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Concept# Critical field

Summary

For a given temperature, the critical field refers to the maximum magnetic field strength below which a material remains superconducting. Superconductivity is characterized both by perfect conductivity (zero resistance) and by the complete expulsion of magnetic fields (the Meissner effect). Changes in either temperature or magnetic flux density can cause the phase transition between normal and superconducting states. The highest temperature under which the superconducting state is seen is known as the critical temperature. At that temperature even the weakest external magnetic field will destroy the superconducting state, so the strength of the critical field is zero. As temperature decreases, the critical field increases generally to a maximum at absolute zero.
For a type-I superconductor the discontinuity in heat capacity seen at the superconducting transition is generally related to the slope of the critical field (H_\text{c}) at the critical temperature (T_\tex

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Single crystals of MgCNi3, with areas sized up to 1 mm(2), were grown by the self-flux method using a cubic anvil high-pressure technique. In low applied fields, the dc magnetization exhibited a very narrow transition into the superconducting state, demonstrating good quality of the grown crystals. The first critical field H-c1, determined from a zero-temperature extrapolation, is around 18 mT. Using the tunnel-diode resonator technique, the London penetration depth was measured with no applied dc field and the Campbell penetration depth was measured with the external dc fields up to 9 T for two different sample orientations with respect to the direction of applied magnetic field. The absolute value of the London penetration depth, lambda(0) = 245 +/- 10 nm, was determined from the thermodynamic Rutgers formula. The superfluid density, rho(s) = lambda(0)/lambda(T), was found to follow the clean isotropic s-wave behavior predicted by the weak-coupling BCS theory in the whole temperature range. The low-temperature behavior of the London penetration depth fits the BCS analytic form as well and produces a value close to the weak coupling one of Delta(0)/(k(B)T(c)) = 1.71. The temperature dependence of the upper critical field H-c2 was found to be isotropic with a slope at T-c of -2.6 T/K and H-c2(0) approximate to 12.3 T at zero temperature. The Campbell penetration depth probes the vortex lattice response in the mixed state and is sensitive to the details of the pinning potential. For MgCNi3, an irreversible feature has been observed in the TDR response when the sample is field cooled and warmed versus zero-field cooled and warmed. This feature possesses a nonmonotonic field dependence and has commonly been referred to as the peak effect. It is most likely related to a field-dependent nonparabolic pinning potential. DOI: 10.1103/PhysRevB.87.094520

High-Temperature Superconductors (HTS) can be superconducting in liquid nitrogen 77 K, holding immense promises for our future. They can enable disruptive technologies such as nuclear fusion, lossless power transmission, cancer treatment devices, and technologies for future transportation.
In the past years, the numerical models to describe the electrical resistivity of REBCO commercial tapes for devices working near and above the critical current, have been shown to be not accurate or very empirical. The resistivity in this regime, in fact, is not very well known. The lack of this knowledge is a significant issue in developing quality simulation tools. The major challenge in retrieving such properties lies in the fact that when I>Ic, heating effects, and thermal instabilities can quickly destroy the conductor if nothing is done to protect it. Moreover, due to the current sharing between the layers, it is difficult to know the amount of current carried by the superconducting layer and its resistivity.
The present work aims to understand better the overcritical current regime combining ultra-fast pulsed current measurements performed on HTS REBCO based coated conductors with Finite Element Modeling. The experimental activities were carried out mostly at EPFL and in part at PM and KIT. The modeling activities were carried out between EPFL and KIT. The major result is a resistivity relationship describing the overcritical current regime to be used in numerical simulations of REBCO tapes.
The first part of the thesis illustrates a post-processing method based on the so-called Uniform Current (UC) model to estimate the REBCO material's resistivity in the overcritical from experimental measurements. Pulsed current measurements as short as 15 us and with current magnitude up to 5 Ic were performed in liquid nitrogen bath 77 K on samples from various manufacturers, without damaging the tapes.
The second part of the thesis discusses a post-processing method based on regularization of data to treat the experimental measurements extracted in the overcritical current regime. The output of this technique is a look-up table that can be shared with interested partners and used in numerical modeling afterward.
The third part of the thesis presents the overcritical current model (rho-\eta\beta): a mathematical relationship of the overcritical current regime based on measurements performed between 77 K and 90 K and in self-field conditions. We compare such models with the power-law model, and we provide a short discussion of the fitting parameters and their typical values.
The last part of the thesis discusses the overcritical current model, based on experimental measurements obtained as outlined above. The model was validated experimentally and used to show that for the case of a superconducting fault current limiter when the power-law model is used to model its electro-thermal response, the device quenches faster than with the overcritical model.
In conclusion, this work can help optimize the use of superconductors and, consequently, the stabilizer. More interestingly, it opens the study of the overcritical current regime, a new exciting aspect of REBCO commercial tapes. This project has received funding from the Swiss Federal Office of Energy SFOE under grant agreement SI/500193-02.

Yihan Jia, Janusz Andrzej Karpinski, Sergiy Katrych

Iron-based superconductors could be useful for electricity distribution and superconducting magnet applications because of their relatively high critical current densities and upper critical fields. SmFeAsO0.8F0.15 is of particular interest as it has the highest transition temperature among these materials. Here we show that by introducing a low density of correlated nano-scale defects into this material by heavy-ion irradiation, we can increase its critical current density to up to 2 x 10(7) A cm(-2) at 5 K-the highest ever reported for an iron-based superconductor-without reducing its critical temperature of 50 K. We also observe a notable reduction in the thermodynamic superconducting anisotropy, from 8 to 4 upon irradiation. We develop a model based on anisotropic electron scattering that predicts that the superconducting anisotropy can be tailored via correlated defects in semimetallic, fully gapped type II superconductors.