Plasmons in high-temperature superconductors

Plasmon spectra of free carriers in high-Tc cuprates and bismates are investigated via their complex dielectric functions, which are accurately determined by combining reflectance and spectroellipsometric measurements. All five superconductors under study exhibit a single optic plasmon located at 0.8 eV for La1.85Sr0.15CuO4, 1.4 eV for YBa2Cu3O7, 1.0 eV for Bi2Sr2CaCu2O8, 1.2 eV for Tl2Ba2Ca2Cu3O10, and 1.5 eV for Ba0.54K 0.46BiO3. The low-energy part of the dielectric loss function of each system is quadratic with frequency: Im(-1/ε{lunate}) = βω2, nearly up to the screened plasma frequency. We propose that this characteristic quadratic frequency dependence is universally obeyed in all high-Tc superconductors. © 1991.