Probing superconducting order in overdoped Ca_xY_1−xBa₂Cu₃O₇ by neutron diffraction measurements of the vortex lattice.

Bragg diffracted intensities and q values for crystalline structures with long repeat distances may be obtained by small angle neutron scattering (SANS) investigations. We have developed a method and an adapted Christen formula [1] for use on TOF instruments to obtain the form factor from the integrated intensity detected in vortex lattices in superconductors [2]. We illustrate this with data obtained from high magnetic field measurements on the high-temperature superconductors YBa₂Cu₃O₇ [3] and Ca_0.15Y_0.85Ba₂Cu₃O₇ [4] at HZB (Berlin). As a result of this, we have shown that in YBa₂Cu₃O₇ and Ca_0.15Y_0.85Ba₂Cu₃O₇ [3,4], there are deviations from the London model at the highest fields measured. Also comparing with Ca_0.04Y_0.96Ba₂Cu₃O₇ results [4], here we present that for the Ca_xY_1-xBa₂Cu₃O₇ compounds, the series of vortex lattice structure transitions have shifted down in field relative to those reported for the undoped compound. We attribute this mainly to the weakening of the 1-D superconductivity in the Cu-O chains by the disorder introduced by doping. The high-field structure of the vortex lattice is similar to recent measurements on the parent compound in fields of 25 T, which indicates that the fundamental d-wave nature of the superconducting gap is unchanged by calcium doping.

[1] D. K. Christen et al., Phys. Rev. B, 15, 4506, (1977).

[2] E. Campillo, et al., J. Appl. Cryst. 55, 1314-1323 (2022).

[3] E. Campillo, et al., Phys. Rev. B, 105, 184508 (2022).

[4] A. Cameron & E. Campillo, et al., arXiv:2208.06706.