A paradox in the crossover of the mechanisms causing the hysteresis in long,thin-film superconductors

We point out an apparent paradox encountered in the region where the temperature of a voltage-sustaining film at the upper critical current crosses its Tin concerning the crossover of the two dominant heating mechanisms currently used to account for the lower critical currents observed on the hysteretic current-voltage characteristics of long, thin-film superconductors at varying high power levels. We explore a possible solution of this paradox by carefully examining the relevant heating models and available experimental observations.     

Nonlinearity in the flux-flow behavior of thin-film superconductors

The nonlinear flux-flow behavior due to the nonequilibrium distribution of quasiparticles predicted by Larkin and Ovchinnikov has been observed in thin films of Al, In, and Sn. From the voltage-current characteristics the inelastic scattering time _E of the electrons was obtained. For Al and In the values of _E are in excellent agreement with the previous results. For Sn the value _E =5×10^–10 sec is slightly larger than the previous results.     

Current hysteresis in the intermediate state of thin-film superconductors

We have studied the current hysteresis at 4.2 K in the intermediate state of superconducting lead films using electrical resistance measurements and high-resolution magnetooptical observations of the magnetic flux structure. At low magnetic fields the liquidlike arrangement of the flux structure in the form of multiquanta flux tubes was found to persist following the application of a high electrical transport current with an average current density of 10⁵–10⁶ A/cm². In this field range areduction of the resistive voltage was observed following the application of a high transport current. This inverse current hysteresis appears to be associated with a shift of the magnetic flux structure in the direction of the current-induced flux motion resulting in an extended fluxfree region along the sample edge where flux tubes enter the film during flux motion. At higher magnetic fields, where the laminar flux structure is the stable configuration, the usual resistive voltage enhancement has been observed following the application of a high transport current. In this regime the laminae rearrange themselves under the influence of a transport current resulting in a preferential orientation perpendicular to the current.Based on work performed under the auspices of the U.S. Atomic Energy Commission.     

Rectification effect in thin-film superconductors in small,oscillatory magnetic fields

We have observed the generation of dc voltages across super conducting films of lead and tin due to the presence of a static magnetic fieldH ₀ upon which a small, oscillatory magnetic field component is superimposed. The voltages are created without an external current passing though the samples. They change sign with a reversal of the static fieldH ₀. The voltages are related to superconductivity and seem to be caused by a nonsymmetric penetration into the specimen of the magnetic field variation. During one cycle of the magnetic field oscillation a small fraction of the total flux entering and leaving the sample appears to be pumped across the specimen in a preferential direction. The thickness of the films ranged between 0.16 and 2.5 µm. The frequency and the amplitudeH ₁ of the oscillatory field was varied from 5 to 120 kHz and from 0 to 2 G rms, respectively. The rectified voltage varies between the different samples and depends uponH ₀, ,H ₁, and the orientation of the film relative to the magnetic field. It generally shows a sharp peak near the critical magnetic field. Small rectified voltages caused by the oscillatory field were observed far above the nominal, critical magnetic fields of the films. Apparently, these voltages were associated with the film edges and the enhancement of the critical field at the edges.Based on work performed under the auspices of the U.S. Atomic Energy Commission.     

Temperature dependence of the vortex nucleation field of thin-film,type II superconductors

The critical supercurrent versus applied field parallel to the surface of Pb-20 wt % In thin films was measured at various temperatures. The temperature dependence of the first vortex nucleation field (the lower critical field of a thin-film superconductor) suggests that the vortex structure is distorted due to surface effects.     

Combined time-resolved magnetooptical and electrical flux detection in thin-film superconductors

The dynamic behavior of the current-induced dissipative flux-flow state in a thin-film type I superconductor was studied by simultaneous stroboscopic magnetooptical flux detection and direct recording of the time-resolved flux-flow voltage. Employing high-resolution magnetooptical flux detection with a high-speed stroboscope, it was possible to visualize globally individual multiquantum flux tubes during their rapid motion across the superconducting Pb film, yielding spatial and temporal resolution of better than 1 μm and 0.1 μsec, respectively. Simultaneously, the temporal structure of the flux-flow voltage was recorded using a highly sensitive signal-averaging procedure, thereby yielding a voltage resolution of about 30 nV at a time resolution of 10 nsec (corresponding to a recording bandwidth of 25 MHz). The recorded temporal voltage structures agreed well with the voltages expected from the velocity profiles of all flux tubes existing simultaneously obtained from the magnetooptical data. The experiments are the first to demonstrate full agreement between both independent flux-detection measurements, clearly confirming the existing theory.     

Meissner shielding currents and magnetic flux penetration in thin-film superconductors

We have observed the magnetic flux penetration at 4.2 K into superconducting lead films of about 4 µm thickness using a high-resolution magnetooptical technique. The specimens had the shape of small squares or long strips. In increasing magnetic field it was found that magnetic flux starts to penetrate into the bulk of the specimens in the form of flux tubes containing about 60 flux quanta. For the samples with the strip geometry the magnetic fieldH*, at which flux penetration into the bulk sets in, was investigated in the presence of an external electrical currentI along the strip superimposed on the Meissner shielding currents. The fieldH* was found to decrease linearly with increasing transport currentI. The magnetic field near a long superconducting cylinder in the presence of both a transverse applied magnetic field and an applied electrical current parallel to the cylinder axis has been calculated. The calculation is valid for a cylinder of elliptical cross section, which approximates a long, flat strip. The experimental observations for the slope H*/I are in good agreement with the calculations.Work performed in part in the Argonne National Laboratory and in part in the Ames Laboratory of the U.S. Atomic Energy Commission.     

Model calculations of flux-tube nucleation in thin-film type I superconductors

In the current-induced resistive state, multiquantum flux tubes are nucleated at the edge of thin-film strips of type I superconductors. The nucleation process can be theoretically described by an extended Gibbs free-energy barrier model. Within this model the size of the nucleated flux tubes is obtained by minimizing the total energy of the flux tube. The dependence of the flux-tube size on the nucleation rate and the transport current is calculated including the influence of the preceding flux tubes and the positive domain wall energy of type I superconductors. The interaction with the preceding flux tubes generates a steplike decrease of the flux-tube size with increasing nucleation frequency, whereas the wall energy establishes a minimum value of the flux-tube size. These calculations are in good agreement with recent experiments on the nucleation process of multiquantum flux tubes.Financial support provided by the Deutsche Forschungsgemeinschaft.     

A simplified expression for low cleavage probability calculation

This article describes an analysis made to develop a simplified stress-based criterion for brittle fracture focussed on the lowest probability of failure. For that, on the basis of fine numerical interpretation of two series of fracture-tests on 16MND5 reactor vessel steel, a number of variables were proposed:

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A stress threshold σ_th below which cleavage cannot occur. This stress is determined by testing on notch tensile specimen at low temperature.

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A minimum toughness K_min(T) required to make a crack unstable. The originality is here to consider this parameter depends on temperature.

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For K_J > K_min(T), a volume susceptible to cleavage, defined as the volume of material subjected to stress exceeding the threshold stress and noted V_th, representative of the fracture probability.

These three variables are explained in the article then used to establish a tentative criterion for expressing the risk of brittle fracture, in the presence or absence of a crack.     

Some results on nonequilibrium thin-film superconductors using the kinetic equation approach

An attempt is made to find the density instability to a superconducting multigap state by using the kinetic equations. This approach is general enough to include both the possibility of anomalous diffusion and the purely kinetic instability proposed by Iguchi and Konno. No instability to small density fluctuations is found. The possibility of an instability involving a nucleation step cannot be ruled out, however. Results for the solutions of the distribution functions in the homogeneous case are also presented, illustrating the jump to the normal state, the extent to which the excited quasiparticles can be described by a few thermodynamic parameters, and the dependence upon the phonon escape time.     

Superconducting current stability in composite superconductors

The maximum transport current in multifilament composite superconductors has been studied. The experimentally obtained values of transport current exceed the normal phase minimum propagation current substantially. The current-voltage characteristics of the investigated samples have been studied. It is shown that the relatively high value of the maximum transport current is attributed, under the given experimental conditions, to the behaviour of the current-voltage characteristic in the region of low values of the electric field. The theory and experiment are in good agreement.     

Quenching stress, quenching current and plastic strain in superconductors

On the basis of macroscopic treatment of plastic yield, the values of maximum mechanical stress and maximum superconducting current have been calculated for a super-conductor subjected to uniform plastic deformation. The training effect due to strain hardening of the material is discussed. These results are compared with an analogous case when the normal transition is caused by the thermo-magnetomechanical instability. The quenching current is shown to depend on the history of field-current application in the presence of plastic yield.     

Symmetric long alkyl chain end-capped anthracene derivatives for solution-processed organic thin-film transistors

Three new anthracene derivatives, 2,6-bis(4-decylphenyl)anthracene (DDPAnt), 2-decyl-5-(2-(5-decylthiophen-2-yl)anthracen-6-yl)thiophene (DDTAnt), and 2,6-bis(4-decyloxy phenyl) anthracene (DDPXAnt) were synthesized by Suzuki cross-coupling reaction. The obtained oligomers were characterized by 1H NMR, FT-IR, Mass, UV-visible spectroscopy, cyclovotammetry, differencial scanning calorimetry, and thermogravimetric analysis. The thermal studies show that these oligomers are stable up to 400 degrees C. The solution processed OTFTs were fabricated using synthesized oligomers by spin-coating and drop casting processes on Si/SiO2. OTFTs based on DDPAnt showed the mobility of 7.6 x 10(-3) cm2/Vs and on/off ratio of 10(5).     

Effect of microwave radiation on the critical current in dirty superconductors

The current density in a dirty superconductor placed in a field of microwave radiation is calculated from Eliashberg's theory for nonequilibrium superconductivity. It is shown that the microwave-induced change in the quasiparticle distribution function f enhances the critical current in two ways: through a term appearing in the self-consistency equation that leads to enhancement of the order parameter; and through a change in the functional form of the supercurrent by a term related to f(). Numerical results are presented describing the behavior of the critical current as a function of the radiation power and frequency and the temperature.Work supported in part by the US National Science Foundation, grant DMR 78-10312.     

Intrinsic intergranular transport current density in high-T c superconductors

The intrinsic, intergranular, critical current densityJ _c(H=0,T≤T _c) in sintered untextured YBCO specimens of different grain size is self-consistently determined using a recently proposed critical state model [1] on ZFC magnetizationM(H, T) and on pulse transportJ _ct(T) measurements. Flux-creep effects, which are significant in these materials, are treated and theJ _c(T) results are corrected to their intrinsic values.     

Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors

Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T _c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J _c, drops 80% from 124 (OT) to 29 A cm^–2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm^–2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm^–2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ _c=(/B ₀)/[(1+B/B ₀)] ⁿ can be used to describe the field dependence of the critical current density, J_c, in the field range 0.2–1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB _c2 for bismuth-based bulk highT _c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.     

Scaling law in voltage — current characteristics of hard superconductors

From scaling laws for pinning we have suggested scaling laws for voltage — current characteristics of type II superconductors. After linearization of J_c dependence on magnetic field B, temperature T and stress ?, they lead to exponential dependence of the voltage E on these parameters. In order to obtain general scaling laws, then the temperature, field and stress dependence of the denominator J₀in E = E₀ exp [(J?J_c)/J₀] should also be determined.The generalization of the instability criterion leads to the conclusion that the prestrain of the superconducting material can enhance or deteriorate the stability of the winding.     

Critical current in high-temperature superconductors with disordered tilt grain boundaries

A model is suggested to describe the effect of tilt grain boundaries with partly random dislocation distribution on the critical current value in high-temperature superconductors. Within this model, the field of grain-boundary stresses σ_αβ acquires a much more pronounced long-range character than in the case of a periodic dislocation arrangement. At large distances x from a tilt grain boundary, σ_αβ ∝ x ^−3/2 (which corresponds to the quasi-equidistant dislocation walls), whereas at small x, we have σ_αβ ∝ x ^−1/2 (which corresponds to randomly arranged dislocation walls). A region with stresses exceeding a certain critical value is treated as the region of normal metal, and, therefore, the critical current passing through this region decreases exponentially. It is shown that the model suggested satisfactorily agrees with experimental data.     

Continuous vortex cutting in type II superconductors with longitudinal current

The consequences of stipulating translational symmetry for a type II superconductor to which longitudinal electric current and longitudinal magnetic field are applied are investigated. The magnetic flux lines must cut each other continuously in order to generate an electric field in this symmetry. We describe the steady state by two interpenetrating vortex lattices moving into and out of the sample. We find for the slab and cylinder geometries that cutting, crossjoining, and subsequent straightening of the flux lines reduce the electric field, as compared with the normal conducting state, by a factor which is of the order of one over the total number of flux lines in the cylinder. We conclude that the much larger voltages observed in cylinders of several millimeters diameter can be explained only by a breakdown of translational symmetry. With translational ¥mmetry, the voltage initially increases as the third power of the current. The resulting vortex configuration is force-free. The transverse flux component increases and the longitudinal component decreases from the axis to the surface, leading to a paramagnetic moment. The drift or oscillation velocity of the flux lines is reduced by the same factor as the electric field. We predict low-frequency oscillations of the vortices near the surface of thin superconducting wires.