9 - NCCO and other cuprates

Summary

NCCO

Nd_2–xCe_xCuO_4–y (NCCO) was the first cuprate superconductor to exhibit n-type conduction. The substitution of Ce⁴⁺ for Nd³⁺ introduces electrons which go to the CuO₂ planes. (There are no Cu–O chains.) T_c peaks at 24 K for x = 0.15. The importance of NCCO is that is can be used to address the question of electron–hole symmetry. The electron doping swells slightly the large Fermi surface, while hole doping in other cuprates shrinks it slightly. In the localized picture with its small Fermi surface, how do the electron and hole Fermi surfaces compare? NCCO has a simpler LDA band structure near E_F than many other cuprates because there is only one CuO₂ plane per unit cell. Fewer bands cross E_F. What are the new states in NCCO the doping introduces along with the new electrons, or do those electrons occupy existing states as in the independent-electron picture? Other trivalent rare earths may be substituted for Nd. Why do the magnetic moments of the rare-earth ions not destroy superconductivity as they do in many other superconductors? A brief review of photoemission studies of NCCO was written by Sakisaka (1994).

Most of the studies of NCCO have been carried out on polycrystalline, i.e., sintered, samples. The earliest study on a single crystal (an epitaxial film), that of Sakisaka et al. (1990a,b), was mentioned in Chapter 6. Since then, and since the review of Sakisaka was written, there have been several more studies on single crystals.