Materials Science Department Seminar

It is commonly believed that the high-Tc superconductivity develops in a Mottinsulating mother compound by doping either hole or electron carriers. However, our recent observation of fairly high-Tc superconductivity (>20 K) in nominally undoped T’- La3+ 2-x RE3+ xCuO4 (RE = rare earth elements, Sm, Eu, Gd, Tb, Lu, and Y) appears to be against this general belief. The purpose of the isovalent RE substitution for La is to stabilize the T’ phase of La2CuO4. There seems to be two possible scenarios regarding the origin of the carriers in these superconductors: (i) oxygen deficiencies at the regular oxygen sites serve as a source of effective electron carriers, and (ii) they are not Mott insulators and have intrinsic carriers. Based on our systematic investigation of the variation of the lattice constants with changing x, where no Cu-O bond stretching due to electron-doping (which is commonly observed in electron-doped T’ and infinite-layer superconductors) was observed, the second scenario is the more plausible. This conclusion is also supported by the results of transport and photoemission experiments. For electron-doped cuprates, the properties of the samples are very sensitive to a small amount of apical oxygen remnant, which is detrimental to superconductivity, but always exists in the as-grown samples before reduction. We believe the larger a0 in the La-based T’ compounds enabled a more thorough removal of the apical oxygen in the reduction process and led to the new cuprate superconductors without Mott-insulating ground state. Another interesting observation closely related to this subject is that electron-hole symmetry appears to be broken in the T-phase materials. We found that, with low growth temperature and appropriate substrate choice, Ce4+ can be incorporated into the K2NiF4 (T) lattice up to x ~ 0.06, which has not yet been realized in bulk synthesis. The results showed the doping of Ce makes T-La2-xCexCuO4 more insulating, which is in sharp contrast to