Spin susceptibility of charge-ordered YBa2Cu3Oy across the upper critical field

The value of the upper critical field H-c2, a fundamental characteristic of the superconducting state, has been subject to strong controversy in high-T-c copper oxides. Since the issue has been tackled almost exclusively by macroscopic techniques so far, there is a clear need for local-probe measurements. Here, we use O-17 NMR to measure the spin susceptibility chi(spin) of the CuO2 planes at low temperature in charge-ordered YBa2Cu3Oy. We find that chi(spin) increases (most likely linearly) with magnetic field H and saturates above field values ranging from 20 T to 40 T. This result is consistent with the lowest H-c2 values claimed previously and with the interpretation that the charge density wave (CDW) reduces H-c2 in underdoped YBa2Cu3Oy. Furthermore, the absence of marked deviation in chi(spin)(H) at the onset of long-range CDW order indicates that this H-c2 reduction and the Fermi-surface reconstruction are primarily rooted in the short-range CDW order already present in zero field, not in the field-induced long-range CDW order. Above H-c2, the relatively low values of chi(spin) at T = 2 K show that the pseudogap is a ground-state property, independent of the superconducting gap.