The nature of the metal-insulator transition in thin films and superlattices of LaNiO3 only a few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate–film interface quality may also affect the observable properties of ultrathin films. Here we report results obtained for near-ideal LaNiO3 films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO3 substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO2 termination). Electronic structure measurements using X-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal-insulator transition.

No abstract

A stereoselective [3+2] cycloaddition of isocyanoesters to methyleneindolinones catalyzed by a quinine-based thiourea-tertiary amine has been successfully developed. Just by tuning the protecting groups on substrates, a variety of optically enriched 3,3'-pyrrolidinyl spirooxindole diastereomers could be obtained in excellent enantioselectivities (up to 99% ee).

No abstract

We report the chiral phosphoric acid catalyzed formal (3 + 2) cycloaddition of 3-substituted 1H-indoles and propargylic alcohols containing a functional directing group (p-NHAc or p-OH). This work represents a straightforward method to synthesize chiral pyrrolo­[1,2-a]­indole bearing a tetrasubstituted carbon stereocenter. The reaction proceeds smoothly with a wide array of substrate tolerance to deliver various chiral pyrrolo­[1,2-a]­indoles in up to 93% yield and 98% ee. The utility of this method is highlighted by the diverse transformations of the products into various indole derivatives.

A highly diastereoselective and enantioselective Michael addition of α-substituted isocyanoacetates with maleimides catalyzed by bifunctional tertiary amine thioureas has been developed. Various chiral succinimide derivatives bearing adjacent quaternary and tertiary stereocenters were prepared in excellent yields (up to 98%), diastereoselectivities (up to 99:1), and enantioselectivities (up to 98% ee). The synthetic utility of chiral succinimide derivatives is also demonstrated in the preparation of h5-HT(1d) receptor agonist motifs.

Temperature-dependent dielectric spectroscopy was used to observe multiple dielectric responses involving four sets of dielectric relaxations (DRs) in molecular beam epitaxy-deposited ultrathin (001) SrMnO3/Nb:SrTiO3 heterojunctions. Two sets of oxygen-vacancy (OV)-related DRs, which were sensitive to oxygen annealing, were attributed to short-range OV hopping and localized Jahn–Teller (JT)-polaron hopping-induced dipolar-type relaxations. Arising from electron-phonon coupling between eg electrons and JT-distortion in Mn3+ ions, the appearance of JT-polarons was closely related to the presence of oxygen vacancies, which leads to transitions from Mn4+ to Mn3+.

No abstract