Direct observation of charge order in underdoped and optimally doped Bi$_{2}$(Sr,La)$_{2}$CuO$_{6+\delta}$ by resonant inelastic x-ray scattering

TL;DR

This study uses resonant inelastic x-ray scattering to directly observe charge order in Bi2201 cuprates, revealing its persistence up to optimal doping and its specific modulation directions, with implications for understanding high-temperature superconductivity.

Contribution

First direct observation of charge density modulation in Bi2201 across a wide doping range using RIXS, clarifying its directional properties and out-of-plane behavior.

Findings

Charge order persists up to optimal doping in Bi2201.

Charge order modulates only along Cu-O bond directions.

No out-of-plane phase correlation observed in single-layer Bi2201.

Abstract

Charge order in underdoped and optimally doped high-$T_\mathrm{c}$ superconductors Bi$_{2}$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ (Bi2201) is investigated by Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS). We have directly observed charge density modulation in the optimally doped Bi2201 at momentum transfer $Q_{\|} \simeq 0.23$ rlu, with smaller intensity and correlation length with respect to the underdoped sample. This demonstrates that short-range charge order in Bi2201 persists up to optimal doping, as in other hole-doped cuprates. We explored the nodal (diagonal) direction and found no charge order peak, confirming that charge order modulates only along the Cu-O bond directions. We measured the out-of-plane dependence of charge order, finding a flat response and no maxima at half integer \emph{L} values. This suggests there is no out-of-plane phase correlation in single layer Bi2201, at variance from YBa$_2$Cu$_3$O$_{6+x}$ and La$_{2-x}$(Ba,Sr)$_x$CuO$_4$. Combining our results with data from the literature we assess that charge order in Bi2201 exists in a large doping range across the phase diagram, i.e. $0.07 \lesssim p \lesssim 0.16$, demonstrating thereby that it is intimately entangled with the antiferromagnetic background, the pseudogap and superconductivity.