Stress-driven structural transformation of Sb-passivated Si(114)

Combined investigation of STM, high-resolution synchrotron photoemission, and density functional theory calculations allowed us to understand the Sb-induced structural-transformation of Si(114)-2 × 1. When 2 ML of Sb is deposited on Si(114)-2 × 1 at room temperature and postannealed at 500 °C, all of the surface Si atoms with dangling bonds are replaced by Sb atoms. Among one-dimensional (1D) structures consisting of Si(114)-2 × 1, such as a dimer with a 6-membered ring (D₆) row, a rebonded-atom (R) row, and a tetramer (T) row [D₆-R-T], the T row is split into a dimer row with a 7-membered ring (D₇) and an R row [D₆-R-D₇-R]. Since the R-D₇-R unit, a building block of Sb/Si(113)2 × 2, is under stress-balance, the Sb/Si(114)-2 × 1 surface is stressed compressively due to the extra D₆ unit. As a result, with additional postannealing at 600 °C, two periods of this 2 × 1 [(D₆-R-D₇-R)-(D₆-R-D₇-R)] are gradually converted to 2 × 2 [(D₆-R-D₆-R)-(R-D₇-R)], where the D₆-R (115) unit is stress-balanced. The corresponding photoemission data obtained from both of the phases show that all of the surface components of the clean surface have disappeared, instead the single Sb–Si interfacial component has appeared, which indicates that the charge transfers from interfacial Si to surface Sb atoms. Finally, the density functional theory calculations have also confirmed that there are two distinct phases determined by the chemical potential of passivating Sb atoms.     

多元醇固—固相变机理的研究

多元醇在一定温度下发生固—固相变,大量文献都报道了它们的固—固相变焓、相变温度以及相图,并认为相变焓与氢键有关.本文以IR 光谱测试结果以及量热实验结果为基础,定量探讨了NPG(neopentylglycol),PG(pentaglycerino),PE(pentaerythritol)的固—固相变焓与氢键的关系,进一步解释了相变的机理:NPG,PG,PE分子在发生固—固相变前,—OH形成分子间氢键,分子具有层状结构.相变后,NPG,PG,PE分子沿着层面移动,移动过程中部分氢键断裂,分子发生相变时所吸收 关键词： 固—固相变 相变焓 氢键 红外光谱     

Dynamics and kinetic roughening of interfaces in two-dimensional forced wetting

We consider the dynamics and kinetic roughening of wetting fronts in the case of forced wetting driven by a constant mass flux into a 2D disordered medium. We employ a coarse-grained phase field model with local conservation of density, which has been developed earlier for spontaneous imbibition driven by capillary forces. The forced flow creates interfaces that propagate at a constant average velocity. We first derive a linearized equation of motion for the interface fluctuations using projection methods. From this we extract a time-independent crossover length ξ_×, which separates two regimes of dissipative behavior and governs the kinetic roughening of the interfaces by giving an upper cutoff for the extent of the fluctuations. By numerically integrating the phase field model, we find that the interfaces are superrough with a roughness exponent of χ= 1.35 ±0.05, a growth exponent of β= 0.50 ± 0.02, and ξ_×∼v^-1/2 as a function of the velocity. These results are in good agreement with recent experiments on Hele-Shaw cells. We also make a direct numerical comparison between the solutions of the full phase field model and the corresponding linearized interface equation. Good agreement is found in spatial correlations, while the temporal correlations in the two models are somewhat different.     

Observation of nonlinear acoustic effects at isotropic solid-solid interfaces

The second harmonic generation of SV shear waves at isotropic solid-solid interfaces is experimentally studied. The amplitude of shear waves is measured for the interfaces of glass-air, glass-iron, glass-copper, and glass-aluminum. The measured angular relation of amplitude of the second harmonic wave is compared with theory and the agreement is reasonably good. The influence of the physical state of the interface on second harmonic generation is also observed. It is found that the second harmonic generation is sensitive to the interface state.     

Formation of oriented structures by laser heating of solid-solid interfaces

Oriented semiconductor structures are produced by laser irradiation of transparent-dielectric—semiconductor interfaces, provided the laser beam intensity is sufficient to melt the semiconductor. The orientation of the structure coincides with that of the initial semiconductor, and does not depend on the orientation of the transparent substrate. Monocrystalline stripes (30 m width) of GaAs and of Ge are formed on sapphire and glass.     

Kinetic roughening and pinning of two coupled interfaces in disordered media

We studied the kinetic roughening dynamic of two coupled interfaces formed in paper wetting experiments at low evaporation rate. We observed three different regimes of impregnation in which kinetic roughening dynamics of coupled precursor and main fronts belong to different universality classes; nevertheless both interfaces are pinned in the same configuration. Reported experimental observations provide a novel insight into the nature of kinetic roughening phenomena occurring in the vast variety of systems far from equilibrium.     

Thermodynamics of stressed solids: Slow deformation and roughening of material interfaces

At every turn in nature we are confronted with complex patterns. Patterns often formed in multiphase systems by an intricate dynamics of mass transport, e.g. diffusion and/or advection, and mass exchange between individual phases. Here we consider instabilities of phase boundaries in idealized stressed multiphase systems. Specifically, we study the growth of small perturbations of surfaces by considering mass transport from regions, where the stress and chemical potential is high, to surrounding regions where the stress and chemical potential is low. We present a linear stability analysis for various stress configurations and their corresponding stability diagrams.     

Measurement of solid-solid transformation rates by positron annihilation

Positron lifetime measurements have been used to obtain the rates of solid-solid phase transformations at 21°C in certain binary mixtures of n-eicosane and n-docosane.     

Kinetics and morphological instabilities of stressed solid-solid phase transformations

An atomistic model of the growth kinetics of stressed solid-solid phase transformations is presented. Solid phase epitaxial growth of (001) Si was used for comparison of new and prior models with experiments. The results indicate that the migration of crystal island ledges in the growth interface may involve coordinated atomic motion. The model accounts for morphological instabilities during stressed solid-solid phase transformations.     

Harmonic generation of an obliquely incident ultrasonic wave in solid-solid contact interfaces

The conventional acoustic nonlinear technique to evaluate the contact acoustic nonlinearity (CAN) at solid-solid contact interfaces (e.g., closed cracks), which uses the through-transmission of normally incident bulk waves, is limited in that access to both the inner and outer surfaces of structures for attaching pulsing and receiving transducers is difficult. The angle beam incidence and reflection technique, where both the pulsing and receiving transducers are located on the same side of the target, may allow the above problem to be overcome. However, in the angle incidence technique, mode-conversion at the contact interfaces as well as the normal and tangential interface stiffness should be taken into account. Based on the linear and nonlinear contact stiffness, we propose a theoretical model for the reflection of an ultrasonic wave angularly incident on contact interfaces. In addition, the magnitude of the CAN-induced second harmonic wave in the reflected ultrasonic wave is predicted. Experimental results obtained for the contact interfaces of A16061-T6 alloy specimens at various loading pressures showed good agreement with theoretical predictions. Such agreement proves the validity of the suggested oblique incidence model.     

Internal friction related to viscous motion of phase interfaces during thermoelastic martensitic transformation

Two internal friction peaks were observed in CuAlNiMnTi polycrystalline shape memory alloy by a partial phase transition method in non-isothermal measurements. The low-temperature internal friction peak arising from the soft mode effect caused by the viscous movement of atoms along phase interfaces was studied in the present study in terms of internal friction fundamentals. An internal friction model related to the peak was established, which was shown to be in good agreement with experimental results.     

Interplay between phase ordering and roughening on growing films

We study the interplay between surface roughening and phase separation during the growth of binary films. Renormalization group calculations are performed on a pair of equations coupling the interface height and order parameter fluctuations. We find a larger roughness exponent at the critical point of the order parameter compared to the disordered phase, and an increase in the upper critical dimension for the surface roughening transition from two to four. Numerical simulations performed on a solid-on-solid model with two types of deposited particles corroborate some of these findings. However, for a range of parameters not accessible to perturbative analysis, we find non-universal behavior with a continuously varying dynamic exponent.Received: 23 July 2003, Published online: 23 December 2003PACS: 68.35.Rh Phase transitions and critical phenomena - 05.70.Jk Critical point phenomena - 05.70.Ln Nonequilibrium and irreversible thermodynamics - 64.60.Cn Order-disorder transformations; statistical mechanics of model systems     

The effect of particle size on first-order solid-solid phase transitions

A detailed equilibrium thermodynamic treatment of the first-order transition of a small solid article from one ordered phase to another is presented. This leads automatically to the conclusion that a change in phase transition temperature with particle size is to be expected, with the smaller the particle the bigger the change. Further consideration of the possible observation of such a change leads to the supposition that substantial superheating and supercooling of such phase transitions are likely to occur and should indeed be clearly observable.     

Surfaces and roughening

Some recent results in the application of statistical mechanics to surfaces are discussed. Only exactly soluable models are described. First, we consider phase separation below the critical temperature in uniaxial ferromagnets and their analogs. We then consider the determination of the equilibrium shape of a crystal having fixed volume, given the orientation-dependent surface tension, using the Wulff construction.     

Conserved dynamics and interface roughening in spontaneous imbibition: A phase field model

The propagation and roughening of a fluid-gas interface through a disordered medium in the case of capillary driven spontaneous imbibition is considered. The system is described by a conserved (model B) phase-field model, with the structure of the disordered medium appearing as a quenched random field . The flow of liquid into the medium is obtained by imposing a non-equilibrium boundary condition on the chemical potential, which reproduces Washburn's equation for the slowing down motion of the average interface position H. The interface is found to be superrough, with global roughness exponent , indicating anomalous scaling. The spatial extent of the roughness is determined by a length scale arising from the conservation law. The interface advances by avalanche motion, which causes temporal multiscaling and qualitatively reproduces the experimental results of Horv'ath and Stanley (Phys. Rev. E 52, 5166 (1995)) on the temporal scaling of the interface. Received 24 November 1999     

An investigation of solid-solid phase transitions in cholesterol and in cholesteryl oleate by positron annihilation

The lifetimes of positrons annihilating in cholesterol and in cholesteryl oleate have been measured as a function of temperature in the vicinity of a previously reported solid-solid transition in each compound. The observed changes in lifetime parameters are discussed in terms of structural changes occurring in these two compounds.