Session V20: Phase Transitions: Structural, Electronic, and Magnetic

Inequivalent Down Atom (3x3) structure in Sn/Ge(111).

The (3x3) phase of Sn/Ge(111) is formed by three Sn atoms in the unit cell, one of them at a higher level than the other two (1-up, 2-down model). Although this model is mostly accepted, it is still controversial because of apparently contradicting experimental results related to the Sn 4d core level line shape. This work reports on high-resolution photoemission spectroscopy (HR-PES) and scanning tunnelling microscopy (STM) experiments on Sn/Ge(111)-(3x3). Our PES data resolve three main components in the Sn 4d core level [1] instead of two found before [2], which are assigned to the three Sn atoms in the unit cell. This indicates that the two down atoms are at slightly different heights, forming an inequivalent-down-atoms (IDA)-(3x3) structure, also confirmed by STM images. These results conclusively solve the long-standing controversy [2] on the interpretation of the Sn 4d core level line shape, and support a model fully consistent with an initial state picture. [1] A. Tejeda et al. Phys. Rev. Lett. In press [2] R.I.G. Uhrberg et al., Phys. Rev. Lett. 85, 1036 (2000) and references there in.   

Formation of $\surd $3$\times \surd $3 structure by depositing Si on Si(111)-(5$\times $2)/Au

By depositing Au on a Si(111) surface at an elevated temperature, 5$\times $2, $\surd $3$\times \surd $3, and 6$\times $6 reconstructions emerge successively as the Au coverage increases. Though great efforts have been made to identify atomic models for each reconstruction, satisfactory result is still lacking. By depositing Si on a 5$\times $2 surface, we have identified the formation of the $\surd $3$\times \surd $3 structure even there was no additional Au being deposited. This observation leads us to speculate (i) the $\surd $3$\times \surd $3 structure has a higher Si density than that of the 5$\times $2 structure, and (ii) the Au density in a single-domain $\surd $3$\times \surd $3 structure, i.e., no domain walls, is roughly equal to that in the 5$\times $2 structure.   

Temperature-induced Self-pinning and Nano-layering of AuSi Eutectic Droplets

A process for self-pinning of AuSi eutectic alloy droplets to a Si substrate, induced by a controlled temperature annealing in ultra-high vacuum, is presented. Surface pinning of AuSi 3D droplets to the Si substrate is found to be a consequence of the readjustment in the chemical composition of the droplets upon annealing, as required to maintain thermodynamic equilibrium at the solid-liquid interface. Structural and morphological changes leading to the pinning of the droplets to the substrate are analyzed using atomic force microscopy, scanning and transmission electron microscopy. Raman spectroscopy measurements performed on the droplets reveal phase separation upon cooling of the droplets, leading to the formation of amorphous Si-rich channels within the core, and the formation of crystalline Si nanoshells on the outside. The mechanism leading to the pinning and surface layering provide new insight into the role of alloying during growth of silicon nanowires and may be relevant to the engineering of nano-scale Si cavities. We shall also present measurements of the diffusion of Au drops on Si(111) obtained by low-energy electron microscopy.   

Adsorbate-Induced Faceting of Ir and Re Surfaces

Since high-index clean metal surfaces typically have lower surface atom densities and higher surface free energies compared to the close-packed surfaces of the same metal they can be used as the basis for surface reconstruction and facet formation studies. In this context experimentally we found recently that on Ir(210) and Re(11$\bar2$1) surfaces strongly interacting adsorbates are able to induce the formation of well defined nanostructures after annealing the system at elevated temperatures. Using density functional theory calculations with the PBE functional and {\it ab initio} atomistic thermodynamics we studied the adsorption of oxygen and nitrogen on the different surface orientations, which are involved in the nanostructures on Ir(210) and Re(11$\bar 2$1). Constructing the corresponding ($p$, $T$)-surface phase diagrams, we find that at experimental pressure conditions ($p_{\mathrm{O}_2}=5\cdot 10^{-10}$\,atm) above 1100\,K for Ir and above 1200\,K for Re the planar surfaces are stable, while lowering the temperature stabilizes the nanofacets found experimentally. While on Ir(210) most nanoscale pyramids consist of smooth and unreconstructed planes, some (110) faces show a stepped double-missing row superstructure, which is only stable on the faceted surface and at higher temperatures.   

Oxygen-induced nanoscale pyramidal faceting of Rh(210) surface

The adsorption of oxygen and nanometer-scale faceting induced by oxygen have been studied on atomically-rough fcc Rh(210) using LEED and AES. The Rh(210) surface remains planar at room temperature after being exposed to oxygen. Upon annealing at temperatures above 600K, Rh(210) covered by $\sim $1ML of oxygen undergoes reconstruction to form 3-sided nanoscale pyramids characterized by two {\{}731{\}} facets and a (2x1)-reconstructed (110) facet. The surface remains faceted for T up to 850K. Oxygen can be completely removed from the faceted surface via CO oxidation at 400K or reaction with H$_{2}$ at room temperature, while preserving the faceted structure. The clean faceted surface remains stable below 600K and irreversibly relaxes to the planar surface at higher temperatures. The clean faceted Rh(210) surface is a potential substrate to study surface reactions whose rates are sensitive to atomic structure and/or nanoscale (facet) size. The results are compared with measurements of oxygen-induced faceting of Ir(210). Work supported by DOE, Office of basic Energy Science   

Order-disorder phase transition of the Cu(001) surface under equilibrium oxygen pressure

Understanding atomic processes involved in catalyzed reactions is of great importance and can be achieved by studies of adsorbate-induced surface structures. Copper catalysts are heavily used in methanol and formaldehyde synthesis, two reactions in which oxygen adsorption is an important intermediate step. To better understand catalytic reactions, it is imperative to both identify and characterize the atomic structure of all phases present on metal surfaces at elevated temperatures and pressures relevant to working catalysts. We will report on our discovery of a new high-temperature oxygen-induced surface phase on Cu (001) using a combination of in situ synchrotron x-ray scattering and first-principles theory. This high-temperature phase is characterized by $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ ML of randomly-distributed vacancies in the topmost Cu layer with a c(2x2)-O adlayer. Below 473 K a reversible transition to a $\left( {2\sqrt 2 \times \sqrt 2 } \right)R45^{\circ}$ missing row phase occurs. The results show that this entropy-driven phase transition occurs through the diffusion of Cu vacancies underneath the oxygen superstructure. This work is supported by DOE BES under Contract \# DE-AC02-06CH11357.   

Ab initio phase diagram of oxygen adsorption on W(110)

Oxygen adsorption on the tungsten (110) surface has been studied experimentally as well as by semi-empirical theoretical approaches. Up to now, no {\em ab initio} modelling of this adsorption process has been done, for which we present a combined density functional theory (DFT) and cluster expansion (CE) study. For the CE all lateral unit cells with up to 12 atoms were considered. The (2x1) and (2x2) adsorption phases are found to be ground states which is confirmed experimentally [1]. On the basis of effective cluster interactions Monte Carlo (MC) simulations were performed in order to access finite temperature effects. Concerning the atomic structure we find excellent agreement to experimental scanning tunneling microscopy studies [2]. The temperature and coverage dependent short range order parameter is analyzed. From the results of the applied DFT, CE and MC approaches an {\em ab inito} surface phase phase diagram can be derived. [1] Wu et al., Physical Review B {\bf 39}, 7595 (1989). [2] Johnson et al., Phys. Rev. Lett., {\bf 71}, 1055 (1993).   

Lattice gas transition of xenon on a fullerite surface

We study a lattice-gas transition of xenon atoms on a honeycomb geometry. It is found from experiments and Monte Carlo simulations that this configuration occurs for xenon adsorbed on a substrate consisting on an array of C60 molecules on Ag(111). At very low coverage the atoms occupy strong-binding 3-fold hollow sites between C60 molecules. In this way, they form a commensurate lattice with nearest neighbor distance 0.58 nm. Using a Lennard Jones model for the Xe-Xe potential, the nearest neighbor interaction strength is U=96.7 K. Using the Ising model we estimate the transition critical temperature to be Tc = 36K. We compare with our results from Monte Carlo simulations based on more realistic interactions.   

Stability of the commensurate monolayer solid of xenon/graphite

A stability analysis based on model calculations of the grand potential finds that the transition from hexagonal incommensurate to commensurate monolayer solid of xenon/graphite is continuous with increasing pressure, in agreement with experimental observations. An atomic-scale interaction model gives an internally consistent account of the thermal expansion of the solid at the 2D sublimation curve and of the chemical potential increase for isothermal compression from monolayer condensation to the commensurate solid. An estimate is given for the corrugation energy of xenon/graphite.   

Molecular dynamics simulations of hexane on graphite at various coverages: the difference explicit hydrogens make

Molecular Dynamics simulations of hexane (C$_{6}$H$_{14})$ adlayers on graphite are carried out for coverages of 0.5 $\le $ \textit{$\rho $ }$\le $ 1 monolayers. The hexanes have explicit hydrogens and the graphite is modeled as an all -- atom, six -- layer structure. Above \textit{$\rho $ }$\cong $ 0.9 the herringbone solid loses orientational order at $T_{1}$ = 140 K $\pm $ 3 K. At \textit{$\rho $} = 0.878 the system presents vacancy patches and $T_{1}$ decreases to ca. 100 K. As coverage decreases further, the vacancy patches become larger and by \textit{$\rho $} = 0.614 the solid is a connected network of randomly oriented domains. All cases show a weak nematic mespohase. The melting temperature is $T_{2}$ = 160 K $\pm $ 3 K and falls to ca. 145 K by \textit{$\rho $} = 0.614. The dynamics and energetics observed demonstrate that the explicit-hydrogen model of hexane is substantially more realistic than the UA approximation.   

Molecular Dynamics study of tetracosane monolayers adsorbed on graphite

We present the results of Molecular Dynamics (MD) simulations of tetracosane (C$_{24}$H$_{50})$ monolayers physisorbed on graphite. C$_{24}$H$_{50}$ molecules have explicit hydrogens and the graphite is represented by six graphene layers. We focus our analysis on the microscopic mechanism of melting, experimentally observed at $T$ = 340 K. We are looking for the pre-melting transformations with emphasis on the correlation between translational disordering of molecules and their internal degrees of freedom. We analyze several order parameters and their fluctuations along the MD trajectories. We show that the all atom representation of C$_{24}$H$_{50}$ is much more sensitive to the model of intramolecular interactions than united atom model. Footprint reduction during melting involves a simultaneous loss of intramolecular and translational order.   

Test of the universality of the scaling energy in alkanes using melting transition of layer adsorbed on graphite

The non-bonded terms together with the torsional energy determine the internal (conformational) degrees of freedom of simulated alkanes. Being able to predict the energy and ordering of conformations correctly is an essential quantity for bringing force field methods into the predictive regime of theoretical methods. To estimate the universality of modern force fields it is helpful to know how different components of classical fields affect the simulated properties. In the case of alkanes, the interplay between torsion potential and the scaling of 1-4 van der Waals and 1-4 charge-charge (QQ) interactions plays a crucial role. It is the purpose of this work to study universality of the scaling factor of 1-4 non-bonded interactions in alkanes. Three alkanes of length from 7 to 30{\AA} (C$_{6}$, C$_{12}$, C$_{24})$, in an all-atom representation have been analyzed.   

Heisenberg Spin Systems with Long Ranged Couplings

Static correlation functions of Heisenberg spin systems with various (non-frustrated) topologies are studied by means of Quantum-Monte Carlo techniques. In particular dimerizations and longer ranged variations of spin couplings and alternations between ferromagnetic and anti-ferromagnetic couplings are considered. The phenomenology of possible ground state phase diagrams is discussed.   

Hysteresis in the anomalous Hall effect of MnAs thin films

We report detailed measurements of the Hall effect in MBE-grown MnAs thin films on $(001)$-GaAs as a function of temperature. Hysteresis of the Hall resistivity is observed for temperatures between 300 and 355~K. Non-linear behavior of the Hall resistivity persists up to 390~K. The appearance of hysteresis at low temperatures can be explained by the emergence of stable, out-of-plane domains due to the shape anisotropy of the contracting $\alpha$-phase. However, the persistence of the hysteresis and the anomalous Hall effect at temperatures significantly above 318~K is not consistent with the complete transformation of the $\alpha$-phase and introduces new questions about the magnetic properties of the $\beta$-phase.