Revisiting Cu 63 NMR evidence for charge order in superconducting La1.885Sr0.115CuO4

The presence of charge and spin stripe order in the La2CuO4-based family of superconductors continues to lead to new insight on the unusual ground-state properties of high-Tc cuprates. Soon after the discovery of charge stripe order at Tcharge≃65 K in Nd3+ co-doped La1.48Nd0.4Sr0.12CuO4(Tc≃6 K) [Tranquada, Nature (London) 375, 561 (1995)NATUAS0028-083610.1038/375561a0], Hunt et al. demonstrated that La1.48Nd0.4Sr0.12CuO4 and superconducting La2-xSrxCuO4 with x∼1/8(Tc≃30 K) share nearly identical NMR anomalies near Tcharge of the former [Phys. Rev. Lett. 82, 4300 (1999)PRLTAO0031-900710.1103/PhysRevLett.82.4300]. Their inevitable conclusion that La1.885Sr0.115CuO4 also undergoes charge order at a comparable temperature became controversial, because diffraction measurements at the time were unable to detect Bragg peaks associated with charge order. Recent advances in x-ray diffraction techniques finally led to definitive confirmations of the charge order Bragg peaks in La1.885Sr0.115CuO4 with an onset at as high as Tcharge≃80 K. Meanwhile, improved instrumental technology has enabled routine NMR measurements that were not feasible two decades ago. Motivated by these new developments, we revisit the charge order transition of a La1.885Sr0.115CuO4 single crystal based on Cu63 NMR techniques. We demonstrate that Cu63 NMR properties of the nuclear spin Iz=-12 to +12 central transition below Tcharge exhibit unprecedentedly strong dependence on the measurement time scale set by the separation time τ between the 90 and 180 radio-frequency pulses; a new kind of anomalous, very broad winglike Cu63 NMR signals gradually emerge below Tcharge only for extremely short τ 4μs, while the spectral weight INormal of the normal NMR signals is progressively wiped out. The NMR linewidth and relaxation rates depend strongly on τ below Tcharge, and their enhancement in the charge ordered state indicates that charge order turns on strong but inhomogeneous growth of Cu spin-spin correlations.