Revisiting Cu63 NMR evidence for charge order in superconducting La1.885Sr0.115CuO4

Here, 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-T_c cuprates. Soon after the discovery of charge stripe order at T_charge≃65 K in Nd³⁺ co-doped La_1.48Nd_0.4Sr_0.12CuO₄(T_c≃6 K), Hunt et al. demonstrated that La_1.48Nd_0.4Sr_0.12CuO₄ and superconducting La_2–xSr_xCuO₄ with x~1/8(T_c≃30 K) share nearly identical NMR anomalies near Tcharge of the former. Their inevitable conclusion that La_1.885Sr_0.115CuO₄ 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 La_1.885Sr_0.115CuO₄ with an onset at as high as T_charge≃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 La_1.885Sr_0.115CuO₄ single crystal based on ⁶³Cu NMR techniques. We demonstrate that ⁶³Cu NMR properties of the nuclear spin I_z=–1/2 to +1/2 central transition below T_charge 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 63Cu NMR signals gradually emerge below T_charge only for extremely short τ≲4μs, while the spectral weight I_Normal of the normal NMR signals is progressively wiped out. The NMR linewidth and relaxation rates depend strongly on τ below T_charge, and their enhancement in the charge ordered state indicates that charge order turns on strong but inhomogeneous growth of Cu spin-spin correlations.