The effect of square wave current was investigated by the voltage-time (V-t curves) measurements at various external magnetic fields and periods (P) in silver doped YBCO. The general sinusoidal behavior of V-t curves was mainly interpreted as a dynamic competition between driving and pinning forces. It is observed that as the period of square wave current is increased, the amplitude of oscillation is also increased. The observed oscillations in the voltage was fitted by an common sinusoidal equation V(t)ımın(ω t+φ). It is also found that there is different phase angle φ values for each cycle. Fast Fourier transform measurements is applied to oscillation period (P_{I}) of the square wave current. The results give us that a physical mechanism is related with charge density waves. Intrinsically, in Ag doped YBCO, the pinned flux line system evokes the general behavior of charge density waves. Due to properties of the converting the square wave current to sinusoidal voltage oscillations, Ag doped YBCO sample can behave as double-integrator for the defined period, amplitude of square wave current and magnetic field values in time.
On the occasion of centenary of superconductivity discovery I recall some facts from the first period and attempts to understand the phenomenon. It turns out that most famous physicists of the first half of XX century have tried to solve the puzzle. Bardeen, Cooper and Schrieffer succeeded in 1957. The BCS theory successfully described all known facts and offered new predictions, which soon have been confirmed experimentally contributing to the widespread acceptance of the theory. It have found applications in nuclear physics, theory of neutron stars and cold atomic gases. The discoveries of new superconductors in the last thirty years show that simple BCS model is not enough to understand new unconventional superconductors. The studies of superconductors develop vividly and still fascinate new generations of physicists working in such diverse fields as material science and string theory.
An atomic force microscope (AFM) was used to locally anodize the serial channels of an inductance-gate type superconducting flux flow transistor (SFFT) as narrow slits with a width of 50 μm, a space of 25 μm, and 12 turns, to improve the transresistance value. Among the serial channels that were anodized with the scanning tip of the AFM in the drain current line, channels 1 and 2 were 7.3 and 7.9 μm wide, and 531 and 461 nm high, respectively. The critical current density in the serial channel of the fabricated SFFT, which was determined using an AFM modification method, was decreased by increasing the gate current. The measured current-voltage curves were compared with the simulated ones. The maximum transresistance value was 0.56 Ω at the drain current of 20 mA when the gate current was 6 mA. The transresistance characteristics of the inductance-gate type SFFT could be more improved than that of the single-channel type SFFT using an inductively coupled plasma lithography method
Magnetoresistivity measurements on a superconducting system of YB_{6} (T_c ≈7.5 K) down to 60 mK at hydrostatic pressures up to 47 kbar are presented. The superconducting transition temperature, as well as the third critical field H_{c3} reveal a linear decrease with increasing pressure with slopes of d ln H_{c3}/dp=-1.1 %/kbar, and d ln T_c/dp=-0.59 %/kbar. From the latter a critical pressure, p_c ≈ 170 kbar, at which T_c vanishes, is determined.
The resistivity, magnetoresistance, and magnetic susceptibility are measured in single crystals of FeTe_{0.65}Se_{0.35} with Cu, Ni, and Co substitutions for Fe. The crystals are grown by Bridgman's method. The resistivity measurements show that superconductivity disappears with the rate which correlates with the nominal valence of the impurity. From magnetoresistance we evaluate doping effect on the basic superconducting parameters, such as upper critical field and coherence length. We find indications that doping leads to two component superconducting behavior, possibly because of local charge depression around impurities.
In this paper we report the formation process and electrophysical properties of 'high temperature superconductor (YBa_2Cu_3O_{7 - x})/photosemiconductor (BiOCl:Ti)' micro- and nanoplates based junction. The energy band model of ≪HTSC-semiconductor≫ junction is proposed.
Using pulsed laser deposition we have grown films of La_{2-x}Sr_xCuO_4 with x in close vicinity of the superconductor-insulator transition, x=0.051 and x=0.048, on SrLaAlO_4 substrates, and of different thickness d (from 25 nm to 250 nm). The X-ray diffraction shows that for each d the films grow with variable degree of compressive in-plane strain, with the largest strain achieved in thinnest films. The resistivity measurements show strong enhancement of superconductivity with increasing strain, so that the onset of superconductivity at temperature as high as 27 K is observed. With increasing strain the character of resistivity changes from the insulating to metallic.
Spin-dependent tunneling through a quantum dot coupled to one ferromagnetic and one superconducting electrodes is studied theoretically in the Andreev reflection (AR) regime. Spectral functions for the system are calculated in terms of the nonequilibrium Green function technique. Effects due to interplay between the Coulomb correlations on the dot and the local phonon mode in the context of the AR transmission are analyzed.
We study the superconductor-insulator transition in Si/Nb/Si trilayers, in which the thickness of Si is fixed at 10 nm, and the nominal thickness of Nb changes in the range between d = 20 nm down to d = 0.3 nm. The transmission electron microscopy indicates the formation of the mixed Nb-Si layer for small d. Both the thickness-induced, and the magnetic-field induced superconductor-insulator transition is observed. The crossing point of the isotherms at the critical field B_{c} decreases with decreasing d, and it is T-independent at temperatures below 300 mK. At larger fields the weak peak in magnetoresistance appears in some of the films.
In the current paper, electrical transport properties of 25 nm thick Nb films sputtered on the photosensitive semiconductor BiOCl were investigated in the temperature range 7.5 K ≤ T ≤8.5 K. The influence of green (532 nm) and red (640 nm) laser excitations on resistive superconducting transitions of the niobium thin films on a silicon glass and BiOCl single crystal substrates were studied. The temperature dependences of the resistivity for Nb are in good agreement with the McMillan model which indicates the strong influence of the inverse proximity effect induced by the interface. The increased influence of the BiOCl/Nb interface under laser excitation corresponds to the raising of the ratio of the density of normal to superconductivity carriers in the Tıghtarrow0 limit and this observation is in agreement with the photoconductivity study of BiOCl single crystals.
Electrical conductivity of nanofluids is one of the physical properties which are intensively investigated by researchers. This paper brings contributions in this research area. Electrical conductivity of nanofluids containing various mass concentration of silicon dioxide (SiO₂) nanoparticles suspended in ethylene glycol (EG) were investigated at various ambient temperatures. Temperature was changed from 20°C to 60°C with 10°C step. Measurements were performed with digital conductivity meter (MultiLine 3410, WTW GmBH, Weilheim, Germany) and it was observed that increase in mass concentration of SiO₂ nanoparticles cause increase in electrical conductivity. The same dependence was observed between temperature and electrical conductivity.
A mixed state in dc-biased thin films of II-type superconductors realizes the Abrikosov magnetic vortices/antivortices, which are the result of the current-self magnetic field penetration into the film at temperatures lower than its critical temperature T_{c}. A nucleation of vortices/antivortices at the superconducting film's edges, their motion perpendicular to the direction of biasing current, and the annihilation in the film's center originates from a current dissipation in the superconductor and expresses itself in experiments as a dc voltage. This work reports on the results of simulation of current density in a 50 μm wide, 100 μm long, and 0.3 μm thick YBa_2Cu_3O_{7 - x} microbridges containing Π-shaped 5 μm wide single channel of easy vortex motion fabricated by means of laser-writing technique. Analyzing a two-dimensional-net of resistors and assuming that, due to the Meissner-Ochsenfeld effect, the magnetic flux penetration into superconducting film is nonlinear, we demonstrate that presence of a Π-shaped channel causes a non-homogeneous distribution of current in the microbridge.
A current-self-induced magnetic field H_{j}, such that H_{c1} < H_{j} < H_{c2} at T < T_{c}, penetrates a thin-film, type-II superconductor forming the Abrikosov magnetic vortex-antivortex pairs in the film's areas of weakest superconductivity. Our atomic force microscopy and scanning tunneling microscopy images confirm that in 50 μm wide, 100 μm long and 0.3 μm thick YBa_2Cu_3O_{7 - x} superconducting devices magnetic flux penetrates first into a 5 μm wide, Π-shaped and partially deoxygenated (x ≈ 0.2) channel for easy vortex motion. When the Lorentz force overcomes pinning force in the channel, the flux starts to move and its drift dissipates energy inducing dc voltage. This work reports on the density of coherently moving vortices along the channel vs. temperature in range from 0.93T_{c} to 0.97T_{c}. Our simulations show that the vortex density vs. temperature dependence extracted from I-V measurements of our devices follows the temperature dependence of magnetic field penetration depth and the coherence length of the superconductor.
The results of investigation of structural and electrical properties of bulk SiC crystals, which were grown by physical vapor transport method with different Ce impurity content added to the SiC source material, are presented. The gradual dosage of cerium from the SiC source and continuous presence of the cerium vapor over the SiC crystallization fronts during the crystal growth processes are confirmed. The cerium influences the overall concentration of structural defects. The increase of the concentration of both, donors and acceptors, and appearance of new shallow donors (15-32 meV) in 4H-SiC crystals are observed.
Paper presents results of experimental studies of electrical conductivity of yttrium aluminum garnet-ethylene glycol (Y₃Al₅O₁₂-EG, YAG-EG) nanofluids, which were prepared by dispersing commercially available nanoparticles manufactured by Baikowski (Annecy, France, ID LOT: 18513) in ethylene glycol. The electrical conductivity was measured using conductivity meter MultiLine 3410 (WTW GmBH, Weilheim, Germany). In turn the temperature was stabilized in a water bath MLL 547 (AJL Electronic, Cracow, Poland). The electrical conductivity of YAG-EG nanofluids with various mass concentrations form 5% to 20% was investigated at different ambient temperatures. The experimental data indicate that changing volume fraction of YAG nanoparticles in ethylene glycol cause change of electrical conductivity of nanofluid. It was also presented that electrical conductivity depends on temperature of materials.
The paper presents experimental investigation on electrical conductivity of thulium oxides-ethylene glycol (Tm₂O₃-EG) nanofluids based on nanoparticles with three different sizes, and prepared in different conditions. Nanofluids were prepared with two-step method with use of the nanoparticles obtained by precipitation method. Measurements were conducted at constant temperature 293.15 K for various mass concentrations from 0% to 20% with 5% step. The electrical conductivity was measured using conductivity meter MultiLine 3410 (WTW GmBH, Weilheim, Germany) and temperature was stabilized in a water bath MLL 547 (AJL Electronic, Cracow, Poland). The results indicate that increase in mass concentration of nanoparticles in base fluid causes increase in electrical conductivity of Tm₂O₃-EG nanofluids. The enhancement in electrical conductivity of nanosuspensions of thulium oxide is dependent on particle size.
A challenging aspect of the usage of patterned nanostructures relates to the development of superconducting devices operating with the Abrikosov vortices in some pinning potential. To provide such a potential we have used thin epitaxial films of Nb with washboard-like nanostructures in the form of grooves or Co stripes. The nanostructures were prepared by focused ion beam milling or focused electron beam induced deposition, respectively. The results of transport measurements affirm the existence of two fluxonic effects, the guided vortex motion and the vortex ratchet effect, both invoked by the nanostructuring. In particular, the effects represent the basis for the development of advanced fluxonic devices using a directional or orientational control of the net vortex motion in Nb films nanostructured by focused particle beam techniques.
The critical currents of commercial bismuth based superconducting tape were determined in the two ways. In the first one the transport critical current density was measured by the four points method using the dc current power supply at the liquid nitrogen temperature. In the second one the critical current densities were obtained from the absorption part of ac susceptibility measurements using the Bean model near the critical temperature. The temperature dependence of the critical current densities was fitted to take advantage of the Ginzburg-Landau strong-coupling limit approach. Using the fit parameters the critical current density at 77 K was calculated. The critical temperature of this tape (T_{c}= 110.5 K) was determined from the ac susceptibility measurements.
We discuss the calculations of the ac Hall conductivity for superconductors with the time reversal symmetry breaking states. In the weak coupling theories these states show vanishing Hall response in one band models, even though one expects otherwise on symmetry grounds. On the other hand, the strong coupling approach based on the anti-de Sitter-conformal field theory correspondence leads to the non-vanishing Hall conductivity. We discuss the possible reasons of the discrepancy. The weak coupling many orbital theory leading to the Hall conductivity with correct temperature dependence is also briefly presented.
Features of electric and thermal conductance, Seebeck coefficient as well as figure of merit in tunnelling through a single level quantum dot connected to external ferromagnetic leads are investigated theoretically using the equation of motion method within the nonequilibrium Green function technique. The influence of the harmonic ac field on the transport characteristics for the considered system is discussed in detail. In particular, it is shown that the photonic field strongly modulates the Seebeck coefficient, thus resulting in multi-peak structure of figure of merit. Thermoelectric phenomenon in a hybrid junction with the dot attached to two ferromagnetic and one superconducting lead is also discussed.
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