Studying the so-called ”hidden” phases of quantum materials—phases that do not exist under equilibrium conditions, but can be accessed with light—reveals new insights into the broader field of structural phase transitions. Using terahertz irradiation as well as hard x-ray probes made available by x-ray free electron lasers (XFELs) provides unique capabilities to study phonon dispersion in these materials. Here, we study the cubic peak of the quantum paraelectric strontium titanate (SrTiO3, STO) below the 110 K cubic-to-tetragonal tran- sition. Our results reveal a temperature and field strength dependence of the transverse acoustic mode in agreement with previous work on the avoided crossing occurring at finite wavevector, as well as evidence of anharmonic coupling between transverse optical phonons and a fully symmetric A1g phonon. These results elucidate previous optical studies on STO and hold promise for future studies on the hidden metastable phases of quantum materials.
This work first compares carbon isotope measurements (δ13C) of particulate matter and fog from locations across the globe to assess how different primary aerosol sources are reflected in the atmosphere. Three field campaigns are then discussed that highlight different aspects of PM formation, composition, and processing. In Tempe, AZ, seasonal and size-dependent differences in the δ13C of total carbon and n-alkanes in PM were studied. δ13C was influenced by seasonal trends, including inversion, transport, population density, and photochemical activity. Variations in δ13C among particle size fractions were caused by sources that generate particles in different size modes.
An analysis of PM from urban and suburban sites in northeastern France shows how both fog and rain can cause measurable changes in the δ13C of PM. The δ13C of PM was consistent over time when no weather events occurred, but particles were isotopically depleted by up to 1.1‰ in the presence of fog due to preferential scavenging of larger isotopically enriched particles. Finally, the δ13C of the dissolved organic carbon in fog collected on the coast of Southern California is discussed. Here, temporal depletion of the δ13C of fog by up to 1.2‰ demonstrates its use in observing the scavenging and deposition of organic PM.
Ultrasonic transducer stacks (modules), which had failed or passed during pulse-echo sensitivity testing, were received from Consortium X. With limited background information on these stacks, the central theme was to determine the origin(s) of failure via the use of thermal and physicochemical characterization techniques.
The optical and scanning electron microscopy revealed that contact electrode layers are discontinuous in all samples, while delaminations between electrodes and pad layer were observed in failed samples. The X-ray diffraction data on the pad PZT revealed an overall c/a ratio of 1.022 ratio and morphotropic boundary composition, with significant variations of the Zr to Ti ratio within a sample and between samples. Electron probe microanalysis confirmed that the overall Zr to Ti ratio of the pad PZT was 52/48, and higher amounts of excess PbO in failed samples, whereas, inductively coupled plasma mass spectrometry revealed the presence of Mn, Al, and Sb (dopants) and presence of Cu (sintering aid) in in this hard (pad) PZT. Additionally, three exothermic peaks during thermal analysis was indicative of incomplete calcination of pad PZT. Moreover, transmission electron microscopy and scanning transmission electron microscopy revealed the presence of parylene at the Ag-pad PZT interface and within the pores of pad PZT (in failed samples subjected to electric fields). This further dilutes the electrical, mechanical, and electromechanical properties of the pad PZT, which in turn detrimentally influences the pulse echo sensitivity.