Abstract
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 HgBa$_{2}$CuO$_{4+\delta}$ ($T_c = 70$ K). At $T=250$ K, above the CDW order temperature $T_\mathrm{CDW} \approx 200$ 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 $T = T_c$, 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 $\boldsymbol{q}_\mathrm{CDW}$, 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.
Highlights
The high-Tc cuprates are doped charge-transfer insulators with lamellar structures that feature the quintessential CuO2 plane [1]
At 250 K, above the previously established temperature TCDW ≈ 200 K that signifies the onset of quasistatic short-range CDW order, we observe significant dynamic correlations that are broadly peaked at about 40 meV and centered at the twodimensional wave vector qCDW
We demonstrate that other single-layer cuprates exhibit approximately the same relative strength of high- to low-temperature energy-integrated charge signal, which points to the universal existence of significant dynamic correlations
Summary
The high-Tc cuprates are doped charge-transfer insulators with lamellar structures that feature the quintessential CuO2 plane [1]. At moderate and intermediate hole doping, these complex oxides exhibit a partial depletion of the density of states at the Fermi level (the pseudogap, PG). The mechanism of CDW formation could be related to Fermi-surface nesting (i.e., a reciprocal-space mechanism) [24,25,26] or to strong electronic correlations that lead to charge separation (i.e., a realspace mechanism) [2,27,28]. The short correlation lengths indicate that disorder might play an important role [31,32], and it is still debated how the CDW in the cuprates
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