Resistivity Measurements of the Strain, Temperature and Angular Dependence of the Upper Critical Field of REBCO Tapes up to 8 T

The design and operation of a miniature strain-board for measuring the resistivity of high temperature superconducting tapes as a function of uniaxial strain (
$\epsilon$
) are described. It was used inside a Physical Property Measurement System (PPMS) that provides access to high fields, variable temperature, and a rotator that can vary the angle of the sample with respect to the field. Resistivity measurements made using the strain-board over the strain range 0% to
$-1.1$
% in-field are presented for a REBCO (REBa
$_{2}$
Cu
$_{3}$
O
$_{7}$
, RE: Rare-earth element) tape from SuperPower with artificial pinning centres (APC). Values of the upper critical field (
$B_{c2}$
) were extracted from the resistivity data. We found that although the temperature and angular dependence of
B
$_{c2}$
were similar to that found in previous studies using ac susceptibility measurements (i.e. the temperature index,
$n = 1.2$
, the anisotropy constant,
$\gamma = 1.4$
and the interlayer spacing
$s = 12$
Å), the upper critical field we found was almost strain independent and therefore quite different. We explain these results by considering the bimodal behaviour of REBCO under strain that is found in single crystal data, and the large parallel shunt the strain-board provided in the measurements. In our resistivity measurements,
$B_{c2}$
is determined by a small percolative supercurrent that preferentially flows through material with the highest critical parameters. Conversely, resistivity measurements at high currents, or ac susceptibility measurements, characterise the material with average or low critical parameters from the broad distribution of critical parameters produced in REBCO tapes under strain.