Abstract

Extensive critical current density measurements are reported as a function of magnetic field , temperature , angle between the applied field and the surface of the tape, and strain , on a REBCO coated conductor. The strain, , at which is maximised, is a function of , , and , which is consistent with weakly emergent behaviour. It is described by the chain model that considers competition between twinned domains with different crystallographic orientations and opposing responses under an applied uniaxial strain.Detailed effective upper critical field data are presented that show universal temperature and strain scaling. They lead to an accurate flux pinning relation for the volume pinning force, , where and and are constants, and are used to help parameterise the scaling behaviour of the angular data more accurately in those cases where cannot be measured directly. We derive approximate analytic in-field expressions that explain how the fraction, , of a-domains amongst the a- and b-domains affects the strain dependence of the critical parameters and conclude that in our tape, = 0.4, and the strain at which is the same in both domains is . We report a sharp peak in as the applied field approaches alignment with the ab-plane and the unusual result that with it, a suppression of the index of transition N also occurs. We find that the effective upper critical field increases as the field angle approaches the ab-plane significantly faster than any available theoretical model for the upper critical field .In addition, we conclude that a weak-emergence description is not limited to high temperature superconductors, but also describes some low-temperature superconductors.

Highlights

  • Flux pinning scaling laws, used to describe low temperature superconductors (LTS) materials, assumed the superconducting material was broadly homogenous so all the grains respond the same manner and the critical parameters, T∗c, B∗c2 and Jc have a similar response to uniaxial strain

  • Features that cannot be described by standard flux pinning scaling with a homogeneous strain response were observed for REBCO: the parabolic behaviour of Jc ; the field, temperature and f dependence of the strain at which Jc is a maximum and the double valued behaviour of Fp,max as a function of B∗c2 when the domain fraction is unequal

  • We have outlined the bimodal chain model to account for the parabolic behaviour of the Jc in REBCO and extended its use to consider the angular dependence of Jc

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Summary

Introduction

We describe in detail the consequences of the chain model that provides a straightforward explanation of how the inverted parabolic strain behaviour of the critical current in twinned REBCO tapes arises from this bimodal behaviour and shows that the model describes the angular Jc data presented here [50]. This chain model provides quite a different explanation to the standard description for LTS where the peak in the strain dependence of the critical current is attributed to a peak in the intrinsic strain dependence of the critical temperature (Tc) and Bc2 (T, εapp) and all the grains show the same dependence (i.e. homogeneous behaviour).

Experimental equipment and procedure
Transport measurements
Scaling with respect to magnetic field and temperature
Scaling with respect to strain
Variable angle Transport Measurements
The bimodal chain model for REBCO
Bimodal chain model for twinned domains in high magnetic fields
Numerical results for the bimodal chain model
The chain model for internal tin Nb3Sn and Jelly-Roll Nb3Al
REBCO data
Discussion
Findings
LTS data
10. Conclusions
Full Text
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