Bayesian Inference of Log Determinants

The log-determinant of a kernel matrix appears in a variety of machine learning problems, ranging from determinantal point processes and generalized Markov random fields, through to the training of Gaussian processes. Exact calculation of this term is often intractable when the size of the kernel matrix exceeds a few thousand. In the spirit of probabilistic numerics, we reinterpret the problem of computing the log-determinant as a Bayesian inference problem. In particular, we combine prior knowledge in the form of bounds from matrix theory and evidence derived from stochastic trace estimation to obtain probabilistic estimates for the log-determinant and its associated uncertainty within a given computational budget. Beyond its novelty and theoretic appeal, the performance of our proposal is competitive with state-of-the-art approaches to approximating the log-determinant, while also quantifying the uncertainty due to budget-constrained evidence.Comment: 12 pages, 3 figure

Discovery of highly polarizable semiconductors BaZrS₃ and Ba₃Zr₂S₇

There are few known semiconductors exhibiting both strong optical response and large dielectric polarizability. Inorganic materials with large dielectric polarizability tend to be wide-band gap complex oxides. Semiconductors with a strong photoresponse to visible and infrared light tend to be weakly polarizable. Interesting exceptions to these trends are halide perovskites and phase-change chalcogenides. Here we introduce complex chalcogenides in the Ba-Zr-S system in perovskite and Ruddlesden-Popper structures as a family of highly polarizable semiconductors. We report the results of impedance spectroscopy on single crystals that establish BaZrS₃ and Ba₃Zr₂S₇ as semiconductors with a low-frequency relative dielectric constant ɛ0 in the range 50–100 and band gap in the range 1.3–1.8 eV. Our electronic structure calculations indicate that the enhanced dielectric response in perovskite BaZrS₃ versus Ruddlesden-Popper Ba₃Zr₂S₇ is primarily due to enhanced IR mode-effective charges and variations in phonon frequencies along 〈001〉; differences in the Born effective charges and the lattice stiffness are of secondary importance. This combination of covalent bonding in crystal structures more common to complex oxides, but comprising sulfur, results in a sizable Fröhlich coupling constant, which suggests that charge carriers are large polarons

Discovery of highly-polarizable semiconductors BaZrS3 and Ba3Zr2S7

There are few known semiconductors exhibiting both strong optical response and large dielectric polarizability. Inorganic materials with large dielectric polarizability tend to be wide-band gap complex oxides. Semiconductors with strong photoresponse to visible and infrared light tend to be weakly polarizable. Interesting exceptions to these trends are halide perovskites and phase-change chalcogenides. Here we introduce complex chalcogenides in the Ba-Zr-S system in perovskite and Ruddlesden-Popper structures as a new family of highly polarizable semiconductors. We report the results of impedance spectroscopy on single crystals that establish BaZrS3 and Ba3Zr2S7 as semiconductors with low-frequency relative dielectric constant (${\epsilon}_0$) in the range 50 - 100, and band gap in the range 1.3 - 1.8 eV. Our electronic structure calculations indicate the enhanced dielectric response in perovskite BaZrS3 versus Ruddlesden-Popper Ba3Zr2S7 is primarily due to enhanced IR mode-effective charges, and variations in phonon frequencies along $\langle 001 \rangle$; differences in the Born effective charges and the lattice stiffness are of secondary importance. This combination of covalent bonding in crystal structures more common to complex oxides results in a sizable Fr\"ohlich coupling constant, which suggests that charge carriers are large polarons.Comment: 22 pages, 5 figure

Erythroid-Specific Transcriptional Changes in PBMCs from Pulmonary Hypertension Patients

Gene expression profiling of peripheral blood mononuclear cells (PBMCs) is a powerful tool for the identification of surrogate markers involved in disease processes. The hypothesis tested in this study was that chronic exposure of PBMCs to a hypertensive environment in remodeled pulmonary vessels would be reflected by specific transcriptional changes in these cells.The transcript profiles of PBMCs from 30 idiopathic pulmonary arterial hypertension patients (IPAH), 19 patients with systemic sclerosis without pulmonary hypertension (SSc), 42 scleroderma-associated pulmonary arterial hypertensio patients (SSc-PAH), and 8 patients with SSc complicated by interstitial lung disease and pulmonary hypertension (SSc-PH-ILD) were compared to the gene expression profiles of PBMCs from 41 healthy individuals. Multiple gene expression signatures were identified which could distinguish various disease groups from controls. One of these signatures, specific for erythrocyte maturation, is enriched specifically in patients with PH. This association was validated in multiple published datasets. The erythropoiesis signature was strongly correlated with hemodynamic measures of increasing disease severity in IPAH patients. No significant correlation of the same type was noted for SSc-PAH patients, this despite a clear signature enrichment within this group overall. These findings suggest an association of the erythropoiesis signature in PBMCs from patients with PH with a variable presentation among different subtypes of disease.In PH, the expansion of immature red blood cell precursors may constitute a response to the increasingly hypoxic conditions prevalent in this syndrome. A correlation of this erythrocyte signature with more severe hypertension cases may provide an important biomarker of disease progression

The principles of design applied to engineering and policy

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1988.Includes bibliographical references.by Stephen Francis Filippone.M.S

Synthesis and Characterization of Chalcogenide Perovskites

Chalcogenide perovskites are both an old and new group of materials. First synthesized in 1957, chalcogenide perovskites are generally sulfur and selenium compounds which contain a 3D network of corner-sharing octahedra. Not until 2015 was this material class recognized as being suitable for any application. Their strong light absorption in the visible to near-IR wavelengths makes them of interest as photovoltaic absorber materials. The potential to deliver on the promise of low-cost thin film photovoltaics is reason enough to study chalcogenide perovskites. Additionally, their structural similarity to lead-halide and oxide perovskites offers a new opportunity to study related questions in those fields such as defect tolerance in lead-halide perovskites and ferroelectricity in oxides. The following projects represent advances in both synthesis and characterization of chalcogenide perovskites. I used computational software to calculate phase diagrams for chalcogenide perovskites. These phase diagrams were calculated in ultra-high vacuum conditions to serve as guides for thin film deposition techniques. I also attempted to synthesis the predicted ferroelectric compound ZnSnS3 by molecular beam epitaxy and solid phase epitaxy. Here I also report on the application of the new “cold sintering process” technique applied to densification of BaZrS3. I explored multiple sintering aides, including organic solvents, sulfur and iodine. I achieved high densification with iodine as a sintering aid. I characterized the dielectric and electronic properties of BaZrS3 and Ba3Zr2S7 single crystals. I used impedance spectroscopy on single crystals to measure their low-frequency relative dielectric constants. I used pump-probe IR reflectivity to measure mobility anisotropy in Ba3Zr2S7 single crystals. I used a Drude model to estimate the ambipolar carrier mobility from IR reflectivity in the and directions of Ba3Zr2S7. This body of work advances the emerging field of chalcogenide perovskites by providing practical guides for thin film synthesis, lessons from thin film and bulk synthesis work, as well and basic property measurements on bulk material, critical for the design of future thin film devices.Ph.D

High densification of BaZrS3 powder inspired by the cold-sintering process

Abstract We report on the sintering of powders of the chalcogenide perovskite BaZrS3 at low and intermediate temperatures. BaZrS3 is the most widely studied chalcogenide perovskite semiconductor. Most experimental results to-date have been made using loose powder and cold-pressed pellets (i.e. green bodies). Straightforward sintering methods to achieve dense ceramics are challenging because BaZrS3 decomposes and oxidizes in air above 550 °C. Here, we take inspiration from the cold-sintering process to develop a low-to-intermediate densification process for chalcogenide perovskites. Typical additives for cold sintering of oxides do not accelerate BaZrS3 sintering. At higher temperature, iodine is remarkably successful as an additive to accelerate densification. We achieve density up to 92% for sintering at 450 °C and 425 MPa. Our sintering process does not cause significant oxidation. Thermogravimetric measurements reveal the formation of an intergranular phase. Impedance spectroscopy measurements reveal that the bulk dielectric properties are unaffected by ball milling and subsequent sintering. Graphical abstrac