J.W. Lin

Non-linear vortex dynamics of (T10.5Pb0.5)(Ba0.2Sr0.8)2Ca2Cu3Oy superconductor in the time window of milliseconds

Abstract The complex AC susceptibility of (T1 0.5 Pb 0.5 )(Ba 0.2 Sr 0.8 ) 2 Ca 2 Cu 3 O y was measured systematically as a function of temperature T , AC amplitude, frequency and DC field H d to study the vortex dynamics in the time window of 10 −4 s–10 −2 s. The dependence of effective barriers of vortex motion on the current density J , H d and T , U ( J , H d , T ), was determined. The current relaxation J ( t ), characteristic curves E ( J ) and irreversibility lines H d ( T ) and critical current densities were studied based upon the U ( J , H d , T ). It is shown that the AC susceptibility technique is a powerful tool in studying vortex dynamics in the time window of milliseconds and estimating the critical current density governed by flux creep.

AC loss of Ag–Bi2223 tapes

Abstract We have studied the effect of flux creep on the AC loss of Ag–Bi2223 tapes by three methods: the transport V – I curve, M – H hysteresis loop and AC susceptibility ( χ – T curve). It was found that the d I /d t , d H /d t and f affected the AC loss apparently in addition to the DC field, respectively. Comparison between the data of the three ways shows a satisfactory agreement with each other. Numerical simulation was conducted to understand the experimental data based on flux creep. It was shown that the dependence of AC losses on f , d H /d t and d I /d t could only be understood by flux creep model rather than any static one.

Critical current and its determination on TlSr2Ca2Ba2Cu3Oy by ac susceptibility measurement

A method is proposed according to which critical current density Jc at various temperatures T and dc fields B is estimated on (Tl0.5Pb0.5)(Ba0.2Sr0.8)2Ca2Cu3Oy based upon the flux diffusion barrier heights U(J,B,T) determined by means of ac susceptibility measurement at different ac field amplitudes, frequencies, T, and B. Jc is also estimated by the dc hysteresis measurement and compared with the former. The high Jc(B,T) and Tc show that this Tl‐based superconductor is one of the more promising high‐Tc materials from a practical standpoint.

Flux-creep mechanism studied in extended windows for the HgBa2Ca2Cu3Oy superconductor

Flux-creep mechanisms are examined by the current density J dependence of the effective barriers U(J), which were determined to be U(J) alpha J(-mu) by three independent measurements i.e. AC susceptibility (ACS), magnetic relaxation (M-t) and hysteresis loop (M-H) for polycrystalline HgBa2Ca2Cu3Oy. It is found that, as J is increased, the U(J) of M-N decreases monotonically and gradually crosses over from mu = 0.43, which connects smoothly with the ACS one with J approximate to 10(2) A/cm(2), to the one with mu = 1.03, which just overlaps with the M-t one with J approximate to 10(6) A/cm(2). This result demonstrates the consistency of the data of the three methods and is considered to be evidence of a crossover between the CDW type and large bundle creep in the vortex-glass phase or collective-creep theories.

Critical current density for superconductors with giant flux creep

Abstract Critical current density Jc is studied for superconductors with giant flux creep. It is shown that there do exist Jc for such superconductors and the Jc is determined by the effective activation energy U of flux diffusion rather than by the force of flux pinning. A procedure is proposed to determine Jc as a function of field Hd, temperature T and criterion fc if U is known. It is found that such Jc are more dependent upon criterion at high frequency fc than at low fc. The Jc(Hd T) curves of HgBa2Ca2Cu3Oy at two criterions are estimated from the AC susceptibility data. The one with lower fc agrees well with the Jc of the hysteresis measurement with the same criterion, whereas the other Jc with higher fc is significantly higher than the former and may decay quickly. The present study not only shows the importance of the criterion for such Jc but also that Hg-1223 is one of the more promising high-Tc materials from a practical standpoint because of its high Jc and Tc.

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.

Field sweeping rate dependent tail of magnetization curve

Abstract SmBa 1.9 Cu 3 O y samples were fabricated by melt-textured growth method. The tail of magnetization hysteresis loop (magnetization curve) was observed. The effect of the magnetic field sweeping rate on the tail was studied. It is found that the faster the sweeping rate the sharper the tail. The experimental results could not be confirmed by numerical simulation, which was based on the model of flux creep with the surface barrier and lower critical field.