Acoustic plasmons and cuprate superconductivity

Acoustic Plasmons in Nickel and Its Modification upon Hydrogen Uptake

In this work, we study, in the framework of the ab initio linear-response time-dependent density functional theory, the low-energy collective electronic excitations with characteristic sound-like dispersion, called acoustic plasmons, in bulk ferromagnetic nickel. Since the respective spatial oscillations in slow and fast charge systems involve states with different spins, excitation of such plasmons in nickel should result in the spatial variations in the spin structure as well. We extend our study to NiHx with different hydrogen concentrations x. We vary the hydrogen concentration and trace variations in the acoustic plasmons properties. Finally, at x=1 the acoustic modes disappear in paramagnetic NiH. The explanation of such evolution is based on the changes in the population of different energy bands with hydrogen content variation.

Gapped Collective Charge Excitations and Interlayer Hopping in Cuprate Superconductors

We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor Sr_{0.9}La_{0.1}CuO_{2}. We detect a plasmon gap of ∼120  meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in Sr_{0.9}La_{0.1}CuO_{2} are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered t-J-V model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping t_{z}. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine t_{z}.

Three-dimensional collective charge excitations in electron-doped copper oxide superconductors

High-temperature copper oxide superconductors consist of stacked CuO₂ planes, with electronic band structures and magnetic excitations that are primarily two-dimensional^1,2, but with superconducting coherence that is three-dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which has been found to be incoherent in the normal state^3,4 within the limited range of momenta accessible by optics. Here we use resonant inelastic X-ray scattering to explore the charge dynamics across all three dimensions of the Brillouin zone. Polarization analysis of recently discovered collective excitations (modes) in electron-doped copper oxides^5-7 reveals their charge origin, that is, without mixing with magnetic components^5-7. The excitations disperse along both the in-plane and out-of-plane directions, revealing its three-dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the distance between neighbouring CuO₂ planes rather than to the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction is responsible for the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought 'acoustic plasmon', which is a branch of distinct charge collective modes predicted for layered systems^8-12 and argued to play a substantial part in mediating high-temperature superconductivity^10-12.

Nonlinear optical observation of coherent acoustic Dirac plasmons in thin-film topological insulators

Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi₂Se₃. Using transient second-harmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac states.