Electronic standing waves on the surface of Bi₂Sr ₂CaCu₂O _8+δ

Scanning tunneling microscopy (STM) measurements have shown that electronic states at the surface of Bi₂Sr₂CaCu₂O _8+δ single crystals form standing wave patterns at temperatures far below the superconducting transition. It has been shown that these patterns are consistent with those expected from the interference of well-defined quasi-particles around the Bi₂Sr₂CaCu₂O _8+δ Fermi surface. We have extended STM measurements of these standing wave patterns to higher temperatures and have found that four unit-cell periodicity patterns observed in the superconducting state persist well above T_C, in the pseudogap state. In this regime, many different experimental studies of the cuprates have long claimed the absence of well-defined quasi-particles. If the pseudogap regime is indeed devoid of any coherent quasi-particles then our results suggest that other phenomena, such as stripe formation, must play a role in formation of these patterns.