Composition rule for Al–transition metal binary quasicrystals

Compositions of Al–transition metal (TM) binary quasicrystals are revisited using the cluster-plus-glue-atom model. These compositions all conform to a unified cluster formula [icosahedron](glue)₁, where the icosahedron is taken from a crystalline approximant. In a given binary system, icosahedral and decagonal quasicrystals are expressed by the same icosahedron but glued, respectively, by one Al and one TM, and their e/a ratios fall close to 1.86 and 1.71–1.79, respectively. Their relative stabilities are discussed by referring to compositions satisfying the fully filled TM-3d states according to Häussler's resonance model.     

Enhancement of spectral shift of plasmon resonances in bimetallic noble metal nanoparticles in core–shell structure

Journal of Nanoparticle Research - In this work, atypical zinc oxide nanorods (ZnO NRs) on quartz substrates were successfully synthesized using simple, low-cost, and environmentally friendly...     

Corrugated-enhanced second harmonic generation in metal–insulator–metal plasmonic waveguides

Nonlinear effects such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal–insulator–metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Symmetric means that the metals at the top and bottom of the dielectric layer are the same and asymmetric means that the metals at the top and bottom of the dielectric layer are different. Two different structures are considered here as plasmonic waveguide for generation of second harmonic and analyzed using finite-difference time domain method. Besides the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per length unit (number of grooves) will be optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation λ = 458 nm is considered. It was shown that field enhancement in symmetric MIM waveguides can result in enhancement of SHG magnitude compared to the literature values and asymmetric device results in more than two orders of magnitude enhancement in SHG compared to symmetric structure. It is also shown that the electric field of second harmonic depends on the thickness of crystal (insulator). So, its thickness is optimized to achieve the highest electric field.     

Waveguiding effect in 2D metal–dielectric–metal grating structure

We have studied the waveguiding effect in a 2D metal–dielectric–metal (MDM) grating structure formed on a quartz substrate. The grating was first formed via e-beam lithography and subsequently covered by Ag/MgF₂/Ag MDM films. At a pitch of 300 nm in both x- and y-directions, low reflectance and transmittance were observed in the UV–VIS range, indicating efficient coupling of normal incident light into waveguiding modes. As evidence, we measured the spectrum of the waveguide from the edge, and the bandwidth of the spectrum was as narrow as ∼74 nm. The bandwidth of the waveguide can be further improved by increasing the MDM stack number. In addition, the bandwidth can also be widened by increasing the pitch of the structure. The physical mechanism underlying the phenomena was analyzed and experimentally confirmed. Such effect could be useful in many applications, such as DFB lasers, solar cells, waveguides, and light emitting devices.     

Surface Plasmon Polaritons in a Graphene–Semiconductor–Graphene Thin Film

Physics of the Solid State - Dispersion properties of surface plasmon polaritons in a semiconductor film with graphene plates in the far-IR range and the possibility of controlling propagation...     

Effect of chemical and magnetic interactions on fractal growth under deposition–diffusion–aggregation in some binary metal systems

We have investigated fractal growth phenomena in three binary metal systems; the Sc-Co, Sc-Cr and Sc-Ni systems. For the Sc-Co and Sc-Cr systems, we describe the observed evolution process of the fractals based on the deposition–diffusion–aggregation model and find that the magnetic interaction can play an important role in influencing the fractal dimension, i.e. the stronger the magnetic moment of the aggregating particles, the smaller the dimension of the resultant fractal. Comparing the observations in the three systems studied, it was found that the chemical interaction between two dissimilar atoms might prevent the formation of the fractal pattern during deposition, for example in the Ni-Sc system . PACS 61.43. Hv; 68.55. -a; 66.30. Ny; 75.50. -y     

Plasmon–Exciton Interaction in Planar Nanostructures with Quantum Dots

Optics and Spectroscopy - In most cases, cylindrical structures with a gradient refractive index (GRIN) are formed on the basis of diffusion technology. The refractive index and the residual...     

Plasmon–Magnon Interaction in the (Graphene–Antiferromagnetic Insulator) System

JETP Letters - Effect of plasmon–magnon interaction on the propagation of a hybrid wave along the graphene surface in the graphene–antiferromagnetic insulator system is studied. Three...     

Photon-assisted tunneling versus tunneling of excited electrons in metal–insulator–metal junctions

Photocurrent measurements in Ag–Al₂O₃–Al metal–insulator–metal junctions under illumination with ultra-short laser pulses reveal that tunneling and internal photoemission of excited electrons are the dominating transport mechanisms. Photon-assisted tunneling is observed under rare conditions that depend critically on the preparation of the interface. The comparison of time-resolved two-pulse correlation measurements with model calculations shows that the photon-induced transport of excited electrons is well described using a one-dimensional many-particle model for two coupled metallic leads, whereas a single-particle model for nonresonant excitation in a rectangular double-minimum potential reveals the signature of photon-assisted tunneling. PACS 73.40.-c; 72.10.-d; 73.40.Gk; 73.50.Pz     

Supercurrent coupling in the Faddeev–Skyrme model

Motivated by the sigma model limit of multicomponent Ginzburg–Landau theory, a version of the Faddeev–Skyrme model is considered in which the scalar field is coupled dynamically to a one-form field called the supercurrent. This coupled model is investigated in the general setting where physical space M$M$ is an oriented Riemannian manifold and the target space N$N$ is a Kähler manifold, and its properties are compared with the usual, uncoupled Faddeev–Skyrme model. In the case N=S²$N=S^2$, it is shown that supercurrent coupling destroys the familiar topological lower energy bound of Vakulenko and Kapitanski when M=R³$M={\mathbb{R}}^3$, and the less familiar linear bound when M$M$ is a compact 3-manifold. Nonetheless, local energy minimizers may still exist. The first variation formula is derived and used to construct three families of static solutions of the model, all on compact domains. In particular, a coupled version of the unit charge hopfion on a three-sphere of arbitrary radius is found. The second variation formula is derived, and used to analyze the stability of some of these solutions. In particular, it is shown that, in contrast to the uncoupled model, the coupled unit hopfion on the three-sphere of radius R$R$ is unstable   for all R$R$. This gives an explicit, exact example of supercurrent coupling destabilizing a stable solution of the usual Faddeev–Skyrme model, and casts doubt on the conjecture of Babaev, Faddeev and Niemi that knot solitons should exist in the low energy regime of two-component superconductors.     

Elastic and inelastic electron transport in metal–molecule(s)–metal junctions

An overview of studies on elastic and inelastic electron transport properties of molecular junction devices is presented. The development of the experimental fabrication and characterization of molecular junctions as well as the corresponding theoretical modeling is briefly summarized. The functions of molecular devices are generally governed by the intrinsic structure–property relationships, and strongly affected by various environment factors including temperature, solvent and intermolecular interactions. Those detailed structural and environmental information could be probed by a powerful tool of inelastic electron tunneling spectroscopy, for which the theoretical modeling becomes particularly important. With many successful examples, it is demonstrated that the combination of theoretical simulations and experimental measurements can help not only to understand the electron–phonon interaction, but more importantly also to accurately determine the real configurations of molecules inside the junctions.     

Heat distribution in Paris–Edinburgh press assemblies through finite element simulations

Knowledge on the temperature distribution in high pressure and temperature devices is important for the interpretation of data and can often not be obtained from experiments. Here, we report on the thermal characteristics of the most employed Paris–Edinburgh press assemblies for in-situ X-ray purposes using finite element calculations. The maximal horizontal T variations in the sample are found to be small and amount maximal 50?K at 2500?K. Temperature differences between the sample and the calibrant material are in the same order of magnitude only if the latter is placed well centred on the sample capsule. The present (pure thermal) calculations can only partly reproduce the discrepancies between experimentally determined input power to T relations (1000?K at 350?W) indicating that different deformation behaviours of assemblies may play a crucial role. Based on the obtained results we present an optimized assembly in terms of sample temperature and gradients.     

Resistive switching effect in metal–oxide–metal structures with ZnO:Li oxide layer

The resistive switching effect in metal–oxide–metal (MOM) structures has been investigated, where the 10% Li-doped ZnO layer was used as an oxide layer, as well as Pt and 20% fluorine doped SnO₂ (SnO₂:F) were used as a bottom electrodes. The current–voltage (I–V) and switching (I–t) characteristics of Ag/ZnO:Li/Pt and Ag/ZnO:Li/SnO₂:F structures were investigated. The unipolar resistive switching is detected in the structures with the Pt, while the use of transparent conductive SnO₂:F electrode instead of Pt, results to the bipolar memory effect.     

Antiferromagnetic exchange coupling in amorphous Fe–Sc alloys

Results of ⁵⁷Fe Mössbauer, AC and DC susceptibility, grazing incidence X-ray diffraction, resistivity and Rutherford-backscattering measurements on the amorphous alloys ${\text{Fe}}_{100 - x} {\text{Sc}}_x (8 \leqslant x \leqslant 70)$ give for the first time convincing evidence for the antiferromagnetic exchange coupling between Fe-moments. The antiferromagnetic coupling between Fe-moments is for $(8 \leqslant x \leqslant 70)$ limited to certain regions (magnetic clusters) of the sample. The nature of the coupling is of the Heisenberg type. For $8 \leqslant x < 20$ , the magnetic coupling between Fe-moments is across nonmagnetic Sc-atoms. The conduction electrons mediate an indirect magnetic interaction between the Fe-moments. The magnetic exchange coupling between Fe-moments across Sc-atoms is negative. The antiferromagnetic coupling between Fe-moments can be explained by the Ruderman-Kittel-Kasuya-Yosida, RKKY, interaction by taking into account the damped oscillatory behavior of the RKKY interactions.     

Electro-optic metal–insulator–semiconductor–insulator–metal Mach-Zehnder plasmonic modulator

The performance of a CMOS-compatible electro-optic Mach-Zehnder plasmonic modulator is investigated using electromagnetic and carrier transport simulations. Each arm of the Mach-Zehnder device comprises a metal–insulator–semiconductor–insulator–metal (MISIM) structure on a buried oxide substrate. Quantum mechanical effects at the oxide/semiconductor interfaces were considered in the calculation of electron density profiles across the structure, in order to determine the refractive index distribution and its dependence on applied bias. This information was used in finite element simulations of the electromagnetic modes within the MISIM structure in order to determine the Mach-Zehnder arm lengths required to achieve destructive interference and the corresponding propagation loss incurred by the device. Both inversion and accumulation mode devices were investigated, and the layer thicknesses and height were adjusted to optimise the device performance. A device loss of <8 dB is predicted for a MISIM structure with a 25 nm thick silicon layer, for which the device length is <3 μm, and <5 dB loss is predicted for the limiting case of a 5 nm thick silicon layer in a 1.2 μm long device: in both cases, the maximum operating voltage is 7.5 V.     

Polar optical phonons in core–shell semiconductor nanowires

We obtain the long-wavelength polar optical vibrational modes of semiconductor core–shell nanowires by means of a phenomenological continuum model. A basis for the space of solutions is derived, and by applying the appropriate boundary conditions, the transcendental equations for the coupled and uncoupled modes are attained. Our results are applied to the study of the GaAs–GaP core–shell nanowire, for which we calculate numerically the polar optical modes, analyzing the role of strain in the vibrational properties of this nanosystem.     

Size-dependent thermal stresses in the core–shell nanoparticles

The thermal stress in a magnetic core–shell nanoparticle during a thermal process is an important parameter to be known and controlled in the magnetization process of the core–shell system. In this paper we analyze the stress that appears in a core–shell nanoparticle subjected to a cooling process. The external surface temperature of the system, considered in equilibrium at room temperature, is instantly reduced to a target temperature. The thermal evolution of the system in time and the induced stress are studied using an analytical model based on a time-dependent heat conduction equation and a differential displacement equation in the formalism of elastic displacements. The source of internal stress is the difference in contraction between core and shell materials due to the temperature change. The thermal stress decreases in time and is minimized when the system reaches the thermal equilibrium. The radial and azimuthal stress components depend on system geometry, material properties, and initial and final temperatures. The magnitude of the stress changes the magnetic state of the core–shell system. For some materials, the values of the thermal stresses are larger than their specific elastic limits and the materials begin to deform plastically in the cooling process. The presence of the induced anisotropy due to the plastic deformation modifies the magnetic domain structure and the magnetic behavior of the system.     

The Andreev conductance in superconductor–insulator–normal metal structures

The Andreev subgap conductance at 0.08–0.2 K in thin-film superconductor (aluminum)–insulator–normal metal (copper, hafnium, or aluminum with iron-sublayer-suppressed superconductivity) structures is studied. The measurements are performed in a magnetic field oriented either along the normal or in the plane of the structure. The dc current–voltage (I–U) characteristics of samples are described using a sum of the Andreev subgap current dominating in the absence of the field at bias voltages U < (0.2–0.4)Δ_c/e (where Δ_c is the energy gap of the superconductor) and the single-carrier tunneling current that predominates at large voltages. To within the measurement accuracy of 1–2%, the Andreev current corresponds to the formula \({I_n} + {I_s} = {K_n}\tanh \left( {{{eU} \mathord{\left/ {\vphantom {{eU} {2k{T_{eff}}}}} \right. \kern-\nulldelimiterspace} {2k{T_{eff}}}}} \right) + {K_s}{{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}} \right. \kern-\nulldelimiterspace} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}\) following from a theory that takes into account mesoscopic phenomena with properly selected effective temperature T _eff and the temperature- and fieldindependent parameters K _n and K _s (characterizing the diffusion of electrons in the normal metal and superconductor, respectively). The experimental value of K _n agrees in order of magnitude with the theoretical prediction, while K _s is several dozen times larger than the theoretical value. The values of T _eff in the absence of the field for the structures with copper and hafnium are close to the sample temperature, while the value for aluminum with an iron sublayer is several times greater than this temperature. For the structure with copper at T = 0.08–0.1 K in the magnetic field B_|| = 200–300 G oriented in the plane of the sample, the effective temperature T _eff increases to 0.4 K, while that in the perpendicular (normal) field B _⊥ ≈ 30 G increases to 0.17 K. In large fields, the Andreev conductance cannot be reliably recognized against the background of single- carrier tunneling current. In the structures with hafnium and in those with aluminum on an iron sublayer, the influence of the magnetic field is not observed.