High-temperature Superconductivity Enables Rich Collective Charge Behavior, Governed by Coulomb Interaction

High-temperature superconductivity, a phenomenon with the potential to revolutionise energy transmission and technology, remains a significant challenge for physicists, particularly in understanding how charge behaves within these materials. Hiroyuki Yamase from the National Institute for Materials Science in Japan, along with colleagues, investigates the complex dynamics of charge carriers in cuprate superconductors, revealing a surprisingly rich collective behaviour. Their work establishes the existence of low-energy acoustic plasmons, consistently observed in both hole and electron-doped materials, and clarifies how charge ordering develops, particularly in electron-doped cuprates. By proposing a unifying scenario, the team addresses discrepancies in current theoretical models and offers a crucial step towards fully understanding and ultimately harnessing the power of these remarkable materials.

Scientists have achieved a detailed understanding of charge dynamics within high-temperature cuprate superconductors, revealing remarkably rich collective behavior governed by strong electronic correlations, layered crystal structure, and long-range Coulomb interactions. The work establishes the emergence of acousticlike plasmons, confirmed through quantitative analyses of resonant inelastic x-ray scattering (RIXS) spectra using an advanced theoretical model, the t-J-V model. These plasmons appear near zero in-plane momentum and exhibit energies significantly below the typical 1 electron volt optical plasmon, a characteristic observed consistently across both hole- and electron-doped cuprates.

Cuprates, Charge Density Waves, and Superconductivity

Research into cuprate superconductors reveals a complex interplay between superconductivity, charge density waves (CDWs), and collective electronic excitations called plasmons. Many cuprates exhibit CDWs, often arranged in stripes, and scientists are investigating whether these CDWs contribute to, compete with, or simply coexist with superconductivity. Theoretical models, including the Hubbard and t-J models, are used to understand these observations and the role of electron interactions. Studies demonstrate that CDWs are common in cuprates, particularly those with electron doping, and their behavior varies significantly with doping level.

The origin of these CDWs is still debated, with proposed mechanisms including spin fluctuations, instabilities in the electronic structure, and charge transfer between layers. The relationship between CDWs and the pseudogap phase, a state preceding superconductivity, is also a key area of investigation. Plasmons, considered potential mediators of electron pairing, couple to other excitations like magnons and phonons, influencing the electronic properties of cuprates. The plasmon spectrum itself changes with doping, and researchers are exploring the significance of out-of-phase plasmon excitations related to the layered structure.

Emerging themes include the role of nematic order, bond-charge orders, and the unique behavior of these materials as marginal Fermi liquids. Future research directions include detailed mapping of CDW phases, investigation of plasmon contributions to pairing, identification of the microscopic origins of CDWs, and exploration of the effects of disorder. Advanced experimental techniques like angle-resolved photoemission spectroscopy (ARPES) and RIXS, alongside sophisticated theoretical modeling, will be crucial for unraveling the mysteries of these materials.

Low-Energy Plasmons in Cuprate Superconductors

Experiments reveal that the energy of these plasmons at zero momentum is approximately 0.7 electron volts, with a nearly flat dispersion, a result of doping effects. Measurements demonstrate that the plasmon dispersion changes dramatically with momentum, softening near zero and exhibiting a V-shaped pattern, highlighting the three-dimensional character of the excitation within the layered structure. Researchers also identified three types of bond-charge excitations: d-wave, s-wave, and d-wave charge-density-wave, accurately characterized by projecting data onto eigenvectors to eliminate unwanted fluctuations. Results demonstrate distinct spectral weight maps for each type of bond-charge excitation, measured at a specific momentum, and reveal positive spectral weight for all three modes. These findings, obtained using parameters appropriate for electron-doped cuprates, facilitate direct comparison with RIXS experiments and provide a crucial step towards understanding the complex interplay of charge dynamics in these materials.

Low-Energy Plasmons and Charge Order Coexistence

Theoretical work has significantly advanced understanding of charge dynamics within high-temperature cuprate superconductors, revealing acousticlike plasmons with energies lower than previously observed optical plasmons. These plasmons occur consistently in both hole- and electron-doped cuprates. Investigations into electron-doped cuprates uncovered a strong inclination towards bond-charge order, resulting in a dual structure where low-energy charge excitations coexist alongside higher-energy plasmons. Similar tendencies toward charge order are also detected in hole-doped cuprates, though current theoretical frameworks struggle to fully explain these observations or the persistent puzzle of charge-stripe order in lanthanum-based cuprates. Researchers propose a unifying scenario suggesting a largely universal behavior of mobile carriers across different doping types, acknowledging limitations in fully capturing the complexities of charge ordering and the need for further investigation of frequency-dependent interactions. Future research may focus on refining theoretical models to better account for these nuances and ultimately provide a more complete description of charge dynamics in these fascinating superconducting materials.