The solution energy of H and He in various interstitial and substitution positions in the hcp lattice of α-Ti has been calculated based on the method of electron density functional. The lowest solution energy of He corresponds to the basal octahedral position and that of H corresponds to the octahedral position (next in energy is the tetrahedral position). The calculated vibration frequencies of H in various positions are used for identification of lines in the vibration spectrum obtained by the method of neutron inelastic scattering. Taking into account these spectra, it can be concluded that hydrogen atoms occupy in the hcp lattice of Ti both the octahedral and tetrahedral positions even at 600 K. The available experimental data do not contradict the conclusion that the octahedral position is more preferable in α-Ti. The energy barriers are estimated for various diffusion paths of H and He.
The current–voltage characteristics of superconductor–insulator–semiconductor (S1–I–S2) tunnel junctions, where superconducting electrode S2 is a thin nanowire, are studied experimentally. The observed blurring of the gap singularities is interpreted as a manifestation of the order parameter quantum fluctuations. We propose a model taking into account the broadening of the density of states due to the interaction of electrons with the Mooij–Schön plasmon mode emerging in a quasi-one-dimensional superconducting channel in the regime of quantum fluctuations of the order parameter. The model gives results that are in a reasonable qualitative agreement with the experimental data.
The equilibrium distributions of the misfit dislocation density rho(z) and elastic stresses epsilon(z) are calculated along the direction of the epitaxial growth of the metamorphic InAlAs/GaAs(001) layer with higher In content (to 87 mol %) and various profiles of varying the composition: step, linear, and root. The calculations are performed using the method based on iteration searching for the minimum total energy of the system. It is shown that the largest differences between various constructions of the buffer layer are observed in the character of distributions rho(z), rather than epsilon(z). Unlike the traditional constructions with a step and linear gradients of the composition, which are characterized by a quite homogeneous distribution of misfit dislocations, in a buffer layer with a root composition gradient, the main part of such dislocations is concentrated in the lower part of the layer near the heteroboundary with a GaAs substrate, and their density sharply decreases by more than one order of value along the layer thickness, achieving the value minimum for all abovementioned constructions. In spite of the fact that the important effect of interacting the dislocations to each other is not taken into account in this work, the calculations enable us to establish the main peculiarities of the distributions rho(z) and epsilon(z) in various metamorphic buffer InAlAs layers, which were observed experimentally before. Thus, this approach can be effectively used when designing optimal constructions of the device metamorphic heterostructures.
We study the effects of electron-hole asymmetry on the electronic structure of helical edge states in HgTe/HgCdTe quantum wells. In the framework of the four-band kp-model, which takes into account the absence of a spatial inversion centre, we obtain analytical expressions for the energy spectrum and wave functions of edge states, as well as the effective g-factor tensor and matrix elements of electro-dipole optical transitions between the spin branches of the edge electrons. We show that when two conditions are simultaneously satisfied—electron-hole asymmetry and the absence of an inversion centre—the spectrum of edge electrons deviates from the linear one, in that case we obtain corrections to the linear spectrum.
The contribution of electron–phonon scattering to conductivity of a quantum cylinder in a lon-gitudinal magnetic field has been studied. It has been shown that the conductivity of the nanotube undergoes Aharonov–Bohm oscillations with variations in the magnetic flux through the nanotube cross section. The formulas describing the temperature dependence of the resistance of the nanostructure both in the case of an isotropic phonon spectrum and with allowance for the effects of phonon confinement have been obtained in the analytical form.
An important role of the morphology of a superconducting layer in the superconducting spin-valve effect has been established. The triplet pairing induced by the superconductor/ferromagnet proximity effect has been experimentally investigated for samples CoO x /Py1/Cu/Py2/Cu/Pb (where Py = Ni 0.81 Fe 0.19 ) with a smooth superconducting layer. The optimization of the parameters of this structure has demonstrated a complete switching between the normal and superconducting states with a change in the relative orientation of magnetizations of the ferromagnetic layers from the antiparallel to orthogonal orientation. A pure triplet contribution has been observed for the sample with a permalloy layer thickness at which the superconducting spin-valve effect vanishes. A direct comparison of the experimental data with the theoretical calculation of the temperature of the transition to the superconducting state has been performed for the first time.
Methods for controlling states of interacting superconducting flux qubits using power-efficient devices of fast single-quantum logic (Josephson nonlinearity cavities) are studied. One- and two-qubit quantum logical operations performed within the conventional control technique using Rabi pulses and using picosecond single unipolar magnetic field pulses are comparatively analyzed. It is shown that all main operations can be implemented with an accuracy of better than 97% due to optimization of the shape and parameters of unipolar control pulses (associated with, e.g., propagation of fluxons in transmission lines). The efficiency of the developed technique for programming a two-qubit quantum processor implementing the simplest Deutsch–Jozsa algorithm is demonstrated.
Volt-ampere characteristics of narrow superconducting titanum wires have been studied experimentally. The narrowest specimens measured by means of high-resistance contacts have revealed a behavior nontrivial for superconductors: the Coulomb blockade. The Coulomb gap size correlates with the frequency of quantum phase slips. The observation confirms the identity of quantum dynamics of charge in the Josephson junctions and in the quasi-one-dimensional superconducting channels in the regime of quantum fluctuations of the order parameter.
High-quality aluminum films on GaAs substrates are studied experimentally. The critical temperature of superconductivity is found to increase markedly with decreasing the film thickness. The observed phenomenon is considered as a manifestation of the quantum confinement effect, which affects both the density of states and the electron–phonon interaction.
The transport properties of two types of quasi-one-dimensional superconducting microstructures were investigated at ultra-low temperatures: the narrow channels close-packed in the shape of meander, and the chains of tunneling contacts "superconductor-insulator-superconductor." Both types of the microstructures demonstrated high value of high-frequency impedance and-or the dynamic resistance. The study opens up potential for using of such structures as current stabilizing elements with zero dissipation.
A numerical and analytical investigation of thermodynamic properties of a magnetized superconducting quantum cylinder has been carried out. The dependence of the difference in the magnetizations of the superconducting and normal phases on the parameters of the nanotube,temperature, and magnetic field has been analyzed. The jump in the heat capacity of the superconducting and normal states at the critical temperature has been calculated. The fluctuation contribution to the thermodynamic properties of the nanotube at a temperature abovethe transition point has been studied.
The specific features of the structure in a polycrystalline anion-deficient strontium ferrite SrFeO3 – δ have been studied at different oxygen contents by the Mössbauer spectroscopy, X-ray diffraction, and scanning electron microscopy. Three compounds with different compositions have been prepared in the dependence on the condition of heat treatment. Within each of the structures, there are several nonequivalent positions of Fe corresponding to different valence states of Fe and different local oxygen environments, the relation and the degree of distortion of which are changed in the dependence on the oxygen content. Based on the Mössbauer data, the oxygen content in each of the structures is estimated. Yet another ideal Sr16Fe16O45 composition of the SrFeO3 – δ compound is proposed for an intermediate composition in addition to those available in the literature.
The dielectric properties of deuterated betainphosphite crystals with a high degree of deuterization in the region of antiferrodisorsion at T = Tc1 and ferroelectric at T = Tc2 phase transitions are studied. We present the description of the temperature behavior of the dielectric constant of Bataipora and deuterated Bataipora in the framework of the thermodynamic model Landau given biquadratic connection between the polar order parameter of the ferroelectric transition and non-polar parameter antiterroristicheskogo transition. It is shown that with increase in the degree of deuterating the relationship between the order parameters is reduced. The increase in temperature of the ferroelectric phase transition at tatarinowii of Bataipora due to the increase of the dielectric constant in symmetric phase above the temperature of phase transition antiterroristicheskogo
Utilization of of superconducting materials for new-generation nanoelectronic devices seems extremely tempting because of the absence of energy dissipation during the electric current flow. However, in small systems, the role of fluctuations can be highly important. In this study, the transport properties of thin superconducting titanium nanoribbons have been experimentally and theoretically investigated. It has been shown that quantum fluctuations of the order parameter differently affect the integral and local characteristics of a quasi-one-dimensional superconductor. In sufficiently thin nanowires, a finite electrical resistance can be observed at lowest temperatures, while the tunneling I–V characteristics only exhibit slightly diffuse gap features and a finite Josephson current. The phenomenon is of fundamental importance for mesoscopic superconductivity and should be taken into account when designing cryoelectronic nanodevices.
We studied the structure, IR absorption spectra, the spectral characteristics of photoluminescence and morphology of cerium- and terbium-doped orthoborates of gadolinium and yttrium obtained by hydrothermal synthesis at 200°C, as well as solid solutions of orthoborates on the basis of yttrium, gadolinium, and lutetium with composition RECe0.01Tb0.1BO3 (RE = Lu0.5Gd0.39, Lu0.5Y0.39, and Y0.5Gd0.39). The X-ray diffraction spectrum of yttrium orthoborate Y1 – x – yCexTbyBO3 is described by a hexagonal lattice with space group P63/m, which, after annealing at 970°C, transforms into a monoclinic lattice with space group C2/c. High-temperature annealing of the studied orthoborates leads to a multiple, more than two orders of magnitude, increase in the luminescence intensity of Tb3+ ions when the samples are excited in the absorption band of cerium ions. This effect is the result of a significant increase in the concentration of Ce3+ ions in the orthoborates at high temperatures. It is shown that the luminescence of terbium ions is due to energy transfer from Ce3+ to Tb3+, which proceeds with high efficiency (∼85%) by the mechanism of dipole-dipole interaction between cerium and terbium.
The current–voltage characteristics of superconductor–insulator–semiconductor (S1–I–S2) tunnel junctions, where superconducting electrode S2 is a thin nanowire, are studied experimentally. The observed blurring of the gap singularities is interpreted as a manifestation of the order parameter quantum fluctuations. We propose a model taking into account the broadening of the density of states due to the interaction of electrons with the Mooij–Schön plasmon mode emerging in a quasi-one-dimensional superconducting channel in the order parameter quantum fluctuation
The numerical simulation was applied to study the temperature dependence of the impurity magnetic susceptibility. The direct exchange interaction of the impurity magnetic moments randomly distributed in space was considered within the Ising model. When the temperature in a system decreases, the magnetic susceptibility behavior in this system ceases to comply with the Curie law, which is associated with the formation of a spin glass phase. It is shown that the expression of the preexponential factor in the formula for the direct exchange interaction considerably influences the temperature dependence of the magnetic susceptibility only in the case of ferromagnetic exchange.
At ultra-low temperatures, the transport properties of two types of quasi-one-dimensional superconducting microstructures were studied: thin channels tightly packed in the form of a meander, and chains of superconductor-insulator-superconductor tunnel contacts. Both types of microstructures demonstrated high values of high-frequency impedance and / or dynamic resistance. The study opens up the possibility of using such structures as current-stabilizing ballast elements with zero dissipation.