Thickness-Dependent Band Gap Modification in BaBiO3_{3}

The material BaBiO$_{3}$ is known for its insulating character. However, for thin films, in the ultra-thin limit, metallicity is expected because BaBiO$_{3}$ is suggested to return to its undistorted cubic phase where the oxygen octahedra breathing mode will be suppresse as reported recently. Here, we confirm the influence of the oxygen breathing mode on the size of the band gap. The electronic properties of a BaBiO$_{3}$ thickness series are studied using \textit{in-situ} scanning tunneling microscopy. We observe a wide-gap ($E_\textrm{G}$~$>$ 1.2 V) to small-gap~($E_\textrm{G}$~$\approx$ 0.07 eV) semiconductor transition as a function of a decreasing BaBiO$_{3}$ film thickness. However, even for an ultra-thin BaBiO$_{3}$ film, no metallic state is present. The dependence of the band gap size is found to be coinciding with the intensity of the Raman response of the breathing phonon mode as a function of thickness

Artificial oxide heterostructures with non-trivial topology

In the quest for topological insulators with large band gaps, heterostructures with Rashba spin-orbit interactions come into play. Transition metal oxides with heavy ions are especially interesting in this respect. We discuss the design principles for stacking oxide Rashba layers. Assuming a single layer with a two-dimensional electron gas (2DEG) on both interfaces as a building block, a two-dimensional topological insulating phase is present when negative coupling between the 2DEGs exists. When stacking multiple building blocks, a two-dimensional or three-dimensional topological insulator is artificially created, depending on the intra- and interlayer coupling strengths and the number of building blocks. We show that the three-dimensional topological insulator is protected by reflection symmetry, and can therefore be classified as a topological crystalline insulator. In order to isolate the topological states from bulk states, the intralayer coupling term needs to be quadratic in momentum. It is described how such a quadratic coupling could potentially be realized by taking buckling within the layers into account. The buckling, thereby, brings the idea of stacked Rashba system very close to the alternative approach of realizing the buckled honeycomb lattice in [111]-oriented perovskite oxides.Comment: Accepted for publication in Journal of Physics: Condensed Matte

Stabilization of the perovskite phase in the Y-Bi-O system by using a BaBiO3_{3} buffer layer

A topological insulating phase has theoretically been predicted for the thermodynamically unstable perovskite phase of YBiO$_{3}$. Here, it is shown that the crystal structure of the Y-Bi-O system can be controlled by using a BaBiO$_{3}$ buffer layer. The BaBiO$_{3}$ film overcomes the large lattice mismatch of 12% with the SrTiO$_{3}$ substrate by forming a rocksalt structure in between the two perovskite structures. Depositing an YBiO$_{3}$ film directly on a SrTiO$_{3}$ substrate gives a fluorite structure. However, when the Y-Bi-O system is deposited on top of the buffer layer with the correct crystal phase and comparable lattice constant, a single oriented perovskite structure with the expected lattice constants is observed.Comment: 8 pages, 7 figures + 4 pages supporting informatio

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Thickness-Dependent Band Gap Modification in BaBiO3

The material BaBiO3 is known for its insulating character. However, for thin films, in the ultra-thin limit, metallicity is expected because the oxygen octahedra breathing mode will be suppressed as reported recently. Here, we confirm the influence of the oxygen breathing mode on the size of the band gap. The electronic properties of a BaBiO3 thickness series are studied using in-situ scanning tunneling microscopy. We observe a wide-gap (EG > 1.2 V) to small-gap (EG ≈ 0.07 eV) semiconductor transition as a function of a decreasing BaBiO3 film thickness. However, even for an ultra-thin BaBiO3 film, no metallic state is present. The dependence of the band gap size is found to be coinciding with the intensity of the Raman response of the breathing phonon mode as a function of thickness

Oxide materials as building blocks for artificial topological insulators

The need for more energy efficient consumer electronics is growing. Topological insulators, a relatively new class of materials, could possibly function as a new building block for the next generation electronics. When a topological insulator is physically connected to a trivial insulator, conducting surface states form at the interface while the bulk of both materials remains insulating. Current experimentally verified topological insulators possess relatively small bulk band gaps that do not exceed the thermal excitation energy at room temperature. Therefore, applications of the desired surface states properties are hindered by the contribution from bulk carriers. Using oxide materials for artificially designed topological insulators potentially enlarges the size of the band gap and provides the possibility of tuning of the material properties by various design principles. In this dissertation, the materials systems BaBiO3 and perovskite Y-Bi-O and fabricated as thin films using pulsed laser deposition. Theoretical calculations of the electronic band structures show the presence of a band inversion, indicating nontrivial topology. However, both compounds suffer from major degradation effects, preventing the use of conventional techniques. Various in situ techniques are, therefore, used such as angle-resolved photoemission spectroscopy, scanning tunneling microscopy and x-ray photoelectron diffraction. Furthermore, it is shown that BaBiO3 accommodates for the 12% lattice mismatch with the SrTiO3 substrate by the formation of a rocksalt structure. Although, BaBiO3 thin films are fabricated with a high quality, accessing the topological insulating state remains difficult. By using the BaBiO3 film as a buffer layer, the energetically unfavorable perovskite phase is stabilized in Y-Bi-O. On this system, linear dispersing states are observed, hinting to the presence of a Dirac cone and thus a topological insulating phase. The use of complex oxides opens up many routes towards the realization of the first oxide topological insulator and applications at room temperature could become feasible

BaBiO3—From single crystals towards oxide topological insulators

BaBiO3 is an oxide perovskite with a wide variety of interesting properties. It was expected that the compound would behave like a metal. However, experiments revealed that BaBiO3 is not metallic, which started an extensive debate about the mechanism responsible for this insulating behavior. The two most important conjectures in this debate are charge disproportionation of the Bi ion into 3+ and 5+ cations and bond hybridization of the Bi 6s and O 2p orbitals. Both mechanisms induce a breathing mode of the oxygen octahedra, which is experimentally observed in single crystals and thin films. Recently, ultra-thin BaBiO3 films were studied with the aim of suppressing the breathing mode, which was expected to result in re-emergence of metallicity. However, this expectation was not confirmed so far. Furthermore, theoretical calculations predict that BaBiO3 becomes a topological insulator (TI) when doped with electrons. Since high-temperature superconductivity was observed when doping the compound with holes, an interface between a superconductor and a TI can be established within the same parent compound. In this Review, we discuss the theoretical and experimental findings concerning the mechanism responsible for the unexpected insulating behavior of BaBiO3 for both single crystals and thin films. An overview is given of the current state of the art and the experimental challenges of achieving an oxide topological insulating state in BaBiO3

Crystal phase control in an YBiO3 thin film by using a Ba BiO3 buffer layer

Topological insulators have a non-trivial band structure, forming gapless surface states when coupled to a normal insulator [1]. Until now, applications are hindered by the competition between the insulating bulk and conducting surface states. Perovskite oxides offer a good alternative, since topological insulating phases are theoretically predicted with band gaps larger than the thermal excitation energy at room temperature [2]. Therefore, promising applications for these materials lie in the elds of quantum computing and spintronics. In YBiO3, a topological insulating phase is predicted for the perovskite crystal structure with yttrium and bismuth located at the A-site and B-site, respectively [3]. However, the fluorite phase is thermodynamically more stable than the perovskite phase as proven when an YBiO3 lm is grown directly on a SrTiO3 substrate. By using a buffer layer, a possibility is given to stabilise the perovskite phase in the YBiO3 film. As buffer layer material BaBiO3 is chosen, since it grows in the perovskite phase and has a comparable lattice constant as predicted for the perovskite YBiO3 structure. By various characterisation techniques, it is shown that BaBiO3 grows as a single oriented perovskite lm in a relaxed state despite the large lattice mismatch with the underlying SrTiO3 substrate. When the YBiO3 is deposited on top of the buer layer, a single oriented perovskite phase is also observed in this lm with the expected lattice constants. These ndings pave a way towards the fabrication of quantum devices for testing the hypothesised topological insulating phase in YBiO3. [1] Y. Ando et al., Journal of the Physical Society Japan 82(10), 102001 (2013). [2] Y. Zhang et al., Phys. Chem. Chem. Phys. 18(11), 8205-8211 (2016). [3] H. Jin et al., Scientic Reports 3, 1651 (2013)