LuFe2O4 has been the most extensively studied among a series of RFe2O4 (R = Sc, Y, In and Dy to Lu) compounds, which possess both dielectric and magnetic orderings originating in equal numbers of Fe2+ and Fe3+ ions in a triangular lattice. We have prepared LuFe2O4 thin films epitaxially grown on a (111)-oriented YSZ (yttria-stabilized zirconia) substrate via a pulsed laser deposition method and found that the resultant thin film comprises a curious self-assembled interface structure. Our structural analysis at an atomic level by using high-angle annular dark-field scanning transmission electron microscopy and energy dispersive X-ray spectrometry reveals that very thin layers of LuFe2O4 lacking Fe–O layers, corresponding to the hexagonal LuFeO3 and Lu2Fe3O7 compositions, are formed at the interface between the c-axis oriented LuFe2O4 thin film and YSZ substrate with a Lu-rich region just on the surface of the substrate. Such an interfacial structure leads to an exchange bias effect peculiarly observed for an interface formed by different types of magnetic materials. The LuFe2O4 thin film itself shows spin glass transition similar to bulk LuFe2O4 with an off-stoichiometric oxygen ratio. Also, a change in electronic transport behavior between Arrhenius-type and variable range hopping schemes has been observed in the temperature dependence of electrical resistivity around the three-dimensional charge ordering temperature.
Doping evolution of the unconventional superconducting properties in BaBiO3-based compounds has yet to be clarified in detail due to the significant change of the oxygen concentration accompanied by the chemical substitution. We suggest that the carrier concentration of an unconventional superconductor, BaPb0.7Bi0.3O3−δ, is controllable without inducing chemical or structural changes using an electric double-layer transistor structure. The critical temperature is found to decrease systematically with increasing carrier concentration.
固有ジョセフソン接合におけるスイッチングダイナミクスに関する論文がSuperconductor Science and Technology誌に掲載されました。
Y. Nomura, R. Okamoto, and I. Kakeya,
“Negative correlation between enhanced crossover temperature and fluctuation-free critical current of the second switch in Bi2Sr2CaCu2O intrinsic Josephson junction”,
Supercond. Sci. Technol., 30 105001(2017).
We have investigated the switching dynamics of the first and second switches in intrinsic Josephson junctions (IJJs) of Bi2Sr2CaCu2O with different maximum Josephson current density Jc to reveal the doping evolution of interaction between IJJs. For the second switch, the crossover temperature between temperature-independent switching similar to quantum tunneling and thermally activated switching is remarkably higher than that for the first switch. Moreover, slightly decreases with increasing Jc, which violates the conventional relation between the crossover temperature and the critical current density. These features can be explained not by a change in the Josephson coupling energy but by a change in the charging energy of the Josephson junction. We argue that the capacitive coupling model explains the increase in the fluctuation in the quantum regime of the second switch and the anti-correlation between and Jc. Furthermore, inductive coupling does not contribute to these peculiar phenomena in the switching dynamics of stacked IJJs.