Experimental observation of copper oxide layers in cuprates by scanning tunneling microscopy

Abstract

The nature of high temperature superconductivity in cuprates has currently been one of major scientific problems in condensed matter physics. In cuprate superconductors, the d -wave pairing symmetry with gap nodes on the Fermi surface and the pseudogap phenomenology have been well documented and regarded as the most important findings in the 30-year study. Due to the very complicated layered structure of cuprates, which consist of alternative superconducting (copper oxide) and non-superconducting (charge reservoir) layers, almost all macroscopic experiments previously performed probe the combined properties of both superconducting layers and non-superconducting layers. Meanwhile, surface-sensitive techniques, including scanning tunneling microscopy and spectroscopy, have mostly conducted on the cleaved reservoir layers, while the copper oxide layer is usually sandwiched between the reservoir layers. Because copper oxide layers are the main building blocks of cuprates, a direct probe of the copper oxide layer should be most desirable, although it is highly challenging experimentally. However, the significant progress has been made in the recent years. In this article we review the recent efforts on the preparation and direct observation of copper oxide layers in cuprates, and shed insight on the relationship between the pseudogap and superconductivity. These results might open new avenue for building up new high temperature superconductors and understanding the pairing mechanism of high temperature superconductivity in cuprates.