STM studies on the hole doping dependence of the hidden order in Pb-doped Bi2201

We have performed STM measurements of the modulation-free Pb-doped Bi2201. In the overdoped (T_c = 7 K) and the optimally doped (T_c = 22 K) samples, the distorted patch arrangement has been observed in low-bias STM images below 50 mV. With decreasing doping level, the size of patches becomes smaller and their arrangement more orderly. In the optimally doped sample, furthermore, the bright atomic-strings have been detected at low bias-voltages below 10 mV. The distorted patch arrangement may be regarded as the two-dimensional superstructure incompletely formed due to the absence of the structural modulation and/or the disturbance from doped holes. The atomic-string indicates the one-dimensional electronic correlation and may be the new type hidden order.