Density wave order with antiphase feature associated with the pseudogap in cuprate superconductor Bi2+xSr2-xCuO6+delta

The strong correlation effect in cuprate superconductors have greatly enriched the phase diagram showing the co-existence of superconductivity with many intertwined orders. One of the prominent issues concerning the superconductivity mechanism is about the pseudogap phase which behaves either as cooperator or competitor for superconductivity and its fundamental reason remains still elusive. Here we report the measurements of scanning tunneling microscopy/spectroscopy (STM/STS) in the model superconducting system Bi2+xSr2-xCuO6+delta with a transition temperature Tc~7 K. Although this system is supposed to be slightly overdoped, a pseudogap feature can be easily observed in the energy region of about 20-60 meV. A modulation of local density of states (LDOS) with a periodicity of about 4a0/3 (a0: Cu-O-Cu bond length) can be easily observed, which is also supported by the Fourier transformation pattern with wavevectors at about (0,+-3pi/2a0) and (+-3pi/2a0,0). Surprisingly, we find that the LDOS exhibits a clear antiphase feature in the pseudogap energy region below and above the Fermi energy, indicating that it is an intrinsic feature of the pseudogap phase. We interpret this modulation and antiphase feature as a possible consequence of the pair density wave due to the Amperean pairing with finite momentum. Our results give a deep insight on the understanding of the pseudogap phase in cuprate superconductors.