"139La and 63Cu NMR investigation of charge order in La2 CuO4+y (Tc=42 K)"

T. Imai: Y. S. Lee; Physical Review B, 03/14/18.

Additional Authors: Y. S. Lee

Abstract:

We report 139La and 63Cu 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 139La 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 63Cu NMR signals are progressively wiped out below Tcharge due to enhanced low-frequency spin fluctuations in charge-ordered domains, but the residual 63Cu 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 139La 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 et al., Phys. Rev. B 96, 224508 (2017) and Arsenault et al., Phys. Rev. B 97, 064511 (2018)], but both charge and spin order take place more sharply in the present case.