elevated temperature (750 K) and are lattice matched to the Ru(0001) substrate. The thicker films, deposited at room temperature and annealed to 880 K, have a relaxed in-plane lattice spacing. All the films present an fcc stacking sequence as determined by LEED intensity versus energy analysis. In all the films, there is hardly any expansion in the surface-layer interlayer spacing. Two types of twin-related stacking sequences of the Pd layers are found on each substrate. On W(110) the two fcc twin types can occur on a single substrate terrace. On Ru(0001) each substrate terrace has a single twin type and the twin boundaries replicate the substrate steps.

The surface relaxation and band structure of Mo(112)

The experimental and theoretical surface band structures of Mo(112) are compared. Thissurface band structure mapping is presented with corrections included for the latticerelaxation of the Mo(112) surface. Quantitative low energy electron diffraction (LEED) hasbeen used to determine the details of the Mo(112) surface structure. The firstlayer contraction is 14.9% by LEED intensity versus voltage analysis and is ingeneral agreement with the 17.6% contraction found from total surface energyoptimization. The electronic band structure is mapped out along – and – of the surface Brillouin zone (SBZ). There is strong evidence of electron–phononcoupling particularly in the region of the Fermi level band crossing at0.54 Å−1.

Nature of Xenon adsorption on graphite: On-top versus hollow site preference

We study the interaction between Xe adatoms and the graphite (0001) surface employing all-electron density-functional theory calculations. Xe adatoms prefer to adsorb in the hollow site, which is consistent with a recent dynamical low-energy electron diffraction intensity analysis. We explain the hollow site preference as a consequence of the greater polarization of Xe adatoms at the hollow site, as well as of the surface C atoms, and hence a greater induced dipole moment, which provides a larger contribution to the attractive interaction. As a consequence, Xe adatoms get closer to the surface in the hollow site. The above-mentioned mechanisms of the interaction between Xe atoms and graphite (0001) are, thus, very similar to the picture recently proposed for rare-gas adatoms on close-packed metal surfaces.

Structural determination of the Bi(110) semimetal surface by LEED analysis andab initiocalculations

The surface structure of Bi(110) has been investigated by low-energy electron diffraction intensity analysis and by first-principles calculations. Diffraction patterns at a sample temperature of $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and normal incidence reveal a bulk truncated $(1\ifmmode\times\else\texttimes\fi{}1)$ surface without indication of any structural reconstruction despite the presence of dangling bonds on the surface layer. Good agreement is obtained between the calculated and measured diffraction intensities for this surface containing only one mirror-plane symmetry element and a buckled bilayer structure. No significant interlayer spacing relaxations are found. The Debye temperature for the surface layer is found to be lower than in the bulk, which is indicative of larger atomic vibrational amplitudes at the surface. Meanwhile, the second layer shows a Debye temperature close to the bulk value. The experimental results for the relaxations agree well with those of our first-principles calculation.

Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations

The surface structure of Bi(111) was investigated by low-energy electron diffraction (LEED) intensity analysis for temperatures between 140 and $313\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and by first-principles calculations. The diffraction pattern reveals a $(1\ifmmode\times\else\texttimes\fi{}1)$ surface structure and LEED intensity versus energy simulations confirm that the crystal is terminated with a Bi bilayer. Excellent agreement is obtained between the calculated and measured diffraction intensities in the whole temperature range. The first interlayer spacing shows no significant relaxation at any temperature while the second interlayer spacing expands slightly. The Debye temperatures deduced from the optimized atomic vibrational amplitudes for the two topmost layers are found to be significantly lower than in the bulk. The experimental results for the relaxations agree well with those of our first-principles calculation.

First-principles investigation of the multilayer relaxation of stepped Cu surfaces

We performed density-functional theory calculations, employing the all-electron full-potential linearized augmented plane-wave (FLAPW) method, for the multilayer relaxations of the vicinal, high-Miller-index Cu(210), Cu(211), and Cu(331) surfaces, as well as for the flat, low-Miller-index Cu(100), Cu(110), and Cu(111) surfaces. Generally, it is expected that the interlayer relaxation-sequence at stepped metal surfaces with $n$ surface atom rows in the terraces exposed to the vacuum show $n\ensuremath{-}1$ contractions (indicated by $\ensuremath{-}$) followed by one expansion (indicated by $+$). However, recent studies based on low-energy electron diffraction (LEED) intensity analysis and all-electron FLAPW calculations suggested that the multilayer relaxation-sequence of the stepped Cu(331) surface, for which $n=3$, behaves anomalously, i.e., $\ensuremath{-}++\ensuremath{\cdots}$, instead of the expected $\ensuremath{-}\ensuremath{-}+\ensuremath{\cdots}$. From the results presented in this work, we did not find any indication of such anomalous behavior for Cu(331) or for any of the investigated stepped Cu surfaces. For the flat surfaces we obtained the expected contraction of the topmost interlayer distance. In the particular case of the Cu(110) surface, a pronounced alternating oscillatory behavior extending over six interlayer distances was found, i.e., $\ensuremath{-}+\ensuremath{-}+\ensuremath{-}+$. For all studied Cu surfaces in the present work, we found a good quantitative agreement between our interlayer relaxations and those obtained by LEED intensity analysis.

TMC(1 0 0) surface relaxation studied with low-energy-electron-diffraction intensity analysis

The atomic structures of TiC(1 0 0), NbC(1 0 0) and ZrC(1 0 0) were determined precisely by low-energy electron diffraction intensity analysis. The surface atomic structure is relaxed; topmost C atoms are displaced outward and metal atoms inward with respect to the truncated bulk atomic positions. It is consistent with the results of the force constant changes in surface phonon experiments. For TiC, the results agree qualitatively with the theoretical work [Price et al., Phys. Rev. Lett. 77 (1996) 3375] and quantitatively with the latest theoretical results based on the first-principles molecular dynamics method [Kobayashi, Jpn. J. Appl. Phys. 39 (2000) 4311–4314, Part 1, No. 7B, 30 July 2000].

The SnO 2(1 1 0)(4 × 1) structure determined by LEED intensity analysis

The (4×1) reconstruction of the SnO 2(1 1 0) surface has been determined by means of low energy electron diffraction (LEED) intensity analysis. The (4×1) reconstruction was obtained by annealing at 1000 K the ion bombarded SnO 2(1 1 0) surface. The intensity versus accelerating voltage ( I– V) curves measured for this surface were compared with calculations performed using the tensor LEED method. As a reference structure, we used the model proposed on the basis of the STM images obtained for the SnO 2(1 1 0)(4×1) superstructure. This model consists of an ordered array of oxygen vacancies in the topmost mixed oxygen–tin layer. The results of the present analysis confirm the STM model. Moreover, we found a buckling in the first layer and lateral displacements of the atoms with respect to the ideal positions around the oxygen vacancies.

TiC(100) surface relaxation studied with low-energy electron diffraction intensity analysis

The atomic structure of TiC(100) was determined precisely by low-energy electron diffraction intensity analysis. The surface atomic structure is relaxed; the topmost C atoms are displaced outward and Ti atoms inward with respect to the truncated bulk atomic positions. The results agree qualitatively with earlier theoretical work [D. L. Price, J. M. Wills, and B. R. Cooper, Phys. Rev. Lett. 77, 3375 (1996)] and quantitatively with the latest theoretical results based on the first-principles molecular dynamics method [K. Kobayashi, Jpn. J. Appl. Phys., Part 1 39, 4311 (2000)].

Surface roughness and LEED crystallography: Analysis of flat and vicinal W(110)

Low-energy electron diffraction intensity vs voltage (LEED I-V) measurements and analysis are used to determine the multilayer surface relaxation of W(110). Measurements and analysis are presented for both flat and vicinal surfaces, demonstrating that surface roughness leads to only small errors in LEED structure determinations. Flat, clean W(110) exhibits first-(d{sub 12}) and second-(d{sub 23}) layer relaxations of {minus}3.0{plus_minus}1.3{percent} and +0.2{plus_minus}1.3{percent}, respectively, relative to the bulk lattice spacing d{sub 0}=2.237h{Angstrom}. This experimentally determined surface relaxation of W(110) is compatible with a recent combined density-functional theory calculation and LEED study [M. Arnold, G. Hupfauer, P. Bayer, L. Hammer, K. Heinz, B. Kohler, and M. Scheffler, Surf. Sci. {bold 382}, 288 (1997)]. Surface roughness (in the present case, uniform atomic height steps) is found to produce a small apparent increase in the measured value of d{sub 12} when determined using standard (flat surface) LEED I-V methodology. However, for low step densities ({lt}20 atoms/step) the apparent change in d{sub 12} is small compared to other sources of error, so it is unlikely that surface roughness is a significant source of error in LEED structure determinations. {copyright} {ital 1999} {ital The American Physical Society}

Surface structure ofCu(100)−c(2×2)−Na: A LEED analysis

The surface structure of the $\mathrm{Cu}(100)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{Na}$ phase formed by adsorption of one-half monolayer Na on Cu(100) has been determined by quantitative analysis of low-energy electron diffraction (LEED) measurements. Na atoms occupy fourfold hollow sites on an essentially unperturbed substrate for adsorption at both 100 and 300 K. The vertical distance between the Na layer and the first Cu layer is determined to be $2.37\ifmmode\pm\else\textpm\fi{}0.04$ \AA{} at 100 K and $2.42\ifmmode\pm\else\textpm\fi{}0.04$ \AA{} at 300 K. An analysis of LEED intensities measured for the clean Cu(100) surface confirms previous reports of a small contraction of the first layer spacing, but does not show an expansion of the second-layer spacing as found in medium energy ion scattering studies and in a previous LEED study. Bulk Debye temperatures found in this study are in remarkably good agreement with previous x-ray studies.

Fe thin-film growth on Au(100): A self-surfactant effect and its limitations

The combination of low-energy electron diffraction intensity analyses and scanning tunneling microscopy was used to investigate the morphology and atomic structure of thin Fe films grown on Au(100) at 400 K. Deposition of only about 0.2 monolayers (ML) Fe is sufficient to lift the reconstruction of the clean substrate. In the initial growth process $(l~1$ ML) place exchanges between Fe and Au lead to almost two-dimensional subsurface Fe film growth with one layer of Au covering the entire film. This way, gold acts as a ``self-surfactant.'' Yet, there are deviations from two-dimensional growth, with a second Fe layer beginning to grow before the first one is fully completed and some substitutional disorder developing in the film because of incomplete place exchange. The amount of gold floating on the surface only gradually decreases with further increasing film thickness. At about 2 ML the surface undergoes a complete restructuring during which short ``wormlike'' chains of atoms form and long-range order is destroyed. Nevertheless, the existence of large terraces of little roughness proves that some surface activity of gold remains. At coverages of several ML, long-range order is reestablished with the Fe film growing in an undistorted bcc arrangement. Although parts of the film are still covered by gold, the surface morphology is not very different from that known for homoepitaxial growth of Fe on Fe(100), i.e., gold has stopped to serve as a ``self-surfactant.''

Structural and electronic properties of chemisorbed oxygen on Rh(111)

Using density-functional theory within the generalized gradient approximation, we investigate the adsorption of oxygen on Rh(111) as a function of coverage $\ensuremath{\Theta}$. At the coverages of 0.25 monolayer (ML) and 0.5 ML oxygen (occupying fcc-hollow sites), the calculated atomic geometries (interlayer spacings, bond lengths, and lateral displacements) are in excellent agreement with those of previous low-energy electron-diffraction intensity analyses. We find a strong coverage dependence of the oxygen-induced buckling relaxations and a substantial overall expansion of the first interlayer spacing, reflecting a weakening of metal-metal bonds between the two outer substrate layers. The work functions of the relaxed structures are presented, and the stability of the adlayers is analyzed. We also predict that the existence of a dense O overlayer $(\ensuremath{\Theta}=1$ ML) with a $(1\ifmmode\times\else\texttimes\fi{}1)$ periodicity is possible, because oxygen will be incorporated as a subsurface adsorbate only when the $(1\ifmmode\times\else\texttimes\fi{}1)$ adlayer is (nearly) completed.

Atomic arrangement on ZrO/W(100) cathode surface—Models for LEED intensity vs. voltage analysis

The ZrO/W(100) thermal field emitter has been widely used for cathodes in electron microscopy because this surface shows a remarkably low work function. In spite of its wide application, the reducing mechanism of the work function has not been clearly understood. On the ZrO/W(100) surface, Zr is considered to be located at an outermost position and O is just below it because the Zr–O pair has an electric dipole whose positive side is Zr, and it makes an electric double layer, which is reasonable for work function reducing. For confirmation of the atomic arrangement, low-energy electron diffraction (LEED) intensity vs. voltage ( I– V) analysis was done. At stage 1 of LEED I– V analysis, the most simple models were built and calculated. The result showed poor possibility of these models. Several more complex models, which should be considered, are shown in this study.