To explore the doping dependence of the recently discovered charge-density-wave (CDW) order in YBa2Cu3Oy, we present a bulk-sensitive high-energy x-ray study for several oxygen concentrations, including strongly underdoped YBa2Cu3O6.44.
Combined with previous data around the so-called 1/8 doping, we show
that bulk CDW order exists at least for hole concentrations (p) in the CuO2 planes of 0.078≲p≲0.132.
This implies that CDW order exists in close vicinity to the quantum
critical point for spin-density-wave (SDW) order. In contrast to the
pseudogap temperature T∗, the onset temperature of CDW order decreases with underdoping to TCDW∼90 K in YBa2Cu3O6.44.
Together with a weakened order parameter this suggests a competition
between CDW and SDW orders. In addition, the CDW order in YBa2Cu3O6.44 shows the same type of competition with superconductivity as a function of temperature and magnetic field as samples closer to p=1/8. At low p
the CDW incommensurability continues the previously reported linear
increasing trend with underdoping. In the entire doping range the
in-plane correlation length of the CDW order in b
axis direction depends only very weakly on the hole concentration, and
appears independent of the type and correlation length of the
oxygen-chain order. The onset temperature of the CDW order is remarkably
close to a temperature T† that marks the maximum of 1/(T1T) in planar Cu63
NQR/NMR experiments, potentially indicating a response of the spin
dynamics to the formation of the CDW. Our discussion of these findings
includes a detailed comparison to the charge stripe order in La2−xBaxCuO4.