Charge fluctuations in cuprate superconductors
  The effect of the lattice periodic potential on superconductivity which was ignored by BCS theory has been investigated. According to the effective mass approximation of band theory, the effect of lattice periodic potential can be embodied in the effective mass of the Ginzburg-Landau (GL) equations. The effective mass has s special property. It can be negative. Negative effective mass leads to many unusual phenomena. The superconducting order parameter shows the period distribution. Its modulate wavevector is proportional to the condensed carrier density, which explains the linear relation between the magnetic peaks displacement epsilon and x for La2-xSrxCuO4. The superconducting phase is always local and separated originally and evolves into global superconducting phase at the certain pairs concentration, which explains why the cuprate superconductors must be insulator at low doped. The doped concentrations of insulator to superconductor transition for La2-xSrxCuO4 is consistent with the experiment results. The relation of the superconducting gap (SG) and the pseudogap (PG) was discussed. Spin density wave state is obtained when the order parameter is homogenous distribution. 
Zhongqing Wu and Xuanjia Zhang