Nanosecond Structural Dynamics during Electrical Melting of Charge Density Waves in 1T-TaS_{2}.

Electrical control of charge density waves has been of immense interest, as the strong underlying electron-lattice interactions potentially open new, efficient pathways for manipulating their ordering and, consequently, their electronic properties. However, the transition mechanisms are often unclear as electric field, current, carrier injection, heat, and strain can all contribute and play varying roles across length scales and timescales. Here, we provide insight on how electrical stimulation melts the room temperature charge density wave order in 1T-TaS_{2} by visualizing the atomic and mesoscopic structural dynamics from quasi-static to nanosecond pulsed melting. Using a newly developed ultrafast electron microscope setup with electrical stimulation, we reveal the order and strain dynamics during voltage pulses as short as 20 ns. The order parameter dynamics across a range of pulse amplitudes and durations support a thermally driven mechanism even for fields as high as 19 kV cm^{-1}. In addition, time-resolved imaging reveals a heterogeneous, mesoscopic strain response across the flake, including MHz-scale acoustic resonances that emerge during sufficiently short pulsed excitation which may modulate the order. These results suggest that metallic charge density wave phases like studied here may be more robust to electronic switching pathways than insulating ones, motivating further investigations at higher fields and currents in this and other related systems.

Plasmonic Nonlinear Energy Transfer Enhanced Second Harmonic Generation Nanoscopy.

Nonlinear optical response is a fingerprint of various physicochemical properties of materials related to symmetry, including crystallography, interfacial configuration, and carrier dynamics. However, the intrinsically weak nonlinear optical susceptibility and the diffraction limit of far-field optics restrict probing deep-subwavelength-scale nonlinear optics with measurable signal-to-noise ratio. Here, we propose an alternative approach toward efficient second harmonic generation (SHG) nanoscopy for SHG-active sample (zinc oxide nanowire; ZnO NW) using an SHG-active plasmonic nanotip. Our full-wave simulation suggests that the experimentally observed high near-field SHG contrast is possible when the nonlinear response of ZnO NW is enhanced and/or that of the tip is suppressed. This result suggests possible evidence of quantum mechanical nonlinear energy transfer between the tip and the sample, modifying the nonlinear optical susceptibility. Further, this process probes the nanoscale corrosion of ZnO NW, demonstrating potential use in studying various physicochemical phenomena in nanoscale resolution.

Accurate quantification of lattice temperature dynamics from ultrafast electron diffraction of single-crystal films using dynamical scattering simulations.

In ultrafast electron diffraction (UED) experiments, accurate retrieval of time-resolved structural parameters, such as atomic coordinates and thermal displacement parameters, requires an accurate scattering model. Unfortunately, kinematical models are often inaccurate even for relativistic electron probes, especially for dense, oriented single crystals where strong channeling and multiple scattering effects are present. This article introduces and demonstrates dynamical scattering models tailored for quantitative analysis of UED experiments performed on single-crystal films. As a case study, we examine ultrafast laser heating of single-crystal gold films. Comparison of kinematical and dynamical models reveals the strong effects of dynamical scattering within nm-scale films and their dependence on sample topography and probe kinetic energy. Applying to UED experiments on an 11 nm thick film using 750 keV electron probe pulses, the dynamical models provide a tenfold improvement over a comparable kinematical model in matching the measured UED patterns. Also, the retrieved lattice temperature rise is in very good agreement with predictions based on previously measured optical constants of gold, whereas fitting the Debye-Waller factor retrieves values that are more than three times lower. Altogether, these results show the importance of a dynamical scattering theory for quantitative analysis of UED and demonstrate models that can be practically applied to single-crystal materials and heterostructures.

Utilization and Costs of Noninvasive Cardiac Tests After Acute Coronary Syndromes: Insights From the Alberta COAPT Study.

BACKGROUND: Although appropriate noninvasive cardiac tests (NICTs) after an acute coronary syndrome (ACS) provide useful prognostic information, inappropriate use leads to inefficient expenditure of existing healthcare resources. By using the Alberta Contemporary Acute Coronary Syndrome Patient Invasive Treatment Strategies (COAPT) Registry, we evaluated the use and costs of NICTs among patients discharged within 1 year after ACS. METHODS: All patients discharged from the hospital with a primary diagnosis of ACS in Alberta between 2004/2005 and 2015/2016 were included. Frequency of NICTs (stress tests [± imaging] and nonstress imaging tests) was determined from linked provincial databases. Costs were obtained from the Alberta Health Care Insurance Plan Medical Procedure List. RESULTS: Of 55,516 patients with ACS, 30,760 had at least 1 NICT (55.4%), with 13,505 (24.3%) having > 1 NICT performed within 1 year. Temporal trends of NICT increased over time (stress tests: P trend < 0.001; nonstress imaging tests: P trend < 0.001). NICT most commonly occurred within the first 4 months after hospital discharge (stress tests at 2 months; nonstress imaging tests at 3-4 months). In 2015/2016, the total estimated costs of NICT were $1.35M, a 22.4% increase from 2004/2005 (1.10M) (P < 0.001), whereas a decrease in incidence of ACS over the same time period was noted (P = 0.008). CONCLUSIONS: Rates of NICT 1 year after ACS are high and increasing over time. Estimated costs of NICT appear to be escalating out of proportion to the ACS growth. Further investigation is warranted because it is speculative whether the increase in NICT and costs results in clinical benefit after ACS.

CONTEXTE: Bien que les différents examens cardiaques non effractifs (ECNE) effectués après un syndrome coronarien aigu (SCA) fournissent des renseignements utiles au pronostic, leur emploi dans des situations inappropriées entraîne un gaspillage des ressources en santé. À l'aide du registre COAPT ( Co ntemporary A cute Coronary Syndrome P atient Invasive T reatment Strategies) de l'Alberta, nous avons évalué l'emploi des ECNE et les coûts qui y sont associés chez les patients qui ont reçu leur congé de l'hôpital dans l'année suivant un SCA. MÉTHODOLOGIE: Tous les patients qui ont reçu leur congé de l'hôpital après un diagnostic primaire de SCA en Alberta entre 2004-2005 et 2015-2016 ont été inclus. La fréquence des ECNE (épreuves d'effort [avec ou sans examen d'imagerie] et examens d'imagerie au repos) a été déterminée à partir des bases de données provinciales couplées. Les coûts ont été établis à partir de la liste des actes médicaux du régime d'assurance-maladie de l'Alberta. RÉSULTATS: Des 55 516 patients ayant présenté un SCA, 30 760 ont subi au moins un ECNE (55,4 %) dans l'année qui a suivi l'événement; 13 505 (24,3 %) d'entre eux ont subi plus d'un ECNE. Les tendances temporelles en matière d'ECNE affichent une hausse (épreuves d'effort : p tendance < 0,001; examens d'imagerie au repos : p tendance < 0,001). Les ECNE ont généralement été effectués au cours des 4 premiers mois après la sortie de l'hôpital (épreuves d'effort, dans les 2 mois; examens d'imagerie au repos, dans les 3 à 4 mois). Le coût total des ECNE effectués en 2015-2016 a été évalué à 1,35 M$, soit une hausse de 22,4 % par rapport à 2004-2005 (1,10 M$) (p < 0,001), tandis que l'incidence des SCA a diminué au cours de la même période (p = 0,008). CONCLUSIONS: Les taux d'ECNE effectués dans l'année suivant un SCA sont élevés et augmentent au fil du temps. Le coût estimatif de ces ECNE semble s'accroître de façon disproportionnée par rapport à la croissance des SCA. Une enquête plus approfondie s'impose; à l'heure actuelle, on ne peut que spéculer quant aux bienfaits cliniques qui découlent de l'augmentation des ECNE effectués après un SCA et des coûts qu'ils engendrent.