Phase control in La-214 epitaxial thin films

The lanthanide (Ln) copper oxides of the general chemical formula Ln(2)CuO(4) take two different crystal structures: K2NiF4 (T) and Nd2CuO4 (T'). La2CuO4 takes the T structure by high-temperature bulk processes. The "thermal expansion mismatch" between the La-O and Cu-O bonds predicts that the T' phase of La2CuO4 can be stabilized at synthesis temperatures below 425degreesC. Such low synthesis temperatures are difficult to access by bulk processes, but easy by thin-film processes. We have surveyed growth parameters in molecular beam epitaxy, and succeeded in the selective stabilization of T- and T'-La2CuO4. From our observations, it turns out that the growth temperature as well as the substrate play a crucial role in the selective stabilization: the T' structure is stabilized at low growth temperatures (< 600degreesC) and with substrates of a(s) < 3.70 Angstrom or a(s) > 3.90 Angstrom, while the T structure is stabilized at high growth temperatures (> 650degreesC) or with substrates of a(s) similar to 3.70 - 3.95 Angstrom. We have also been attempting hole (Ca, Sr, and Ba) and electron (Ce) doping into both of T- and T'-La2CuO4. In T-La2CuO4, hole doping produces the well-known LSCO and LBCO. Surprisingly, contrary to the empirical law, electron doping is also possible up to x similar to 0.06 - 0.08, although the films do not show superconductivity. In T'-La2CuO4, electron doping produces superconducting T'-(La,Ce)(2)CuO4 with T-c similar to 30 K, although hole doping has as yet been unsuccessful.