Observation of an extended charge density wave boundary beyond the LTT phase in optimally doped LBCO

Charge density wave (CDW) correlation in cuprates has been deemed a key ingredient to understand high-Tc superconductivity (SC). Since CDW is intertwined with other phases in a complicated way, determining the phase diagram of CDW is of critical importance. Here we focus on an archetypal La-based cuprate La_2-xBa_xCuO₄ (LBCO) that has shown an interesting CDW behavior. The onset of CDW appears correlated with the low-temperature tetragonal (LTT) phase transition in the underdoped region whereas there is a big discrepancy above x=1/8. Particularly at the optimal doping of x=0.155, CDW was reported to occur at more than 20 K below the LTT transition. This contrasting behavior has not been well understood. Here, we present a resonant soft X-ray scattering (RSXS) study of CDW in optimally-doped LBCO, where we utilized a novel TES spectrometer with unprecedented sensitivity. Surprisingly, we detected short-ranged CDW up to T ~ 115 K, which is much higher than not just the previously reported CDW onset temperature but also the LTT transition temperature. We also observed behavior that suggests a positive correlation between CDW and SC, which are thought to be in general competing with each other. In this presentation, we will further discuss the implications of these new findings.