NSRRC Activity Report 2023

Physics and Materials Science 021 C uprates are a class of high-temperature superconductors that exhibit complex phase diagrams with competing electronic orders, such as antiferromagnetism, charge order, and superconductivity. At extremely high doping levels, the conventional understanding is that electronic interactions in cuprates become weak, and the system can be described using Fermi liquid theory. In this regime, the behavior of electrons near the Fermi surface can be described well with quasi-particles that obey Fermi–Dirac statistics, and the system exhibits metallic behavior such as electrical conductivity and a well-defined Fermi surface. Nevertheless, Yingying Peng (Peking University, China) and her collaborators studied the existence of charge order correlation in heavily overdoped La 2-x Sr x CuO 4 thin films beyond the superconducting dome. This charge ordering behavior exhibits a periodicity of approximately six lattice units and correlation length of approximately 20 lattice units. This charge ordering shows similar in-plane momentum, polarization dependence, and dispersive excitations as seen for underdoped cuprates, however, the maximum intensity differs along the c direction and persists up to 300 K. This discovery challenges the conventional understanding of extremely overdoped cuprates as Fermi liquid metals without exotic electronic orders. The research team suggests that charge order prevails in the overdoped metallic regime and requires a reassessment of understanding overdoped cuprates as weakly correlated Fermi liquids. The team investigated the charge order correlation in heavily overdoped La 2 − x Sr x CuO 4 thin films beyond the superconducting dome utilizing resonant inelastic X-ray scattering (RIXS) at TPS 41A . Figure 1 provides a comprehensive overview of the experimental observations and measurements obtained through RIXS, supporting the team’s argument regarding prevailing Prevailing Charge Order Found Beyond Superconducting Dome The discovery of charge order correlation in heavily overdoped cuprate thin films prompts a reassessment of overdoped cuprates as weakly correlated Fermi liquids and suggests unexplored physics at high doping levels. Fig. 1 : Observation of charge order correlation by RIXS in overdoped metallic La 2−x Sr x CuO 4 (x = 0.45). The UHB and LHB refer to upper Hubbard band and lower Hubbard band, respectively. (a) Schematic plot of the RIXS process at O K edge. (b) A typical RIXS spectrum at q // = (0.12, 0), displaying the elastic peak, phonon, charge, and orbital excitations. Inset: Resistance curve displaying the metallic nature of the sample. (c) Integrated intensity of elastic peaks for positive and negative ( H , 0 ) and ( H, H ) directions, using σ polarization. Red and blue curves are Lorentzian peak fits to the data with a polynomial background (gray dashed lines). (d) Polarization measurements with σ- and π-polarized light, collected at 33 and 250 K. (e) Reciprocal-space image. The blue shaded region is the accessible momentum-transfer range of O K edge. The green and blue lines indicate the momentum cuts, and red dots indicate the observed CO peaks. (f) XAS spectra near the Zhang-Rice singlet (ZRS) absorption peak with σ polarization at normal incidence. Incident energy dependence of the integrated intensity of elastic peak and orbital excitation, normalized to the value at the ZRS peak. [Reproduced from Ref. 1]