Charge order and fluctuations in Bi₂Sr_2−xLa_xCuO_6+δ revealed by ^63,65Cu-nuclear magnetic resonance

The discovery of a magnetic-field-induced charge-density-wave (CDW) order in the pseudogap state via nuclear magnetic resonance (NMR) studies has highlighted the importance of “charge” in the physics of high transition-temperature (T_c) superconductivity in copper oxides (cuprates). Herein, after briefly reviewing the progress achieved in the last few years, we report new results of ^63,65Cu-NMR measurements on the CDW order and its fluctuation in the single-layered cuprate Bi₂Sr₂−_xLa_xCuO₆₊δ. The NMR spectrum under both in- and out-of-plane magnetic fields above H = 10 T indicates that the CDW replaces the antiferromagnetic order before superconductivity appears, but disappears before superconductivity is optimized. We found that the CDW onset temperature T_CDW scales with the pseudogap temperature T ⁺. Comparison between ⁶³Cu and ⁶⁵Cu NMR indicates that the spin–lattice relaxation process is dominated by charge fluctuations in the doping regions where the CDW appears as well as at the pseudogap end point (T ⁺ = 0). These results suggest that charge orders and fluctuations exist in multiple doping regions and over a quite wide temperature range.