La139 and Cu63 NMR investigation of charge order in La2CuO4+y (Tc=42 K)

We report La139 and Cu63 NMR investigation of the successive charge order, spin order, and superconducting transitions in superoxygenated La2CuO4+y single crystal with stage-4 excess oxygen order at Tstage≃290 K. We show that the stage-4 order induces tilting of CuO6 octahedra below Tstage, which in turn causes La139 NMR line broadening. The structural distortion continues to develop far below Tstage, and completes at Tcharge≃60 K, where charge order sets in. This sequence is reminiscent of the the charge-order transition in Nd codoped La1.88Sr0.12CuO4 that sets in once the low-temperature tetragonal phase is established. We also show that the paramagnetic Cu63 NMR signals are progressively wiped out below Tcharge due to enhanced low-frequency spin fluctuations in charge-ordered domains, but the residual Cu63 NMR signals continue to exhibit the characteristics expected for optimally doped superconducting CuO2 planes. This indicates that charge order in La2CuO4+y does not take place uniformly in space. In addition, unlike the typical second-order magnetic phase transitions, low-frequency Cu spin fluctuations as probed by La139 nuclear spin-lattice relaxation rate do not exhibit critical divergence at Tspin(≃Tc) =42 K. These findings, including the spatially inhomogeneous nature of the charge-ordered state, are qualitatively similar to the case of La1.885Sr0.115CuO4 [Imai , Phys. Rev. B 96, 224508 (2017)2469-995010.1103/PhysRevB.96.224508 and Arsenault , Phys. Rev. B 97, 064511 (2018)10.1103/PhysRevB.97.064511], but both charge and spin order take place more sharply in the present case.