Microelectromechanical systems (MEMS) incorporating active piezoelectric layers offer integrated actuation, sensing, and transduction. The broad implementation of such active MEMS has long been constrained by the inability to integrate materials with giant piezoelectric response, such as Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT). We synthesized high-quality PMN-PT epitaxial thin films on vicinal (001) Si wafers with the use of an epitaxial (001) SrTiO₃ template layer with superior piezoelectric coefficients (e_31,f = –27 ± 3 coulombs per square meter) and figures of merit for piezoelectric energy-harvesting systems. We have incorporated these heterostructures into microcantilevers that are actuated with extremely low drive voltage due to thin-film piezoelectric properties that rival bulk PMN-PT single crystals. These epitaxial heterostructures exhibit very large electromechanical coupling for ultrasound medical imaging, microfluidic control, mechanical sensing, and energy harvesting.

Bulk polycrystals. The highest Hc2 film also exhibits a significant upward curvature of Hc2(T), and temperature dependence of the anisotropy parameter γ(T) = Hc2 || / Hc2 ⊥ opposite to that of single crystals: γ(T) decreases as the temperature decreases, from γ(Tc) ≈ 2 to γ(0) ≈ 1.5. This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2, which offers special opportunities for further Hc2 increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of Hc2(T) observed suggest that MgB2 can be made into a versatile, competitive high-field superconductor.

To investigate the critical thickness of ferroelectric $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ (BTO) films, we fabricated fully strained $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3∕\mathrm{BTO}∕\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ heterostructures on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates by pulsed laser deposition with in situ reflection high-energy electron diffraction. We varied the BTO layer thickness from $3\text{to}30\mathrm{nm}$ . By fabricating $10\times 10\mu {\mathrm{m}}^2$ capacitors, we could observe polarization versus electric-field hysteresis loops, which demonstrate the existence of ferroelectricity in BTO layers thicker than $5\mathrm{nm}$ . This observation provides an experimental upper bound of $5\mathrm{nm}$ for the critical thickness. The BTO thickness-dependent scaling of the remanent polarization agrees with the predictions of recent first-principle simulations [J. Junquera and P. Ghosez, Nature 422, 506 (2003)].

Anatase ${\mathrm{Ti}}_{\text{0.96}}{\mathrm{Co}}_{\text{0.04}}{\mathrm{O}}_2$ films were grown epitaxially on ${\mathrm{SrTiO}}_3$ (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, $P_{{\mathrm{O}}_2},$ during the growth was systematically varied. As $P_{{\mathrm{O}}_2}$ decreased, the growth behavior was changed from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped ${\mathrm{TiO}}_2$ films.