Anomalous electronic transport in quasicrystals and related complex metallic alloys

We analyse the transport properties in approximants of quasicrystals α-AlMnSi, 1/1-AlCuFe and for the complex metallic phase λ-AlMn. These phases present strong analogies in their local atomic structures and are related to existing quasicrystalline phases. Experimentally, they present unusual transport properties with low conductivities and a mix of metallic-like and insulating-like characteristics. We compute the band structure and the quantum diffusion in the perfect structure without disorder and introduce simple approximations that allow us to treat the effect of disorder. Our results demonstrate that the standard Bloch–Boltzmann theory is not applicable to these intermetallic phases. Indeed their dispersion relations are flat, indicating small band velocities, and corrections to quantum diffusion, which are not taken into account in the semi-classical Bloch–Boltzmann scheme, become dominant. We call this regime the small velocity regime. A simple relaxation time approximation to treat the effect of disorder allows us to reproduce the main experimental facts on conductivity qualitatively and even quantitatively.     

三角晶格磁振子晶体带结构的优化研究

用平面波展开法数值计算了椭圆柱散射体正三角排列的二维磁振子晶体带结构.结果显示,同样的填充率下,带隙的宽度和中心频率随着椭圆两半径之比s的变化出现明显的变化,椭圆柱散射体在三角晶格中可以打开低频带隙或使低频带隙加宽.     

Breakdown of metallic conductivity in quasicrystals

In the icosahedral phases i-AlCuFe, i-AlPdMn and i-AlPdRe, the electrical conductivity is in the same range as for doped semiconductors. Strong similarities are observed between the direct and tunneling conductivity for the i-AlPdRe phase and for disordered systems on both sides of the metal–insulator (MI) transition.     

Electronic transport in quasicrystals

Mobility of electrons in quasicrystals is considered in the framework of the fractional Fermi surface (FS) model, i.e., a multiconnected FS with many electron-hole pockets. The Mott law for the variable range hopping conductivity is obtained when intervalley scattering processes with small momentum transfer are taken into account. The transition to the power-law temperature dependence is discussed.     

Band-structure determination for finite 3-D photonic crystals

The partial band structure from a finite photonic crystal is determined using a model based on light diffraction and the transfer-matrix formalism. The predictions from such a model are compared to an experimental measurement of the bands in the LU direction of a face centered cubic colloidal crystal. Then, both the theoretical predictions and the experimental measurements are compared with the usual band-structure calculation based on a plane-wave expansion with perfectly periodic boundary conditions. As in measurements performed in the past, discrepancies between the predictions of this later model and the experimentally determined bands are observed. On the contrary, using the model presented based on light propagation through a finite crystal, where no periodicity is imposed in the direction perpendicular to any of the set of planes considered to determine a specific branch of the band structure, we found a very good agreement between the experimentally determined and the predicted bandwidths.     

Formation of dissipative structures in crystals during heat and electron transport

The surface restructuring (faceting) of solids subjected to longitudinal electric-field and temperature gradients has been studied experimentally and theoretically. Tungsten crystals and wires preheated with a direct current in vacuum or a hydrogen atmosphere to a temperature higher than half the melting temperature are studied by electron microscopy and metallography. The processes of formation of bulk defects and of a regular surface structure are found to correlate. For the first time, these processes are analyzed in terms of synergetics.     

Quantum transport studies of electron accumulation layers in InSe bulk crystals

Summary Experiments show that the Hall resistivityρ _xy of InSe bulk crystals is quantized into integer multiples ofh/e ². Quantum Hall effect in InSe is explained as a result of plane defects in InSe crystals. The electrons in bulk InSe are localized at these defects at low temperatures forming regions with two-dimensional conductivity. Concentration of electrons in these regions isN _2D=2·10¹¹ cm⁻² and their mobility μ=2·10⁴cm²/V·s. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

平面波展开法计算二维磁振子晶体带结构

磁振子晶体是光子晶体或声子晶体在磁性材料领域内的替代品,是近来的一个研究热点. 本文提出了磁振子晶体领域内的一种平面波展开法,其较传统的平面波展开法能节约一半以上的计算时间. 采用此方法,数值计算了由Fe/EuO二种铁磁材料构成的二维磁振子晶体带结构. 数值计算结果表明,在一定的体积填充率下,有自旋波带隙的出现;影响磁振子晶体带隙结构形成的主要因素是有效场中的交换作用场,其他作用场的影响相对很小. 关键词： 磁振子晶体 带隙 平面波展开法     

Imaging metallic multilayer structures through ultrafast optically driven excited electron transport

Received: 20 September 1998 / Revised version: 21 December 1998     

Transverse electron transport in layered metallic systems: Giant magnetoresistance and injection of spins

The distribution functions for electrons differing in the sign of the spin projection and belonging to different layers (ferromagnetic and nonmagnetic) are determined from the system of Boltzmann kinetic equations. These functions make it possible to derive integral equations for electrochemical potentials for arbitrary ratios of characteristic lengths (layer thicknesses and momentum-and diffusion mean free paths) and to describe transverse electron transport both in the bulk and in the surface regions of the multilayer structure. The expressions for the effective contact resistance are derived and the value of the transverse ohmic resistance of the structure is found, as well as its spin-dependent part determined by the values of injection factors γ. The values of nonequilibrium spin polarizations, which are also connected with coefficients γ, are determined. The values of γ are calculated for various relations between the characteristic parameters of the given system and for various types of magnetic order.     

Coupled metallic ring gap waveguide

Employing the surface plasmon polaritons (SPPs), a kind of coupled metallic squareness ring waveguide structure is presented. Its properties has been analyzed with the finite different time domain method and the coupling length has been derived from the coupled mode theory. It is demonstrated that the SPPs excited by the light with different wavelength will come out from different output port due to different coupling length. By appropriately designing the structure, it can be utilized to realize some optics devices such as multiple-wavelength sorter and beam splitter. This will break through the diffraction limit of traditional optical devices.     

Holographic method for the determination of photo-induced electron and hole transport in electro-optic crystals

A new experimental method based on holographic techniques is proposed, which allows the determination of electron and hole contributions to the photo-induced charge transport in highly insulating electro-optic crystals. Experimental results for transition metal doped LiNbO₃- and LiTaO₃- crystals are presented.     

The influence of shape and orientation of scatterers on the photonic band gap in 2D metallic photonic crystals

Using the revised plane wave method, we have calculated the photonic band structures of 2D metallic photonic crystals composed of parallel metallic rods in air background and air holes drilled in metal background. We discuss the maximization of gap-to-midgap ratio as a function of scatterer parameters with different shapes and orientations in three types of lattices.     

Band gap formation and multiple scattering in photonic quasicrystals with a Penrose-type lattice

This Letter presents a study of the local density of states (LDOS) in photonic quasicrystals. We show that the LDOS of a Penrose-type quasicrystal exhibits small additional band gaps. Among the band gaps, some exhibit a behavior similar to that typical of photonic crystals, while others do not. The development of certain band gaps requires large-size quasicrystals. It is explained by the long-range interactions involved in their formation. Moreover, the frequencies where the band gaps occur are not necessarily explained using single scattering and should therefore involve multiple scattering.     

New experimental results for electron transport in weak accumulation layers of ZnO crystals

Previous Hall measurements on (0001) and (0001&#x0304;) faces of ZnO have shown a Hall mobility oscillating as a function of Hall surface electron density in the range between N_SH=10⁶ and 10¹¹ cm^?2. Here we report on new results obtained by a field effect arrangement for free surfaces in UHV. With donors from H exposure or by illumination weak accumulation layers (n_sh <10¹¹ cm^?2) are established. The field effect shows oscillations in surface conductivity as a function of gate voltage. Also the combination of a field effect with a Hall effect measurement reveals discrete values of Hall surface electron density n_sh. Various pretreatments do not change the periodicity of these oscillations. Necessary preconditions are a temperature below 130 K, a Hall surface electron density below 3 × 10¹² cm^?2 and a source-drain field of a few V/cm. A model regarding impurity levels in the space charge layer relates the results of the field effect measurements to the results of the Hall effect measurements.