X. Leng

History effect in inhomogeneous superconductors

A model was proposed to account for a different kind of history effect in the transport measurement of a sample with inhomogeneous flux pinning coupled with flux creep. The inhomogeneity of flux pinning was described in terms of alternating weak pinning (lower j c ) and strong pinning regions (higher j c ). The flux creep was characterized by logarithmic barrier. Based on this model, we numerically observed the same clockwise V-I loops as reported in references. Moreover, we predicted behaviors of the V-I loop at different sweeping rates of applied current dI/dt and magnetic fields B a , etc. Electric transport measurement was performed in Ag-sheathed Bi 2 - x Pb x Sr 2 Ca 2 Cu 3 O y tapes immersed in liquid nitrogen with and without magnetic fields. A V-I loop at certain dI/dt and B a was observed. It is found that the area of the loop is more sensitive to dI/dt than to B a , which agrees well with our numerical results.

Second peak and critical current for YBa1.9Na0.1Cu3Oy

Abstract Systematic measurement of the AC susceptibility (ACS) has been taken for the Na doped YBa2Cu3Oy (YBNCO) crystals as a function of temperature T, DC field Bd, amplitude Bac and frequency f of AC field. Its critical current density jc as a function of T, Bd, Bac and Ec was determined, where Ec is criterion of jc. Influence of criterion Ec on jc was studied by varying Bac and f, which allows one to determine the effective activation energy as a function of j. It is shown that the sweeping rate of the applied field affects the criterion Ec and thus the jc apparently. Hence the magnetic jc may be not comparable to each other unless the rate of applied field was indicated clearly.

Effect of Na doping on flux pinning of YBa1.9Na0.1Cu3O7−δ

Abstract We have prepared Na-doped YBa 2 Cu 3 O y (YBa 1.9 Na 0.1 Cu 3 O y +40 mol% Y211) (YBNCO) and Na-free YBa 2 Cu 3 O y (YBCO) samples by the melt-textured growth method to study the effect of doped Na ion on flux pinning. The ac susceptibility curves as well as the hysteresis loops were measured for the samples. Then the effective pinning energy ( U ( T , H dc , J )), irreversibility line ( H irr ( T )) and critical current density ( j c ( H dc )) were determined, where T , H dc and J are temperature, dc magnetic field and current density, respectively. We found that, with Na doping, the H irr ( T ) line shifted to lower temperatures while the J c ( H dc ) and U ( T , H dc , J ) became smaller. Hence Na ions play a negative role in the flux pinning of YBCO. The appearance of the second peak in the J c ( H dc ) curves and the enhancement of anisotropy in YBNCO further support this finding.

Second peak effect in SmBa2Cu3Oy

Abstract Two SmBa 2 Cu 3 O y samples with Y211 (Sm1 and Sm2) were fabricated by the melt-textured growth method to study the effect of heat treatment and argon atmosphere on the second peak effect (SP). Magnetization–temperature ( M – T ) measurement at 10 G field cooling shows that the sample Sm1 has a transition temperature T c =80 K but the Sm2 has higher T c (=90 K) and higher irreversibility fields than Sm1. Magnetization versus field ( M – B curves) at various temperatures T in fields up to 7 T was measured. The SP of the two samples was observed and compared over a range of temperatures. Discussion of flux pinning and the effect of Ar and 211 is made.

Influence of process on flux pinning for MTG SmBa2Cu3Oy

Abstract Two SmBa 2 Cu 3 O y samples (Sm1 and Sm3) with different processes were fabricated by the melt-textured growth method to study the effect of heat treatment and argon atmosphere on flux pinning. Magnetic measurement shows that Sm1 is of lower transition temperature, lower irreversibility field and more pronounced second peak effect. But Sm3 has higher T c , higher H irr and less pronounced SP. It was found that the flux pinning of SM1 is stranger than that of SM3 in wide range of temperature. It evidences that the irreversibility field and critical current density are independent of each other. Discussion of flux pinning and the effect of Ar and 211 is made.

Flux motion in new superconductor MgB2

Abstract Flux motion and its influence on the properties of the new discovered superconductor MgB 2 was studied by electric transport resistance as well as by ac susceptibility measurements. Different bias current I was applied perpendicular and parallel to applied dc field B d up to 7 T. The upper critical field B c2 and irreversibility field B irr were determined as functions of temperature T and bias current I . It is found that the larger the bias current I the smaller the B irr , and B c2 is observed to be affected by flux motion. The B irr ( T ) and B c2 ( T ) data determined by ac susceptibility measurement confirm the transport result. These results suggest that the influence of flux motion should be taken into account if resistively determined parameters for MgB 2 are considered. The present study also implies that the flux dynamics of MgB 2 is more like as that of high temperature superconductors rather than of low temperature superconductors.

Determination of critical current density in flux creep state for MgB2

Abstract We propose a method to determine critical current density jc for MgB2 with flux creep from the real part of AC susceptibility (ACS), which facilitates the determination of jc at different DC fields Bd in temperature range as wide as 5–38 K. Influence of criterion Ec on jc was studied by varying amplitude (Bac) and frequency (f) of AC field. The result shows that it is not proper to obtain temperature dependence of jc by measuring only the peak temperature of imaginary part of ACS.

RETRACTED: Dip effect and surface barrier in single crystal YBa2Cu3Ox

Abstract This article has been retracted at the request of the chief editor and the author. Reason: Experimental raw data presented in the paper is the property of Dr Sean Ling, and was used without his prior permission and has at this point, not yet been formally published by Dr Sean Ling.

RETRACTED: Dip effect and its disappearance

Abstract This article has been retracted at the request of the chief editor and the author. Reason: Experimental raw data presented in the paper is the property of Dr Sean Ling, and was used without his prior permission and has at this point, not yet been formally published by Dr Sean Ling.

Study on the limiting factor of critical current in Ag–Bi2223 tapes

Abstract The silver sheathed tapes of Ag–Bi 2− x Pb x Sr 2 Ca 2 –Cu 3 O 7− y (Ag–Bi2223) were fabricated by usual process. The limiting factor of the critical current density j c of the tapes was studied. V – I characteristic was performed for the tapes immersed in liquid nitrogen with and without magnetic fields. Hysteresis loop of V – I curve was observed. A model based on polycrystalline superconductors was proposed to account for this kind of hysteresis. The result shows that in our Ag–Bi2223 tapes the weak link is the key factor limiting the global critical current. The measurement of V – I loop can be an effective method to check whether there exist harmful weak links in polycrystalline samples.