The charge-density-wave (CDW) instability in the underdoped, pseudogap part of the cuprate phase diagram has been a major recent research focus, yet measurements of dynamic, energy-resolved CDW correlations are still in their infancy. We report a high-resolution resonant inelastic X-ray scattering (RIXS) study of the underdoped cuprate superconductor HgBa2CuO4+δ (Tc=70 K). At T=250 K, above the CDW order temperature TCDW≈200 K, we observe significant dynamic CDW correlations at about 40 meV. This energy scale is comparable to both the superconducting gap and the previously reported low-energy pseudogap. At T=Tc, a strong elastic CDW peak appears, but the dynamic correlations around 40 meV remain virtually unchanged. In addition, we observe a new feature: dynamic correlations at significantly higher energy, with a characteristic scale of about 160 meV. A similar scale was previously identified in other experiments as a high-energy pseudogap. The existence of three distinct features in the charge response is highly unusual for a CDW system, and suggests that charge order in the cuprates is closely related to the pseudogap phenomenon and more complex than previously thought. We further observe the paramagnon dispersion along [1,0], across the two-dimensional CDW wavevector qCDW, which is consistent with magnetic excitations measured by inelastic neutron scattering. Unlike for some other cuprates, our results point to the absence of a discernible coupling between CDW and magnetic excitations.
@article{arxiv.1907.10047,
title = {Unusual dynamic charge-density-wave correlations in HgBa$_2$CuO$_{4+\delta}$},
author = {B. Yu and W. Tabis and I. Bialo and F. Yakhou and N. Brookes and Z. Anderson and Y. Tang and G. Yu and M. Greven},
journal= {arXiv preprint arXiv:1907.10047},
year = {2020}
}