Density fluctuations in strange metals*

Metals exhibit plasmon excitations, which are collective modes one can think of as sound waves in the electron density (as opposed to the atomic density). The so-called "strange metals" are bizarre phases of matter that fail to exhibit well-defined quasiparticles but somehow are still good conductors, leading one to wonder what degree of freedom is actually carrying the charge. A sensible question to ask is, Do strange metals exhibit plasmons? In this talk I will describe momentum-resolved EELS (M-EELS) measurements of several strange metals, notably Bi2Sr2CaCu2O8+x which is also a high-temperature superconductor. I will show that plasmon excitations are barely defined in these materials, which instead exhibit an incoherent continuum of charge fluctuations with no particular length or time scale. These fluctuations obey a simple, power law form, suggesting some kind of scale-invariant phase is present, though the data are not consistent with a (textbook) quantum critical point. I will discuss efforts by theorists to explain this phenomenon using holographic
approaches based on the AdS-CFT correspondence.