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Optical control of magnons in two-dimensional (2D) materials promises new functionalities for spintronics and magnonics in atomically thin devices. Here, we report control of magnon dynamics, using laser polarization, in a ferromagnetic van der Waals (vdW) material, Fe_3.6Co_1.4GeTe₂. The magnon amplitude, frequency, and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy. We show substantial (over 25%) and continuous modulation of magnon dynamics as a function of incident laser polarization. Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption. This implies that pump laser pulses modify the local spin environment, which enables the launch of magnons with tunable dynamics. Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations. Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices. 

Abstract Nitrous oxide (N₂O) is a greenhouse gas and stratospheric ozone‐depleting substance with large and growing anthropogenic emissions. Previous studies identified the influx of N₂O‐depleted air from the stratosphere to partly cause the seasonality in tropospheric N₂O (aN₂O), but other contributions remain unclear. Here, we combine surface fluxes from eight land and four ocean models from phase 2 of the Nitrogen/N₂O Model Intercomparison Project with tropospheric transport modeling to simulate aN₂O at eight remote air sampling sites for modern and pre‐industrial periods. Models show general agreement on the seasonal phasing of zonal‐average N₂O fluxes for most sites, but seasonal peak‐to‐peak amplitudes differ several‐fold across models. The modeled seasonal amplitude of surface aN₂O ranges from 0.25 to 0.80 ppb (interquartile ranges 21%–52% of median) for land, 0.14–0.25 ppb (17%–68%) for ocean, and 0.28–0.77 ppb (23%–52%) for combined flux contributions. The observed seasonal amplitude ranges from 0.34 to 1.08 ppb for these sites. The stratospheric contributions to aN₂O, inferred by the difference between the surface‐troposphere model and observations, show 16%–126% larger amplitudes and minima delayed by ∼1 month compared to Northern Hemisphere site observations. Land fluxes and their seasonal amplitude have increased since the pre‐industrial era and are projected to grow further under anthropogenic activities. Our results demonstrate the increasing importance of land fluxes for aN₂O seasonality. Considering the large model spread, in situ aN₂O observations and atmospheric transport‐chemistry models will provide opportunities for constraining terrestrial and oceanic biosphere models, critical for projecting carbon‐nitrogen cycles under ongoing global warming. 

This article explores and characterizes the pandemic urbanism of NYC during the first year of COVID-19. It analyzes changes to the sidewalk and the urban lifestyle using a novel method of remote ethnography: the integrated use of Zoom video conferencing and GPS smartphone tracking to interview participants as they walked and filmed the city. The dataset, composed of transcripts, videos, and routes, was analyzed to reveal recurring themes and visualized through individual Scrollytelling maps combined into one Supermap. The findings are broken down into: 1) changes to the sidewalk, including fewer people, more outdoor sports activity, signs of social distancing, signs of closure, more bikes, and construction; and 2) lifestyle changes, including longings for the urban lifestyle, new-formed solidarity, a renewed appreciation for the local neighborhood, an undercurrent of “moving out” of the city or “moving up” to a better neighborhood, and a difference between Manhattan and the boroughs. 

Abstract Optical manipulation of coherent phonon frequency in two-dimensional (2D) materials could advance the development of ultrafast phononics in atomic-thin platforms. However, conventional approaches for such control are limited to doping, strain, structural or thermal engineering. Here, we report the experimental observation of strong laser-polarization control of coherent phonon frequency through time-resolved pump-probe spectroscopic study of van der Waals (vdW) materials Fe 3 GeTe 2 . When the polarization of the pumping laser with tilted incidence is swept between in-plane and out-of-plane orientations, the frequencies of excited phonons can be monotonically tuned by as large as 3% (~100 GHz). Our first-principles calculations suggest the strong planar and vertical inter-atomic interaction asymmetry in layered materials accounts for the observed polarization-dependent phonon frequencies, as in-plane/out-of-plane polarization modifies the restoring force of the lattice vibration differently. Our work provides insightful understanding of the coherent phonon dynamics in layered vdW materials and opens up new avenues to optically manipulating coherent phonons. 

The COVID-19 pandemic, travel restrictions, and social distancing measures have made it difficult to observe, monitor, or manage urban life. To capture the experience of being in New York City during the first year of the COVID-19 pandemic, we used a novel method of remote ethnography to interview people who were walking the city. We developed the Walkie-Talkie Map to collect and present these interviews, enabling website visitors to see what the subject saw as they walked the route of their choice. Visitors can interactively scroll through the interview and have access to additional visualizations and imagery that contextualize the main narrative. Visitors are thus able to vicariously experience what it was like to be in New York City at the outset of the COVID-19 epidemic. This work provides a case study on how to perform observational research when geographic and bodily distance has become the norm. We discuss the advantages and limitations of our method and conclude with its contributions to the study of cities and for others looking to conduct remote observational research in different fields of knowledge. The Walkie-Talkie maps can be found on this url: https://www.socialdistancing.tech.cornell.edu/what-is-a-walike-talkie