Floquet spectrum of electrons in graphene superlattice subjected to the ac electric field was investigated. The ac-field vector was assumed to oscillate perpendicularly to the superlattice axis. In the vicinity of extra Dirac points of superlattice band structure the gap was shown to be dynamically induced by the acfield. Such effects are due to the nonadditivity of the electron spectrum of graphene superlattice.

In this work, a simulation of the CZTSSe (Cu2ZnSn(S,Se)4) solar cell with Al/ZnO:Al/ZnO(i)/CdS/CZTSSe/Mo structure have been studied using the SCAPS-ID (Solar Cell Capacitance Simulator in one Dimension). The simulation results have been validated with real experimental results. Next, a novel structure is proposed in which a back surface field (BSF) layer is inserted in order to boost the solar cell performance, a CZTSSe layer has been used as (BSF) layer. The efficiency of CZTSSe solar cell increases from 12,3 to 15,3 % by inserting a BSF layer. Finally, an optimization of different physical parameters (Thickness and doping concentration) of BSF layer has been done, and the optimum values are determined. In this research work, the proposed structure of CZTSSe solar cells with optimum parameters showed higher functional properties. The maximum value of efficiency achieved was 16,98 % with <$EJ sub sc ~=~36,31> mA/cm<^>2, <$EV sub ac ~=~0,69> V, and FF = 69,60 % under 1,5G AM illumination.

We present the Condon domain phase diagram for a silver single crystal measured in magnetic fields up to 28 T and temperatures down to 1,3 K. A standard ac method with a pickup coil system is used at low frequency for the measurements of the de Haas - van Alphen effect (dHvA). The transition point from the state of homogeneous magnetization to the inhomogeneous Condon domain state (CDS) is found as the point where a small irreversibility in the dHvA magnetization arises, as manifested by an extremely nonlinear response in the pickup voltage showing threshold character. The third harmonic content in the ac response is used to determine with high precision the CDS phase boundary. The experimentally determined Condon domain phase diagram is in good agreement with the theoretical prediction calculated by the standard Lifshitz - Kosevich formula.

The measurement of the complex impedance response and accompanied power absorption <$E P( omega )> in the radiofrequency and microwave ranges represents a most popular experimental method to investigate pinning mechanisms and the vortex dynamics in type-II superconductors. In the theory, the pinning potential (PP) well for a vortex must be a priori specified in order to subsequently analyze the measured data. We have theoretically solved the inverse problem at T = 0 K and exemplify how the coordinate dependence of a PP can be determined from a set of experimental curves <$EP( omega | j sub 0>) measured at subcritical dc currents <$E0~<<~j sub 0 ~<<~j sub c> under a small microwave excitation <$Ej sub 1 ~<< <<~j sub c> with frequency <$Eomega>. We furthermore elucidate how and why the depinning frequency <$Eomega sub p>, which separates the non-dissipative (quasi-adiabatic) and the dissipative (high-frequency) regimes of small vortex oscillations in the PP, is reduced with the increase of <$Ej sub 0>. The results can be directly applied to a wide range of conventional superconductors with a PP subjected to superimposed dc and small microwave ac currents at <$ET~<< <<~T sub c>.

Індекс рубрикатора НБУВ: В368.31

Рубрики:

Надпровідність. Надпровідники

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 
Повний текст  Наукова періодика України 

We synthesized (CuO, CaO2, and BaO)y/<$Eroman {Cu sub 0,5 Tl sub 0,5 Ba sub 2 Ca sub 2 Cu sub 3 O} sub {10- delta }> (y = 0,5 %, 10 %, 15 %) composites by solid-state reaction and characterized by x-ray diffraction, scanning electron microscopy, dc-resistivity, and Fourier transform infrared spectroscopy. Frequency and temperature dependent dielectric properties such as real and imaginary part of dielectric constant, dielectric loss, and ac-conductivity of these composites are studied by capacitance and conductance measurement as a function of frequency (10 kHz to 10 MHz) and temperature (78 to 300 K). The x-ray diffraction analysis reveals that the characteristic behavior of <$Eroman {Cu sub 0,5 Tl sub 0,5 Ba sub 2 Ca sub 2 Cu sub 3 O} sub {10- delta }> superconductor phase and its structure is nearly undisturbed by doping of nanoparticles. The scanning electron microscopy images show the improvement in the intergranular links among the superconducting grains with increasing nanoparticles concentration. Microcracks are healed up with the inclusion of these nanoparticles and superconducting volume fraction is also increased. The dielectric properties of these composites strongly depend upon the frequency and temperature. The zero resistivity critical temperature and dielectric properties show opposite trend with the addition of nanoparticles in <$Eroman {Cu sub 0,5 Tl sub 0,5 Ba sub 2 Ca sub 2 Cu sub 3 O} sub {10- delta }> superconductor matrix.

Ferromagnetic nickel (Ni) nanoparticles were added in <$E roman {(Cu sub 0,5 Tl sub 0,5 )Ba sub 2 Ca sub 2 Cu sub 3 O sub {10- delta}}> (CuTl-1223) superconducting matrix to get Nix/CuTl-1223; x = 0 - 1,00 wt % nanoparticles-superconductor composites. Temperature- and frequency-dependent dielectric properties of CuTl-1223 superconducting phase with different contents of Ni nanoparticles were studied. Different dielectric parameters such as dielectric constant (<$E {epsilon prime} sub r>, <$E {epsilon symbol Т} sub r>), dielectric tangent loss (tan <$E delta>) and ac conductivity (<$E sigma sub ac>) were determined from experimentally measured capacitance and conductance at different frequencies from 10 kHz to 10 MHz and at different operating temperatures from 78 to 290 K. The values of <$E {epsilon prime} sub r> and <$E {epsilon symbol Т} sub r> were found maximal at smaller frequencies and started to decrease at higher frequencies. The value of <$E sigma sub ac> is high at high frequency unlike to <$E {epsilon prime} sub r> and <$E {epsilon symbol Т} sub r>, which is due to release of space charges at high frequencies. Peaks in tan <$E delta> graphs represent the resonance phenomenon at certain frequencies in these samples. Non-monotonic behavior in variation of dielectric parameters with temperature of Nix/CuTl-1223 samples was observed particularly at high temperatures which was due to thermal instability of the system at high temperatures.

We investigated the effect of uniaxial pressure (<$E 0~symbol Ш~1000> bars) applied parallely to the ac electric field on dielectric properties of PLZT-x/65/35 (<$E 2~symbol Г~x~symbol Г~13>) ceramics. There was revealed a significant effect of the external stress on these properties. The application of uniaxial pressure leads to the change of the peak intensity of the electric permittivity (<$E epsilon>), of the frequency dispersion as well as of the dielectric hysteresis. The peak intensity <$E epsilon> becomes diffused/sharpened and shifts to a higher/lower temperatures with increasing the pressure. It was concluded that the application of uniaxial pressure induces similar effects as increasing the Ti ion concentration in PZT system. We interpreted our results based on the domain switching processes under the action of combined electromechanical loading.

Excitation of ferromagnetic resonance (FMR) by an ac current has been observed in macroscopic ferromagnetic films for decades and typically relies on the ac Oersted field of the current to drive magnetic moments into precession and classical rectification of ac signals to detect the resonance. Recently, current-driven ferromagnetic resonances have attracted renewed attention with the discovery of the spin-transfer torque (STT) effect due to its potential applications in magnetic memory and microwave technologies. Here STT associated with the ac current is used to drive magnetodynamics on the nanoscale that enables FMR studies in sample volumes smaller by a factor of 1000 compared to conventional resonance techniques. In this paper, we briefly review the basics of STT-FMR technique and the results of various STT-FMR experiments.

Recently developed ac ZnS-powder electroluminescence devices have flexibility (thickness is about 60 <$E mu>m) and can be multisegment, multicolor, as well as rolled and bent. All colors (white, blue, blue-green, green, and orange) have been investigated for improvement of their operational characteristics. Many factors including the type of phosphor, formation method, contacts, initial input power (peak voltage and frequency), type of input power, and environment safety which define the panel's properties are discussed.

We investigated the properties of a system where the itinerant electrons coexist and interact with the preformed local pairs. Using the nonperturbative continuous unitary transformation technique we show that Andreev-type scattering between these charge carriers gives rise to the enhanced diamagnetic response and is accompanied by appearance of the Drude peak inside the pseudogap regime ω ≤ 2Δ_pg. Both effects are caused by the short-range superconducting correlations above the transition temperature T_c. In fact, the residual diamagnetism has been detected by the torque magnetometry in the lanthanum and bismuth cuprate superconductors at temperatures up to ~ 1,5T_c. In this work we show how the superconducting correlations can be observed in the ac and dc conductivity.

For certain orientations of Josephson junctions between two <$E p sub x>-wave or two <$E d>-wave superconductors, the subgap Andreev bound states produce a <$E 4 pi>-periodic relation between the Josephson current <$E I> and the phase difference <$E phi :~I~inf~sin ( phi "/" 2)>. Consequently, the ac Josephson current has the fractional frequency <$E eV "/" h bar> where <$E V> is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current between unconventional superconductors is carried by single electrons, rather than by Cooper pairs. The fractional ac Josephson effect can be observed experimentally by measuring frequency spectrum of microwave radiation from the junction.

The frequency dispersion of the loss tangent (tg<$E delta>) and the ac conductivity (<$E sigma sub roman ac>) of amorphous films prepared by evaporation of TlInS2 has been investigated at frequencies <$E f~=~5~cdot~10 sup 4~-~3,5~cdot~10 sup 7> Hz. It is shown that, at <$E f~>>~10 sup 6> Hz, relaxation losses take place. It is established that the hopping conduction near the Fermi level occurs in TlInS2 amorphous films at frequencies up to <$E 3~cdot~10 sup 6> Hz. The density of localized states at the Fermi level, the mean time for phonon-assisted tunneling, and the hopping distance have been evaluated for polymorphic TlInS2 films. For frequencies above <$E 10 sup 7> Hz, <$E sigma sub roman ac (f)> ~ <$E f sup 2>. Such a behavior is caused by optical transitions in TlInS2 amorphous films.

The nonlinear response to an ac magnetic field with and without parametric pumping was studied experimentally on a rod-like beryllium single crystal at low temperatures in a quantizing magnetic field applied parallel to the hexagonal axis. At low temperatures (<$E symbol Г~3> К) giant parametric amplification of the nonlinear response is observed in narrow windows near de Haas - van Alphen (dHvA) beating antinodes where the amplitude of the magnetic oscillations is maximal. This effect is of threshold character with respect to the amplitude of the dHvA oscillations and occurs as a result of a diamagnetic phase transition of a strongly correlated electron gas into nonuniform state with formation of Condon domain structure. It is shown that the nonuniform phase appears periodically in a magnetic field. The components of rectified signal field reproduce the envelope of H - T critical curves and can be used for reconstruction of diamagnetic phase diagrams.

Magnetization, ac initial magnetic susceptibility, resistance, magnetoresistance, thermal expansion and magnetostriction measurements were performed for R_{1 - x}Sr_xMnO₃ (R = Sm, x = 0,33, 0,40, 0,45; R = Nd, x = 0,33, 0,45) compounds. For all compounds in the T_C region we have observed a large volume contraction DELTA V / V approx 0,1 % and unusual behavior of the volume magnetostriction w, namely, a peak of anomalous magnitude of negative volume magnetostriction on the w(T) curve. We have obtained a giant negative volume magnetostriction ~ 5 ^. 10^-4 at a relatively low magnetic field B = 0,9T and ~ 10^-3 at a high magnetic field B = 13T for Sm samples. The magnetostriction of Nd compounds is one order of magnitude less. For all compounds the giant magnetostriction is accompanied by colossal negative magnetoresistance equal to 78, 72, and 44 % at B = 0,9T for Sm compounds with x = 0,33, 0,40, and 0,45, respectively. All of the observed properties are explained in the framework of an electronic phase separation model.

X-ray diffraction and magnetic using dc and ac methods measurements of the polycrystalline and nanosize REMnO3 (RE = Pr, Nd) powdered samples have been performed. The nanosize manganites were synthesized with a sol-gel method at different (800, 850 and <$E900~symbol Р roman C>) temperatures. The average size of synthesized nanoparticles (from 56 to 89 nm) and polycrystalline powders (above 200 nm) was estimated using the x-ray diffraction data. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature with smaller values of the lattice parameters in the nanosamples. The temperature-dependent ac magnetic susceptibilities show a sharp high-temperature peak connected with Mn magnetic moments ordering. The low-temperature maximum of magnetic susceptibility is proposed to be due to the polarization of the rareearth sublattice by an effective exchange field of the Mn ordered sublattice. The antiferromagnetic ordering of Mn sublattice and paramagnetic Curie temperatures as well as the magnetic moment values for the nanosize samples were found to be smaller than those for polycrystalline sample.

Індекс рубрикатора НБУВ: В371.236 + В377.3

Рубрики:

Магнітні властивості твердих тіл. Магнетики

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 
Повний текст  Наукова періодика України 

The magnetoimpedance (MI) effect has a potential for the development of high performance magnetic sensors. For sensor applications, off-diagonal configuration is preferable when the MI element is excited by ac current and the output is detected from the coil. In the present work, the off-diagonal sensor design was advanced by utilising a complex waveform excitation produced by a microcontroller and applied to a multiple wire MI element. For optimised excitation with a waveform close to a positive half sine form and characteristic frequency of 8 MHz the field resolution of about 60 mV/Oe was achieved. The pulse excitation does not require an additional bias since it includes controllable low frequency components. The concept of microcontroller driven sensor element could be attractive for the development of intellectual sensors.

The article has investigated the removal of water borne pathogens using dielectrophoresis (DEP) filter which is energized by varying the frequency of the applied potential from 10 kHz to 2 MHz with different voltage levels of 5; 10; 15 and 20 V. Separate experiments are conducted in artificially contaminated water samples with Escherichia coli, Staphylococcus aureus and Vibrio cholera up to 2 h. The impact of signal frequency and voltages on DEP based water treatment system has been analyzed statistically. Results have demonstrated that an ac signal of 20 V with frequency range of 500 kHz to 2 MHZ is suitable to remove the tested bacterial population and the rate of removal of E. coli is the highest with a dielectrophoretic filtration efficiency of 77,1 %.

Выяснен вопрос касательно повышения надежности вспомогательных машин электропоездов переменного тока при эксплуатации в реальных условиях. Предложен инженерный метод определения влияния несимметрии питающего напряжения и отклонения его от номинального значения на нагревание изоляции вспомогательных машин. Установлено, что в случае, когда вспомогательные машины электропоездов переменного тока работают при коэффициенте несимметрии напряжений 10 % и более и отклонении питающего напряжения от номинального порядка 0,6 в относительных единицах, то возможен перегрев их изоляции, даже если она будет иметь класс Н. Впервые освещен вопрос о суммарном нагревании изоляции при таких сочетаниях граничных параметров качества энергии, когда каждый из них способствует увеличению нагревания изоляции по сравнению с номинальным режимом системы "вращающийся фазорасщепитель - вспомогательные машины". Проведенные исследования позволяют установить граничные значения параметров качества питающей электроэнергии (коэффициента несимметрии, отклонения питающего напряжения от номинального значения), при которых дополнительный перегрев изоляции данного класса под влиянием указанных факторов не будет превышать установленного значения.

The static and dynamic aspects of magnetic behavior of the complex of iron with 1-nitroso-2-naphthol Na[FeO6(C10H6N)3] as a function of temperature, frequency and magnetic field have been analysed. The ac susceptibility (<$E chi sub ac>) and dc magnetization (<$E M sub dc>) were measured over a temperature range of 1,5 - 200 K in external magnetic field up to 90 kOe and over a frequency range of 95 - 2000 Hz. The experimental data indicate the absence of magnetic long range order in this complex. The magnetization does not reach a saturation in field of 90 kOe at 1,5 K. At low temperatures, the following characteristic peculiarities of magnetic behavior of complex studied have been found, namely : cusp-like anomalies in the ac susceptibility and zero field coold (ZFC) magnetization at <$E T sub roman cusp~=~17> K; frequency dependence of the reusp temperature; remanence and time-dependent relaxation of ZFC magnetization. Attempts were made to compare the <$E chi sub ac> and <$E M sub dc> anomalies in complex studied with those of other magnetic systems. Comparison of characteristic peculiarities of magnetic behavior of the spin-glasses and superparamagnets was also performed. Although superparamagnetic behavior should not be completely ruled out as an explanation of the freezing phenomena in sample studied, the analysis of experimental results strongly suggests that the spin-glass-like behavior is a more consistent explanation.

Розроблено математичну модель вентильного двигуна з висококоерцитивними постійними магнітами, в основі якої лежить метод вторинних джерел, що дав змогу завдання визначення розподілу магнітної проникності у феромагнітних елементах двигуна звести до розв'язання системи інтегральних рівнянь для густини простого шару й густини об'ємних магнітних зарядів. Запропоновано ітераційний метод знаходження магнітної проникності з урахуванням її нелінійної залежності.