Spin susceptibility of charge-ordered YBa ₂ Cu ₃ O _<i>y</i> across the upper critical field

Abstract

The value of the upper critical field <i>H</i>_<i>c</i>2, a fundamental characteristic of the superconducting state, has been subject to strong controversy in high-<i>T</i>_<i>c</i> 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 ¹⁷O NMR to measure the spin susceptibility [Formula: see text] of the CuO₂ planes at low temperature in charge-ordered YBa₂Cu₃O _<i>y</i> We find that [Formula: see text] increases (most likely linearly) with magnetic field <i>H</i> and saturates above field values ranging from 20 T to 40 T. This result is consistent with the lowest <i>H</i>_<i>c</i>2 values claimed previously and with the interpretation that the charge density wave (CDW) reduces <i>H</i>_<i>c</i>2 in underdoped YBa₂Cu₃O _<i>y</i> 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.