Quasicrystals in a monodisperse system

We investigate the formation of a two-dimensional quasicrystal in a monodisperse system, using molecular dynamics simulations of hard-sphere particles interacting via a two-dimensional square-well potential. We find that more than one stable crystalline phase can form for certain values of the square-well parameters. Quenching the liquid phase at a very low temperature, we obtain an amorphous phase. By heating this amorphous phase, we obtain a quasicrystalline structure with fivefold symmetry. From estimations of the Helmholtz potentials of the stable crystalline phases and of the quasicrystal, we conclude that the observed quasicrystal phase can be the stable phase in a specific range of temperatures.     

用EXAFS测定准晶Al-Mn合金中Mn原子的近邻结构

测定了晶态Al₆Mn,准晶Al₄Mn和准晶Al₆Mn的Mn原子K吸收EXAFS谱(包括Mn原子K的近限吸收谱)。研究了准晶Al₄Mn和准晶Al₆Mn中Mn原子周围的第一近邻结构,讨论了Mn原子周围第一近邻Al原子的分布特征。在准晶Al₆Mn和准晶Al₄Mn中,Al-Mn间原子平均距离约为2.52?,小于晶态中Al-Mn间平均距离,围绕Mn原子的配位数约为 关键词：     

Picosecond and femtosecond pulsed laser ablation and deposition of quasicrystals

A Nd:glass laser with pulse duration of 250 fs and 1.3 ps has been used to evaporate a Al₆₅Cu₂₃Fe₁₂ quasicrystalline target. The gaseous phase obtained from the ablation process has been characterised by several techniques such as emission spectroscopy, quadrupole mass spectrometry and ICCD imaging, used to study the plume composition, energy and morphology. The results show that the ablation processes in the short-pulse regimes are very different to the nanosecond one. In particular the plume angular distribution shows a characteristic high cosine exponent and the composition is completely stoichiometric and independent from the laser fluence. Furthermore the mass spectra indicate the presence of clusters, both neutral and ionised and the emission from the target suggest a rapid thermalisation leading to the melting of the surface. To clarify the ablation process some films have been deposited, on oriented silicon, at different experimental conditions and analysed by scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis and X-ray diffraction. The analyses show the presence of nanostructured films retaining the target stoichiometry but consisting of different crystalline and non crystalline phases. In particular the nanostructure supports the hypothesis of the melting of the target during the ablation and a mechanism of material ejection is proposed for both picosecond and femtosecond regimes.     

Topological states in quasicrystals

With the rapid development of topological states in crystals, the study of topological states has been extended to quasicrystals in recent years. In this review, we summarize the recent progress of topological states in quasicrystals, particularly focusing on one-dimensional (1D) and 2D systems. We first give a brief introduction to quasicrystalline structures. Then, we discuss topological phases in 1D quasicrystals where the topological nature is attributed to the synthetic dimensions associated with the quasiperiodic order of quasicrystals. We further present the generalization of various types of crystalline topological states to 2D quasicrystals, where real-space expressions of corresponding topological invariants are introduced due to the lack of translational symmetry in quasicrystals. Finally, since quasicrystals possess forbidden symmetries in crystals such as five-fold and eight-fold rotation, we provide an overview of unique quasicrystalline symmetry-protected topological states without crystalline counterpart.     

An empirical study of the Mn partial density of states in icosahedral Al70Pd20Mn10

Using the line-shape analysis of the Mn 2p_3/23d3d Auger transitions and by observing the Mn 3p→3d transitions in electron energy-loss spectroscopy, we have studied the local density of electronic states of the crystalline cubic and the quasicrystalline icosahedral phases of Al₇₀Pd₂₀Mn₁₀. We find that, in the cubic phase the Mn 3d states coincide with the Fermi level E_F, but in the quasicrystalline state the Mn 3d band is fully occupied and shifted to lower binding energies causing a significant reduction in the density of states at E_F.     

Shift and broadening of surface polaritons of sapphire upon deposition of a quasicrystalline film

The influence of a quasicrystalline Al-Pd-Re film on the shift and broadening of surface polaritons of a substrate (sapphire) has been studied. Measurements have been performed both on a sample containing only the quasicrystalline phase and on a sample which, in addition to the quasicrystalline phase, contains the crystalline (metallic) phase. The complex dielectric function of the films in the mid-IR region (650–800 cm⁻¹) has been estimated.     

Nanoepitaxy on quasicrystal surfaces

In film growth on quasicrystalline surfaces, the epitaxy-imposed ordering cannot compete with the stable bulk phases of thick films due to absence of translational order in the structure of the substrate. Energetically, this renders the formation of crystalline domains in the native structure of the film material more favorable, while their global orientation is prescribed by the quasicrystalline order. We present experimental results on the dissociative chemisorption of oxygen at the decagonal surface of Al₇₀Co₁₅Ni₁₅ as well as molecular-dynamics simulations of the diffusion of adatoms on the surface of the partially covered substrate.     

IR reflectance spectra and optical constants of Al-Cu-Fe quasicrystalline thin films

The optical properties of Al-Cu-Fe quasicrystalline films and, for comparison, crystalline films of similar composition are studied using middle-and far-IR reflectance spectroscopy. Measurements are performed with 0.1-to 0.3-μm-thick films on sapphire substrates. The complex dielectric function of the films is calculated from experimental data. It is found that the real part of the dielectric function is negative for the crystalline films but positive and weakly frequency dependent, except in the range near 245 cm^?1, for the quasicrystalline films. The optical conductivity of the quasicrystalline films does not feature the Drude peak observed for crystalline films and exhibits a peak at 245 cm^?1, which can be assigned to optical phonon excitations and is absent for crystalline films.     

Epitaxial Bi allotropes on quasicrystal surfaces as templates for adsorption of pentacene and fullerene

The growth of Bi on surfaces of Al-based quasicrystals yields a quasicrystalline monolayer which is followed by the formation of crystalline islands of various forms depending on coverage, deposition flux and substrate temperature. We have used the Bi thin films consisting of both crystalline and quasicrystalline allotropes as substrates to study the deposition of C(60) and pentacene molecules. Scanning tunneling microscopy (STM) reveals substrate-dependent differences in molecular adsorption.     

Endothermic quasicrystalline-to-crystalline phase transition in Al6CuMg4

Electron microscopic and differential scanning calorimetric (DSC) studies have been performed to investigate the phase transitions in the rapidly solidified Al₆CuMg₄ alloy system. We present evidence, for the first time, that anendothermic quasicrystalline to crystalline transition occurs in this system around 340°C. This is an unexpected feature—for conventional wisdom would lead one to expect exothermic behaviour as was seen in Al₈₆Mn₁₄—and points to the fact that stability of certain quasicrystalline phases may be much larger than hitherto expected. Some comments on the recently reported large quasicrystal for the Al₆CuLi₃ system are also made in the light of our observation.     

Morphological instability induced by the interaction of a particle with a solid-liquid interface

Motivated by recent experimental findings, we investigate the possible occurrence and characteristics of quasicrystalline order in two-dimensional mixtures of point dipoles with two sorts of dipole moments. Despite the fact that the dipolar interaction potential does not exhibit an intrinsic length scale and cannot be tuned a priori to support the formation of quasicrystalline order, we find that configurations with long-range quasicrystallinity yield minima in the potential energy surface of the many particle system. These configurations emanate from an ideal or perturbed ideal decoration of a binary tiling by steepest descent relaxation. Ground state energy calculations of alternative ordered states and parallel tempering Monte-Carlo simulations reveal that the quasicrystalline configurations do not correspond to a thermodynamically stable state. On the other hand, steepest descent relaxations and conventional Monte-Carlo simulations suggest that they are rather robust against fluctuations. Local quasicrystalline order in the disordered equilibrium states can be strong.Received: 15 September 2004, Published online: 26 November 2004PACS: 61.44.Br Quasicrystals - 75.50.Kj Amorphous and quasicrystalline magnetic materials - 82.70.Dd Colloids     

似生物膜系统之六角液相

首度在二维溶致型层状液晶系统中，发现了液相与固相之间存在一“六角方向性”的新状态．这个状态显示了的六角长程有序与似液相的位置相关连特性之特征，而与熟知的二维融化理论所预期的现象有显著的差异．     

Computer simulation of pulsed laser processing of amorphous Si

The paper introduces a 3D computer simulation model of the melting and recrystallization process of amorphous Si induced by pulsed laser irradiation. The model takes into account the temperature dependence of thermal and optical properties of crystalline, amorphous and liquid Si. The melting process is described by introducing for each volume element of melt pool the characteristic times of beginning of melting, end of melting and nucleation of a stable nucleus. The solution of heat equations of liquid and solid phases also provides one with the nucleation rates and temperatures. These data enable one to discriminate whether amorphous or crystalline phases are really allowed to be formed. Two examples of computer simulation are carried out to show the outputs of the model. Received: 7 February 2000 / Accepted: 28 March 2000 / Published online: 9 August 2000     

NANOSTRUCTURED Zr-BASED ALLOYS-PHASE FORMATION AND MECHANICAL PROPERTIES

Newly developed nanostructured Zr/Ti-Al-TM multiphase alloys can provide a large bandwidth of interesting properties, such as mechanical properties. Bulk materials with nanocrystalline/ amorphous and (nano)quasicrystalline/ amorphous microstructure with different volume fractions of nanophases and with different grain sizes can be obtained by slowly cooling the melt as well as by solid state reactions. Multiphase structures are realized either by partial de-vitrification of bulk glass-forming alloys or by defined addition of inert compounds upon alloying. Special preparation techniques e.g. copper mould casting and subsequent controlled annealing and mechanical alloying combined with hot consolidation of powders are described. The phase formation and transformation processes and the thermal stability of such materials in dependence on alloy composition and processing parameters are discussed in detail. Currently, the exploration of properties with respect to potential applications of these nanostructured alloys is still at the beginning. First investigations on the contributions of different phases/ volume fractions to the overall mechanical behaviour will be shown. At room temperature, the deformation behaviour of amorphous/crystalline bulk samples is governed by contributions of all existing phases yielding a high strength of the material.     

Crystalline phase transitions and acoustic phonons behaviour of polymorphic ethanol

Ethanol shows a complex scenario of different crystalline phases in the temperature range between its glass transition at T_g = 97 K and its melting point at T_m = 159 K. Brillouin spectroscopy has revealed capable of observing and assessing acoustic phonon changes related to these phase transitions between different crystalline phases (either a plastic-crystal phase or different monoclinic phases). By combining this technique and calorimetric experimental data, we are able to corroborate the existence of at least two stable and two metastable (monoclinic) crystalline phases of pure ethanol.     

Two-step melting of the vortex solid in layered superconductors with random columnar pins

We consider the melting of the vortex solid in highly anisotropic layered superconductors with a small concentration of random columnar pinning centers. Using large-scale numerical minimization of a free-energy functional, we find that melting of the low-temperature, nearly crystalline vortex solid (Bragg glass) into a vortex liquid occurs in two steps as the temperature increases: the Bragg glass and liquid phases are separated by an intermediate Bose glass phase. A suitably defined local melting temperature exhibits spatial variation similar to that observed in experiments.     

Triple point on the melting curve and polymorphism of nitrogen at high pressure

Raman spectra of solid and fluid nitrogen to pressures up to 120 GPa and temperatures up to 2500 K reveal that the melting line exhibits a maximum near 70 GPa, followed by a triple point near 87 GPa, after which the melting temperature rises again. Fluid nitrogen remains molecular over the entire pressure range studied, and there is no sign of a fluid-fluid transition. Solid phases obtained on quenching from the melt above 48 GPa are identical to the recently discovered iota and zeta' phases. We find that kinetics plays a major role in the experimentally observed phase changes and account for the metastability of various crystalline molecular phases and the existence of an amorphous single bonded eta-N.     

Two-dimensional quasicrystals of decagonal order in one-component monolayer films

By using Monte Carlo simulations we demonstrate that atomic monolayers formed on a crystalline surface may exhibit quasicrystalline decagonal order. It is shown that the stability of two-dimensional quasicrystals (QCs) is determined by the misfit between the adsorbate and the surface lattice and by the corrugation of the surface potential. QCs may be stable at the ground state or develop from the compressed commensurate c(2 x 2) structure, via the first-order phase transition at finite temperatures. The decagonally ordered phase melts into a partially ordered liquidlike phase, which then disorders via a continuous Ising-like transition.     

The role of the “Casimir force analogue” at the microscopic processes of crystallization and melting

Melting (crystallization), a phase transition from a crystalline solid to a liquid state, is a common phenomenon in nature. We suggest a new factor, “the Casimir force analogue”, to describe mechanisms of melting and crystallization. The Casimir force analogue is a force occurring between the surfaces of solid and liquid phases of metals caused by different energy density of phonons of these phases. It explains abrupt changes in geometry and thermodynamic parameters at a melting point. “The Casimir force analogue” helps to estimate latent melting heat and to gain an insight into a solid–liquid transition problem.     

Diffuse scattering data acquisition techniques

Techniques are presented for acquiring and reducing X-ray diffuse scattering data from disordered crystalline materials. These methods are part of a comprehensive approach to study disorder in novel quasicrystalline phases as a function of temperature, but can be applied without further adaptation to periodic crystalline systems. By fully exploiting the possibilities of modern two-dimensional X-ray detector systems - using imaging plates or charge coupled devices (CCD) - large volumes of reciprocal space can be measured in a quantitative and rapid way. For this purpose, the classical rotation method for collecting integrated Bragg intensities is extended for acquiring quasi-continuous diffuse diffraction data. A new high-temperature furnace and helium beam path, designed for the diffraction geometry of the rotation method, are integral parts of the diffraction system. New methods are presented for handling the reduction of diffuse diffraction data from area detectors. One of the key techniques is the reconstruction of arbitrary slices and volumes in reciprocal space from a single series of rotation images taken from an arbitrarily oriented single-crystal (reciprocal space mapping).