Accoustic and optical plasmon excitations in cuprate and ruthenate “strange metals” studied by EELS, RIXS, and optical spectroscopy

Conventional metals show at low temperature a scattering rate which is quadratic in temperature or energy. In "strange metals", the scattering rate is enhanced at low energies leading to a linear dependence due to correlation effects. This is possibly related to strong quantum fluctuations which are also supposed to mediate superconductivity in cuprates and Sr2RuO4. There are theories based on holographic calculations which predict an over-damping of plasmons due to a low-energy continuum. These theories are supported by recent EELS measurements in reflection (R-EELS). The results are at variance with our early EELS experiments in transmission (T-EELS) and RIXS data on various cuprates and more recent T-EELS data on Sr2CuO4. In all cases we see well-defined optical plasmons which decay into particle-hole excitations only for large momentum in the range of the classical Lindhard continuum. The dispersion of the optical plasmon can be well described within the RPA without a mass renormalization. In contrast, the acoustic plasmon dispersion in p-type and n-type cuprates, studied by RIXS can be explained using a mass renormalization of m*=2-4, also detected by ARPES at low energies. These conflicting results can be described by an energy dependent renormalization of the charge carriers, being large at low energy due to a coupling to spin excitations and turning small in the energy range of the optical plasmon well above the spin fluctuation energy. In addition, recent results on the plasmon width are discussed. They strongly support an origin based on interband transitions and not on correlation effects.

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