Resonant x-ray scattering study of charge-density wave correlations in YBa2Cu3O6+x

We report the results of a comprehensive study of charge-density wave (CDW) correlations in untwinned YBa2Cu3O6+x single crystals with 0.4 <= x <= 0.99 using CuL3 edge resonant x-ray scattering (RXS). Evidence of CDW formation is found for 0.45 <= x <= 0.93 (hole-doping levels 0.086 less than or similar to p less than or similar to 0.163), but not for samples with x <= 0.44 (p less than or similar to 0.084) that exhibit incommensurate spin-density-wave order, and in slightly overdoped samples with x = 0.99 (p similar to 0.19). This suggests the presence of two proximate zero-temperature CDW critical points at p(c1) similar to 0.08 and p(c2) similar to 0.18. Remarkably, pc(2) is close to the doping level that is optimal for superconductivity. The CDW reflections are observed at incommensurate in-plane wave vectors (delta(a), 0) and (0, delta(b)) with delta(a) less than or similar to delta(b). Both delta(a) and delta(b) decrease linearly with increasing doping, in agreement with recent reports on Bi-based high-T-c superconductors, but in sharp contrast to the behavior of the La2-x(Ba,Sr)(x)CuO4 family. The CDW intensity and correlation length exhibit maxima at p similar to 0.12, coincident with a plateau in the superconducting transition temperature T-c. The onset temperature of the CDW reflections depends nonmonotonically on p, with a maximum of similar to 160 K for p similar to 0.12. The RXS reflections exhibit a uniaxial intensity anisotropy. Whereas in strongly underdoped samples the reflections at (delta(a), 0) are much weaker than those at (0, delta(b)), the anisotropy is minimal for p similar to 0.12, and reversed close to optimal doping. We further observe a depression of CDW correlations upon cooling below T-c, and (for samples with p >= 0.09) an enhancement of the signal when an external magnetic field up to 6 T is applied in the superconducting state. For samples with p similar to 0.08, where prior work has revealed a field-enhancement of incommensurate magnetic order, the RXS signal is field independent. This supports a previously suggested scenario in which incommensurate charge and spin orders compete against each other, in addition to individually competing against superconductivity [Blanco-Canosa et al., Phys. Rev. Lett. 110, 187001 (2013)]. We discuss the relationship of these results to prior observations of stripe order in La2-x(Ba,Sr)(x)CuO4, the pseudogap phenomenon, superconducting fluctuations, and quantum oscillations, as well as their implications for the mechanism of high-temperature superconductivity.