Phase coexistence between the (√3 × √3)R30° - β and (1 × 1) phases on Pb/Ge(1 1 1)

We have characterized the phase transition between the (1 × 1) and (√3 × √3)R30° - β phases on Pb/Ge(1 1 1) using low energy electron microscopy (LEEM). We show that the transition is first-order and that, in the coexistence region of the two phases, the dominant mechanism for phase separation changes critically with Pb coverage, from nucleation and growth at 1.33 ML (saturation coverage of the β phase) to spontaneous domain switching due to thermal fluctuations of the local Pb density for slightly smaller coverage. As the Pb coverage decreases, the concentration of vacancies in the β phase increases, making additional possible Pb adsorption sites available. The larger resulting local density fluctuation of Pb becomes comparable to the density difference of the two phases, manifesting itself in the observed domain switching.     

The dense α-√3 × √3Pb/Si(1 1 1) phase: A comprehensive STM and SPA-LEED study of ordering, phase transitions and interactions

The T-θ phase diagram for the system Pb/Si(1 1 1) was determined in the coverage range 6/5 ML < θ < 4/3 ML from complementary STM and SPA-LEED experiments. This coverage is within the range where a “Devil’s Staircase” (DS) has been realized. The numerous DS phases answer conflicting information in the Pb/Si(1 1 1) literature and update the previously published phase diagram. The measurements reveal the thermal stability of the different linear DS phases with the transition temperature found to be a function of phase period. Because of additional complexity in the experimental system (i.e. two-dimensionality and 3-fold symmetry) the linear DS phases transform at higher temperature into commensurate phases of 3-fold symmetry HIC (historically named “hexagonal incommensurate phase”). Different types of HIC phases have been discovered differing in the size of the supercell built out of √3 × √3 domains separated by domain walls of the √7 × √3 phase. The detailed structures of these HIC phases (coverage, binding site, twist angle, etc.) have been deduced from the comparison of STM images and diffraction patterns. After heating the system to even higher temperature the HIC phase transforms into the disordered phase. For sufficiently high coverage a SIC (“striped incommensurate phase” which is also built from √3 × √3 domains but meandering √7 × √3 domain walls) is observed which also disorders at high temperatures.     

New (sqrt3 xsqrt 3)R30 degrees phase of Pb on Ge(111) and its consequence for the melting transition

Depending on the preparation method, we find two different structures of the Pb/Ge(111) system at a nominal coverage of 4 / 3 monolayer that exhibit different melting points. One is the well studied beta phase that melts at 270 degrees C, but the other is a new and metastable phase that melts at 330 degrees C. Using surface x-ray diffraction the atomic structure of both phases is found to be surprisingly similar. The difference in melting points can be explained by the distribution of the excess Pb present on the surface, which has a direct effect on the vacancy density. We propose a modified phase diagram, in which the melting temperature of the beta phase depends strongly on coverage.     

Impact of surface phase transitions and structure on surface diffusion profiles of Pb and Bi over Cu(1 0 0)

Surface diffusion of Pb and Bi over Cu(1 0 0) surfaces has been studied by scanning Auger microscopy techniques. The diffusion profiles of Pb and Bi have been found to be quite different. The results show that three major factors control the shapes of the surface diffusion profiles: (a) First order phase transitions, which lead to phase coexistence over specified coverage ranges, tend to produce abrupt changes in coverage versus distance profiles; (b) profiles can be affected by the existence of significant differences in the diffusion coefficients of the various phases present, and by the possibility (c) of important changes in diffusivity within a given phase, as a function of coverage, due to interactions between the diffusing atoms. In addition, it has been shown that the strong connection between diffusion profile shapes and the 2D phase diagram allows certain features of the 2D phase diagram to be determined from diffusion profiles.     

Mixed PbSi dimer chains on Si(1 0 0): a first-principles study

It has been a common belief that the one-dimensional structures observed by STM at low coverage of Pb on Si(1 0 0) are buckled Pb-Pb dimer chains. However, using first-principles density functional calculations, we found that it is energetically favorable for Pb adatoms to intermix with Si atoms to form mixed dimer chains on Si(1 0 0), instead of Pb-Pb dimer chains as assumed in previous studies. Up to a Pb coverage of 0.125 ML, mixed PbSi dimer chain is 0.19 eV per Pb atom lower in energy than Pb dimer chain.     

Intrinsic character of the (3 x 3) to (square root 3 x square root 3) phase transition in Pb/Si(111)

We have investigated the (3 x 3) to (square root 3 x square root 3) reversible phase transition in Pb/Si(111) by means of variable temperature scanning tunneling microscopy and density functional first-principles calculations. By tracking exactly the same regions of the surface with atomic resolution in a temperature range between 40 and 200 K, we have observed the phase transition in real time. The ability to prepare and track exceptionally large domains without defects has allowed us to detect the intrinsic character of the phase transition at temperatures around 86 K. This intrinsic character is in full agreement with our first-principles calculations. Moreover, our results show that the hypothesis that point defects play a fundamental role as the driving force, reported for similar systems, can be discarded for Pb/Si(111).     

Ordered high coverage NO overlayers on Pd(111)

High coverage NO overlayers on Pd(111) were studied by LEED and vibrational infrared spectroscopy under equilibrium adsorption conditions. Three ordered, coincident site superlattices were found to exist at NO coverages of 0.5, 0.625 and 0.75 monolayers (ML). Structural models have been proposed to give a consistent interpretation to both the LEED and vibrational infrared data. In particular, the 0.625 ML structure can be regarded as phase/antiphase domains of low density NO separated by domain walls containing NO at a higher local density. Infrared reflection-absorption data indicates that relaxation of the domain wall structure produces a tilted NO species.     

Surface alloys, overlayer and incommensurate structures of Bi on Cu(1 1 1)

Using surface X-ray diffraction, we have determined the structure of three different sub-monolayer phases of Bi on Cu(1 1 1). In contrast to an early report, we find that at a coverage of 1/3 monolayer a substitutional surface alloy is formed with a (√3 × √3)R30° unit cell. For increasing coverage, de-alloying occurs, leading to an overlayer structure at a coverage of 0.5 ML in which the Bi atoms form zigzag chains. The surface contains three domains of this phase. Finally, at a slightly higher coverage of 0.53 ML, the unit cell is compressed in one direction, leading to a uniaxial-incommensurate phase with three rotational domains.The structure determination includes relaxations in the topmost layers and therefore allows a detailed comparison of the most important bond distances. This shows that an increased charge density of the Cu(1 1 1) surface is the main driving force for the different phases.     

用可调探测深度电子能量损失谱与俄歇电子能谱研究Pb/Ni(001)界面

本工作用可调探测深度电子能量损失谱(ELS)与俄歇电子能谱(AES)研究Pb在Ni(001)表面的生长过程。发现Pb是一层一层地在表面生长的,即按Franck-van der Merwe(F-M)模式生长。当Pb的覆盖度大于1单层(ML)时,Pb的6s能带对应的电子能量损失峰开始出现,当Pb的覆盖度为3ML时,Pb的体等离激元的损失峰已相当明显。在Pb的蒸镀过程及随后的整个退火过程中,Pb的体等离激元峰,6s能带峰和Ni的3p能带峰的峰位与峰宽均保持与纯金属相同的值,也没有出现新的体等离激元峰。由此说明P     

Monte carlo simulations of growth modes of Pb nanoislands on Si(111) surface

The growth of Pb islands on a Si(111) surface exhibits many interesting properties. For example, the self-assembled process results in a homogeneous distribution of Pb islands with uniform height. The dependence of this height on coverage and temperature can be expressed as a phase diagram [1]. In this paper we develop a model of the growth process that reflects the main features of the experimental observations and determines the key processes of quantum dot formation in a Pb/Si(111) system. The growth of islands is simulated by the Monte Carlo method. With suitably chosen parameters the model is able to reconstruct the phase diagram, via the dependence of the dynamics of Pb atoms on area and height. These dependencies are attributed to stress energy and quantum size effects.     

Mechanism and dynamics of the CO-induced lifting of the Pt(100) surface reconstruction

The first atomistic simulations of the CO-induced lifting of the Pt(100)-hex reconstruction have been performed. During this phase transformation the surface changes back to bulk-terminated Pt(100)-(1 x 1), whereby the surface atom density decreases by approximately 20%. The simulations reveal a mechanism collective in nature, indicating that restructuring proceeds through ejection of chains of Pt atoms. These chains explain the anisotropy as seen in scanning tunneling microscopy experiments. The restructuring rate depends nonlinearly on the CO coverage, but the absence of local clustering of CO excludes an explanation in terms of elementary reaction kinetics as proposed previously.     

Structure determination of Pb/Ge(111)-β-(√3 × √3)R30° by dynamical low-energy electron diffraction analysis and first-principles calculation

We have determined the atomic structure of the Pb/Ge(111)-β-(√3 × √3)R30° surface, which was shown to exhibit a large Rashba spin splitting in a metallic surface state by dynamical low-energy electron diffraction analysis. The Pb coverage for the optimized atomic structure is 4/3 with one Pb atom located at every third H(3) site of the bulk-truncated Ge(111) surface and the other three near the T(1) sites but slightly displaced towards the T(4) sites. The determined atomic structure agrees well with the energetically optimized one obtained from the first-principles calculation. The calculation also revealed that the potential for the Pb atoms on the H(3) sites is very soft along the surface normal, suggesting that their vertical position is distributed within a range of about 0.2-0.3 ?. The previously proposed phase transition associated with the surface melting is discussed.     

"Devil's staircase" in Pb/Si(111) ordered phases

With scanning tunneling microscopy we have found that ordered phases in Pb/Si(111) are one of the best examples of the "devil's staircase" phase diagram. Phases within a narrow coverage range (1.2相似文献   

A valence band photoemission study of Pb adsorption on Rh(1 0 0) and Rh(1 1 0)

We have studied the adsorption of Pb on the Rh(1 0 0) and (1 1 0) surfaces by photoemission and low energy electron diffraction (LEED), and tested the chemical properties by adsorption of CO. Pb forms two distinct c(2 × 2) phases on Rh(1 0 0), according to the temperature of the substrate. The phase formed below about 570-620 K, denoted α-c(2 × 2), reduces the coverage of adsorbed CO but does not affect the valence band spectrum of the molecule. The phase formed above this temperature, denoted β-c(2 × 2), also reduces the coverage of adsorbed CO but the valence band spectrum of the adsorbed CO is strongly affected. The two phases are also characterised by a slightly different binding energy of the Pb 5d_5/2 level, 17.54 eV for the α phase and 17.70 for the β phase. The Pb/Rh(1 1 0) surface shows two ordered Pb induced phases, c(2 × 2) and p(3 × 1). CO adsorbs on the first with reduced heat of adsorption and with a valence band spectrum that is strongly altered with respect to CO adsorbed on clean Rh(1 1 0), but does not adsorb on the p(3 × 1) structure at 300 K. We compare the present results with previous results from related systems.     

Self-assembly of paired nanoribbons

Self-assembly of surface phase domains is a promising route to fabricate stable nanometer-scale structures. This Letter reports a novel labyrinth structure of orthogonal nanoscale ribbons of Cu4Pb3 ordered-alloy on Cu(100) formed by electrochemical deposition. The labyrinth develops as loops of Cu4Pb3 ribbons elongate as closely spaced paired stripes. The structure is explained in terms of elastic interactions between anisotropic surface stress domains, wherein stripes of different phase variants form attractive dipoles. An energetic analysis determines the physical conditions necessary for the structure to form.     

处于力学不稳定条件下的受激有限核的动力学演化与确定论混沌

以208Pb为例,研究了不同热力学稳定条件下有限核体系单体耗散的非线性特征。首先,给出了该原子核在不同温度下的压强密度相图以及相图中的力学不稳定区(Spinodal region),在相图中属于不同稳定性区域的温度和密度值下,抽取了208Pb中各核子的坐标和动量,然后,将其作为量子分子动力学(QMD)模型的初始值,模拟受激有限原子核体系仅在平均场作用下的时空演化,分析不同初始分布所对应的系统在时空演化过程中核子的空间分布、密度涨落等随时间的演化特征,重点对比分析了对应于相图中不同区域的初始208Pb原子核在演化过程中的不同表现特征;通过对定义在事件空间和定义在相空间的准Lyapunov指数的计算,清楚地显示,在力学不稳定的条件下,平均场动力学对于核子空间分布的敏感依赖性,定量地揭示了反应动力学中单体耗散的非线性混沌特征,进一步证实了中能重离子碰撞中多重碎裂的混沌机制。     

Photoelectron spectroscopy study of Pb/Ag(1 1 1) in the submonolayer range

The vacuum deposition of Pb onto Ag(1 1 1) gives rise to two different surface structures depending on coverage and deposition temperature. At room temperature (RT), low energy electron diffraction (LEED) reveals a sharp reconstruction completed at 1/3 Pb monolayer (ML). Beyond, a close-packed Pb(1 1 1) incommensurate overlayer develops. At low temperature (LT, ∼100 K) the incommensurate structure is directly observed whatever the coverage, corresponding to the growth of close-packed two-dimensional Pb(1 1 1) islands. Synchrotron radiation Pb 5d core-level spectra clearly demonstrate that in each surface structure all Pb atoms have essentially a unique, but different, environment. This reflects the surface alloy formation between the two immiscible metals in the reconstruction and a clear signature of the de-alloying process at RT beyond 1/3 ML coverage.     

Probing dynamics of a phase transition of two-dimensional nano-domains with STM imaging and manipulation

A reversible, temperature-driven structural surface phase transition of Pb/Si(1 1 1) nano-domains is studied with a variable-temperature scanning tunneling microscope (STM). Finite-size effects of the transition are clearly demonstrated. Most importantly, structural fluctuations in the low-temperature phase can be induced by the direct interaction between the tip atoms and the surface atoms. The structural changes reveal dynamics in the low-temperature phase. Amazingly, the largest size of the domains that can be manipulated decreases with decreasing sample temperature.     

Atomic chain ordering with ultra-long periods: Pb/Si(5 5 7)

As shown previously, Pb on vicinal Si(5 5 7) refacets the surface into a (2 2 3) facet orientation at a Pb coverage of 1.31 ML. This facet formation is electronically stabilized by Fermi nesting and leads to one-dimensional conductance. Electronic correlation seems to be responsible also for the periodic arrangement of atomic Pb chains which decorate the step edges at concentrations exceeding 1.31 ML, up to a concentration of 1.5 ML. Instead of random step decoration, periodicities up to six (2 2 3)-terrace widths (28 lattice constants, 93 Å) have been found. These depend inversely on excess Pb concentration and end at a concentration of 1.52 ML when all steps are decorated with a line density equal to the Si density at steps.These one-dimensional periodicities can be explained assuming that split-off states from surface bands are completely filled by two electrons per Pb atom with corresponding gap opening. This behavior is reminiscent of the formation of charge density waves with tunable wavelengths as a function of excess Pb concentration, and indicates strong electron correlation in this strongly anisotropic 2d system. The alternative, simple band filling within a rigid band model is expected to destabilize the (2 2 3) facet structure upon further adsorption of Pb, which has not been observed.