Intermediate stages of a transformation between a quasicrystal and an approximant including nanodomain structures in the Al-Ni-Co system

Transition states between decagonal quasicrystal and periodic approximants are studied in the Al-Ni-Co system at a measured composition of Al_71.3Ni_11.3Co _17.4 by high-resolution transmission electron microscopy and electron diffraction. The nanodomain structures appearing after annealing at 1270 K show periodic fluctuations coherently embedded in domains with the coarse order of a one-dimensional quasicrystal. Further annealing at lower temperatures changes the features of nanodomain structures and results in an increase in more periodic structures. These can be strongly disordered and full of defects but tiling analysis and electron diffraction patterns show that they correspond to locked phason strain values of two closely related periodic approximants. We conclude that the periodic approximants do not result from a continuous increase in phason strain but from the growth of seeds with a locked phason strain.     

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.     

Diffuse scattering and phason fluctuations in the Zn-Mg-Sc icosahedral quasicrystal and its Zn-Sc periodic approximant

We report on the absolute scale measurement of the x-ray diffuse scattering in the ZnMgSc icosahedral quasicrystal and its periodic approximant. Whereas the diffuse scattering in the approximant is purely accounted for by thermal diffuse scattering, an additional signal is observed in the quasicrystal. It is related to phason fluctuations as indicated by its Q(2)(per) dependence. Moreover, when compared to previous measurements carried out on the i-AlPdMn phase, we find that the amount of diffuse scattering is smaller in the i-ZnMgSc phase, in agreement with larger phason elastic constants in this phase. This is confirmed by the observation of a large number of weak Bragg peaks having a high Q(per) reciprocal space component.     

Self-assembly of monatomic complex crystals and quasicrystals with a double-well interaction potential

For the study of crystal formation and dynamics, we introduce a simple two-dimensional monatomic model system with a parametrized interaction potential. We find in molecular dynamics simulations that a surprising variety of crystals, a decagonal, and a dodecagonal quasicrystal are self-assembled. In the case of the quasicrystals, the particles reorder by phason flips at elevated temperatures. During annealing, the entropically stabilized decagonal quasicrystal undergoes a reversible phase transition at 65% of the melting temperature into an approximant, which is monitored by the rotation of the de Bruijn surface in hyperspace.     

高压下Al65Co20Mn15合金准晶相得截获及其稳定性的研究

 本文利用高压熔态淬火方法，对Al₆₅Co₂₀Mn₁₅合金进行了研究。首次发现在4.4 GPa压力下淬火的样品中有准晶T相形成。使用电子衍射和X射线衍射对准晶态进行了鉴别。用高温X射线衍射进行了热稳定性研究，发现Al₆₅Co₂₀Mn₁₅合金中准晶T相得晶化温度约为600 ℃。     

State Sampling Dependence of Hopfield Network Inference

The fully connected Hopfield network is inferred based on observed magnetizations and pairwise correlations.We present the system in the glassy phase with low temperature and high memory load.We find that the inference error is very sensitive to the form of state sampling.When a single state is sampled to compute magnetizations and correlations,the inference error is almost indistinguishable irrespective of the sampled state.However,the error can be greatly reduced if the data is collected with state transitions.Our result holds for different disorder samples and accounts for the previously observed large fluctuations of inference error at low temperatures.     

Phase fluctuations in a strongly disordered s-wave NbN superconductor close to the metal-insulator transition

We explore the role of phase fluctuations in a three-dimensional s-wave superconductor, NbN, as we approach the critical disorder for destruction of the superconducting state. Close to critical disorder, we observe a finite gap in the electronic spectrum which persists at temperatures well above T(c). The superfluid density is strongly suppressed at low temperatures and evolves towards a linear-T variation at higher temperatures. These observations provide strong evidence that phase fluctuations play a central role in the formation of a pseudogap state in a disordered s-wave superconductor.     

Phonon Transmission and Thermal Conductance in Fibonacci Wire at Low Temperature

We investigate the phonon transmission and thermal conductance in a general Fibonacci quasicrystal by the model of lattice dynamics and the technique of transfer matrix. It is found that quasiperiodic distribution of masses may greatly destroy the phonon transport at both low and high frequencies and thus may affect the thermal conductance. The thermal conductance increases with temperature at low temperatures and displays saturation with further increase of the temperature. Such saturation behaviour is preserved even when the mass ratio of atoms in the Fibonacci chain is changed.     

Reconstruction of the Fermi surface in the pseudogap state of cuprates

Reconstruction of the Fermi surface of high-temperature superconducting cuprates in the pseudogap state is analyzed within a nearly exactly solvable model of the pseudogap state, induced by short-range order fluctuations of the antiferromagnetic (AFM), spin-density wave (SDW), or a similar charge-density wave (CDW) order parameter, competing with the superconductivity. We explicitly demonstrate the evolution from “Fermi arcs” (on the “large” Fermi surface) observed in the ARPES experiments at relatively high temperatures (when both the amplitude and phase of the density waves fluctuate randomly) towards the formation of typical “small” electron and hole “pockets,” which are apparently observed in the de Haas-van Alphen and Hall resistance oscillation experiments at low temperatures (when only the phase of the density waves fluctuate and the correlation length of the short-range order is large enough). A qualitative criterion for the quantum oscillations in high magnetic fields to be observable in the pseudogap state is formulated in terms of the cyclotron frequency, the correlation length of fluctuations, and the Fermi velocity. The text was submitted by the authors in English.     

Phase-fluctuating 3D Bose-Einstein condensates in elongated traps

We find that in very elongated 3D trapped Bose gases, even at temperatures far below the BEC transition temperature T(c), the equilibrium state will be a 3D condensate with fluctuating phase (quasicondensate). At sufficiently low temperatures the phase fluctuations are suppressed and the quasicondensate turns into a true condensate. The presence of the phase fluctuations allows for extending thermometry of Bose-condensed gases well below those established in current experiments.     

High-temperature surface oxidation of the decagonal AlCoNi quasicrystal

The interaction of oxygen with the 10-fold-symmetry surface of the decagonal Al_72.9Co_16.7Ni_10.4 quasicrystal at high temperatures was investigated by low-energy-electron diffraction and Auger electron spectroscopy. The results are consistent with a well-ordered aluminum-oxide layer possessing a hexagonal antiphase domain structure with a limited lateral size of about 35 Å. We claim that the separation distances of the domain boundaries, separating domains of equal orientation, are primarily a consequence of the preferential cluster nucleation on decagonal Al-Co-Ni. The domains are azimuthally oriented along one direction of the two sets of five twofold-symmetry axes lying on the decagonal surface in accordance with the local symmetry of the quasicrystal surface, while the size of the domains can be explained in terms of self-size-selecting arguments.     

Bridging room-temperature and high-temperature plasticity in decagonal Al–Ni–Co quasicrystals by micro-thermomechanical testing

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behaviour of decagonal Al–Ni–Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25–500 °C), strain rates and sample sizes accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity: at room temperature, cracking controls deformation; at 100–300 °C, dislocation activities control the plastic deformation exhibiting serrated flows and a constant flow stress; at 400–500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5–3.5 GPa and enhanced ductility of over 15% strains between 100 and 500 °C. This study improves understanding of quasicrystal plasticity in their low- and intermediate-temperature regimes, which was poorly understood before, and sheds light on their applications as small-sized structural materials.     

Electronic conduction in quasicrystals at low temperatures

At low temperatures, a perfect quasicrystal is in the “critical” state of metal-insulator transition. A power-law temperature dependence of conductivity, which was experimentally observed at T<5 K in the icosahedral phase of Al-Pd-Re, was obtained using the critical wave functions. Mott’s hopping law was also observed in the Al-Pd-Re samples and explained by the delocalization of electronic states in the momentum space.     

Temperature dependence of bound state spectra in field theory

We analyze the behaviour of kinks and semiclassical bound states at finite temperatures by applying quantum statistics to the fluctuations which determine the quantum dynamics of these states. We consider two theories in one space dimension — the ?⁴ theory with a dynamical symmetry breaking and the Gross-Neveu model. For the ?⁴ theory, the one-loop temperature corrections are obtained by using temperature-dependent Green function techniques. We show that the same result can be obtained by applying quantum statistics to the fluctuations around the kink. For the Gross-Neveu model, the temperature dependence of the bound states, which correspond to time-independent field configurations, is obtained. We show that for every bound state there exists a critical temperature at which this state breaks up into its constituents. This critical temperature increases with the number of constituents of the bound state.     

Single-mode operation regime for 12-fold index-guiding quasicrystal optical fibers

We have studied the extent of the single-mode operation regime for high symmetric index-guiding quasicrystal fibers by analyzing the validity of the effective V parameter, which is used to determine the single-mode cutoff for photonic crystal fibers. We demonstrate that this parameter can also be applied, without any approximations, to a high symmetric 12-fold Stampfli quasicrystal made of silica. We explain this result in terms of both intrinsic-quasicrystal defect and photonic crystal constituent units. We also analyze the extent of the second-order mode operation to further confirm the cutoff between the single- and multimode operations.     

Thermal stability and data retention of resistive random access memory with HfO_x/ZnO double layers

As an industry accepted storage scheme, hafnium oxide(HfO_x) based resistive random access memory(RRAM)should further improve its thermal stability and data retention for practical applications. We therefore fabricated RRAMs with HfO_x/ZnO double-layer as the storage medium to study their thermal stability as well as data retention. The HfO_x/ZnO double-layer is capable of reversible bipolar switching under ultralow switching current( 3 μA) with a Schottky emission dominant conduction for the high resistance state and a Poole–Frenkel emission governed conduction for the low resistance state. Compared with a drastically increased switching current at 120℃ for the single HfO_x layer RRAM, the HfO_x/ZnO double-layer exhibits excellent thermal stability and maintains neglectful fluctuations in switching current at high temperatures(up to 180℃), which might be attributed to the increased Schottky barrier height to suppress current at high temperatures. Additionally, the HfO_x/ZnO double-layer exhibits 10-year data retention @85℃ that is helpful for the practical applications in RRAMs.     

Intrinsic stability of quasicrystals under the generation of Frenkel pairs

Under irradiation metastable quasicrystals undergo a phase transition to an amorphous state. This transition can be reversed by annealing. As in normal crystalline materials the phase transition is considered to be triggered by generation and recombination of vacancies and interstitial atoms (Frenkel pairs). We have classified the possible Frenkel defects in a metastable monatomic quasicrystal with respect to geometric and energetic properties. With numerical simulation we have studied the behaviour of the quasicrystal under a load of Frenkel defects for various defect concentrations. We find three ranges of behaviour: up to 5% defects per atom the structure remains icosahedral, in a middle range it stays disordered icosahedral or it becomes either disordered or perfect crystalline, depending on the implementation of the defects. If there are more than 10% defects the structure becomes irreversibly amorphous. We finally compare our results with experimental data.     

Fluctuations and long-range order in finite-temperature pion condensates

We study the stability of the neutral and charged pion-condensed phases of nuclear matter against fluctuations of the order parameter. At finite temperatures pion condensates with an order parameter varying in only one dimension are, as we show, prohibited, while such condensates are allowed at zero temperature. Condensates that vary in two and three dimensions can be stable at all temperatures. Another allowed state, which may be favored energetically, is a quasi-ordered one-dimensional condensate characterized by long-range pion field correlations decaying only algebraically in space; insufficient experimental resolution may, however, limit one's ability to distinguish such a one-dimensional structure from true one-dimensional long-range order. Finally, we calculate the normal modes and the pion propagator in a charged one-dimensional running-wave condensate, explicitly illustrating the effect of long-range Coulomb forces on the order-parameter fluctuations.     

Energy levels and their correlations in quasicrystals

Quasicrystals can be considered, from the point of view of their electronic properties, as being intermediate between metals and insulators. For example, experiments show that quasicrystalline alloys such as AlCuFe or AlPdMn have conductivities far smaller than those of the metals that these alloys are composed from. Wavefunctions in a quasicrystal are typically intermediate in character between the extended states of a crystal and the exponentially localized states in the insulating phase, and this is also reflected in the energy spectrum and the density of states. In the theoretical studies we consider in this review, the quasicrystals are described by a pure hopping tight binding model on simple tilings. We focus on spectral properties, which we compare with those of other complex systems, in particular, the Anderson model of a disordered metal. We discuss ‘strong‘ and ‘weak’ quasicrystals, which are described by different universal laws. We find similarities and universal behaviour, but also significant differences between quasiperiodic models and models with disorder. Like weakly disordered metals, the quasicrystal can be described by the universal level statistics that can be derived from random matrix theory. These level statistics are only one aspect of the energy spectrum, whose very large fluctuations can also be described by a level spacing distribution that is log-normal. An analysis of spectral rigidity shows that electrons diffuse with a bigger exponent (super-diffusion) than in a disordered metal. Adding disorder attenuates the singular properties of the perfect quasicrystal, and leads to improved transport. Spectral properties are also used in computing conductances of such systems, and to attempt to resolve the experimental enigmas such as whether quasicrystals are intrinsically conductors, and if so, how conductances depend on the structure.     

Structure of Al deposits on nanometer-size islands of aluminum-oxide

Because of the immense structural mismatch between a crystal and a quasicrystal, the aluminum-oxide domains that grow on the pentagonal surface of icosahedral Al–Pd–Mn at high temperatures are in the order of a few nm large. Here, we exploit the small lateral extension of the oxide domains to grow crystalline Al particles in the same size-region by vapor deposition on them. Low-energy-electron diffraction and secondary-electron imaging investigations show that the nanocrystals expose their (1 1 1) faces parallel to the pentagonal surface of the quasicrystal, while the in-plane orientation of the crystallites is random. Spot-profile analysis of the diffracted beams indicate that the Al nanocrystals grow in 3 nm large domains up to a deposition thickness of 51 monolayers.