Vol 126, No 6 (2018)

new scenario for the occurrence of a Fano resonance in the transmission probability of electron waveguides is investigated using a coupled-channel theory. Both a quantum dot and an antidot with either short- or finite-range interaction are embedded in the electron waveguide. Particularly, when the Fano resonance occurs close to the mobility edge (channel threshold), it is shown that Γ~U₁₂^4/3, where Γ is the resonance width and U₁₂ is the coupling strength between bound state and continuum. This is in contrast to the usual result Γ ~U₁₂², which is valid when the resonance occurs far from the mobility edge. Furthermore, it is shown that increasing the size of both dot and antidot leads to larger resonance width.

We consider photoionization of a system bound by the central potential V(r). We demonstrate that the high energy nonrelativistic asymptotics of the photoionization cross section can be obtained without solving the wave equation. The asymptotics can be expressed in terms of the Fourier transform of the potential by employing the Lippmann–Schwinger equation. We find the asymptotics for the screened Coulomb field. We demonstrate that the leading corrections to this asymptotics are described by the universal factor. The high energy nonrelativistic asymptotics is found to be determined by the analytic properties of the potential V(r). We show that the energy dependence of the asymptotics of photoionization cross sections of fullerenes is to large extent model-dependent. We demonstrate that if the fullerene field V(r) is approximated by the function with singularities in the complex plane, the power drop of the asymptotics is reached at the energies which are so high that the cross section becomes unobservably small. The preasymptotic behavior with a faster decrease of the cross sections becomes important in these cases.

A family of attacks on the BB84 protocol of quantum key distribution is explicitly constructed under which the lower bound of fundamental entropy uncertainty relations is attained in the asymptotic limit of long sequences. All attacks are parameterized by a single parameter Q, which has the meaning of error probability on the receiver side, is known to the eavesdropper, but is reliably unknown to legitimate users. The situation on the receiver side looks like a scheme with classical Bernoulli tests with unknown parameter Q. For the eavesdropper, the situation also looks like a Bernoulli scheme—tossing a coin with quantum states, where each message of the eavesdropper leads to a quantum state that is uniquely determined by the outcome of measurements on the receiver side. A statistical interpretation is given to the estimate of the error probability for Q and the key secrecy parameter ε_δ,n. It is shown that, for a given length n of a series of tests, the width δ of the confidence interval actually determines the accuracy of the estimate for the parameter Q and, accordingly, the key secrecy level—the value of the secrecy parameter \({\varepsilon _{\delta ,n}} = 2{e^{ - 2{\delta ^2}n}}\).

We have studied the neutrino–electron processes in a matter with an external magnetic field of an arbitrary strength. Invariant squares of S-matrix elements, which have been obtained for such reactions using the technique based on the density matrix of a particle propagating in an external magnetic field, are valid in an arbitrary frame of reference moving along the magnetic field lines. The transition probabilities obtained can easily be generalized to the processes of interaction of a neutrino with other charged leptons and protons. The probabilities of the processes have been integrated over the transverse momenta of charged particles for the rates of neutrino–electron reactions as well as the energy and momentum transferred in them from the medium to neutrinos. The expressions obtained are written in the unified form for all neutrino–electron processes.

In this paper, we have reported the influence of annealing treatment on structural, optical, electrical, and thermoelectric properties of MBE-grown ZnO on Si substrate. After growth, a set of as grown ZnO was annealed in oxygen environment at 500–800°C and another set was annealed in different environments (vacuum, oxygen, zinc, and vacuum + zinc) at 600°C for one hour in a programmable furnace. X-ray diffraction (XRD) results demonstrated that all annealed samples exhibited a major diffraction peak related to (002) plane. The full width at half maximum (FWHM) of this plane decreased and crystalline size increased for oxygen annealed sample and it increased when samples were annealed in zinc, vacuum, and successively annealed in vacuum and zinc. Further, photoluminescence spectrum revealed that the intensity of band edge emission increased and defect emission decreased as annealing temperature (oxygen environment) increased while it decreased for rest of annealing ambient. It is suspected that annealing in oxygen environment causes compensation of the oxygen vacancies by the incoming oxygen flux, while annealing in zinc and vacuum generates more oxygen vacancies. Hall and Seebeck measurements are also consistent with these arguments.

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.

The dichroic effect (“rotated” polarization) in the reflectivity from a magnetically ordered sample is experimentally studied at the station PHASE of the Kurchatov Synchrotron Radiation Source. The experiments are performed for the Gd_0.23Co_0.77 film, which has a compensation temperature T_comp ≈ 433 K, using linearly polarized radiation of the photon energy of 7930 eV (L₂ absorption edge of gadolinium) at room temperature. The developed theory of reflectivity accounted for the magnetic contributions to the scattering amplitude predicts the appearance of a peak for the orthogonal (to the incident polarization) polarization of the reflected radiation near the critical angle of the total external reflection. The experiment reveals the significant difficulties because of the incomplete σ polarization of the synchrotron beam, the beam instability, and so on. Therefore, a rotated-polarization peak has been detected near the critical angle but at the limits of the measurement accuracy. In principle, our experimental technique could be an alternative to circular polarization experiments, which are widely used at synchrotrons to study magnetic ordering. However, as we have shown, it makes high demands of the radiation source parameters.

The microstructures of amorphous and polycrystalline ferroelectric Hf_0.5Zr_0.5O₂ films are studied by X-ray spectroscopy and ellipsometry. EXAFS spectra demonstrate that the amorphous film consists of an “incompletely mixed” solid solution of metallic oxides HfO₂ and ZrO₂. After rapid thermal annealing, the mixed Hf_0.5Zr_0.5O₂ oxide films have a more ordered polycrystalline structure, and individual Hf and Zr monoxide islands are formed in the films. These islands are several nanometers in size and have a structure that is similar to the monoclinic structure of HfO₂ and ZrO₂. The presence of the HfO₂ and ZrO₂ phases in the Hf_0.5Zr_0.5O₂ films is also detected by ellipsometry.

We have measured the time of the optical response of a 350-GHz radiation detector based on a superconductor–insulator–normal metal–insulator–superconductor tunnel structure with a suspended normal-metal bridge integrated into a planar log-periodic antenna. The transient characteristics of the detector were recorded when irradiated by a fast cryogenic blackbody source with a rise time of the order of microseconds. For this purpose, a short intense heating pulse was fed to an emitting NiCr film radiation source with a low heat capacity. The measured response time was 1.8 ± 0.5 μs at a bolometer electron temperature of 0.17 K, with a detection sensitivity of 10^–17–10^–18 W Hz^–1/2 being potentially achievable.

We have studied experimentally the oscillatory process in the dusty plasma in the glow discharge stratum. It is shown that the oscillations are induced by the dust–acoustic instability, as a result of which dust–acoustic solitons are excited. The motion of dust particles in the electric field of solitons has been analyzed. It has been established that a soliton leads to a large-scale unidirectional transport of charged particles in the direction of its motion. The experimental results have been interpreted theoretically using the MHD model of the plasma.

A model describing the ionization of atoms and ions in a cluster under irradiation by a short laser pulse has been constructed. It is shown that the electron-impact ionization weakly affects the final charge composition, and the main mechanism of ion formation in the cluster is the over-barrier ionization by an electric field. The electric field acting on atoms and ions is the result of combined action of the external laser field and the intrinsic electric field of the cluster. The key parameters of the cluster beam and the laser pulse, which determine the properties of the charge composition of the cluster plasma, have been established.