Temperature and thickness evolution and epitaxial breakdown in highly-strained BiFeO3 thin films

We present the temperature- and thickness-dependent structural and morphological evolution of strain induced transformations in highly-strained epitaxial BiFeO3 films deposited on LaAlO3 (001) substrates. Using high-resolution X-ray diffraction and temperature-dependent scanning-probe-based studies we observe a complex temperature- and thickness-dependent evolution of phases in this system. A thickness-dependent transformation from a single monoclinically distorted tetragonal-like phase to a complex mixed-phase structure in films with thicknesses up to ~200 nm is the consequence of a strain-induced spinodal instability in the BiFeO3/LaAlO3 system. Additionally, a breakdown of this strain-stabilized metastable mixed-phase structure to non-epitaxial microcrystals of the parent rhombohedral structure of BiFeO3 is observed to occur at a critical thickness of ~300 nm. We further propose a mechanism for this abrupt breakdown that provides insight into the competing nature of the phases in this system.Comment: 7 figure

Dielectric and pyroelectric susceptibilities of epitaxial ferroelectric thin films

The dielectric and pyroelectric properties of ferroelectrics have been utilized extensively in a variety of applications such as memories, transducers, and sensors. Traditionally, such applications have utilized bulk or single crystal versions of ferroelectrics but modern electronic devices increasingly require micro-fabricated architectures on thin-fi lms. Despite spectacular advances in our ability to synthesize high-quality thin fi lms, much remains to be understood regarding the evolution of their dielectric and pyroelectric properties. In the present work, we investigate the dielectric and pyroelectric susceptibilities of ferroelectric thin films with PbZr1-xTixO3 (PZT) as a model system using a combination of phenomenological Ginzburg-Landau-Devonshire (GLD) modeling, epitaxial thin- film growth, and ex situ characterization techniques such as x-ray di ffraction, piezoresponse force microscopy, and electrical measurements. We show that large dielectric and pyroelectric susceptibilities can be obtained in tensile strained PZT thin fi lms due to the displacement of 90° domain walls and thermal expansion mismatch with the substrate. The primary advances made herein that enable this are the development of a polydomain GLD model to predict ferroelectric susceptibilities, the development of a hard-mask technique for the fabrication epitaxial capacitor structures, and the implementation of a phase-sensitive technique for the accurate measurement of pyroelectric properties. We then proceed to study the eff ect of composition and strain gradients on the ferroelectric properties of PZT. A combined theoretical and experimental study indicates that the graded heterostructures possess exotic structural and ferroelectric properties that are largely determined by the structure of the initial growth layer. We show that obtain large residual strains and novel ferroelectric properties (such as large built-in electric elds and low dielectric susceptibility) can be obtained in graded structures that are not found in the single layer variants. Finally, we present preliminary results of pyroelectric energy conversion in a micro-fabricated device utilizing the concepts developed in this work. Analysis of reversible pyroelectric cycles indicate that large temperature ramp rates and power densities can be obtained for thermal to electrical energy conversion in thin films as compared to prior experiments on bulk ferroelectric ceramics

Impact of the Lectin Chaperone Calnexin on the Stress Response, Virulence and Proteolytic Secretome of the Fungal Pathogen Aspergillus fumigatus

Calnexin is a membrane-bound lectin chaperone in the endoplasmic reticulum (ER) that is part of a quality control system that promotes the accurate folding of glycoproteins entering the secretory pathway. We have previously shown that ER homeostasis is important for virulence of the human fungal pathogen Aspergillus fumigatus, but the contribution of calnexin has not been explored. Here, we determined the extent to which A. fumigatus relies on calnexin for growth under conditions of environmental stress and for virulence. The calnexin gene, clxA, was deleted from A. fumigatus and complemented by reconstitution with the wild type gene. Loss of clxA altered the proteolytic secretome of the fungus, but had no impact on growth rates in either minimal or complex media at 37uC. However, the DclxA mutant was growth impaired at temperatures above 42uC and was hypersensitive to acute ER stress caused by the reducing agent dithiothreitol. In contrast to wild type A. fumigatus, DclxA hyphae were unable to grow when transferred to starvation medium. In addition, depleting the medium of cations by chelation prevented DclxA from sustaining polarized hyphal growth, resulting in blunted hyphae with irregular morphology. Despite these abnormal stress responses, the DclxA mutant remained virulent in two immunologically distinct models of invasive aspergillosis. These findings demonstrate that calnexin functions are needed for growth under conditions of thermal, ER and nutrient stress, but are dispensable for surviving the stresse

Evaluating LDL-C control in Indian acute coronary syndrome (ACS) patients- A retrospective real-world study LDL-C control in ACS

Background: Low-density lipoprotein-cholesterol (LDL-C) is an independent risk factor for atherosclerotic cardiovascular disease (ASCVD) progression. Although lipid lowering therapies remain the cornerstone of secondary ACSVD prevention, there exists residual dyslipidemia. The current study aimed to evaluate the real-world experience related to the treatment patterns and LDL-C control in Indian Acute Coronary Syndrome (ACS) patients. Methods: This was a real-world, descriptive, retrospective, observational, and multicentric study conducted across India. The data was collected for 1 year following the ACS event. The change in the levels of LDL-C from the baseline to the follow-up visits and the control of LDL-C, the change in lipid profile, lipoprotein levels, treatment patterns for lipid-lowering, and tolerability of existing treatments were evaluated. Results: Overall, 575 patients were included from 11 centers across India. The mean age of the patients was 52.92 years, with male predominance (76.35%). Although there was a significant reduction in the mean levels of LDL-C from the baseline [(122.64 ± 42.01 mg/dl to 74.41 ± 26.45 mg/dl (p < 0.001)], it was observed that despite high-intensity statin therapy, only 20.87% patients managed to achieve target LDL-C of <55 mg/dL and 55.65% were unable to reach LDL-C levels of <70 mg/dl one year after the event. Six patients reported adverse events without treatment discontinuation. Conclusion: The majority of the patients received high-intensity statins and did not attain target LDL-C levels, suggesting LDL-C control after an ACS event requires management with novel therapies having better efficacy as recommended by international and national guidelines

Real-time observation of local strain effects on nonvolatile ferroelectric memory storage mechanisms.

We use in situ transmission electron microscopy to directly observe, at high temporal and spatial resolution, the interaction of ferroelectric domains and dislocation networks within BiFeO3 thin films. The experimental observations are compared with a phase field model constructed to simulate the dynamics of domains in the presence of dislocations and their resulting strain fields. We demonstrate that a global network of misfit dislocations at the film-substrate interface can act as nucleation sites and slow down domain propagation in the vicinity of the dislocations. Networks of individual threading dislocations emanating from the film-electrode interface play a more dramatic role in pinning domain motion. These dislocations may be responsible for the domain behavior in ferroelectric thin-film devices deviating from conventional Kolmogorov-Avrami-Ishibashi dynamics toward a Nucleation Limited Switching model

Real-time observation of local strain effects on nonvolatile ferroelectric memory storage mechanisms.

We use in situ transmission electron microscopy to directly observe, at high temporal and spatial resolution, the interaction of ferroelectric domains and dislocation networks within BiFeO3 thin films. The experimental observations are compared with a phase field model constructed to simulate the dynamics of domains in the presence of dislocations and their resulting strain fields. We demonstrate that a global network of misfit dislocations at the film-substrate interface can act as nucleation sites and slow down domain propagation in the vicinity of the dislocations. Networks of individual threading dislocations emanating from the film-electrode interface play a more dramatic role in pinning domain motion. These dislocations may be responsible for the domain behavior in ferroelectric thin-film devices deviating from conventional Kolmogorov-Avrami-Ishibashi dynamics toward a Nucleation Limited Switching model