Mapping the softening of the breathing phonons to elucidate the structure of charge fluctuations in YBCO

Recent experimental and theoretical studies in cuprate high Tc superconductors (HTS) have linked charge density fluctuations (CDF), precursors of charge density waves, to the strange metal phase. While their existence is now firmly established, there is still much to discover about their spatial structure and their connection with breathing phonons. In particular, a recent paper has attributed the softening of these phonons to “quasi-circular charge correlations” instead of CDF. To address these questions, we will employ RIXS on YBCO films and analyze CDW, CDF and breathing phonons as a function of the azimuthal angle, spanning from the (H,0) to the (0,K) direction. This will enable us to track the spatial distribution of the CDF peak and determine which charge modulation is responsible for the softening of the breathing phonons. Ultimately, our findings will confirm whether CDF indeed drive the unconventional phenomenology of the strange metal phase.