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We have studied the dislocation structures that occur in films of Ag, Au, and Ag0.5Au0.5 alloy on a Ru(0 0 0 1) substrate. Monolayer (ML) films form herringbone phases while films two or more layers thick contain triangular patterns of dislocations. We use scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to determine how the film composition affects the structure and periodicity of these ordered structures. One layer of Ag forms two different herringbone phases depending on the exact Ag coverage and temperature. Low-energy electron microscopy (LEEM) establishes that a reversible, first-order phase transition occurs between these two phases at a certain temperature. We critically compare our 1 ML Ag structures to conflicting results from an X-ray scattering study [H. Zajonz et al., Phys. Rev. B 67 (2003) 155417]. Unlike Ag, the herringbone phases of Au and AgAu alloy are independent of the exact film coverage. For two layer films in all three systems, none of the dislocations in the triangular networks thread into the second film layer. In all three systems, the in-plane atomic spacing of the second film layer is nearly the same as in the bulk. Film composition does, however, affect the details of the two layer structures. Ag and Au films form interconnected networks of dislocations, which we refer to as “trigons.” In 2 ML AgAu alloy, the dislocations form a different triangular network that shares features of both trigon and moire structures. Yet another well-ordered structure, with square symmetry, forms at the boundaries of translational trigon domains in 2 ML Ag films but not in Au films.

Using low-energy electron microscopy, we show that de-wetting of Ag and Cu films on Ru(0 0 0 1) occurs by 3-D islands migrating across step edges in the “downhill” direction. We have observed islands thicken by more than 50 atomic layers in this manner. The island migration allows 3-D growth to occur in a way that avoids the nucleation barrier associated with forming 2-D islands on top of 3-D islands. Indeed, without substrate steps this nucleation barrier is not surmounted, and no 3-D islands are formed in the films studied.

The influence of oxygen and sulfur on the dislocation patterns of the strained two monolayer Cu film on Ru was observed by scanning tunneling microscopy. Both oxygen and sulfur adsorption lead to the formation of vacancies that aggregate over existing dislocations in the film, thereby modifying the dislocation structure. With increasing adsorbate coverage the overall dislocation structure and pattern are transformed. The atomic mechanisms and general nature of this transformation can be explained in terms of generic dislocation reactions. This interpretation is also supported by atomistic simulations.

The effect of uniaxial applied stress on dislocation networks present in the atomic surface layer of Au(111) was studied. The measurements were made using a novel instrument combining ultrahigh vacuum scanned-probe microscopy with an in-situ stress-strain testing machine. The technique provides microscopic information, up to atomic resolution, about the large scale plasticity of surface layers under applied loads. The herringbone reconstruction of the Au(111) surface is a classic example of a strain stabilized dislocation network. We find that under 0.5% uniaxially applied compressive strain a dramatic restructuring of the network takes place. The three-fold orientational degeneracy of the system is removed and threading edge dislocations are annihilated.

Using scanning tunneling microscopy (STM) and first-principles local-spin-density-approximation calculations to study submonolayer films of $\mathrm{Co}{}_{1\ensuremath{-}c}\mathrm{Ag}{}_{c}/\mathrm{Ru}(0001)$ alloys, we have discovered a novel phase-separation mechanism. When the Ag concentration $c$ exceeds 0.4, the surface phase separates between a dislocated, pure Ag phase and a pseudomorphically strained $\mathrm{Co}{}_{0.6}\mathrm{Ag}{}_{0.4}$ surface alloy. We attribute the phase separation to the competition between two stress relief mechanisms: surface alloying and dislocation formation. The agreement between STM measurements and our calculated phase diagram supports this interpretation.

To study the dynamics of alloy phase transformations, we deposited submonolayer amounts of Sn onto Cu(111) surfaces. The subsequently observed formation of bronze alloy was found to be surprisingly complex. First, the Sn forms 2D clusters with a 2×2 structure and typical diameters of the order of 100 nm. Then the Sn clusters move across the Cu surface and shrink in size, leaving dilute bronze in their wake. The curious motion of the Sn clusters can be interpreted as an example of chemomechanical energy transduction on a surprisingly small length scale. Thus the Sn clusters are microscopic engines and one can discuss their fundamental mechanical properties. A speculative comparison of the self-propelled motion of the Sn clusters with other isothermal engines, such as molecular biological machines, is considered.

We have studied the growth of Cu islands on rutile TiO 2 (110)-(1×1) using scanning tunneling microscopy for Cu coverages of 0.03–1.25 monolayers (ML). The formation of three-dimensional islands at all the coverages reflects the relatively high mobility of Cu atoms on TiO 2 at room temperature (diffusion constant ≥4×10 −10 cm 2 /s) and the weak interaction between Cu and TiO 2 compared with CuCu bonding. At low coverages ( 0.5 ML), the average island size scales with coverage, but this increase in island size is primarily due to an increase in height, not diameter. Although larger islands can be formed by annealing, the average size of the islands is nearly independent of coverage for any given annealing temperature. We propose two general schemes that can simultaneously explain the increase in island density with increasing coverage and the narrow island size distribution. In the first scheme, the rate at which adatoms attach to existing islands drops rapidly as the island size increases. This could be due to the existence of strain fields that accommodate the lattice mismatch at the interface between the Cu islands and the TiO 2 substrate. Adatoms rejected by the islands are then available for nucleation of new islands. In the second scheme, the rate at which adatoms reach existing islands drops rapidly as the Cu coverage increases. This could be related to continued nucleation of islands at defects in the TiO 2 surface.

Real-time light scattering measurements of coherent island coarsening during SiGe/Si heteroepitaxy reveal unusual kinetics. In particular, the mean island volume increases superlinearly with time, while the areal density of islands decreases at a faster-than-linear rate. Neither observation is consistent with standard considerations of Ostwald ripening. Modification of the standard theory to incorporate the effect of elastic interactions in the growing island array reproduces the observed behavior.

Structural and thermodynamic properties of thermally equilibrated Pd-Au/Ru(0001) surface alloys are studied within the framework of lattice models based upon first-principles total energy calculations and scanning-tunneling-microscopy imaging of atomic structure. Pd-rich alloys form pseudomorphic solid-solution phases with pronounced chemical short-range order (SRO), consistent with the prediction of a low-temperature order-disorder transition to a ({radical}(3){times}{radical}(3) )-R30{degree} ground state structure. Calculated and measured SRO parameters are found to be in excellent agreement and are quantitatively described by a lattice-model Hamiltonian dominated by short-ranged interactions. {copyright} {ital 1999} {ital The American Physical Society}

Vacancy islands formed during room-temperature exposure to sulfur of a submonolayer Ag film on Ru(0001) order spontaneously to form a triangular lattice at an island area fraction just above 20%. A normal mode decomposition of the thermal vibrations of this vacancy island crystal, measured with time-resolved scanning tunneling microscopy, obtains Lame coefficients in the range of 10 8 N m −2 . We show that these estimates are consistent with stabilizing forces derived from long-range elastic interactions between the islands.

Strained thin films often contain ordered networks of misfit dislocations which can determine their chemical and mechanical properties. We consider the reaction of sulfur with two-monolayer films of Cu on Ru(0001). These films contain a network of parallel partial dislocations, separating regions of fcc and hcp stacking, with threading edge dislocations where partial dislocations meet. Sulfur reacts with the threading dislocations and dissociates them. The increase in the threading dislocation density is accommodated by modifying the dislocation network.

The evolution of strain driven coherent islands is examined using sensitive real time stress measurements during heteroepitaxial growth of Si{sub 1{minus}x}Ge{sub x}/Si(001), combined with {ital ex situ} microscopy. We show that the sequence of morphological transitions at low mismatch strain is qualitatively identical to that for pure Ge heteroepitaxy on Si(001). In particular, films with strains less than 1{percent} undergo Stranski-Krastanov-like island-on-layer growth, followed by an extended regime of [501]-faceted hut clusters that eventually transform into higher aspect ratio dome clusters. The hut and dome islands are fully coherently strained and do not exhibit lateral composition modulation. Quantitatively, the relevant island length scales are significantly increased at low strain. Scaling of the morphological transitions with strain is directly demonstrated using the real time stress data. We further show that the apparent formation of a ripplelike surface morphology at low strain is actually a consequence of kinetic limitations on adatom diffusion, and does not necessarily signify the presence of a surface instability. {copyright} {ital 1999} {ital The American Physical Society}

The interaction of a model strained metal film with oxygen is studied by scanning tunneling microscopy. Two monolayers of copper on Ru(0001) present a well-defined dislocation network composed of threading dislocations and Shockley partial dislocations separating areas of fcc and hcp stacking. We find that oxygen first reacts with these threading dislocations. Then, for exposures up to ~0.4 L O 2 , new threading dislocation arrays appear on the surface. With the addition of more oxygen, the mesoscopic structure of the film changes from a striped array of Shockley partials to a disordered array of triangular fcc regions bounded by dislocations, as oxygen proceeds to etch away the hcp areas of the copper film. © 1998 Published by Elsevier Science B.V. All rights reserved.

The rate of capture by stable Co islands on Ru(0001) of additionally deposited Cu atoms is quantified using scanning tunneling microscopy, kinetic Monte Carlo simulations, and diffusion equation analyses. We find strong dependence of the capture rates on Co-island size, larger islands showing larger capture rates, qualitatively distinct from self-consistent mean-field predictions. The observed size dependence is shown to reflect larger island-free areas surrounding bigger islands, i.e., a strong correlation between island sizes and separations neglected in mean-field treatments.

We have used scanning tunneling microscopy to study grain boundary motion in oxygen films adsorbed on Ru(0001). Starting from an analysis of atomically resolved images of the thermal motion of oxygen atoms within the grain boundaries, we are able to successfully predict the future of complicated configurations of grains involving several hundred thousand atoms over several hours. The detail to which theory can reproduce experimental histories shows the precision to which previously hypothetical models of surface kinetics can now be verified. {copyright} {ital 1998} {ital The American Physical Society}

Indentation fracture and continuous nanoscratch testing were used in this study to determine the effects of compressive residual stresses on the fracture of thin sputter-deposited tantalum nitride films. Some films were tested in the as-deposited condition while others were vacuum annealed at 300 C. The only discernible change in structure was a surface rearrangement of atoms into parallel arrays of striations on the vacuum annealed samples revealing a high compressive residual stress state after deposition. Comparison of results and application of mechanics-based models showed that these stresses had a strong effect on the fracture of the as-deposited films. The models also provided a good measure of the residual stress levels and interfacial strain energy release rates.

We report a new mechanism of self-organization that can lead to robust surface ordering. We have quantitatively analyzed the thermal motion of holes created by sulfur atoms in a silver monolayer on a ruthenium surface, which we observed in real time with scanning tunneling microscopy. We find that the stability of the array of holes is determined by the arrangement and structure of misfit dislocations in the film.

Real-time stress measurements during Si{sub 8 }Ge{sub 2}/Si(001) heteroepitaxy, combined with {ital ex situ} microscopy, are used to examine islanding dynamics under conditions of relatively low strain and high adatom mobility, where morphological evolution bypasses dislocation formation. We show that growth in this regime proceeds similarly to growth of Ge/Si(001) (i.e., at high strain, low temperature), but with the length scales expanded by the reduced strain. This greatly facilitates measurement of the coupled kinetics of morphological evolution and stress relaxation. {copyright} {ital 1997} {ital The American Physical Society}

We have used sensitive real-time measurements of film stress during Si1-xGex molecular beam epitaxy to examine strain relaxation due to coherent island formation, and to probe the kinetics of Ge surface segregation. We first describe our novel curvature-measurement technique for real-time stress determination. Measurements of the relaxation kinetics during high temperature Si79Ge21 growth on Si (001) are reported in which formation of highly regular arrays of [501]-faceted islands produce 20% stress relaxation. An island shape transition is also observed that reduces the effective stress by up to 50% without dislocations. Nonuniform composition profiles due to Ge surface segregation during growth of planar alloy films are determined with submonolayer thickness resolution from the real-time stress evolution. Up to two monolayers of Ge can segregate to the growth surface.

We present results of a detailed scanning tunneling microscopy (STM) study of submonolayer nucleation and growth of two-dimensional Au islands during deposition on clean versus S-precovered Ru(0001), at room temperature. In the presence of tiny amounts of S, a dramatic increase in the mean island densities is observed, corresponding to a decrease in the effective mobility of Au on Ru(0001). As imaged also by STM, the structure of the S adlayer changes during Au deposition, from a dilute lattice-gas, at low Au coverages, to an increasingly denser sequence of ordered superstructure p(2 × 2), ( 3 × 3 - R 30° ) and c(2 × 4) domains, with increasing Au coverage. These findings suggest that the deposited Au atoms displace S adatoms, compressing S domains, a phenomenon consistent with a net repulsive interaction between neighboring Au and S mediated by the Ru substrate.

Scanning tunneling microscopy (STM) images of Ag films grown on Ru(0001) reveal a periodic unit cell with a structural transformation at silver coverages slightly less than one monolayer. Dislocations in the overlayers are similar to the Au(111) herringbone structure; however, Ag/Ru(0001) exhibits two novel differences from previously observed herringbone structures. First, the unit cell dimensions change dramatically as a function of coverage. Second, STM images of Ag/Ru(0001) are inverted relative to those for Au(111). We show that this image inversion is due to substantial relaxations perpendicular to the surface resulting from local strains in the silver overlayer. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.

It is shown that a pair of hexagonal dislocation structures can be used as a basis for understanding the herringbone pattern and the node networks that have been observed in the structure of several (111) metal surfaces and overlayers. The hexagons are composed of the dissociation patterns of perfect edge dislocations, which are derived from the well-known concepts of the Thompson tetrahedron. At each corner there is a unit threading dislocation; the actual location of this dislocation alternates between the inner and outer Shockley partial dislocation. One of the hexagons will form when there is an excess of vacancies in the surface layer, the other when there are excess atoms. The hexagons can each be reduced to give a corresponding threefold node also terminated by threading dislocations. From such a model, structural characteristics of (111) surface reconstructions such as the presence of perfect edge dislocations on only one side of the stacking fault, can be understood. Whether the herringbone pattern or the node array will be observed in a particular case will depend on the details of the specific systems. In particular, the balance between the mechanisms of the dislocation pattern formation and the equilibrium configuration may be important. The threading dislocations play a critical role in these processes.

In this study, short bar fracture toughness samples were used to determine the effects of radiation and hydrogen on the resistance to fracture of vitreous silica and four commercial borosilicate glasses. The results showed that neither exposure to 10{sup 10} rads of {gamma}-radiation nor saturation to a room-temperature-equivalent exposure of 408 atm of hydrogen had an effect on fracture resistance. Nevertheless, when samples were saturated with hydrogen and then exposed to radiation, resistance to fracture increased significantly in some of the glasses. The increase in the resistance to fracture correlated directly with the increase in concentration of hydroxyl and hydride groups created by exposure to explain and hydrogen. The concentration of these groups differs with glass composition and accounts for the difference in behavior between the types of glass studied.

We describe a light scattering technique for measuring the real-time evolution of thin film and surface morphology. By using spectroscopic detection, the technique requires no motion of the sample during the measurement, which makes it compatible with many processing geometries. Results from the growth of strained heteroepitaxial layers of SixGe1−x on Si(001) are presented to demonstrate the technique.

We find that less than 0.01 monolayer of S can enhance surface self-diffusion on Cu(111) by several orders of magnitude. The measured dependence of two-dimensional island decay rates on S coverage (theta(S)) is consistent with the proposal that Cu3S3 clusters are responsible for the enhancement. Unexpectedly, the decay and ripening are diffusion limited with very low and very high theta(S) but not for intermediate theta(S). To explain this result we propose that surface mass transport in the intermediate region is limited by the rate of reaction to form Cu3S3 clusters on the terraces.

persists to at least 9 ML of Ag on Ru(0001) and presumably applies for the bulk heterophase Ag� 111� =Ru� 0001� interface. Our STM observations show that this interface reconstruction effectively relieves the 7% misfit strain between the Ag and Ru bulk lattices. We use atomistic modeling to determine the arrangement of atoms at the interface, thereby revealing the dislocation structure associated with this interface reconstruction. Finally, we consider why this type of reconstruction occurs at the Ag� 111� =Ru� 0001� interface, but not the Cu� 111� =Ru� 0001� interface. Our observations demonstrate that the structure of heterophase interfaces can be very complex and that we are far from a complete understanding of the issues determining even equilibrium metal-metal interface structures.

Deposition of S onto a monolayer of $\mathrm{Ag}/\mathrm{Ru}(0001)$ transforms the herringbone pattern of the clean Ag film into a strikingly regular array of 2D-vacancy islands [K. Pohl et al., Nature (London) 397, 238 (1999)]. Time-resolved scanning tunneling microscopy reveals that this nanometer-scale restructuring occurs by a cooperative mechanism involving the sequential formation of triangular regions with fcc and hcp stacking. Using a 2D Frenkel-Kontorova model, we can simulate the creation of these triangular building blocks via basic dislocation motions and reactions.

Annealed submonolayer CoAg/Ru(0001) films form an alloy with a structure that contains droplets of Ag surrounded by Co [G. E. Thayer, V. Ozolins, A. K. Schmid, N. C. Bartelt, M. Asta, J. J. Hoyt, S. Chiang, and R. Q. Hwang, Phys. Rev. Lett. 86, 660 (2001)]. To understand how surface stress contributes to the formation of this structure, we use scanning tunneling microscopy to extract atomic displacements at the boundaries between regions of Co and Ag. Comparing our measurements to Frenkel-Kontorova model calculations, we show how stress due to lattice mismatch contributes to the formation of the alloy droplet structure. In particular, we quantitatively evaluate how competing strain and chemical energy contributions determine surface structure.

Using scanning tunneling microscopy on Cu/Ru(0001) thin films we have located the depth at which the cores of misfit dislocations lie below the film surface. The procedure is based on matching areas with unknown structure to areas with a known stacking sequence in the same film. Our results show that dislocations occur not only at the Cu/Ru interface, but also at various levels within the Cu films. Our analysis method should be applicable to the characterization of dislocation structures in other ultrathin film systems.

We have studied the formation kinetics of the copper-tin alloy bronze when tin is deposited on the (111) surface of copper at room temperature. Low-energy electron microscopy and atomic-resolution scanning tunneling microscopy reveal that bronze forms on the surface by a complicated, unanticipated cooperative mechanism: Ordered two-dimensional tin islands containing several hundred thousand atoms spontaneously sweep across the surface, leaving bronze alloys in their tracks. We propose that this process, driven by surface free energy, is a version of the “camphor dance” observed on liquid surfaces, and should be a general mechanism of surface alloying when surface diffusion is faster than exchange into the substrate.

Structural and thermodynamic properties of equilibrated ultrathin Pd-Au, Cu-Ag, and Co-Ag alloys on Ru(0001) were studied and are comparatively discussed. These studies provide a framework for a detailed understanding of the energetics of surface alloy formation.Additionally, the differences between the two bulk immiscible systems (Cu-Ag and Co-Ag) and the bulk miscible Pd-Au system are discussed.

Prior to the advent of the scanning tunneling microscope (STM), most experimental studies measured average properties such as overall chemical composition and long-range periodic structure using techniques such as electron- and photon-based spectroscopies and diffraction. This article reviews selected contributions that STM has made in the understanding of metal-on-metal epitaxy. This is presented not from a historical perspective but rather by highlighting key scientific issues that STM has and will continue to impact. The paper is divided into three topic areas: nucleation, structure, and composition. This is done only for organizational purposes and not to imply that they can be viewed distinctly. There exist fundamental interactions between all of these phenomena that lead to the rich behavior in metal films highlighted throughout the article. One of the great contributions of STM has been precisely to discover these links. 106 refs.

Interdiffusion profiles across a linear interface separating two metals that are miscible as monolayers [Co and Cu on a Ru(0001) surface] have been measured as a function of time. The interface remains sharp even after substantial intermixing, in dramatic contrast to bulk interdiffusion experiments between miscible metals. The unusual concentration profiles are explained by a combination of adatom surface diffusion and exchange of adatoms with atoms in the monolayer films. A simple Monte Carlo simulation of the adatom exchange mechanism quantitatively agrees with observations.

In this study, continuous microscratch testing was used to determine the effects of compressive residual stresses on the fracture of thin tantalum nitride films. The films were sputter deposited at room temperature onto single crystal sapphire substrates to a thickness of 600 nm. Some films were left in the as-deposited condition while the remaining films were vacuum annealed at 300°C. The only discernible change in structure was a surface rearrangement of atoms into parallel arrays of striations on the vacuum annealed samples revealing a high compressive residual stress in the films. These stresses had a strong effect on fracture as shown by the as-deposited films which buckled readily during scratch tests forming uniform-width and circular blisters while the vacuum annealed films exhibited a much lower susceptibility to fracture. Application of mechanics-based models for film buckling defined the levels of compressive residual stress and their effect on interfacial fracture.

We employed nanoindentation, continuous microscratch testing, and high resolution TEM to determine the effect of structure on the properties and resistance to fracture of thin polycrystalline aluminum films deposited onto single crystal sapphire substrates at 25°C and 250°C. These films had a nominal thickness of 90 nm and a grain size of 160 nm. The elastic and plastic properties were similar for both films. The elastic moduli superimposed, increasing from bulk aluminum values at the surface to sapphire values at the interface. Hardness values also superimposed, but were constant through the film thickness at a value between aluminum and sapphire. In contrast, susceptibility to fracture varied markedly between the films with the 25°C film exhibiting abrupt failure along the film-substrate interface while the 250'C film gave no indication of fracture in the film, along the interface, or in the substrate under the conditions tested.

Resists used for soft x-ray projection lithography must achieve high resolution and exhibit both high sensitivity and reasonable exposure latitude. Additionally, to prevent the deposition of highly-absorbing molecular species on multilayer-coated reflective optics, photon-induced desorption from the resist must be minimized. Chemically amplified resists show promise, exhibiting excellent sensitivity and resolution for e-beam and keV x-ray exposures[1-4], although they have not yet been characterized at wavelengths appropriate to soft x-ray projection lithography (SXPL). One often-noted issue regarding these resists is that the diffusion length of the photogenerated catalysts may limit their ultimate resolution[4]. Characterization of the diffusion length, and its dependence on the post-exposure bake temperature, has not been performed at SXPL wavelengths, however. Lastly, the beam-induced evolution of neutral and ionic molecular species from resists is a well-known phenomenon [5], although such desorption has not been studied for incident extreme ultraviolet (XUV) and soft x-ray radiation.

The sensitivity, ion photodesorption, and lithographic performance of selected novolac-based chemically amplified resists have been studied at an exposure wavelength of 140 Å. Flood exposures of the resits AZ PF514, AZ PN114, and SAL 601 yield D(0.9) values of 2.5-3.5 mJ/cm(2) for 0.25-µm-thick films. Contrast values range from 3 for AZ PN114 to 5 for SAL 601. Photodesorption of fragment ions induced by 140-Å radiation has been examined in AZ PN114 by using time-of-flight mass spectrometry and compared with poly(methylmethacrylate) (PMMA). Mass-integrated ion desorption yields from AZ PN114 are found to be ∼ 90 times less per exposure than from PMMA. Soft-x-ray projection imaging in AZ PF514 and SAL 601 has been characterized by use of a multilayer-coated 20 × Schwarzschild objective and a transmissive Ge/Si mask illuminated by a laser plasma source.

Sensitivity, lithographic performance, photoabsorption, and photodesorption of chemically amplified novolac‐based resists have been studied at an exposure wavelength of 140 A and are compared to poly(methylmethacrylate) (PMMA). Monochromatic exposures of the resists AZ PF514, AZ PN114, and SAL 601 yielded D0.9 values of 2.5–3 mJ/cm2 for 0.25 μm thick films. Contrast values ranged from 3 for AZ PN114 to 5 for SAL 601. Photoabsorption measurements of supported AZ PN114 films at 140 A yield an absorption coefficient of 4.4±0.1 μm−1. Photodesorption of fragment ions induced by 140 A radiation has been studied in PMMA and AZ PN114 using time‐of‐flight mass spectrometry. It is found that H+, CH2+, CH3+, H2O+, CHO+, C3H5+, and COOCH3+ dominate the ion mass spectra photodesorbed from PMMA, while H+, CH3+, H2O+, and CHO+ dominate the ion mass spectra for AZ PN114. The mass‐integrated ion desorption yield from AZ PN114 is three times less than that measured for PMMA per photon or 90 times less when expressed per ex...

We have used low-energy electron diffraction to study the disordering transition of the (√3 ×√3)R30° structure of Al/Si(111). If this transition were continuous, one would predict that it would be governed by the critical exponents of the three-state Potts model. However, for Al coverages close to 13 of a monolayer on a substrate on which no Al has been previously deposited, we find a transition much more abrupt than that of the three-state Potts model. Consequently the effective specific heat exponent determined from measurements of the integrated diffracted intensity versus temperature is much larger than the exponent of the three-state Potts model. The transition appears to be first-order. However, repeated cycles through the transition region degrade the diffraction pattern and result in a broadened transition. A roughness induced finite-size effect is proposed as an explanation for a previous report that the transition is second-order.

A high‐resolution LEED system with optimum operating parameters for use below 100 eV has been designed and tested. A field‐emission‐based electron gun with an angular divergence of 2 mrad, and a large sample detector distance of 150 mm provide the major improvements in resolution. Measured beam widths (Δk/2π FWHM) for both specular and nonspecular beams are approximately 0.002 A−1 in the energy range of 50–100 eV.

Ces types de surfaces sont observes aux facettes, ou phase separee, de maniere reversible quand la temperature est abaissee a travers la transition (1×1) a (7×7). Des mesures sur des echantillons desorientes de 1 o permettent une bonne comparaison avec une theorie recemment proposee

Using scanning tunneling microscopy we have observed thermally induced dislocation glide in monolayer Cu films on Ru(0001) at room temperature. The motion is governed by a Peierls barrier that depends on the detailed structure of the dislocations, in particular upon whether the threading dislocations that terminate them are dissociated or not. Calculations based on the Frenkel-Kontorova model reproduce the threading dislocation structure and provide estimates of the Peierls barrier and dislocation stiffness which are consistent with experiment.