F.Y. Lin

Effect of electric field relaxation on the V-I curve

The effects of sweeping rate of applied current (dI/dt) on the characteristic (V - I) and electric field relaxation (V - t) curves have been investigated numerically as well as experimentally. The calculation is based on the nonlinear electric field diffusion equation, while the experiment is conducted by electric transport measurements of V - I and V - t curves on three Ag-Bi-2223 tape samples. It is found that the V - I curves shift toward smaller current with increasing dI/dt and V decays with time apparently. We show that a certain dI/dt causes a highly spatially varied electric field which is the natural result of the fact that flux lines can only diffuse over barriers with certain velocity. We also show that the electric field diffusion causes the two phenomena in the bulk sample: (1) the V - I curve is affected by the sweeping rate of the applied current; (2) the resistance relaxes with time.

Temperature dependence of the distribution of the thermally activated energy barriers in Tl2Ba2CaCu2O8 film

The effects of frequency and ac amplitude on ac susceptibility have been measured for a thin Tl2Ba2CaCu2O8 film in the range 100 Hz-100 kHz in magnetic field 0.52 T. A phenomenological equation with an asymmetrical distribution of thermally activated energy barriers has been used to analyse these frequency and amplitude dependences of the ac susceptibility (,hac) in the vicinity of the peak temperature of ´´. We obtain the effective energy barrier U against amplitude hac (current density j): U hac-0.38. This U(j) relationship shows that the flux lines are in the 3D collective creep regime. Therefore, we conclude that the effective energy barrier is in fact an average of the barriers distribution, and the distribution function is a distinguished asymmetrical one in this 3D collective creep regime.

Dynamic response of Ag–Bi2223 tapes in applied fields by electric transport measurement

Abstract Electric transport measurement of Ag-sheathed Bi 2−x Pb x Sr 2 Ca 2 Cu 3 O y tapes in magnetic fields H was conducted by the four-probe method to study the effect of H and sweeping rate of applied current d I/ d t on critical current. It is found that H and d I/ d t apparently affect the critical current and the V–I characteristic curve. To understand the experimental result, numerical simulation was carried out based on the collective creep model of the vortex glass. The numerical result is in agreement qualitatively with the experiment. It is shown that the influence of the applied field, temperature and vortex pinning on the normalized critical current can be reduced to a single critical current parameter n .

Dip effect in χ′ of ac susceptibility in Bi2Sr2CaCu2O8 single crystal

Abstract We present a direct evidence of the softening of the shear modulus constant c 66 by a measurement of the real part χ ′( T ) of ac susceptibility. A dip in χ ′( T ) for a single crystal of Bi 2 Sr 2 CaCu 2 O 8 has been found at an intermediate field region between 50 and 3000 G. We showed that the dip effect was due to the softening of the shear modulus constant c 66 , equivalent to the peak effect in resistivity measurement. The dependence of the peak temperature T p in χ ″ on magnetic field and current density shows the transition between vortex solid and vortex liquid. Thus, the dip in χ ′( T ) is illustrated as a precursor to vortex lattice melting in this narrow field region, indicative of lattice softening prior to melting. In the region between T d and the melting temperature T m , the flux lines move by a type of plastic deformation, and the pinning energy barrier U has been obtained as: U ∝[1−( J / J c ) ν ] H μ with ν , μ >0.

Current distribution and ac susceptibility response of a thin superconducting disc in an axial field: a theoretical approach

Within the framework of the thermally activated process of the flux line or flux line bundles, and by time integration of the 1D equation of motion of the circulating current density ( ,t ), which is suitable for thin superconducting films (R >>d , ), we present numerical calculations of the current profiles, magnetization hysteresis loops and ac susceptibility n = ´n +i ´´n for n = 1, 3 and 5 of a thin disc immersed in an axial time-dependent external magnetic field Ba (t ) = Bdc +Bac cos(2 t ). Our calculated results are compared with those of the critical state model (CSM) and found to prove the approximate validity of the CSM below the irreversibility field. The differences between our computed results and those of the CSM are also discussed.

The effect of the geometric factor on the V-I curve of Tl2212 film

A numerical procedure is proposed for calculating the current density j and electric field E from measurements of the electric transport inside a sample with rectangular cross section by solving the non-linear electric field diffusion equation based upon the collective creep model. It is shown that the sample geometry greatly affects E and j and thus the V-I curve. To verify our numerical prediction, the V-I curve and critical current of Tl2Ba2CaCu2O8+x films with a rectangular cross section in various magnetic fields and temperatures were measured. Comparison shows that the numerical results agree well with the experimental data.

Harmonic response of textured YBa2Cu3O7 superconductor

Abstract Harmonic AC susceptibilities as a function of temperature ( T ), AC field frequency ( f ), amplitude ( B ac ) in DC field ( B dc ) have been studied numerically as well as experimentally. The numerical calculation is based on nonlinear vortex diffusion. The numerical result shows that there exists a second harmonic, although very weak, while a third harmonic is strong. The location of the third harmonic shifts toward low temperature while its height increases with either decreasing f or increasing B ac . These numerical results are in agreement with experimental results observed for textured YBa 2 Cu 3 O 7 and in contrast with prediction of the Bean critical state model or self-organized criticality. It is also shown that the height of the third harmonic peak depends on B ac and f more sensitively than that of the fundamental one.

The effects of field and temperature on V-I curve for Ag-Bi2223 tape

Abstract Electric transport measurements and numerical simulation were made to observe effect of field H on V-I curve and critical current (I c ) for Ag-sheathed Bi 2-x Pb x Sr 2 Ca 2 Cu 3 O y tapes (Ag-Bi2223). It is shown that the effect is mainly dynamic.

Distribution of barriers of vortex matter at different temperatures in single crystal Bi2212

Abstract Complex ac susceptibility, systematically measured in different temperatures, dc fields, ac field amplitudes and frequencies, is analyzed to extract activation barrier U and time scale t 0 of vortex diffusion for single crystal Bi2212. The phase transition line between the vortex solid and the liquid states is determined according to the jump of t 0 as well as the change of U . The analysis, based on the phenomenological equation with an asymmetrical distribution of thermal activated barriers, shows for the first time that U in plastic deformation region is of a statistical distribution dependent on temperature, whereas there is no such a distribution for the vortex liquid state.

Critical current density in the Tl2212 film

Abstract A method of numerical simulation combined with experimental curves of AC susceptibility (ACS) is proposed to determine the temperature dependent critical current density J c0 (T) without flux creep. Based on this method and ACS curves, we have determined the J c0 (T) of the Tl2212 thin film.