Abstract
We report and NMR investigation of the successive charge order, spin order, and superconducting transitions in superoxygenated single crystal with stage-4 excess oxygen order at K. We show that the stage-4 order induces tilting of octahedra below , which in turn causes NMR line broadening. The structural distortion continues to develop far below , and completes at K, where charge order sets in. This sequence is reminiscent of the the charge-order transition in Nd codoped that sets in once the low-temperature tetragonal phase is established. We also show that the paramagnetic NMR signals are progressively wiped out below due to enhanced low-frequency spin fluctuations in charge-ordered domains, but the residual NMR signals continue to exhibit the characteristics expected for optimally doped superconducting planes. This indicates that charge order in 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 nuclear spin-lattice relaxation rate do not exhibit critical divergence at () K. These findings, including the spatially inhomogeneous nature of the charge-ordered state, are qualitatively similar to the case of [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.
- Received 27 December 2017
- Revised 4 February 2018
DOI:https://doi.org/10.1103/PhysRevB.97.104506
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