How charge order weakens superconductivity in YBCO

The upper critical field, Hc2, 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 Hc2 values in high-Tc copper-oxides, particularly in the context of the competition between superconductivity and charge density wave (CDW) order in YBa2Cu3Oy. We have addressed this question by using for the first time a local probe, 17O NMR, to measure the spin susceptibility spin of the CuO2 planes at low temperature in charge-ordered YBa2Cu3Oy. The central result of this study is the observation of an essentially linear increase in spin up to a magnetic field in the range of 20 to 40 T (depending on the hole-doping level), followed by a constant value. These saturation fields agree remarkably well with Hc2 values determined in the work of [G. Grissonnanche et al., Nat. Commun. 5, 3280 (2014) and EMFL News n°3 (2014)], showing a minimum around the hole doping p = 0.12, where the CDW is strongest (Figure). This result is consistent with the interpretation that the CDW reduces Hc2 in underdoped YBa2Cu3Oy. Our data also yields quantitative information on the electronic density of states that should be benchmarked against theories and other measurements such as specific heat. We find that the “residual” spin at Hc2 in the zero-temperature limit is a small fraction (~16%) of spin of near-optimally doped YBCO. This shows that the non superconducting ground state has a large pseudogap, distinct from the superconducting gap. Second, the absence of a visible impact of the field-induced CDW transition on spin suggests that the observed modifications of Hc2 and of the Fermi-surface topology do not occur abruptly but rather gradually as a function of field. They must essentially be rooted in the presence of short-range (but static) CDW correlations already in zero field.