Temperature dependent time delay of the voltage response to supercritical current pulses in superconducting aluminium

Current pulses larger than the critical current I_c are passed through narrow superconducting aluminium strips. The time delay between the start of the pulse and the onset of the voltage response is measured as a function of current amplitude for various temperatures (0.84<T/T_c<0.98). We find the delay time to be dependent on the temperature as (1 - T/T_c)^α with α changing from a value of - 0.5 for I/I_c close to 1 to a value of - 1.0 for I/I_c larger than 3.     

Critical currents in polycrystalline thin films of high-T c superconductors

Summary It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T _c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (=SIS) ?sandwich? between each grain leads to a crossover fromI _c∼(1−T/T _c) toI _c∼(1−T/T _c)^3/2, for temperatures nearT _c (whereI _c is the critical current,T the absolute temperature, andT _c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (=SNS) sandwich the dependenceI _c∼(1−T/T _c)² is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Diamagnetic shielding and Meissner effect in the high Tc superconductor Sr0.2La1.8CuO4

SQUID susceptibility measurements of the high T_c superconductor Sr_0.2La_1.8CuO₄, with a 40 K resistive onset, are reported for fields from 1 μT to 4.5 T (10 mG to 45 kG). Data show an average lower critical field of 30 mT at 5 K, but with nonlinearities appearing as low as 100 μT. Zero field cooling and subsequent application of a field shows a diamagnetic shielding effect reaching 100% in the lowest fields with a 37 K onset and 31 K midpoint. The observed Meissner effect in single phase material exceeds 40% but decreases with oxygen annealing even though T_c increases.     

Current-induced resistive state of superconducting tin whiskers with indium impurities

We have investigated the current-induced step-like structure in theV?T andV?I characteristics of tin whiskers with a mean free pathl for the electrons which is reduced by indium impurities (up to 3.6 at %). The extrapolation of theV?I characteristics beyond the first voltage step shows a zero-voltage interceptI ₀≈0.5I _c (I _c=critical current). From the differential resistance beyond the first voltage step a lengthL _An1 can be obtained which is proportional tol ^1/2. These results can be understood by assuming phase-slip centers in the whisker which carry a time-averaged supercurrent of approximately 0.5I _c above the critical current, and at which a voltage is developed by the “healing” of nonequilibrium quasiparticles.     

Global ac susceptibility of low pinning high-T c crystals near T c

We have studied the hysteretic loss and the shielding capability of Bi₂Sr₂CaCu₂O₈and YBa₂Cu₄O₈ crystals as a function of ac field (μ ₀ H ≤ 1mT), frequency, and temperature at small dc fields (μ₀ H _a < 1 mT) near the critical temperature T _c . We show that the global ac field dependence of the susceptibility can be described in terms of a geometrical barrier model in agreement with local permeability results. At temperatures very near T _c , however, we observe a crossover to diffusive behavior. The frequency dependence of the temperature of the dissipation maximum at finite dc fields shows a thermally activated behavior.     

Influence of experimental conditions on the electric field effect in the ceramic (BiPb)2Sr2Ca2Cu3Ox

An experimental study is reported of the influence of temperature (T), electric field polarity (±E), as well as of changes in the electrode/insulator/superconductor (E/I/S) measuring system on the field effect in the ceramic (BiPb)₂Sr₂Ca₂Cu₃O_x. It has been established that at 77 K and for E⩾60 MV/m the critical current I _c and conductivity of the sample increase for I>I _c, irrespective of the field polarity. For lower fields and a negative electrode potential the conductivity in an electric field may decrease. The field effect decreases with increasing temperature, to practically vanish near T _c where the sample is still in superconducting state. Experiments carried out with more complex systems E/I/M/I/S and E/I/M/S (M stands for a metallic foil) support the conclusion that it is the external electric field that is responsible for the observed effects. Fiz. Tverd. Tela (St. Petersburg) 39, 1967–1970 (December 1997)     

I–V characteristics and the electric-field effect in melt-grown YBa2Cu3Ox/Y2BaCuO5 HTSC crystals

The effect of an electric field E=120 MV/m in the electrode-insulator-superconductor system on I–V curves obtained at 77 K on two types of single-crystal samples cut from monolithic superconducting YBa₂Cu₃O_x/Y₂BaCuO₅ has been studied. The Y211 nonsuperconducting phase in the ingot was in the form of precipitates ≈1 μm in size. It has been found that an electric field applied to samples with a comparatively low Y211 content (volume fraction 8%) does not affect the critical current I _c while reducing the resistance R at currents slightly above I _c . In samples containing more than 35% Y211 phase, electric field results in an increase of I _c and a decrease of R for I>I _c . Data on the critical temperature T _c and the temperature dependence of I _c have also been obtained. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 7, 2000, pp. 1172–1175. Original Russian Text Copyright ? 2000 by Smirnov, Orlova, Sengupta, Goretta.     

A specific heat and magnetization study of highT c ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor La₂CuO₄ and the high temperature superconductors La_1.8Sr_0.2CuO₄ (T _c =37.2 K) and YBa_1.9K_0.1Cu₃O_6.9 (T _c =91.5 K) in the range 1.5–30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of La_1.8Sr_0.2CuO₄ the thermodynamical critical field belowT _c has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol^?1 K^?1 was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible withT _c =36 K when using McMillan's formula forT _c . A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K.     

Flux pinning by discontinuous columnar defects in 74 MeV Ag-irradiated YBa2Cu3O7 coated conductors

Linear damage tracks are created in production-quality YBCO coated conductors by irradiation with 61–74 MeV Ag ions. The ion tracks are observed by transmission electron microscopy to be elongated but discontinuous. The in-field transport critical current (I_c) is enhanced significantly for fields applied parallel to the irradiation direction with a broad peak appearing in the magnetic field-angle dependence of the critical current, coinciding with the irradiation direction. The zero-field I_c is typically reduced somewhat, however annealing at 200 °C partially restores this and enhances the in-field I_c even for field parallel to the irradiation direction. Lower energies tend to produce a weaker peak, but also retain the zero-field I_c to a greater extent, consistent with a trend of greater discontinuity in the ion tracks.     

Thickness dependence of Ic and Jc of LTG-SmBCO coated-conductor on IBAD-MgO tapes

We have reported SmBa₂Cu₃O_y (SmBCO) films on single crystalline substrates prepared by low-temperature growth (LTG) technique. The LTG-SmBCO films showed high critical current densities in magnetic fields compared with conventional SmBCO films prepared by pulsed laser deposition (PLD) method. In this study, to enhance critical current (I_c) in magnetic field, we fabricated thick LTG-SmBCO films on metal substrates with ion-beam assisted deposition (IBAD)-MgO buffer and estimated the I_c and J_c in magnetic fields.All the SmBCO films showed c-axis orientation and cube-on-cube in-plane texture. T_c of the LTG-SmBCO films were 93.1–93.4 K. J_c and I_c of a 0.5 μm-thick SmBCO film were 3.0 MA/cm² and 150 A/cm-width at 77 K in self-field, respectively. Those of a 2.0 μm-thick film were 1.6 MA/cm² and 284 A/cm-width respectively. Although I_c increased with the film thickness increasing up to 2 μm, the I_c tended to be saturated in 300 A/cm-width. From a cross sectional TEM image of the SmBCO film, we recognized a-axis oriented grains and 45° grains and Cu–O precipitates. Because these undesired grains form dead layers, I_c saturated above a certain thickness. We achieved that I_c in magnetic fields of the LTG-SmBCO films with a thickness of 2.0 μm were 88 A/cm-width at 1 T and 28 A/cm-width at 3 T.     

Effect of nitrogen on some properties of vapor-deposited Nb3Sn

Measurements of critical currentsI _c, transition temperaturesT _c, as well as transmission electron microscopy, scanning electron microscopy and X-ray analysis have established a correlation between critical currentI _c, the tape heating temperature during deposition, and the presence of a fine dispersion of precipitates. As the tape heating current (temperature) increased from 3 to 5 Amps,I _c increased from 20 to 430 Amps/3 mm tape width at 50 kG. A fine precipitate of the Nb₂N phase was observed for highest tape heating temperatures. It is concluded that the precipitate particles may act as flux-pinning centers in the Nb₃Sn layer.     

表面与界面的绝缘铁磁理论

采用Green函数技术,我们讨论了温度从T=0直到T=T_c范围内铁磁体表面和界面的磁化强度随温度的变化情况,所得到表面磁化强度的结果与其他作者的结果一致;而在界面计算中,我们发现界面的T_c与体内的不同,并且找到了一个关系式:T_cc^A,T_c^B)。 关键词：     

Different types of vortex pinning contribution to critical current density in YBa2Cu3O7−δ /YSZ quasi-multilayer

The quasi-multilayer films of YBa₂Cu₃O_7−δ /YSZ (YSZ denotes Yttria Stabilized Zirconia) are prepared by means of pulsed laser deposition (PLD), and a systematic study of the magnetic-field and temperature dependence of the critical current density J _c(H,T) for the YBCO/YSZ quasi-multilayer film is presented. Angular-dependent J _c(H,T) measurements have demonstrated that the growth control strategy is very effective in preventing the vortex motion at high fields and high temperatures. The temperature dependence of isotropic and anisotropic contribution to J _c is investigated in order to evaluate the strength of the defects. It is suggested that at high applied fields (such as 7 T), the pinning contribution of the YBCO/YSZ quasi-multilayer is dominated by the anisotropic disorders, while at intermediate-low fields (such as 1 and 3 T) the pinning contribution is determined by both isotropic and anisotropic disorders.     

Hall breakdown in a modulation-doped GaAs/AlAs heterostructure

The effect of dc current I _dc on the electron transport in a GaAs quantum well with AlAs/GaAs superlattice barriers is studied experimentally at a temperature of 4.2 K in magnetic fields up to 2 T. A sharp increase in resistance R _xx is observed in magnetic fields higher than the critical field B _c. The value of B _c is found to decrease with increasing the current I _dc. The phenomenon observed in the experiment is qualitatively explained by the electric breakdown of superlattice barriers under the action of the Hall field.     

Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters T _c and I _c of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7I _c) in liquid nitrogen (77 K) is studied. Voltage-current characteristics U(I), as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by ～30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.     

2H-Nb0.9Ta0.1Se2中磁通涡旋输运的标度行为

测量了2H-Nb_0.9Ta_0.1Se₂单晶样品在不同电流下电压随磁场变化的曲线V(H),并从V(H)曲线得到V(I)数据.使用标度关系V=α(I-I_c)^β进行了拟合,得到了临界电流随磁场I_c(H)和微分电阻随磁场R_d的变化关系.在微分电阻随磁场变化的曲线中,电流较大时,靠近上临界磁场H_c2附近出现一个强峰,而在低电流下,该峰消失.同 关键词： 磁通涡旋 标度定律 临界电流峰效应     

Ll2超导体的Tc表达式

通过对L1₂超导体的研究,我们得出了这类超导体临界温度的表达式T_c=(15.9T_BV(B)G_A)/(M^1/2V(L1₂)G_B)。其中T_B是B组元的超导转变温度,V(B)是B组元的原子体积,V(L1₂)为L1₂化合物的平均原子体积,M是L1₂化合物的平均原子量,G_A,G_B分别为A组元、B组元的Gordy电负性值。 关键词：     

Addition of nanometer SiC in the silver-sheathed Bi2223 superconducting tapes

Nanometer SiC particles were added to (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi2223) precursor powders and the powders were transformed into silver-sheathed superconducting tapes by the standard powder-in-tube (PIT) technique. The influence of SiC on the melting temperature, high-T_c (Bi2223) phase formation, microstructure and transport property of the tapes was studied by means of DTA, XRD, SEM/EDX, electrical and magnetic measurements. The results showed that SiC addition lowered the melting temperature of the superconductor powders and consequently, decreased the optimum sintering temperature of the tapes. However, the addition did not affect the Bi2223 phase formation and critical temperature (T_c) of the tapes up to 1.00 wt.% of SiC. Most importantly, the addition of a small amount of SiC (0.15 wt.%) improved the critical current (I_c) and I_c behavior in magnetic field as a result of the enhancement in density, grain alignment, grain connectivity and flux pinning of the tapes.     

吸气溅射Mo1-xSix薄膜的超导性能

对于四个Mo_1-xSi_x薄膜样品进行了超导转变温度、临界温度附近的临界磁场以及在4.2K下的临界电流的测试,并且测量了室温(300K)及低温(8K)下的电阻率,结果显示四个样品具有非晶的特征,其中Mo₇₈Si₂₂薄膜样品有较好的非晶特性。 关键词：     

Generating power of a thermoelectric generator under periodically alternating temperature gradients

The thermo-emf ΔV and thermoelectric current ΔI generated by imposing a temperature gradient alternating at a period of T on a thermoelectric (TE) generator were measured as a function of t, where t is the lapsed time and 1/T was varied from 0 to 1/30 s^-1. A TE generator was sandwiched between two Peltier modules connected in series. The alternating temperature gradients were produced by imposing an alternating voltage V on two Peltier modules, where V was varied from 1.0 to 3.7 V. Both ΔV and ΔI generated by the TE generator oscillate at a period of T but their amplitudes tend to increase monotonically with an increase of V. The effective thermo-emf ΔV_eff and current ΔI_eff calculated from ΔV and ΔI increase abruptly with an increase of 1/T and have a local maximum at 1/T=1/120 or 1/240 s^-1. The generating power ΔW_eff(=ΔV_effΔI_eff) tends to increase proportionally with an increase of input power W_input, owing to the increase in the temperature difference. The rate of ΔW_eff to W_input at 1/T=1/240 s^-1 reached approximately 3.2 times as large as that obtained for the steady temperature gradient corresponding to 1/T=0 s^-1. It was thus found that the generating power of the TE generator operating under the temperature gradient alternating at an optimum period is remarkably increased compared to that of a TE generator working under a conventional steady temperature gradient. PACS 72.15.Jf; 84.60.Rb; 85.30De