Nonuniversal temperature dependencies of the low-frequency ac magnetic susceptibility in high-Tcsuperconductors

Abstract

The complex ac magnetic susceptibilities (χ n = χ' n + iχ n ") of high T c superconductors in absence of dc fields have been studied by numerically solving the non-linear diffusion equation for the magnetic flux, where the diffusivity is determined by the resistivity. In our approach the parallel resistor model between the creep and flux flow resistivities is used, so that the crossover between different flux dynamic processes (thermally activated flux flow, flux creep, flux flow) can naturally arise. For this reason we remark that, as the frequency increases, the presence of a different non linearity in different regions of the I-V characteristic determines nonuniversal temperature dependencies of the χ n , i.e. the χ n are found to be not universal functions of a frequency and temperature dependent single parameter. Moreover, the actual frequency dependent behavior is also shown to be strictly related to the particular pinning model chosen for the simulations. Indeed, for large values of the reduced pinning potential (U/KT≥220) and for increasing frequency, a transition has been observed between dynamic regimes dominated by creep and flux flow processes. On the other hand, for smaller reduced pinning potentials, a transition from the thermally activated flux flow (Taff) to the flow regime occurs. In qualitative agreement with available experimental data but in contrast with previously used simpler models, the amplitude of the peak of the imaginary part of the first harmonic is shown to be frequency dependent. Moreover the frequency dependence of its peak temperature shows large discrepancies with approximated analytical predictions. Finally, the shape of the temperature dependencies of the higher harmonics are found to be strongly affected by the frequency.

Evidence for Irradiation Triggered Nonuniform Defects Distribution in Multiharmonic Magnetic Susceptibility of Neutron Irradiated YBa2Cu3O7− δ

“…The effect of the irradiation is to wipe out this structure both in χ 2 ' and χ 2 ''. The large dip at 85 K, which is predicted by theoretical models [17][18][19], and the smaller one at 90 K, which we attribute to the intragrain contribution are not more visible in 7 V. Sandu Page 7 Evidence for Irradiation Triggered Nonuniform Defect Distribution in Multiharmonic χ 2 ' at the highest fluence, whereas the tiny, positive peak is enhanced and seems to become the tail of a large peak shifted at much lower temperatures ( Fig. 2. b).…”

Effect of spacer layer on flux-pinning properties of iron-based superconductors

“…2a shows the temperature dependence of the third harmonic |χ 3 | at H dc = 0 T for both NdFeAsO 0.86 F 0.14 and FeSe 0.88 samples. The former shows just an intense and single sharp peak, pointing out to an efficient pinning inside the materials [40], while the latter is characterized by a broad and asymmetric distribution related to a different flux pinning mechanism. This broad peak suggests the presence of only an intra-grain phase.…”

“…In fact, the high harmonic components ac detect only the non-linear components in the signal, separate and amplify the signals due to different superconducting phases present in the sample [41][42][43][44][48][49][50][51], even when there are small amount of superconducting phases or evanescent superconducting phases are present [52,53]. The latter are connected to changes of the effective flux-diffusivity tuned by the flux-pinning interaction [39,40].…”

“…Multi-harmonic ac-susceptibility is a powerful tool to probe flux-pinning properties [39]. In fact, the high harmonic coefficients, χn, are very sensitive to the non-linear flux-pinning interaction in the superconducting phases [40][41][42][43][44].…”

A study of coupling loss on bi-columnar BSCCO/Ag tapes through ac susceptibility measurements

“…In this region, as the frequency rises, the maximum value (χ ′′ max ) increases and shifts at higher field amplitudes. Such increasing is due to a larger contribution of resistive effects with respect to the critical state 21 . In order to compare the experimental curves with the analytical results for the a.c. susceptibility, the data reported in bility as calculated in the framework of the critical state model, respectively for a slab 22,23 , a thin strip 24 and a thin disk 25 .…”