Incipient ferroelectricity in 2.3% tensile-strained CaMnO3 films

Epitaxial CaMnO3 films grown with 2.3% tensile strain on (001)-oriented LaAlO3 substrates are found to be incipiently ferroelectric below 25 K. Optical second harmonic generation (SHG) was used for the detection of the incipient polarization. The SHG analysis reveals that CaMnO3 crystallites with in-plane orientation of the orthorhombic b axis contribute to an electric polarization oriented along the orthorhombic a (resp.\ c) axis in agreement with the predictions from density functional calculations

Crystalline perovskite epitaxial growth on germanium (001) by atomic layer deposition

Crystalline perovskites (ABO3) have aroused widespread attention in material science due to their multiple properties. This research uses atomic layer deposition (ALD) to achieve perovskite oxides (ABO3) deposition on Ge (001) for gate oxide applications in microelectronics devices. In particular, this work is mainly focused on the study of crystalline Sr-based perovskites SrMO3, where M = Ti, Zr, Hf. In this research work, the mechanism for the initial growth of perovskites on Ge by ALD has been studied. High resolution scanning transmission electron microscopy (STEM) images have shown that both of molecular beam epitaxy (MBE)-grown BaTiO3 films and ALD-grown SrHfO3 films have the same interface structure, which has a 2×1 periodicity and with the alkaline earth metal (AEM) atoms between the Ge dimer rows. This result indicates that the ALD growth proceeds by forming the same Zintl-template layer that is purposely formed in MBE through formation of a 0.5-monolayer (ML) exposure to the AEM. The in situ XPS analysis has shown the same surface core level shift (SCLS) behavior results from half-cycle Sr or Ba precursor dosing on a bare Ge (001) surface as is observed following 0.5 -ML Sr or Ba exposure on Ge by MBE. These observations support the conclusion drawn from the STEM images. Based on the previous study of SrTiO3 (STO) and SrHfO3 (SHO) on Ge (001), there is a trade-off between dielectric constant and leakage current in STO and SHO. This research has also studied SrHfxTi1-xO3 (SHTO) films with different Hf content x to see how composition and lattice constant affected the crystallization behavior. Crystalline SrZrO3 films have also been deposited by ALD on Ge. The C-V and I-V measurements indicate that the SrZrO3 yield the best results for dielectric properties compared to STO, SHO and SHTO. A new combined approach of oxygen plasma pre-treatment, Zintl template formation and atomic deuterium post treatment has been applied on this work to minimize the interface trap density, which has achieved 8.56×1011cm-2eV-1.Chemical Engineerin

Robust synthetic control

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 63-65).In this thesis, we present a robust generalization of the synthetic control method. A distinguishing feature of our algorithm is that of de-noising the data matrix via singular value thresholding, which renders our approach robust in multiple facets: it automatically identifies a good subset of donors, functions without extraneous covariates (vital to existing methods), and overcomes missing data (never been addressed in prior works). To our knowledge, we provide the first theoretical finite sample analysis for a broader class of models than previously considered in literature. Additionally, we relate the inference quality of our estimator to the amount of training data available and show our estimator to be asymptotically consistent. In order to move beyond point estimates, we introduce a Bayesian framework that not only provides practitioners the ability to readily develop different estimators under various loss functions, but also equips them with the tools to quantitatively measure the uncertainty of their model/estimates through posterior probabilities. Our empirical results demonstrate that our robust generalization yields a positive impact over the classical synthetic control method, underscoring the value of our key de-noising procedure.by Dennis Shen.S.M

Developing models for the assessment and the design of the in situ remediation of contaminated sediments

The sediments in natural environment serve as sinks for contaminants from historical release, particularly hydrophobic organic compounds (HOC) and heavy metals. In-situ remediation, including monitored natural recovery (MNR), in-situ treatment (e.g. sorbing amendment) and in-situ capping, is one of the few alternative economically viable options with a proven record of success for sediment remediation. Modeling is often used to compare in-situ remedial approaches and design a system of meeting long term remedial goals. The fate and transport of contaminants in a remediation system is commonly modeled using a generalized advection-dispersion-reaction equation with potentially different physical and chemical properties in each layer. An analytical solution was developed with computational efficiency and unconditional stability for the multi-layered transport problem with linear processes and was shown to be more convenient for sensitivity analyses and parameter estimation and implement. A numerical model, CapSim, has been developed to model the transport and fate under more general conditions. Several important processes in sediment environments, such as nonlinear and kinetically limited sorption, steady and periodic advection, bioturbation, consolidation and deposition, are incorporated in the model. The current model also allows description of multiplied coupled chemical reactions. It builds on a simpler numerical model of Lampert (2009). It allows assessment of the transport and fate of chemicals under the most important dynamic sediment processes. Performance reference compounds (PRC) are often used to support passive sampling as a means of monitoring sediment processes and in situ remedial processes. An analytical solution was developed for modeling the release of PRC and uptake of target compounds in cylindrical passive sampling system. In the presence of nonlinear sorbents such as activated carbon, the interpretation and application of PRCs is more difficult. The fate and transport model CapSim was used to simulate the behavior of PRCs and target compounds in a passive sampling system with activated carbon. The impacts from the non-linear sorption of the compounds in activated carbon as well as the competitive sorption between an isotope-labeled PRC and the non-labeled compound are discussed.Chemical Engineerin

Formation of a stable deacagonal quasicrystalline Al-Pd-Mn surface layer

We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn quasicrystal overlayer on the 5-fold symmetric surface of an icosahedral Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by x-ray photoelectron diffraction, low-energy electron diffraction and electron backscatter diffraction. This overlayer is also characterized by a reduced density of states near the Fermi edge as expected for quasicrystals. This is the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn is obtained, studied and compared to its icosahedral counterpart.Comment: Submitted to Phys. Ref. Lett. (18 July 2001

New Phase Induced by Pressure in the Iron-Arsenide Superconductor K-Ba122

The electrical resistivity rho of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases linearly with pressure. At pressures above around 1.0 GPa, a second sharp anomaly is detected at a lower temperature T_0, which rises with pressure. We attribute this second anomaly to the onset of a phase that causes a reconstruction of the Fermi surface. This new phase expands with increasing x and it competes with superconductivity. We discuss the possibility that a second spin-density wave orders at T_0, with a Q vector distinct from that of the spin-density wave that sets in at T_N.Comment: Two higher K concentrations were added, revealing a steady expansion of the new phase in the T-P phase diagra