Spin susceptibility of charge-ordered YBa 2 Cu 3 O y across the upper critical field

Significance The upper critical field H c 2 is a fundamental, and technologically important, property that measures the ability of a superconductor to withstand magnetic fields. Recently, there has been a controversy regarding the values H c 2 in high- T c copper oxides, particularly in YBa 2 Cu 3 O y when a charge density wave (CDW) is present. Here, taking advantage of nuclear magnetic resonance, a local probe that can measure the intrinsic spin susceptibility χ spin of the superconducting planes, we observe an essentially linear increase in χ spin up to a point in the range of 20–40 T, followed by a constant value. This saturation point is in accord with previous measurements of H c 2 . Our data suggest that the CDW drastically reduces H c 2 in YBa 2 Cu 3 O y .