Density of states modulation in the pseudogap state of high-$T_c$   superconductors
  The density of states modulation recently observed by the scanning tunneling microscopic experiment in the pseudogap state of high-$T_c$ superconductors is explained by the pairing assisted particle transitions under perturbation of a periodic pairing interaction. In such a transition process, a particle with momentum ${\bf k}$ firstly picks up another particle of inverse spin to produce a pair leaving a hole. Under the perturbation of periodic pairing interaction of modulation wave vector ${\bf Q}$, the pair absorbs a momentum ${\bf Q}$ and then breaks into two single-particles: one propagates with momentum ${\bf k+Q}$, and another one fills in the hole. The transition is significant at low energies within the pseudogap since where the pairing excitations most favorably survive. We calculate the Fourier component of the modulated density of states using two different models. Both of the theoretical results are consistent with the experiment. 
Xin-Zhong Yan