using resonant inelastic x-ray scattering*

High-temperature cuprate superconductors consist of two-dimensional CuO₂ planes, with anisotropic superconductivity and critical field [1]. The interlayer correlation is considered significant to promote the critical temperature [2]. In recent years, the electronic collective mode defined as plasmon has been discovered in both electron and hole doped cuprates, which has a three-dimensional dispersion and considered to be important for superconductivity[MOU1]  [3,4,5]. Here we performed an extensive study on La_2-xSr_xCuO₄ by using resonant inelastic x-ray scattering (RIXS) at O K-edge, and observed the plasmon persisting up to a highly overdoped level of x=0.45. We have also modulated the doping level by constructing the interfacial superconducting [La_2-xSr_xCuO₄/La₂CuO₄] superlattice. Our results reveal a smooth evolution of the plasmon from overdoped x~0.45 to underdoped x~0.07 region with increasingly lower energies and incoherence. Our observation of plasmon in the metallic phase of cuprates suggests that plasmon is not an imperative component for superconductivity.

Reference

[1] Shen, Z. X et al. Rev. Mod. Phys. 75, 473 (2003).

[2] Ajay et al. Physica C. 274, 73-80 (1997).

[3] Hepting, M. et al. Nature 563, 374–378 (2018).

[4] Abhishek Nag et al. Phys. Rev. Lett. 125, 257002 (2020).

[5] A. Singh et al. arXiv:2006.13424 (2020).