Ultrafast Carrier Relaxation Dynamics in a Nodal-Line Semimetal PtSn4.

Topological Dirac nodal-line semimetals host topologically nontrivial electronic structure with nodal-line crossings around the Fermi level, which could affect the photocarrier dynamics and lead to novel relaxation mechanisms. Herein, by using time- and angle-resolved photoemission spectroscopy, we reveal the previously inaccessible linear dispersions of the bulk conduction bands above the Fermi level in a Dirac nodal-line semimetal PtSn4, as well as the momentum and temporal evolution of the gapless nodal lines. A surprisingly ultrafast relaxation dynamics within a few hundred femtoseconds is revealed for photoexcited carriers in the nodal line. Theoretical calculations suggest that such ultrafast carrier relaxation is attributed to the multichannel scatterings among the complex metallic bands of PtSn4 via electron-phonon coupling. In addition, a unique dynamic relaxation mechanism contributed by the highly anisotropic Dirac nodal-line electronic structure is also identified. Our work provides a comprehensive understanding of the ultrafast carrier dynamics in a Dirac nodal-line semimetal.

Association Between Park Use and Moderate-to-Vigorous Activity During COVID-19 Years among a Cohort of Low-Income Youth.

Neighborhood parks are important venues to support moderate-to-vigorous (MVPA) activity. There has been a noticeable increase promoting physical activity among youth in neighborhood parks. This paper aims to assess the association between park use and MVPA among low-income youth in a large urban area. We recruited a cohort of 434 youth participants during the COVID pandemic years (2020-2022) from low-income households in Washington, D.C. We collected multiple data components: accelerometry, survey, and electronic health record data. We explored the bivariate relationship between the accelerometer-measured daily MVPA time outcome and survey-based park use measures. A mixed-effect model was fitted to adjust the effect estimate for participant-level and time-varying confounders. The overall average daily MVPA time is 16.0 min (SD = 12.7). The unadjusted bivariate relation between daily MVPA time and frequency of park visit is 1.3 min of daily MVPA time per one day with park visits (p < 0.0001). The model-adjusted estimate is 0.7 daily MVPA minutes for 1 day with park visit (p = 0.04). The duration of a typical park visit is not a significant predictor to daily MVPA time with or without adjustments. The initial COVID outbreak in 2020 resulted in a significant decline in daily MVPA time (- 4.7 min for 2020 versus 2022, p < 0.0001). Park visit frequency is a significant predictor to low-income youth's daily MVPA time with considerable absolute effect sizes compared with other barriers and facilitators. Promoting more frequent park use may be a useful means to improve low-income youth's MVPA outcome.