Search

Hexagonal ABO3 oxides ( A, B = cation) are a class of rich materials for realizing novel quantum phenomena. Their hexagonal symmetry, oxygen trigonal bipyramid coordination, and quasi-two dimensional layering give rise to properties distinct from those of the cubic ABO3 perovskites. As bulk materials, most of the focus in this class of materials has been on the rare-earth manganites, RMnO3 ( R = rare earth); these materials display coupled ferroelectricity and antiferromagnetic order. In this review, we focus on the thin-film manifestations of the hexagonal ABO3 oxides. We cover the stability of the hexagonal oxides and substrates which can be used to template the hexagonal structure. We show how the thin-film geometry not only allows for further tuning of the bulk-stable manganites but also allows for the realization of metastable hexagonal oxides such as the RFeO3 that combine ferroelectricity with weak ferromagnetic order. The thin-film geometry is a promising platform to stabilize additional metastable hexagonal oxides to search for predicted high-temperature superconductivity and topological phases in this class of materials.

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproduciblemetal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinnerfilms with sharp MITs are discussed, including the Volmer-Weber type growth mode due to thehigh energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2substrate, which we quantify using low angle annular dark field scanning transmission electronmicroscopy in conjunction with electron energy loss spectroscopy. We find that controlling islandcoalescence on the (001) surface and minimization of cation interdiffusion by using a low growthtemperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2films with abrupt MIT behavior. VC 2015 AIP Publishing LLC.

We demonstrate the ability to tune the thermal conductivity of homoepitaxial SrTiO3 films depositedby reactive molecular-beam epitaxy by varying growth temperature, oxidation environment, and cationstoichiometry. Both point defects and planar defects decrease the longitudinal thermal conductivity(k33), with the greatest decrease in films of the same composition observed for films containingplanar defects oriented perpendicular to the direction of heat flow. The longitudinal thermal conductivitycan be modified by as much as 80%—from 11.5W m1K1 for stoichiometric homoepitaxialSrTiO3 to 2W m1K1 for strontium-rich homoepitaxial Sr1þdTiOx films—by incorporating (SrO)2Ruddlesden-Popper planar defects.VC 2015 AIP Publishing LLC.

We measured the optical properties of epitaxial thin films of the metastable hexagonal polymorph of LuFeO3 by absorption spectroscopy, magnetic circular dichroism, and photoconductivity. Comparison with complementary electronic structure calculations reveals a 1.1 eV direct gap involving hybridized Fe 3dz2+O 2pz→Fe d excitations at the Γ and A points, with a higher energy direct gap at 2.0 eV. Both charge gaps nicely overlap the solar spectrum.

The effect of dimensional confinement on the optical band gap of SrTiO3 is investigated by periodically introducing one extra SrO monolayer every n SrTiO3 layers. The result is the n = 1–5 and 10 members of the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. Spectroscopic ellipsometry, optical transmission, and cathodoluminescence measurements reveal these Srn+1TinO3n+1 phases to have indirect optical band gaps at room temperature with values that decrease monotonically with increasing n. First-principles calculations suggest that as n increases and the TiO6 octahedra become connected for increasing distances along the c-axis, the band edge electronic states become less confined. This is responsible for the decrease in band gaps with increasing n (for finite n) among Srn+1TinO3n+1 phases.

We demonstrate the strain release of LaAlO3 epitaxial films on SrTiO3 (001) by inserting ultrathin “SrAlOx” buffer layers. Although SrAlOx is not a perovskite, nor stable as a single phase in bulk, epitaxy stabilizes the perovskite structure up to a thickness of 2 unit cells (uc). At a critical thickness of 3 uc of SrAlOx, the interlayer acts as a sliding buffer layer, and abruptly relieves the lattice mismatch between the LaAlO3 film and the SrTiO3 substrate, while maintaining crystallinity. This technique may provide a general approach for strain relaxation of perovskite films far below the thermodynamic critical thickness.

The stability of an amorphous material depends on how fast and by what mechanism crystallization occurs. Based on crystallization rate measurements through optical reflectivity changes in supercooled methanol thin films, it is observed for the first time that there is a definitive and detectable change of the crystallization mechanism at the glass transition temperature T(g). For methanol glasses below T(g)=103.4 K, crystallization occurs as an interface controlled, one-dimension process at frozen-in embryo sites, while in the deep supercooled liquid phase above T(g) crystallization is diffusion controlled in two dimensions with a constant nucleation rate and an activation energy of 107.8(+/-4.7) kJ/mol.