Relaxation Of Top Layer Research Articles

Effects of surface compliance and relaxation on the frictional properties of lamellar materials

We describe the results of atomic-level stick-slip friction measurements performed on chemically-modified graphite, using atomic force microscopy (AFM). Through detailed molecular dynamics simulations, coarse-grained simulations, and theoretical arguments, we report on complex indentation profiles during AFM scans involving local reversible exfoliation of the top layer of graphene from the underlying graphite sample and its effect on the measured friction force during retraction of the scanning tip. In particular, we report nearly constant lateral stick-slip magnitudes at decreasing loads, which cannot be explained within the standard framework based on continuum mechanics models for the contact area. We explain this anomalous behavior by introducing the effect of local compliance of the topmost graphene layer, which varies when interaction with the AFM tip is enhanced. Such behavior is not observed for non-lamellar materials. We extend our discussion toward the more general understanding of the effects of the top layer relaxation on the friction force under pushing and pulling loads. Our results may provide a more comprehensive understanding of the effectively negative coefficient of friction recently observed on chemically-modified graphite.

Pulsed Laser Deposition of epitaxial SrTiO 3 films: Growth,structure and functional properties

Langmuir ion probe and in situ RHEED were applied to study the effects of low oxygen pressure on SrTiO 3 (STO) film growth by Pulsed Laser Deposition (PLD). Contributions of different oxygen fluxes were analysed and parameters of STO epitaxial film growth were evaluated using physical model of adiabatic expansion of the ablation products and its interaction with ambient gas. Film surface undergoes reconstruction at growing temperatures > 600 °C indicating complete or partial relaxation of top layer without changing growth mechanism of smooth multilayered film. All films have a tetrahedral lattice distortion in the direction of growth that varying with deposition temperature and oxygen pressure. STO lattice distortion is the relevant factor in determining both agility and dielectric loss for tuneable microwave devices. Annealing in oxygen at 1100 °C improves significantly functional properties of STO films, but only the layers deposited under the pressure lower than 10 − 3 Pa possesses low dielectric losses in combination with high agility.

Ab initiostudy of the geometric dependence ofAgPdsurface segregation

The role of surface orientation and surface relaxation on the segregation profile of $\mathrm{AgPd}$ random alloys is investigated. The calculations are performed using the full charge density exact muffin-tin orbitals method in combination with the coherent potential approximation. The equilibrium concentration profiles for the first three close-packed surfaces are established as a function of bulk composition and temperature. In $\mathrm{AgPd}$ alloys the surface relaxation is significant only on the (100) and (110) facets. For these facets our study shows that the top layer relaxation has negligible effect on the segregation profile. The present results obtained for ideal surface geometries are compared to former theoretical and experimental data. Strong $\mathrm{Ag}$ segregation to the top surface layer is found for all surfaces and all bulk concentrations. At the same time, the behavior of the subsurface layer in $\mathrm{Pd}$ rich alloys depends significantly on the surface orientation.

Surface relaxation and surface stress of 4d transition metals

Using the density functional theory formulated within the framework of the exact muffin–tin orbitals method, we present a systematic study of the top layer relaxation and surface stress of 4d transition metals. Our calculations predict layer contractions for most surfaces. We also find that the relaxations of the close packed surfaces decrease with increasing atomic number through the 4d series. We propose that the relaxation is mainly due to the reduction of the number of sp electrons in the surface layer relative to bulk. The surface stress is found to be very sensitive to the relaxation and, therefore, an accurate determination of the layer relaxation is necessary for obtaining reliable values for the surface stress. Comparing the top layer relaxations for the close packed surfaces, we see essential deviations between data derived in different ab initio calculations. At the same time, the overall trend for the present surface stress of 4d metals is in reasonable agreement with recent full-potential data.

Electronic and magnetic properties of bulk and(100)and(111)surfaces ofMnPt3: Anab initiostudy

Electronic and magnetic properties of bulk and close-packed surfaces of the intermetallic compound ${\mathrm{MnPt}}_{3}$ have been investigated by means of the exact muffin-tin orbital method. The present ab initio bulk parameters compare well with the available experimental data. Theoretical ground state of the bulk ${\mathrm{MnPt}}_{3}$ is ferromagnetic, in perfect agreement with the observation. Ferromagnetic configuration is found to be stable on the $(111)$ free surface, while on the $(100)$ surface an antiferromagnetic state is obtained. Top layer relaxation of the $(100)$ surface is shown to have no influence on the antiferromagnetic ground state. Total energy differences between ferro- and antiferromagnetic structures on the surfaces are significantly smaller than in the bulk, which may have implications concerning the Kerr effect of ${\mathrm{MnPt}}_{3}$. Using the linear muffin-tin orbital method we identify prominent structures in the density of states of the surface layers.

Surface relaxation and surface stress of Au(1 1 1)

Changes in surface stress and in the top-layer expansion of Au(1 1 1) electrodes in sulfuric acid have been measured as a function of electrode potential by combining surface stress and X-ray diffraction measurements. Both are linear functions of interfacial charge in the electrode potential range of changing anion coverage. Over this range the surface stress changes by −0.5 N m −1 (compressive direction), while the outward top-layer relaxation decreases from +1.5% to +0.2%. The surface stress changes can be rationalized in terms of a jellium model, while ab initio simulations are needed to explain the top-layer expansion. These simulations yield +1.3% relaxation for the uncharged gold surface, in good agreement with the X-ray diffraction measurements. They also demonstrate that the outward relaxation of the surface is curbed in the presence of an electron withdrawing adsorbate (Cl), which mimics the effects of positive surface charging.

DIRECT DETERMINATION OF RELAXATION PARAMETER FROM LEED INTENSITY PATTERNS WITH THE SINGLE OVERLAYER RELAXED STRUCTURE MODEL

Using the analytical formulae for Low Energy Electron Diffraction (LEED) intensity patterns in the layer doubling method, we have shown that the relaxation parameter for single overlayer relaxed structure can be solved. Although it is difficult to obtain analytically, numerical method based on linear approximation successfully retrieves the parameter value for simulated pseudo-experimental data. This method uses the intensity pattern along the geometry and the chemistry of the bulk to solve the parameters and it may be thought of as a precursor to a full-fledged method for direct determination of surface structure from LEED pattern. Application of this method on the experimental data of Rh(110), which is known to be a two-overlayer relaxed structure, did not yield a single cluster of relaxation parameters. However, qualitative feature of contraction of top layer relaxation parameter has been properly retrieved at different energies.

Temperature-dependent surface relaxations of Ag(111)

The temperature dependent surface relaxation of Ag(111) is calculated by density-functional theory. At a given temperature, the equilibrium geometry is determined by minimizing the Helmholtz free energy within the quasiharmonic approximation. To this end, phonon dispersions all over the Brillouin zone are determined from density-functional perturbation theory. We find that the top-layer relaxation of Ag(111) changes from an inward contraction (-0.8 %) to an outward expansion (+6.3%) as the temperature increases from T=0 K to 1150 K, in agreement with experimental findings. Also the calculated surface phonon dispersion curves at room temperature are in good agreement with helium scattering measurements. The mechanism driving this surface expansion is analyzed.

Relaxation of magnetic transition-metal surfaces

We present a model which allows the calculation of the relaxation rate of magnetic transition-metal surfaces from the variation of the total energy upon atomic displacements. We apply this model to calculate the top-layer relaxation of ferromagnetic Fe(001) and antiferromagnetic Cr(001) surfaces. In the tight-binding scheme the relaxation rate is related to the interatomic Green functions and p-d, p-f, p-g like hopping integrals between neutral atoms. We obtain in both cases a contraction of the top layers and we deduce in a self-consistent way the corresponding magnetic moments. Finally, we discuss the modifications which occur with respect to the paramagnetic case and compare with available experimental results [J. Sokolov et al. Phys. Rev. B 33 (1986) 1397].

EPITAXIAL GROWTH OF Au ON Pt(011) STUDIED WITH X-RAY PHOTOELECTRON DIFFRACTION

Au films 0.24 to 5.9 monolayers thick were evaporated on Pt(011), and the growth mode was investigated with X-ray photoelectron diffraction. At 0.5 monolayer coverage most Au atoms are found in the second layer below the surface while a few atoms diffuse as far as four layers deep even though Au is not miscible with Pt in the bulk at low concentrations. With increasing coverage the missing rows of the Pt(011) (2×1) reconstruction are gradually filled by replaced Pt atoms and further Au atoms, but the top layer relaxation is not changed significantly. At high coverages Au grows epitaxially as Au(011) on the Pt(011) surface and exhibits a poorly developed missing-row reconstruction.

Trends of the surface relaxations, surface energies, and work functions of the 4d transition metals.

Density-functional-theory calculations of the surface energies, surface relaxations, and work functions for a number of low-index surfaces of the 4d transition metals from Y to Ag are presented. The calculations were done for seven-layer slabs using the full-potential linear-muffin-tin-orbital method. Agreement with experimentally obtained results is very good for the cases where experimental data are available. Roughly parabolic behavior is found for the top-layer relaxation and the surface energy across the series. The study of trends makes it possible to extract the relevance of different models and to interpret the results within a simple physical picture. The trend in top-layer relaxation can be understood in a model that combines the effects of the inward electrostatic force due to Smoluchowski smoothing with the directed forces due to the localized d bonds. The calculated surface energies can be explained by a bond-cutting model, but only if the change of the bond strength with coordination number is taken into account. We find that popular models that relate the surface energy to quantities such as the cohesive energy are flawed because their estimate of the surface energy erroneously includes contributions of the free-atom spin polarization and orbital structure. Several predictions of the surface dependence of work functions, surface energies, and surface relaxations are given.

Theoretical study of the surface energy and surface relaxation of the W(001) surface

The structural properties of the W(001)(1\ifmmode\times\else\texttimes\fi{}1) surface are calculated using a fully self-consistent pseudopotential linear combination of atomic orbitals method. The surface energy [3.09 eV/(surface atom)], the top-layer relaxation (5.9%), and the relaxation energy (0.059 eV) we obtained are in good agreement with available experimental data and recent all-electron full-potential linear augmented-plane-wave results. The results demonstrate that the method can treat the structural properties of transition-metal surfaces accurately.

Trends in metal surface relaxation

New LEED (low-energy electron diffraction) results are reported for Fe{310} and Fe{210} surfaces, including multilayer relaxation of atomic planes both perpendicular and parallel to the surface. The structures of six different surfaces of iron are now known. A comparison of the results yields relaxation trends: top-layer relaxation is found to increase monotonically as the surfaces become more open; for the higher-index surfaces {211}, {310} and {210} “decay curves” of relaxation as functions of depth into the surface show a surface-independent decay length (the depth at which the crystal returns to a bulk-like arrangement) of approximately 2 Å.

A LEED study of Cu(111) (1 × 1)-Ni using reliability (R) factors

An epitaxial monolayer of nickel was deposited onto a clean, flat copper (111) surface and a data set of I(V) curves collected at three diffraction geometries. Comparison between theory and experiment using the Pendry R factor gives the relaxation of the topmost layer to be -2% +or- 1% of an interlayer spacing or -0.04 Å +or- 0.02 Å. From this result a nearest-neighbour distance for Ni-Cu is calculated which is about mid-way between that for Ni-Ni and Cu-Cu. Decreasing the surface Debye temperature of the nickel layer to 242K from the bulk value of 456K improves agreement between theory and experiment. The pendry R factor is found to be particularly sensitive to this parameter. The result for the top-layer relaxation using the Zanazzi and Jona R factor is -3% +or- 2% or -0.06 Å +or- 0.04 Å.