Probing superconducting order in overdoped Cax Y1-xBa2Cu3 O7 by neutron diffraction measurements of the vortex lattice

We present small-Angle neutron scattering studies of the magnetic vortex lattice (VL) in Ca0.04Y0.96Ba2Cu3O7 up to a field of 16.7 T and Ca0.15Y0.85Ba2Cu3O7 up to 25 T to investigate the general behavior of the superconducting gap in YBCO-based compounds at high magnetic field. We find in these overdoped compounds that the series of VL structure transitions have shifted down in field relative to those reported for the undoped compound. The hole doping by calcium is expected to alter the Fermi velocity and it reduces the upper critical field of the system. However, we attribute the VL changes mainly to the weakening of the 1D superconductivity in the Cu-O chains by the disorder introduced by doping. The high-field structure of the VL is similar to recent measurements on the parent compound in even higher fields of 25 T, which indicates that the fundamental d-wave nature of the superconducting gap is unchanged by calcium doping. This is corroborated by the temperature dependence of the VL form factor, which also shows the same d-wave behavior as observed in other cuprates. We also argue that they might be the explanation of Pauli paramagnetic effects in the field dependence of the VL form factor.