Resonant x-ray scattering study of charge-density wave correlations in YBa2Cu2O6+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 Cu L3 edge resonant x-ray scattering (RXS). Evidence of CDW formation is found for 0.45 ⩽ x ⩽ 0.93 (hole-doping levels 0.086 ≲ p ≲ 0.163), but not for samples with x ⩽ 0.44 (p ≲ 0.084) that exhibit incommensurate spin-density-wave order, and in slightly overdoped samples with x = 0.99 (p ∼ 0.19). This suggests the presence of two proximate zero-temperature CDW critical points at Pc1 ∼ 0.08 and pc2 ∼ 0.18. Remarkably, pc2 is close to the doping level that is optimal for superconductivity. The CDW reflections are observed at incommensurate in-plane wave vectors (δa, 0) and (0, δb) with δa < δa. Both δa and δb decrease linearly with increasing doping, in agreement with recent reports on Bi-based high-Tc; superconductors, but in sharp contrast to the behavior of the La2-x (Ba,Sr)xCuO4 family. The CDW intensity and correlation length exhibit maxima at p ∼ 0.12, coincident with a plateau in the superconducting transition temperature Tc. The onset temperature of the CDW reflections depends nonmonotonically on p , with a maximum of ∼160 K for p ∼ 0.12. The RXS reflections exhibit a uniaxial intensity anisotropy. Whereas in strongly underdoped samples the reflections at (δa, 0) are much weaker than those at (0, δb), the anisotropy is minimal for p ∼ 0.12, and reversed close to optimal doping. We further observe a depression of CDW correlations upon cooling below Tc, 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 ∼ 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)xCuO4 , the "pseudogap" phenomenon, superconducting fluctuations, and quantum oscillations, as well as their implications for the mechanism of high-temperature superconductivity. [ABSTRACT FROM AUTHOR]