The European Physical Journal B

We have studied the multipole polarizability of a graded spherical particle in a nonuniform electric field, in which the conductivity can vary radially inside the particle. The main objective of this work is to access the effects of multipole interactions at small interparticle separations, which can be important in non-dilute suspensions of functionally graded materials. The nonuniform electric field arises either from that applied on the particle or from the local field of all other particles. We developed a differential effective multipole moment approximation (DEMMA) to compute the multipole moment of a graded spherical particle in a nonuniform external field. Moreover, we compare the DEMMA results with the exact results of the power-law graded profile and the agreement is excellent. The extension to anisotropic DEMMA will be studied in an Appendix.

In a variety of samples, conductivity is a limiting factor regarding the resolution of dielectric loss peaks of interest. One approach to eliminating the signal that originates from conductivity is the use of insulating layers on one or both electrodes. For the typical case, it is shown that insulation layers add errors rather than improving the situation.

First principles calculations of the total energy of Imma states have found instabilities in states near the β-Sn phase and in states near the simple hexagonal (sh) phase of Si crystal. In agreement with experiment the two instability ranges narrow the stable range between them and also in agreement with experiment the instabilities force first-order transitions to both the β–Sn and sh phases when the pressure is held constant, the experimental condition. The transition pressures to the β-Sn and sh phases for a non-vibrating crystal model are found to be 96 and 110 kbar respectively. These pressure values are considerably lower than the experimental values, but we show that lattice vibrations will increase the equilibrium-state pressures. We find widespread occurrence of instability in the equilibrium states of the three phases and show the presence of three kinds of instability. Near and up to the sh phase structure we find the unusual case of stability at constant volume, but, as observed, instability at constant pressure p. Two special computational procedures are discussed, which locate the unstable ranges of structure. One is based on finding phases from minima of total energy E at constant V and the other finds phases from minima of the Gibbs free energy G at constant p. When the minima cease to exist the Imma phase is unstable.

We study quantum transport in honeycomb lattice ribbons with either armchair or zigzag edges. The ribbons are coupled to semi-infinite linear chains serving as the input and output leads and we use a tight-binding Hamiltonian with nearest-neighbor hops. The input and output leads are coupled to the ribbons through bar contacts. In narrow ribbons we find transmission gaps for both types of edges. The appearance of this gap is due to the enhanced quantum interference coming from the multiple channels in bar contacts. The center of the gap is at the middle of the band in ribbons with armchair edges. This particle-hole symmetry is because bar contacts do not mix the two sublattices of the underlying bipartite honeycomb lattice when the ribbon has armchair edges. In ribbons with zigzag edges the gap center is displaced to the right of the band center. This breakdown of particle-hole symmetry is the result of bar contacts now mixing the two sublattices. We also find transmission oscillations and resonances within the transmitting region of the band for both types of edges. Extending the length of a ribbon does not affect the width of the transmission gap, as long as the ribbon’s length is longer than a critical value when the gap can form. Increasing the width of the ribbon, however, changes the width of the gap. In ribbons with zigzag edges the gap width systematically shrinks as the width of the ribbon is increased. In ribbons with armchair edges the gap is not well-defined because of the appearance of transmission resonances. We also find only evanescent waves within the gap and both evanescent and propagating waves in the transmitting regions.

We theoretically study the spin pump effects of the rotating magnetic field on the spin current through two coupled quantum dots. Owing to the interdot coupling, two molecular states with different bands can be formed, resulting asymmetric spin current peaks. The possibility of manipulating the spin current is explored by tuning the strength, the frequency, and the direction of the rotating magnetic field. The number and location of the spin current peaks can be controlled by making use of various tunings. Furthermore, the normal 2π period of the spin current with respect to the magnetic flux can be destroyed by the interdot coupling.

We provide an alternative means of electric field control for spin manipulation in the absence of magnetic fields by transporting quantum dots adiabatically in the plane of two-dimensional electron gas. We show that the spin splitting energy of moving quantum dots is possible due to the presence of quasi-Hamiltonian that might be implemented to make the next generation spintronic devices of post CMOS technology. Such spin splitting energy is highly dependent on the material properties of semiconductor. It turns out that this energy is in the range of meV and can be further enhanced with increasing pulse frequency. In particular, we show that quantum oscillations in phonon mediated spin-flip behaviors can be observed. We also confirm that no oscillations in spin-flip behaviors can be observed for the pure Rashba or pure Dresselhaus cases.

The normal state of Iron chalcogenide superconductors show a range of unconventional features. Bad-metallic resistivity and proximity to insulating state manifest themselves in spectral and transport responses. In particular, obervation of low-energy pseudogap feature in the normal state raises the issue of the nature of processes underpinning its emergence as well as its relation to unconventional superconductivity. Here, using the LDA+DMFT method, we show how correlation-induced orbital-selective pseudogap-like physics underpin these incoherent features in stoichimetric and electron-doped FeSe superconductor. We discuss the pseudogap regime microscopically, along with implications for the superconductive instability.

Population cycles in small rodents of the north are modeled by noise with memory. Multiannual lemming density fluctuations are presented as a pulse sequence. These pulses correspond to the peaks of lemming density. The memory is presented as some delay time after each pulse. During this time the next pulse is forbidden. Parameter of periodicity, average period, correlation function and parameter of synchronization are calculated for different places of North America. Examples of equations modeling population dynamics of lemmings (or their predators) are considered. The model of connected oscillators gives the qualitative explanation of synchronization effects and relation between synchronization and periodicity. These results have implication for the testing of hypotheses regarding lemming cycles.

Mesoscopic transport through an ultrasmall quantum dot (QD) coupled to two single-wall carbon nanotube (SWCN) leads under microwave fields (MWFs) is investigated by employing the nonequilibrium Green’s function (NGF) technique. The charging energy and junction capacitances influence the output characteristics sensitively. The MWFs applied on the leads and gate induce novel photon-assisted tunnelling, strongly associated with the density of states (DOS) of the SWCN leads. The SWCN leads act as quantum wires, and the compound effect induces nonlinear current behavior and resonant tunnelling in a larger region of energy scale. Negative differential conductance (NDC) is clearly observed, as the source-drain junction capacitances C L , and C R are large enough. The multi-resonant NDC oscillation appears due to the charging and photon-electron pumping effects associated with the contribution of multi-channel quantum wires.

The effect of vacancy defects on electrons transport behavior of the alpha-armchair graphyne nanoribbons has been studied by density-functional tight-binding and non-equilibrium Green’s function methods. Three different widths of the nanoribbons with 6, 7 and 8 atoms and four types of vacancy defects contain one, two, three and four missing atoms were selected in this study. In relaxed structures, the structural changes around the defects are observed. Some of the free hands form new atomic chains containing 6 or 7 atoms. Comparing with perfect devices, the current decreases at the defective devices with 8 atoms width, whereas, it increases for devices with 6 atoms width. By calculating the density of states, transmission spectrums and molecular energy spectrums for devices with 6-atoms widths, there is a resonance state for DDOS and T(E) peaks in the QV device, while the peak of the density of states and transmission spectrums does not match in the SV1 device. Also, the results show that HOMO-LUMO gap energy in the SV1 device is much more than the perfect and QV devices. For devices with 8 atoms width, the transmission spectrums are reduced for all defects due to the lower density of the energy level of molecular energy. However, the orbital distribution of LUMO state in the device with the defect is localized but for the perfect structure, both the LUMO and the HOMO orbital distribution are quite delocalized.