  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 ($T_{\rm c}$) superconductivity in copper oxides
(cuprates). Herein, after briefly reviewing the progress achieved in the last
few years, we report new results of $^{63,65}$Cu-NMR measurements on the CDW
order and its fluctuation in the single-layered cuprate
Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$. 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
$T_{\rm CDW}$ scales with the pseudogap temperature $T^{\rm *}$. Comparison
between $^{63}$Cu and $^{65}$Cu 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.
