Paper detail

Phase control in La-214 epitaxial thin films

The lanthanide (Ln) copper oxides of the general chemical formula Ln2CuO4 take two different crystal structures: K2NiF4(T) and Nd2CuO4(T&#39;). La2CuO4 takes the T structure by high-temperature bulk processes. The &#34;thermal expansion mismatch&#34; between the La-O and Cu-O bonds predicts that the T&#39; phase of La2CuO4 can be stabilized at synthesis temperatures below 425 deg. 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&#39;-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&#39; structure is stabilized at low growth temperatures (< 600 deg.) and with substrates of a < 3.70 Ang or a > 3.90 Ang, while the T structure is stabilized at high growth temperatures (> 650 deg) or with substrates of a ~ 3.70 - 3.85 Ang. We have also been attempting hole (Ca, Sr, and Ba) and electron (Ce) doping into both of T- and T&#39;-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 ~ 0.06 - 0.08, although the films do not show superconductivity. In T&#39;-La2CuO4, electron doping produces superconducting T&#39;-(La,Ce)2CuO4 with Tc ~ 30 K, although hole doping has as yet been unsuccessful.