Evidence for onset of charge density wave in the La-based perovskite superconductors

Recent studies have shown that structural instabilities play a vital role in high-T-c superconductivity. We have used a local structural probe, extended X-ray absorption fine structure (EXAFS), to investigate the instability of the CuO2 plane in La-based high-T-c perovskite systems. Temperature-dependent Cu K-edge EXAFS spectra measured on single crystals of La1.85Sr0.15CuO4 (LSCO) and La2CuO4.1 (LCO) show a characteristic temperature (T-ls) where the local structure od the system diverges from the average one. This temperature coincides with a crossover temperature where the system shows anomalous metal-insulator transition in transport measurements. We have discussed the EXAFS results as an evidence for a particular phase transition of the system with segregation of localized and itinerant charge carriers in a short coherence charge density wave (CDW). A phase diagram for this CDW has been proposed.