The cuprate high temperature superconductors develop spontaneous charge density wave (CDW) order below a temperature $T_{CDW}$ and over a wide range of hole doping (p). An outstanding challenge in the field is to understand whether this modulated phase is related to the more exhaustively studied pseudogap and superconducting phases. To address this issue it is important to extract the energy scale $Delta_{CDW}$ associated with the charge modulations, and to compare it with the pseudogap (PG) $Delta_{PG}$ and the superconducting gap $Delta_{SC}$. However, while $T_{CDW}$ is well-characterized from earlier works little has been known about $Delta_{CDW}$ until now. Here, we report the extraction of $Delta_{CDW}$ for several cuprates using electronic Raman spectroscopy. Crucially, we find that, upon approaching the parent Mott state by lowering $p$, $Delta_{CDW}$ increases in a manner similar to the doping dependence of $Delta_{PG}$ and $Delta_{SC}$. This shows that CDW is an unconventional order, and that the above three phases are controlled by the same electronic correlations. In addition, we find that $Delta_{CDW} approx Delta_{SC}$ over a substantial doping range, which is suggestive of an approximate emergent symmetry connecting the charge modulated phase with superconductivity.
تحميل البحث