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 charge correlations are still in their infancy. Such information is crucial in order to help discern the connection between CDW and pseudogap phenomena, and to understand the extent to which charge correlations in general shape the phase diagram. We report a resonant inelastic x-ray scattering study of the underdoped cuprate superconductor (). At 250 K, above the previously established temperature 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 two-dimensional wave vector . This energy scale is comparable to both the superconducting gap and the low-energy pseudogap. At 70 K, we observe a quasistatic CDW peak at , but the dynamic correlations around 40 meV remain virtually unchanged, and we identify a new feature: dynamic correlations well above the optic phonon range that are broadly peaked in the 150–200 meV range. A similar energy scale was previously identified in other experiments as a high-energy pseudogap. The observation of three distinct features in the charge response is highly unusual for a CDW system and suggests that charge order in the cuprates is complex. 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. Intriguingly, the two energy scales identified here are also comparable to those of the superconducting glue function extracted from other spectroscopic techniques, consistent with a dual charge and magnetic nature of the pairing glue. We further determine the paramagnon dispersion along [1, 0], across , and find it to be consistent with magnetic excitations measured by inelastic neutron scattering. Unlike for some other cuprates, our result points to the absence of a discernible coupling between charge and magnetic excitations.
- Received 4 July 2019
- Revised 13 April 2020
- Accepted 29 April 2020
DOI:https://doi.org/10.1103/PhysRevX.10.021059
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Unraveling the connection between superconductivity, patterns of charge organization, and partial energy gaps in the cuprates is a major thrust in quantum materials research. The elusive superconducting pairing glue might be associated with dynamic charge correlations, which are fluctuations of the electronic density, but momentum- and energy-resolved measurements with the required energy resolution have not been possible until recently. Here, we report one of the first resonant inelastic x-ray scattering studies of cuprate charge dynamics with meaningful energy resolution to resolve such correlations.
We study Hg-Ba-Cu-O, arguably the best model system among the hundreds of cuprate compounds because of its high structural symmetry and high optimal superconducting transition temperature of nearly 100 K. In addition to the well-known static charge modulation, we find an unexpected large charge-fluctuation scale, which provides comprehensive insight into the electronic properties of the cuprates by connecting these new findings to a range of prior observations regarding the pseudogap phenomenon and superconducting pairing. Our results are broadly consistent with a recent phenomenological model for the cuprates.