Plasmon dispersion in bilayer cuprate superconductors

Abstract

The essential building blocks of cuprate superconductors are two-dimensional CuO2 sheets interspersed with charge reservoir layers. In bilayer cuprates, two closely spaced CuO2 sheets are separated by a larger distance from the subsequent pair in the next unit cell. In contrast to single-layer cuprates, prior theoretical work on bilayer systems has predicted two distinct acoustic plasmon bands for a given out-of-plane momentum transfer. Here we report random phase approximation (RPA) calculations for bilayer systems which corroborate the existence of two distinct plasmon bands. We find that the intensity of the lower-energy band is negligibly small in most parts of the Brillouin zone, whereas the higher-energy band carries significant spectral weight. We also present resonant inelastic x-ray scattering (RIXS) experiments at the O K-edge on the bilayer cuprate Y0.85Ca0.15Ba2Cu3O7 (Ca-YBCO), which show only one dispersive plasmon branch, in agreement with the RPA calculations. In addition, the RPA results indicate that the dispersion of the higher-energy plasmon band in Ca-YBCO is not strictly acoustic but exhibits a substantial energy gap of approximately 250 meV at the two-dimensional Brillouin zone center. Published by the American Physical Society 2024

Optical Absorption in Tilted Geometries as an Indirect Measurement of Longitudinal Plasma Waves in Layered Cuprates

“…By using realistic values of plasma frequencies in cuprates, one sees that . As such, this scale is two orders of magnitude smaller than the momenta usually accessible in RIXS [ 25 , 26 , 27 , 28 , 29 ] or EELS [ 30 , 31 , 32 ] experiments, which are not sensitive to the relativistic regime and probe the plasmon dispersion given by Equation ( 20 ).…”

“…In addition, we provide an analytical expression for the peak frequency as a function of both tilting angle and light polarization, which can be used to derive from a single set of measurements the relevant scales for plasma excitations in these systems. It is worth stressing that in recent years, after charged plasmons were detected for the first time with high-resolution RIXS [ 25 , 26 , 27 , 28 , 29 ] and EELS [ 30 , 31 , 32 ] experiments, an intense discussion emerged on the nature of charge fluctuations in these correlated materials [ 31 , 32 ]. The all-optical measurement proposed here is in principle a bulk probe; it is not affected by the lack of sensitivity at small momenta associated with plasmon measurements via charge-detecting probes and allows for precise control over the momentum value, which can be problematic, e.g., with EELS [ 32 ].…”

“…[ 18 , 19 , 20 ], this mechanism leads to a mixing of the longitudinal and transverse responses inside the material, making the distinction between plasmons and polaritons blurred at momenta smaller than a scale set by the anisotropy between in-plane and out-of-plane plasma frequencies. Since usually , the effect is relevant for non-linear Josephson plasmonics in the THz regime [ 21 , 22 , 23 , 24 ], but does not affect, e.g., the measurements of plasmons in RIXS [ 25 , 26 , 27 , 28 , 29 ] or EELS [ 30 , 31 , 32 ], which usually measure momenta in a fraction of the Brillouin zone. In the present manuscript, we investigate an additional consequence of the above-mentioned mixing, showing how even linear optics can be used to disentangle the longitudinal-transverse mixing in a reflection or transmission geometry, which highlights the emergence inside the material of a longitudinal response induced by an external transverse electromagnetic wave.…”