Finite-temperature scaling of quantum coherence near criticality in a spin chain

We explore quantum coherence, inherited from Wigner-Yanase skew information, to analyzequantum criticality in the anisotropic XY chain model at finite temperature. Based on theexact solutions of the Hamiltonian, the quantum coherence contained in a nearest-neighborspin pairs reduced density matrix ρ is obtained. The first-order derivative of thequantum coherence is non-analytic around the critical point at sufficient low temperature.The finite-temperature scaling behavior and the universality are verified numerically. Inparticular, the quantum coherence can also detect the factorization transition in such amodel at sufficient low temperature. We also show that quantum coherence contained indistant spin pairs can characterize quantum criticality and factorization phenomena atfinite temperature. Our results imply that quantum coherence can serve as an efficientindicator of quantum criticality in such a model and shed considerable light on therelationships between quantum phase transitions and quantum information theory at finitetemperature.     

Thin film semiconductor islands and numerical experiment

The distribution functions of island nuclei of germanium melt drops on Si(100) substrate over lateral sizes and heights in the presence of a surface linear defect are obtained in a numerical experiment simulating the initial stage of the first-order phase transition. A similar model of heterogeneous condensation of silicon carbide vapor is discussed. Quasi-linear kinetic partial differential equations are solved by the efficient numerical method of stochastic analogue of nonequilibrium processes. The Volmer-Weber mechanism of cluster formation from melt islands, their crystallization, and island structure formation on the substrate are considered.     

Quantum Coherence and Correlation in Spin Models with Dzyaloshinskii-Moriya Interaction

We study quantum coherence and quantum correlation for detecting quantum phase transition (QPT) by means of quantum renormalization group (QRG) in various spin chain models with Dzyaloshinskii-Moriya (DM) interaction, including XXZ model with DM interaction, Ising model with DM interaction and XY model with DM interaction. It is found that through enough QRG iterations, l ₁ norm quantum coherence and one-norm geometric quantum discord can effectively characterize QPT. We also discuss the effect of DM interaction and anisotropy on quantum coherence and quantum correlation.     

Raman scattering in InN films and nanostructures

Studies of lattice dynamics devoted to wurtzite InN are presented. Raman scattering experiments on both InN thin films and nanometric islands grown by Metal–Organic Vapor Phase Epitaxy (MOVPE) were performed at room temperature. From the Raman spectra recorded from InN films under hydrostatic pressure up to 13 GPa, linear pressure coefficients and the corresponding Grüneisen parameters for both E₂ and A₁(LO) phonons were extracted for the wurtzite structure up to 11 GPa, close to the starting pressure of the hexagonal to rock-salt phase transition of InN. Spectra at higher pressure suggest that InN undergoes a gradual phase transition, and the reverse transition exhibits a strong hysteresis effect during the downstroke. Then, we discuss recent results on large single InN islands grown on GaN buffer layers, obtained by spatially resolved micro-Raman measurements. The magnitude of the residual strain is estimated, using a recent determination of phonon deformation potentials. It is found to vary linearly as a function of island height.     

Glassy features of a Bose glass

We study a two-dimensional Bose-Hubbad model at a zero temperature with random local potentials in the presence of either uniform or binary disorder. Many low-energy metastable configurations are found with virtually the same energy as the ground state. These are characterized by the same blotchy pattern of the, in principle, complex nonzero local-order parameter as the ground state. Yet, unlike the ground state, each island exhibits an overall random independent phase. The different phases in different coherent islands could provide a further explanation for the lack of coherence observed in experiments on Bose glasses.     

Two-dimensional Abrikosov-vortex microclusters: Shell structure and melting

Melting of two-dimensional Abrikosov-vortex microclusters in a type-II superconductor island with thickness less than the coherence length has been studied. Equilibrium configurations corresponding to local and global minima of potential energy for clusters with N=1–50 particles are calculated. The temperature dependences of the structure and of mean-square radial and angular vortex displacements are investigated. It is shown that vortex microclusters melt in two stages: first the frozen-out phase transfers to a state corresponding to rotational reorientation of crystalline shells with respect to one another, followed by a transition to a state with no radial order at a substantially higher temperature. The reason for this is that the barrier to shell rotation is significantly lower than that to radial breakdown of shells. Fiz. Tverd. Tela (St. Petersburg) 39, 1005–1010 (June 1997)     

Nanowedge island formation on Mo(1 1 0)

The deposition of ultrathin Fe films on the Mo(1 1 0) surface at elevated temperatures results in the formation of distinctive nanowedge islands. The model of island formation presented in this work is based on both experiment and DFT calculations of Fe adatom hopping barriers. Also, a number of classical molecular dynamics simulations were carried out to illustrate fragments of the model. The islands are formed during a transition from a nanostripe morphology at around 2 ML coverage through a Bales-Zangwill type instability. Islands nucleate when the meandering step fronts are sufficiently roughened to produce a substantial overlap between adjacent steps. The islands propagate along the substrate [0 0 1] direction due to anisotropic diffusion/capture processes along the island edges. It was found that the substrate steps limit adatom diffusion and provide heterogeneous nucleation sites, resulting in a higher density of islands on a vicinal surface. As the islands can be several layers thick at their thinnest end, we propose that adatoms entering the islands undertake a so-called “vertical climb” along the sides of the island. This is facilitated by the presence of mismatch-induced dislocations that thread to the sides of the islands and produce local maxima of compressive strain. Dislocation lines also trigger initial nucleation on the surface with 2-3 ML Fe coverage. The sides of the nanowedge islands typically form along low-index crystallographic directions but can also form along dislocation lines or the substrate miscut direction.     

The influence of mode coupling on the non-linear evolution of tearing modes

On the basis of tearing mode theory a simple but physically explicit model of the evolution of toroidally coupled rotating magnetic islands has been developed. The basic mechanism identified by the model in the island evolution is the locking in phase of rotating islands that leads to rapid destabilisation of an initially stable mode. Destabilisation of marginally stable (2, 1) and (3, 1) modes is analysed in several scenarios. It is shown that mode coupling is an effective way of destabilising a m=3 island in a low- plasma. The numerical examples presented show the individual roles of coupling, inertia and a resistive wall. The model was applied for the analysis of MHD observations of an ASDEX discharge. Received 4 May 1999 and Received in final form 23 July 1999     

Formation and evolution of the phase composition and associated properties during the growth of thin films

A complete system of equations describing the evolution of the phase composition of islands in the Ostwald ripening stage during the growth of thin films is derived and a solution is found. The distribution function of islands of solid solutions is obtained for different growth mechanisms. A general approach to controlling the phase composition and associated properties (electrical, optical, strength, and so on) in the multicomponent systems formed is developed. The investigation is conducted for the example of thin multicomponent films. It is shown that the greatest possibilities for controlling these properties are at the Ostwald ripening stage and in the presence of nonlinear phenomena such as self-oscillation or self-organization. A relation is found between the size and composition of the new-phase nuclei which form during the condensation of films of solid solutions. A system of equations describing the evolution of the distribution function of properties in island films of solid solutions as a function of the external parameters of the system is derived and solved. It is shown that a strong dependence of the composition on the external parameters appears for islands with radius R⩽10⁻⁸ m, irrespective of the type of material. A diagram of the coexistence of the corresponding properties in concentration-temperature coordinates is constructed for island films of stoichiometric compounds; this makes it possible to determine the necessary conditions for obtaining such films. It is predicted that various properties in multicomponent systems which undergo a first-order phase transition can vary periodically in time and space. Zh. Tekh. Fiz. 67, 112–120 (October 1997)     

Finite-temperature phase-locking transition in three-dimensional arrays of Josephson coupled Bose–Einstein condensates

We consider theoretically a phase-locking transition in Bose–Einstein condensate in an optical lattice in the regime where system can realized as a three-dimensional Josephson junction array. The coherence between adjacent Bose condensates (trapped in the valleys of the periodic potential) caused by the Josephson tunneling can lead to a phase transition with a global phase coherence at certain critical temperature. Using a model Hamiltonian of Josephson weakly coupled Bose condensates we calculate the critical temperature for the three-dimensional system placed in a simple cubic lattice and discuss the result in the context of system parameters and possible experiments.     

Global Migration Can Lead to Stronger Spatial Selection than Local Migration

The outcome of evolutionary processes depends on population structure. It is well known that mobility plays an important role in affecting evolutionary dynamics in group structured populations. But it is largely unknown whether global or local migration leads to stronger spatial selection and would therefore favor to a larger extent the evolution of cooperation. To address this issue, we quantify the impacts of these two migration patterns on the evolutionary competition of two strategies in a finite island model. Global migration means that individuals can migrate from any one island to any other island. Local migration means that individuals can only migrate between islands that are nearest neighbors; we study a simple geometry where islands are arranged on a one-dimensional, regular cycle. We derive general results for weak selection and large population size. Our key parameters are: the number of islands, the migration rate and the mutation rate. Surprisingly, our comparative analysis reveals that global migration can lead to stronger spatial selection than local migration for a wide range of parameter conditions. Our work provides useful insights into understanding how different mobility patterns affect evolutionary processes.     

Equilibrium shape and size of supported heteroepitaxial nanoislands

We study the equilibrium shape, shape transitions and optimal size of strained heteroepitaxial nanoislands with a two-dimensional atomistic model using simply adjustable interatomic pair potentials. We map out the global phase diagram as a function of substrate-adsorbate misfit and interaction. This phase diagram reveals all the phases corresponding to different well-known growth modes. In particular, for large enough misfits and attractive substrate there is a Stranski-Krastanow regime, where nano-sized islands grow on top of wetting films. We analyze the various terms contributing to the total island energy in detail, and show how the competition between them leads to the optimal shape and size of the islands. Finally, we also develop an analytic interpolation formula for the various contributions to the total energy of strained nanoislands.     

Growth mechanisms for wire-like epitaxial gold silicide islands on Si(1 1 0) surfaces

Epitaxial islands grown on various substrates are usually strained because of differences in lattice constants of the materials of the island and the substrate. Shape transition in the growth of strained islands has been proposed as a mechanism for strain relief and a way to form self-organized quantum wires. Shape transition usually leads to an elongated island growth. However, an elongated island growth may also be due to an anisotropic diffusion of material, the anisotropy being imposed by the symmetry of the substrate surface. In the present example, growth of gold silicide wire-like nanostructures on a Si(1 1 0) surface has been investigated by photoemission electron microscopy (PEEM). Growth of elongated unidirectional gold silicide islands, with an aspect ratio as large as 12:1, has been observed by PEEM following gold deposition on the Si substrate and subsequent annealing at the Au-Si eutectic temperature. Distribution of the width and the length of the gold silicide islands as a function of island area shows a feature similar to that for the shape transition. However, detailed investigations reveal that the elongated growth of gold silicide islands is rather mainly due to anisotropic diffusion of gold due to the twofold symmetry of the (1 1 0) surface of the Si substrate.     

The effect of oxygen islanding on Co and H2 oxidation on Pt(111)

A Pt(111) surface partially covered with ¹⁶O islands was postdosed with ¹⁸O₂ to increase the island size under conditions where oxygen atom mobility is limited. Subsequently, these islands were titrated with CO and H₂. Preferential production of species containing ¹⁸O was not observed during any stage of the reaction. Assuming there is no extensive mixing of ¹⁶O and ¹⁸O isotopes within the islands, the results indicate no preferential reaction at island perimeters. The data are consistent with a model in which reaction proceeds uniformly over the whole island but the concentration of the mobile species (CO, H) inside the island is controlled by the island structure. This model qualitatively accounts for several apparent rate anomalies reported for CO and H₂ oxidation.     

Island formation and condensation of a chemisorbed overlayer

The condensation of a chemisorbed overlayer from a lattice gas into a particular ordered structure in discussed in terms of the lattice-gas theory of Lee and Yang. The formation of islands of ordered structure is identified with the condensation phenomenon predicted by the theory. The phase diagram (transition temperature versus coverage) based on the theory of a two-dimensional Ising ferromagnet in zero magnetic field is constructed for the condensation of a lattice gas system with net attractive interactions between the particles. It is demonstrated that critical points at coverages other than θ = 0.5 are achieved for overlayer systems with unit meshes larger than (1 × 1). Low-energy electron diffraction results of the thermal disordering (island dissolution) for oxygen chemisorbed on W(110) are compared with the theory, and the effect of substrate surface heterogeneity on the phase diagram is discussed.     

EFFECTS OF ISLAND-EDGE EXCHANGE BARRIER ON 2D PATTERN FORMATION IN SURFACTANT-MEDIATED EPITAXY

The nucleation and growth of two-dimensional islands in a surfactant-mediated epitaxy system have been studied by computer simulation. To improve the recent results published in the literature, we use a configuration-dependent energy barrier for the exchange process at the island edge in our model. The simulations produce fractal islands at high temperatures or low deposition fluxes, and a transition to regular compact islands occurs at lower temperatures or higher fluxes, in good agreement with the recent experimental results. The barrier for the island-edge exchange has quite a strong effect on the island density as a function of temperature and flux. A small change of the island-edge exchange barrier induces a large variation of the island density in the low-temperature or high-flux region. The flux-dependent island density shows a clear scaling-law behaviour in the intermediate-flux region. The scaling exponent increases evidently as the island-edge exchange barrier increases.     

薄膜生长中的表面动力学(Ⅱ)

本文较全面地从理论上研究了薄膜生长过程中原子在表面上的各种动力学表现，涉及的内容包括亚单层生长时，原子在表面上的扩散，粘接，成核，以及已经形成的原子岛之间的相互作用，兼并，失稳，退化等一系列过程。在第一部分(即0—6章，刊登在《物理学进展》23卷1期上)介绍了薄膜生长动力学的基础之后，从第7章开始我们侧重研究一个在向同性和各向异性表面都普遍成立的原子岛尺寸大小及密度分布的标度定律；建立了一套研究在各向同性和各向异性表面上二维原子岛退化过程的广义动力学标度理论。基于对层间质量扩散通道的研究，本文提出了两种不同的原子下跃机制，既任意跃迁机制和选择跃迁机制，并做了进一步的实验验证。利用这一新的层间质量扩散机制，我们成功地解释了实验上观测到的三维原子岛的退化规律。更加有趣的是，本文讨论了应力对岛的形状和各种动力学规律的影响。在最后我们还提出了一个利用凝聚能转化来控制二维原子岛生长的方法，其目的是希望能够找到一种人为有效地在表面上制备低维量子结构的方法。     

Insulator-metal transition in a conservative system: An evidence for mobility coalescence in island silver films

Aging, which manifests itself as an irreversible increase in electrical resistance in island metal films is of considerable interest from both academic as well as applications point of view. Aging is attributed to various causes, oxidation of islands and mobility of islands followed by coalescence (mobility coalescence) being the main contenders. The effect of parameters like substrate temperature, substrate cleaning, residual gases in the vacuum chamber, ultrasonic vibration of the substrate, suggest that the mobility coalescence is responsible for the aging in island metal films. Electron microscopy studies show evidence for mobility of islands at high substrate temperatures. The comparison of aging data of island silver films deposited on glass substrates in ultra high vacuum and high vacuum suggests that the oxidation of islands, as being responsible for aging in these films, can be ruled out. Further, under certain conditions of deposition, island silver films exhibit a dramatic and drastic fall in electrical resistance, marking the insulator-metal transition. This interesting transition observed in a conservative system — after the stoppage of deposition of the film — is a clear evidence for mobility coalescence of islands even at room temperature. The sudden fall in resistance is preceded by fluctuations in resistance with time and the fluctuations are attributed to the making and breaking of the percolation path in the film.     

“Islands in Sea” and “Lakes in Mainland” phases and related transitions simulated on a square lattice

We describe the Island-Mainland (IM) transition on a square lattice. Black squares are randomly dropped on a white background, with increasing concentration p. We define black (white) clusters as groups of black (white) squares, connected through edges or corners on a quasi-2D system. At a certain p the black islands in white sea (IS) phase crosses over to a mixed phase (MP). At still larger p the MP phase crosses over to the lakes in mainland (LM) phase with white lakes in a black mainland. We show that the Euler number, defined as difference between number of white clusters and number of black clusters goes through extrema near these transition points. The phenomenon is supported by experimental observations.     

薄膜生长中的表面动力学(Ⅱ)

本文较全面地从理论上研究了薄膜生长过程中原子在表面上的各种动力学表现 ,涉及的内容包括亚单层生长时 ,原子在表面上的扩散 ,粘接 ,成核 ,以及已经形成的原子岛之间的相互作用 ,兼并 ,失稳 ,退化等一系列过程。在第一部分 (即 0～ 6章 ,刊登在《物理学进展》2 3卷 1期上 )介绍了薄膜生长动力学的基础之后 ,从第 7章开始我们侧重研究一个在向同性和各向异性表面都普遍成立的原子岛尺寸大小及密度分布的标度定律 ;建立了一套研究在各向同性和各向异性表面上二维原子岛退化过程的广义动力学标度理论。基于对层间质量扩散通道的研究 ,本文提出了两种不同的原子下跃机制 ,既任意跃迁机制和选择跃迁机制 ,并做了进一步的实验验证。利用这一新的层间质量扩散机制 ,我们成功地解释了实验上观测到的三维原子岛的退化规律。更加有趣的是 ,本文讨论了应力对岛的形状和各种动力学规律的影响。在最后我们还提出了一个利用凝聚能转化来控制二维原子岛生长的方法 ,其目的是希望能够找到一种人为有效地在表面上制备低维量子结构的方法。