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Register a new userFlux pinning mechanism in NdFeAsO0.82F0.18 superconductor: Thermally activated flux flow and charge carrier mean free path fluctuation pinning
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The flux pinning mechanism of NdO0.82F0.18FeAs superconductor made under high pressure, with a critical temperature, Tc, of 51 K, has been investigated in detail in this work. The field dependence of the magnetization and the temperature dependence of the magnetoresistivity were measured in fields up to 13 T. The field dependence of the critical current density, Jc(B), was analyzed within the collective pinning model. A crossover field, Bsb, from the single vortex to the small vortex bundle pinning regime was observed. The temperature dependence of Bsb(T) is in good agreement with the delta-l pinning mechanism, i.e., pinning associated with fluctuations in the charge-carrier mean free path, l. Analysis of resistive transition broadening revealed that thermally activated flux flow is found to be responsible for the resistivity contribution in the vicinity of Tc. The activation energy U0/kB is 2000 K in low fields and scales as B (-1/3) over a wide field range. Our results indicate that the NdO0.82F0.18FeAs has stronger intrinsic pinning than Bi-2212 and also stronger than MgB2 for H > 8 T.
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We study the temperature dependence of the resistivity as a function of magnetic field in superconducting transition (Tconset - TcR=0) region for different Bi2Sr2CaCu2O8+{delta} superconducting samples being synthesized using sol-gel method. The superconducting transition temperature (TcR=0) of the studied samples is increased from 32 K to 82K by simply increasing the final sintering temperature with an improved grains morphology. On the other hand, broadening of transition is increased substantially with decrease in sintering temperature; this is because Tconset is not affected much with grains morphology. Further broadening of the superconducting transition is seen under magnetic field, which is being explained on the basis of thermally activated flux flow (TAFF) below superconducting transition temperature (Tc). TAFF activation energy (U0) is calculated using the resistive broadening of samples in the presence of magnetic field. Temperature dependence of TAFF activation energy revealed linear temperature dependence for all the samples. Further, magnetic field dependence is found to obey power law for all the samples and the negative exponent is increased with increase in sintering temperature or the improved grains morphology for different Bi-2212 samples. We believe that the sintering temperature and the ensuing role of grain morphology is yet a key issue to be addressed in case of cuprate superconductors.
m-H loops for virgin and neutron irradiated bulk and powder samples of MgB_{2} were measured in the temperature range 5-30 K in magnetic field B<= 1 T. The irradiation at thermal neutron fluences 9*10^{13} and 4.5*10^{14} cm^{-2} caused very small enhancement of m-H loops at lower temperatures (T<20 K), whereas the effect at high temperatures was unclear due to difficulty in achieving exactly the same measurement temperature prior and after irradiation. However, the irradiation at 4.5*10^{15} cm^{-2} produced clear enhancement of m-H loops (hence J_{c}) at all investigated temperatures, which provides the evidence for the enhancement of flux pinning in MgB_{2} due to ion tracks resulting from n+^{10}B reaction. The potential of this technique for the enhancement of flux pinning in high temperature superconductors is briefly discussed.
Demanding microwave applications in a magnetic field require the material optimization not only in zero-field but, more important, in the in-field flux motion dominated regime. However, the effect of artificial pinning centers (APC) remains unclear at high frequency. Moreover, in coated conductors the evaluation of the high frequency material properties is difficult due to the complicated electromagnetic problem of a thin superconducting film on a buffered metal substrate. In this paper we present an experimental study at 48 GHz of 150-200 nm YBa$_2$Cu$_3$O$_{7-x}$ coated conductors, with and without APCs, on buffered Ni-5at%W tapes. By properly addressing the electromagnetic problem of the extraction of the superconductor parameters from the measured overall surface impedance $Z$, we are able to extract and to comment on the London penetration depth, the flux flow resistivity and the pinning constant, highlighting the effect of artificial pinning centers in these samples.
We have studied flux-pinning effects of MgB$_2$ superconductor by doping (Fe, Ti) particles of which radius is 163 nm on average. 5 wt.% (Fe, Ti) doped MgB$_2$ among the specimens showed the best field dependence of magnetization and 25 wt.% one did the worst at 5 K . The difference of field dependence of magnetization of the two increased as temperature increased. Here we show experimental results of (Fe, Ti) particle-doped MgB$_2$ according to dopant level and the causes of the behaviors. Flux-pinning effect of volume defects-doped superconductor was modeled in ideal state. During the study, we had to divide M-H curve of volume defect-dominating superconductor as three discreet regions for analyzing flux pinning effects, which are diamagnetic increase region, $Delta$H=$Delta$B region, and diamagnetic decrease region. As a result, flux-pinning effects of volume defects decreased as dopant level increased over the optimal dopant level, which was caused by decrease of flux-pinning limit of a volume defect. And similar behaviors are obtained as dopant level decreased below the optimal dopant level, which was caused by the decreased number of volume defects. Comparing the theory with experimental results, deviations increased as dopant level increased over the optimal dopant level, whereas the two was well matched on less dopant level than the optimal dopant level. The behavior is considered to be caused by segregation of volume defects. On the other hand, the property of over-doped specimens dramatically decrease as temperature increases, which is caused by double decreases of flux-pinning limit of a volume defect and segregation effect.
The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. Investigated are the cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by focused ion beam. The generic feature of the current-voltage curves of the films consists in instability jumps to the normal state at some instability current density $j^ast$ as the vortex lattice reaches its critical velocity $v^ast$. While $v^ast(B)$ monotonically decreases for as-grown films, the irradiated films exhibit a non-monotonic dependence $v^ast(B)$ attaining a maximum in the low-field range. In the case of nanopatterned films, this broad maximum is accompanied by a much sharper maximum in both, $v^ast(B)$ and $j^ast(B)$, which we attribute to the commensurability effect when the spacing between the vortex rows coincides with the location of the grooves. We argue that the observed behavior of $v^ast(B)$ can be explained by the pinning effect on the vortex flow instability and support our claims by fitting the experimental data to theoretical expressions derived within a model accounting for the field dependence of the depinning current density.
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