Irreversibility line and flux pinning properties in iron-based high-Tc superconductor SmFeAsO0.85

The irreversibility line and flux pinning properties of high-T_c superconductor SmFeAsO_0.85 were studied using DC magnetization data. Polycrystalline SmFeAsO_0.85 was prepared in a high pressure synthesis apparatus under the pressure of 6 GPa. The results of DC susceptibility showed the superconducting transition at about 55 K. A critical current density J_c(B) was calculated using Bean’s critical state model. At low temperatures (20 K), J_c(B) showed a relatively high value with weak dependence on an applied magnetic field. At higher temperatures, a stronger dependence of the magnetic field was observed, which resulted from decrease in a critical current density probably due to the flux creep effect. The irreversibility line (IL) agreed well with the flux creep theory of Matsushita et al. A comparison of normalized pinning force density with the theoretical models showed that the irreversible behavior in SmFeAsO_0.85 is dominated mainly by normal point pinning (δT_c) and surface pinning mechanisms.     

Critical current density of a YBa2Cu3O7 film: Comparison between experiment and collective pinning theory

With the vibrating reed technique we have measured the critical current densityJ _c of a Y 123 200 nm thin film as a function of magnetic field perpendicular to the CuO₂ planes (O T<B8 T) and temperature (20KT60 K). At fieldsB<0.1 TJ _c is magnetic field independent and decreases at higher fields. A comparison with theory indicates that a crossover from a single pinning to a small bundle collective pinning regime may explain the observed behaviour. According to our estimate the main pinning centers are weak point pins due to oxygen vacancies. From the temperature dependence ofJ _c atB0 we obtain a temperature dependence of the thermodynamical critical fieldB _c (1–T/T _c )² forT20 K which agrees with the anomalous temperature dependence ofB _c2 observed recently in highly anisotropic high temperature superconductors.     

Point-contact measurements of CeB6 and CeCu6 in high magnetic fields

Epitaxial YBa₂Cu₄O₇-thin films with thec-axis oriented perpendicular to the film plane were prepared bydc-sputtering from a single stoichiometric target on (100) SrTiO₃-substrates. Typical values of the inductively measured superconducting transitions were about 90 K with a width less than 0.5 K. Critical current densities were measured on 5 to 10 m wide strips as function of magnetic field and temperature. The temperature dependences ofj _c follow a universal functionj _c(B,T)=j _c ^* (T=0,B)·(1–T/T _c (B)) with =1.5±0.1. ForB=0 andT=77 K we obtainedj _c =4·10⁶ A/cm². The field dependence of the resistive transitions was measured with the magnetic field parallel to thec-axis. The slope of the upper critical fieldB _c2 (T) was determined for different criteria. The carrier concentration evaluated from Hall-effect measurements was found to decrease linearly from one per unit cell at 240 K with decreasing temperature extrapolating nearly through zero forT=0. Highly resolved angular dependent measurements of the critical current density withB perpendicular to the current but tilted from thec-axis show a very strong and sharp enhancement ofj _c for the magnetic field parallel to the (CuO₂)-layers (Bc). Additionally to this phenomenon, which is caused by an intrinsic pinning mechanism due to the layered structure of high-T _c -superconductors the influence of the anisotropy of the upper critical field onj _c (B, T, ) is evident nearT _c .     

Irradiation effects on the YBa2Cu3O7-δ superconducting compound

The effects of neutron irradiation on the magnetic and superconducting properties of the YBa₂Cu₃O_7- (Y123) superconducting compound have been investigated. No significant change of the superconducting transition temperature (T _c) was found. The effect of irradiation on the magnetization critical current density (J _c) was studied. Bean's critical state model was used to estimate J _c. Around 40% increase in J _c was found. The temperature dependence of J _c was also studied.     

Temperature dependence of the critical current density in Y-Ba-Cu-O thin films

We present a study of the temperature dependence of the critical currentJ _c of several dc magnetron sputtered thin Y-Ba-Cu-O films on single crystalline SrTiO₃, ZrO₂ and Al₂O₃ substrates. Near the critical temperature T_c it is found thatJ _c(1–T/T_c)ⁿ withn=3 for the SrTiO₃ and ZrO₂ substrates, whilen=1·3 for the Al₂O₃ substrate. The temperature dependence in our samples approximately agrees with standard theories for weak links or with the Ambegaokar-Baratoff equation.     

High-quality YBCO d.c. SQUIDs based on bicrystal substrates

Summary An analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of ac-axis-oriented YBCO film on a SrTiO₃ bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle θ=20° are reported. The SQUID shows a critical temperatureT _c∼89 K and a high critical current densityJ _c∼3·10⁶ A/cm² atT=4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependenceJ _c(T) shows a linear behaviour at small reduced temperatures and a depressedJ _c value close toT _c. High-quality flux-voltage curves have been found upT=87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of θ make YBCO bicrystal GBJs very useful for applications in electronics. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

SQUID法和Campbell法测量超导材料的研究

本文对比研究了超导材料磁测量中的SQUID法和Campbell法; 并用高压PIT法制备的超导材料MgB₂作为测量样品,用两种方法,测量了超导样品的临界电流密度, 分别得到了样品的J_c-B关系曲线; SQUID法测量样品的外磁场可以达到6 T, 此时材料已经处于失超状态,此方法测得的结果是样品各个小区域结果的平均值, SQUID还可以用来进一步标度材料的钉扎力行为,研究材料磁特性. Campbell法测量只能测量到外磁场强度为0.4 T,外磁场的交流部分的频率可以达到800 Hz, 用这种测量方法得到的是整块样品的电流,由于测量计及材料内部微观结构缺陷等影响电流传输因素, 所测结果小于直流磁化法,但更切近材料实际电流,能用来深入研究材料内部结构差别对材料电性能的影响.     

Influence of transport current and thermal fluctuations on the resistive properties of HTSC+CuO composites

Measurements of the temperature dependence of the electrical resistance R(T) below the superconducting transition temperature have been performed at different values of the transport current in HTSC+CuO composites modeling a network of weak S-I-S Josephson junctions (S—superconductor, I—insulator). It has been shown experimentally that the temperature dependence R(T) at different values of the transport current is adequately described by means of the mechanism of thermally activated phase slippage developed by Ambegaokar and Halperin for tunnel structures. Within the framework of this model we have numerically calculated the temperature dependence of the critical current J _c(T) as defined by various criteria. Qualitative agreement obtains between the measured and calculated temperature dependences J _c(T). Fiz. Tverd. Tela (St. Petersburg) 41, 969–974 (June 1999)     

Electromagnetic properties of bulk Bi-Pb-Sr-Ca-Cu-O superconductor at low magnetic fields

Magnetization and critical current density measurements have been performed on a Bi_1.7Pb_0.3Sr₂Ca₂Cu₅O _y bulk material as the functions of magnetic field and temperature. The diamagnetic shielding effect has been discussed from the initial magnetization curve. The lower critical field of bulk granular superconductor (H _g) and the lower critical field of superconducting grains (H _c1) are estimated from magnetization curves. They are both linearly decreasing functions of temperature: (dH _g/dT) and (dH _c1/dT) are –0.4 and –1.8 G/K respectively. The transport critical current density drops drastically by a factor of 4 at a magnetic field of about 20G. The magnetization J _c of superconducting grains derived from the remanent magnetization is about 10⁶ A/cm² at 77 K, zero field, much greater than the transport J _c. The experimental results reveal that the transport J _c is dominated by weaklink current between grains. The magnetization J _c versus temperature has been obtained from the remanent flux at zero magnetic field and is a linearly decreasing function of temperature within experimental error.     

The effective activation energy Ueff(T,B,J) in Hg-1223 single phase superconductors

We review the methods of calculating the effective activation energy U_eff(T,B,J) for both transport measurements and magnetic decay, together with some theoretical models. Then, we apply these methods to our Hg-1223 single-phase superconductor to obtain the activation energy. Transport results give that the magnetic field and temperature dependence of the U_eff can be well described as U₀B^−α(1−T/T_c)^m. Magnetic relaxation shows that the current density dependence of U(J) can be scaled onto a single curve, which can be considered as the activation energy at some temperature T₀. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J, are responsible for the magnetic decay data scaling onto a single curve at various temperatures. As temperatures close to zero and near T_c, thermally assisted flux motion model is no longer valid since other processes dominate.     

Breakdown of superconductivity in current-carrying indium whiskers

Like whiskers of tin andtin-indium alloys indium whiskers showV-T andV-I characteristics with a large transition width and regular voltage steps. Applied HF-radiation leads to additional steps of zero slope in theV-I characteristics (current steps) at voltages related to the applied frequency by the Josephson relation, supporting the assumption that phase-slip processes at Josephson frequency are related to the appearance of voltage steps in the transition curves without radiation. At temperatures very close to the critical temperatureT _c0 theV-I characteristics beyond the first voltage step show a differential resistance (dV/dI)₁ and a ratioI ₀/I _c which grow with increasing temperature, contrary to tin andtin-indium alloy whiskers (I _c : critical current,I ₀: extrapolated zero voltage intercept). A probable explanation for this observation is, that in indium whiskers very close toT _c0 nonequilibrium quasiparticles relax by branch crossing processes due to inelastic electron phonon scattering or gap anisotropy.     

柠檬酸掺杂的MgB2超导体钉扎机理的研究

本文研究了柠檬酸掺杂的MgB₂超导材料的J_c-B行为及其钉扎机理.在纯MgB₂多晶样品中,δT_c钉扎起主要作用,而在掺杂的样品中,则是δl钉扎和δT_c钉扎共同作用,并且δl钉扎机理占主要作用,其贡献比重随着掺杂量的增加而增加.从J_c-B行为和钉扎行为的分析都可以得到 关键词： 柠檬酸 2')" href="#">MgB₂ T_c钉扎')" href="#">δT_c钉扎 l钉扎')" href="#">δl钉扎     

钛离子辐照对MgB2超导薄膜的载流能力和磁通钉扎能力的影响

利用混合物理化学气相沉积法(hybrid physical-chemical vapor deposition, HPCVD)可以制备出高性能的MgB₂超导薄膜, 再对薄膜进行钛(Ti)离子辐照处理.经过辐照处理后的样品被掺入了Ti元素, 与未处理的干净MgB₂样品相比,其超导转变温度没有出现大幅度的下降, 而在外加磁场下的临界电流密度得到了明显的提高,同时样品的上临界磁场也得到了提高. 在温度5 K, 外加垂直磁场为4 T的情况下, Ti离子辐照剂量为1× 10¹³/cm²的样品的临界电流密度达到了1.72× 10⁵ A/cm², 比干净的MgB₂要高出许多,而其超导转变温度仍能维持在39.9 K的较高水平.     

Effect of annealing temperature on some physical properties of MgB2 by using the Hall probe ac-susceptibility method

A commercially available powder of MgB₂ is used as starting material for the examination of the influence of the annealing temperature on the properties of this intermediate-T_c superconductor. We performed scanning electron microscopy (SEM) and Hall ac-susceptibility measurements as a function of temperature and ac-field amplitude on samples annealed at 650, 750, 850 and 950 °C. The imaginary part of ac-susceptibility measurements is used to calculate both the inter-granular critical current density, J_c(T_p) and density of pinning force, α_j(0). It was observed that all T_c, J_c(T_p) and α_j(0) exhibit a non-monotonic behavior on the annealing temperature range studied in this work. T_c is measured to be 39.85±0.02 K and J_c(T_p) is estimated to be as high as 60 A/cm² at 39.2 K for the sample annealed at 850 °C. The peak temperature, T_p, in the imaginary part of the ac-susceptibility curves shifts to lower temperatures with both decreasing the annealing temperature and increasing the amplitude of the ac-magnetic fields. A comparison of the experimental ac-susceptibility data with theoretical critical-state models that are currently available is performed. SEM investigations showed that the grain size increases, and the grain connectivity improves when the annealing temperature increases up to 850 °C. The possible reasons for the observed changes in transport, microstructure and magnetic properties due to annealing temperature are discussed.     

Effects of neutron irradiation on superconducting properties of orthorhombic Y–Ba–Cu oxides

Abstract

We have studied the effects of fast neutron (E>0.1 MeV) irradiation at reactor (～ 360 K) and low (～ 20 K) temperatures on the superconducting properties of polycrystalline orthorhombic YBa₂Cu₃O_7?y . Measurements were made on the superconducting critical temperature T_c , critical current J_c , Meissner effect and magnetic field dependence of J_c . The T_c drops by an irradiation at reactor temperature and J_c increases with increasing fluence. On the other hand with the irradiation at low temperature, T_c rises and J_c increases. Results of observation of Meissner effect and the magnetic field dependence of J_c are consistent with the behavior of T_c and J_c .     

AC response of YBa2Cu3O7 thin film superconductor

Low-field ac measurements of magnetic susceptibility of YBa₂Cu₃O₇ high-temperature superconducting thin film were carried out over a wide range of temperatures and ac magnetic field amplitudes. A strong field dependence of the real χ′ and imaginary χ″ components was observed. The field dependence of the imaginary component is used to extract the temperature dependence of the critical current density in the sample using the critical state model. The exponent β of the power law relation J_c ～ (1 ? T/T_c)^β was determined from ac-susceptibility data and different values were found. Experimental results are compared with predictions of some existing theoretical models describing the ac response of superconducting thin film in perpendicular ac field.     

Transport properties of long monofilamentary Bi(2223) tapes

Summary Monofilamentary Bi(2223) tapes withJ _c(77 K, 0 tesla) up to 30 000 A/cm² have been prepared by cold rolling using the powder-in-tube method. An optimization of the precursor powders has led to a higher phase purity after the reaction heat treatment. The deformation process has been optimized in order to increase the oxide density and to reduce sausaging effects on the oxide thickness. The transport properties of these tapes have been studied in a wide range of temperature (4.2K-T _c) and magnetic fields (up to 28 tesla). The critical-current values at 77 K fields of 0.5 T and 1 T parallel to the tape surface are 10 000 A/cm² and 5400 A/cm², respectively. At 4.2 K theJ _c value decreases from 1.6·10⁵ A/cm² at 0 T to 6·10⁴ A/cm² at 15 T. At fields higher than 15 tesla a very low field dependence ofJ _c has been found, regardless of the tape orientation. Transport properties have also been studied by cutting small sections of the tape in order to investigate the local critical-current distribution. It has been found that, even in rolled tapes of good quality (J _c (77 K, 0 T)>20000 A/cm²), theJ _c distribution is homogeneous: the critical current density increases gradually from the centre of the tape to the sides, the latter exhibiting much higherJ _c (46000 A/cm²) than in the centre (18000 A/cm²). Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

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.     

Nonequilibrium phase transition in stochastic lattice gases: Simulation of a three-dimensional system

We report results of computer simulations of a three-dimensional lattice gas of interacting particles subject to a uniform external fieldE. The dynamics of the system is given by hoppings of particles to nearby empty sites with rates biased for jumps in the direction ofE. As for the two-dimensional system we find that here too there exists a critical temperature,T _c (E) such that forT < T _c (E) the systems orders in a very anisotropic phase with striplike typical configurations parallel to the field.T _c (E) increases withE but substantially less strongly than in two dimensions. There is a break in the slope of the saturation current atT _c (E). Our data are consistent with the critical exponent being mean field.     

Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu _x Sr_1–x Te at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureT _N=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent =–0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toT _N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: becomes negative and decreases continuously towards –1 atxx _c. This concentration dependence of was previously observed in the diluted ferromagnetic system Eu _x Sr_1–x S. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationx _c reveal spin-glass behavior in the specific heat. In addition, the dependence ofT _N on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryB _c(T)/B_c(T=0) vs.T _N(B)/T_N(B=0) which is independent of concentration.