2015/09/30 by Sean Vig, Anshul Kogar, Matteo Mitrano +9 · 1 citation
Physics and Astronomy · #cond-mat.supr-con
paper · pdf · doi:10.21468/scipostphys.3.4.026
published as SciPost Phys. 3, 026 (2017) · 26 pages, 10 sections, 7 figures, and an appendix
arxiv created 2017/09/12 · arxiv updated 2017/10/12
One of the most fundamental properties of an interacting electron system is its frequency- and wave-vector-dependent density response function, χ(\bf q,ω). The imaginary part, χ''(\bf q,ω), defines the fundamental bosonic charge excitations of the system, exhibiting peaks wherever collective modes are present. χ quantifies the electronic compressibility of a material, its response to external fields, its ability to screen charge, and its tendency to form charge density waves. Unfortunately, there has never been a fully momentum-resolved means to measure χ(\bf q,ω) at the meV energy scale relevant to modern elecronic materials. Here, we demonstrate a way to measure χ with quantitative momentum resolution by applying alignment techniques from x-ray and neutron scattering to surface high-resolution electron energy-loss spectroscopy (HR-EELS). This approach, which we refer to here as "M-EELS," allows direct measurement of χ''(\bf q,ω) with meV resolution while controlling the momentum with an accuracy better than a percent of a typical Brillouin zone. We apply this technique to finite-q excitations in the optimally-doped high temperature superconductor, Bi2Sr2CaCu2O8+x (Bi2212), which exhibits several phonons potentially relevant to dispersion anomalies observed in ARPES and STM experiments. Our study defines a path to studying the long-sought collective charge modes in quantum materials at the meV scale and with full momentum control.