Spin susceptibility of charge-ordered YBaCuO across the upper critical field.

The value of the upper critical field , a fundamental characteristic of the superconducting state, has been subject to strong controversy in high- 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 NMR to measure the spin susceptibility [Formula: see text] of the CuO planes at low temperature in charge-ordered YBaCuO We find that [Formula: see text] increases (most likely linearly) with magnetic field and saturates above field values ranging from 20 T to 40 T. This result is consistent with the lowest values claimed previously and with the interpretation that the charge density wave (CDW) reduces in underdoped YBaCuO Furthermore, the absence of marked deviation in [Formula: see text] at the onset of long-range CDW order indicates that this [Formula: see text] 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 [Formula: see text], the relatively low values of [Formula: see text] at [Formula: see text] K show that the pseudogap is a ground-state property, independent of the superconducting gap.