High-temperature electronic and thermal properties of icosahedral quasicrystals

We discuss the high-temperature electronic and thermal properties of an icosahedral quasicrystal within the framework of the fractional multicomponent Fermi-surface model. When intervalley electron-phonon scattering sets in above a characteristic temperature T^∗ of the order of the Debye temperature ΘD the quasicrystal becomes more “metallic”. In this regime the electrical conductivity and the electronic contribution to the thermal conductivity vary as T and T², respectively. We predict that at elevated temperatures the electronic specific heat will vary faster than γT and the low-frequency Drude-type component of the optical conductivity σ₁(ω) will gain weight.     

Continuity properties of the electronic spectrum of 1D quasicrystals

In this paper we consider operatorsH(,x) defined onl ²() by

Electrochromic materials: Microstructure,electronic bands,and optical properties

Crystals of all the well known electrochromic metal (Me) oxides are composed of MeO₆ octahedra in various corner-sharing and edge-sharing arrangements. Cluster-type and columnar microstructures, based on the MeO₆ units, exist in electrochromic films. The coordination of the ions leads to schematic electronic band structures that, at least for the (defect) perovskite and rutile lattices, are able to explain the presence or absence of cathodic and anodic electrochromism. Small polaron absorption prevails in disordered oxides, while crystalline tungsten oxide can show free-electron effects.     

Unconventional properties of dislocations in quasicrystals

Dislocations in quasicrystals are defined as the intersections of dislocations in a high dimensional lattice with an irrational cut which figures the physical space. This definition confers to them a number of unusual geometrical properties which can be studied either by suitable extensions of the Volterra process, or by topological approaches, which often offer complementary points of view and are presented in this paper. Amongst these unusual properties, the production of stacking faults under shear at low temperature, reshuffling processes on stacking faults, and properties of non-commutativity which could have some incidences on the interplay between dislocations in deformation processes are mentioned.Dedicated to Dr. Frantiek Kroupa in honour of his 70^th birthday.Unité de Recherche Associée 009 du CNRS, associée aux Universités de Paris VI at Paris VIIWe are very grateful to Dr. Vladimir Dmitrienko for discussions and useful remarks.     

Thermodynamics of equilibrium properties of quasicrystals

Thermodynamics of equilibrium properties of crystals has been generalized to the case of quasicrystals. Based on it, relations between direct and converse effects, and some relations between the coefficients measured under different conditions have been derived. By group representation theory, physical property tensors have been derived for two-dimensional pentagonal, octagonal, decagonal and dodecagonal, and three-dimensional icosahedral and cubic quasicrystals.     

Isotropic and anisotropic physical properties of quasicrystals

Since quasicrystals have positional and orientational long-range order, they are essentially anisotropic. However, the researches show that some physical properties of quasicrystals are isotropic. On the other hand, quasicrystals have additional phason degrees of freedom which can influence on their physical behaviours. To reveal the quasicrystal anisotropy, we investigate the quasicrystal elasticity and other physical properties, such as thermal expansion, piezoelectric and piezoresistance, for which one must consider the contributions of the phason field. The results indicate that: for the elastic properties, within linear phonon domain all quasicrystals are isotropic, and within nonlinear phonon domain the planar quasicrystals are still isotropic but the icosahedral quasicrystals are anisotropic. Moreover, the nonlinear elastic properties due to the coupling between phonons and phasons may reveal the anisotropic structure of QCs. For the other physical properties all quasicrystals behave like isotropic media except for piezoresistance properties of icosahedral quasicrystals due to the phason field.     

Local electronic and electrical properties of functionalized graphene nano flakes

Based on experimental findings models of amorphous graphene related carbon materials were generated using graphene nano flakes. On the optimized structures detailed local electronic properties were investigated using density functional theory. The electrical conductivities of all these models were also estimated using an in-house program based on tight-binding method. The calculated electrical conductivity values of all the models agreed well with the trend of calculated energy gap and graphitic character.     

Electronic properties of quasicrystals an experimental review

The electronic properties of a large number of icosahedral-crystal systems have been studied experimentally. These systems include alloys of the simple metals and those that contain transition metals. Some of the icosahedral phases (i-phases) are thermally stable while others are metastable; and their degree of structural order varies within each of the stability classes. The importance of sample quality to the exposition of intrinsic properties is emphasized, particularly for systems with high resistivities. As a result, experiments on single-phase samples from the diversity of systems studied have shed light on the physics of quasicrystalline alloys. Comparison made with the approximant and amorphous phases have provided important insights to the understanding of quasicrystallinity, randomness, and atomic-potential effects on electronic properties. Reduction in the electronic density of states (DOS) at the Fermi level relative to the free-electronic value is observed in the stable i-phase systems studied to date. Examples are the GaMgZn, AlCuLi, AlCuFe, and AlCuRu systems. Although the simple-metal i-alloys such as AlCu (Mg, Li) and (Al, Ga)MgZn within their metastable phase fields are found to have nearly free-electron DOS, the anomalies in their transport behaviour and compositional dependence of electronic properties have revealed the important role of the Fermi-surface-Jones-zone boundaries (FS-JZB) interaction in these measurements. Also, central to the FS-JZB interaction criterion of phase stability is the existence of a pseudogap in the DOS which is most evident in the stable i-phases. Unusual transport behaviour has been reported for the structurally ordered i-phases that contain transition metals. These materials exhibit semi-metallic transport behaviour characterized by a high resistivity (～4000–30000 μΩcm), a large resistivity ratio ρ(4·2 K)/ρ(300 K) ranging from 1·5 to 4 as well as a low carrier concentration of 10^?2 to 10^?3 electron per atom. Adding to the anomalous electron transport are the strong temperature dependences of the Hall coefficient and thermopower leading to a change of sign. Further understanding of the ordered i-phases has been advanced through studies of amorphous phases and crystalline counterparts of i-phases known as rational approximants. Based on these studies, the key findings are reported: (i) even for simple-metal systems in the weak scattering regime, differences in the electronic properties between the structural phases begin to emerge as the FS-JZB interaction increases in strength, (ii) semi-metallic conductivity and other anomalous transport properties are also observed in the approximant-crystals that contain transition metals (e.g. AlMnSi, AlFeCu), (iii) on the other hand, disordered i-phases and amorphous phases containing transition metals are found to possess metallic-glass-like properties. Experimental results reported indicate that the electronic properties of an ordered icosahedral quasicrystal are similar to those of a crystal in the presence of a strong FS-JZB interaction. Quite surprisingly, the elastic scattering time of electrons in the sp-d band ordered phases is found to be much longer than that in the nearly-free-electron i-phases. The propensity towards electron localization is ascribed to the realization of a potentially strong FS-JZB interaction in an ordered structure, particularly that with the icosahedral point group symmetry. The data have also provided evidence for the existence of rapidly varying electronic structures within the pseudogap of a semi-metallic i-crystal and its crystal analogue, despite the short electronic mean free path. The electron transport at low temperature can be described by the weak localization and electron-electron interaction effects. Contact with theories on quasiperiodic systems is also made. Finally, results from the stable decagonal crystals are discussed.     

Dislocations and mechanical properties of icosahedral quasicrystals

In this article we interpret the mechanical properties of icosahedral quasicrystals with the dislocation theory. After having defined the concept of dislocation in a periodic crystal, we extend this notion to quasicrystals in the 6-dimensional space. We show that perfect dislocations and imperfect dislocations trailing a phason fault can be defined and observed in transmission electron microscopy (TEM). In-situ straining TEM experiments at high temperature show that dislocations move solely by climb, a non-conservative motion-requiring diffusion. This behavior at variance with that of crystals which deform mainly by glide is explained by the atypical nature of the atomic structure of icosahedral quasicrystals.     

Short-range order and singularities of the electronic structure of icosahedral quasicrystals

For icosahedral phases of the Al-Cu-Fe system, components of the electrical conductivity, magnetic susceptibility, Hall effect, and heat capacity associated with thermally induced charge carriers for the first time have been considered jointly over a wide temperature range. It has been shown that the full range of thermal effects can be understood in the framework of the unified concept, which is based on an inhomogeneous system of two-level electronic excitations. A model of the inhomogeneous electronic state and the mechanism of its formation with the dominant role of short-range order have been proposed.     

Transition-metal-substituted indium thiospinels as novel intermediate-band materials: prediction and understanding of their electronic properties

Results of density-functional calculations for indium thiospinel semiconductors substituted at octahedral sites with isolated transition metals (M=Ti,V) show an isolated partially filled narrow band containing three t2g-type states per M atom inside the usual semiconductor band gap. Thanks to this electronic structure feature, these materials will allow the absorption of photons with energy below the band gap, in addition to the normal light absorption of a semiconductor. To our knowledge, we demonstrate for the first time the formation of an isolated intermediate electronic band structure through M substitution at octahedral sites in a semiconductor, leading to an enhancement of the absorption coefficient in both infrared and visible ranges of the solar spectrum. This electronic structure feature could be applied for developing a new third-generation photovoltaic cell.