Gauging urban resilience in the United States during the COVID-19 pandemic via social network analysis.

Social distancing policies and other restrictive measures have demonstrated efficacy in curbing the spread of the COVID-19 pandemic. However, these interventions have concurrently led to short- and long-term alterations in social connectedness. Comprehending the transformation in intracity social interactions is imperative for facilitating post-pandemic recovery and development. In this research, we employ social network analysis (SNA) to delve into the nuances of urban resilience. Specifically, we constructed intricate networks utilizing human mobility data to represent the impact of social interactions on the structural attributes of social networks while assessing urban resilience by examining the stability features of social connectedness. Our findings disclose a diverse array of responses to social distancing policies regarding social connectedness and varied social reactions across U.S. Metropolitan Statistical Areas (MSAs). Social networks generally exhibited a shift from dense to sparse configurations during restrictive orders, followed by a transition from sparse to dense arrangements upon relaxation of said orders. Furthermore, we analyzed the alterations in social connectedness as demonstrated by network centrality, which can presumably be attributed to the rigidity of policies and the inherent qualities of the examined MSAs. Our findings contribute valuable scientific insights to support informed decision-making for post-pandemic recovery and development initiatives.

Orientation Switching of Single Molecules on Surface Excited by Tunneling Electrons and Ultrafast Laser Pulses.

We investigate the orientation switching of individual azobenzene molecules adsorbed on a Au(111) surface using a laser-assisted scanning tunneling microscope (STM). It is found that the rotational motion of the molecule can be regulated by both sample bias and laser wavelength. By measuring the switching rate and state occupation as a function of both bias voltage and photon energy, the threshold in sample bias and the minimal photon energy are derived. It has been revealed that the tip-induced local electrostatic potential remarkably contributes to the reduction in hopping barrier. We also find that the tunneling electrons and photons play distinct roles in controlling rotational dynamics of single azobenzene molecules on the surface, which are useful for understanding dynamic behaviors in similar molecular systems.

Identifying Few-Molecule Water Clusters with High Precision on Au(111) Surface.

Revealing the nature of a hydrogen-bond network in water structures is one of the imperative objectives of science. With the use of a low-temperature scanning tunneling microscope, water clusters on a Au(111) surface were directly imaged with molecular resolution by a functionalized tip. The internal structures of the water clusters as well as the geometry variations with the increase of size were identified. In contrast to a buckled water hexamer predicted by previous theoretical calculations, our results present deterministic evidence for a flat configuration of water hexamers on Au(111), corroborated by density functional theory calculations with properly implemented van der Waals corrections. The consistency between the experimental observations and improved theoretical calculations not only renders the internal structures of absorbed water clusters unambiguously, but also directly manifests the crucial role of van der Waals interactions in constructing water-solid interfaces.