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The interaction between K atoms and oxygen molecules on solid surfaces is of topical interest to oxidation–reduction processes in K–O2 batteries. Alkali metals have one ns electron in their valence shell, making them highly chemically reactive toward oxidizing reactants. Mechanistic information on the oxygen reduction by K at the atomic level is scarce despite its key role in defining the alkali metal–O2 battery performance. Here, we use scanning tunneling microscopy and density functional theory to investigate the reduction of a single oxygen molecule by K atoms codeposited on the Ag(111) surface. Our study provides fundamental chemical information on the binary and collective interactions between the O2 and K atoms on metal surfaces. 

This paper addresses the challenge of constructing finite element curl div complexes in three dimensions. Tangential-normal continuity is introduced in order to develop distributional finite element curl div complexes. The spaces constructed are applied to discretize the quad curl problem, demonstrating optimal order of convergence. Furthermore, a hybridization technique is proposed, demonstrating its equivalence to nonconforming finite elements and weak Galerkin methods. 

Abstract Although the existing causal inference literature focuses on the forward-looking perspective by estimating effects of causes, the backward-looking perspective can provide insights into causes of effects. In backward-looking causal inference, the probability of necessity measures the probability that a certain event is caused by the treatment given the observed treatment and outcome. Most existing results focus on binary outcomes. Motivated by applications with ordinal outcomes, we propose a general definition of the probability of necessity. However, identifying the probability of necessity is challenging because it involves the joint distribution of the potential outcomes. We propose a novel assumption of monotonic incremental treatment effect to identify the probability of necessity with ordinal outcomes. We also discuss the testable implications of this key identification assumption. When it fails, we derive explicit formulas of the sharp large-sample bounds on the probability of necessity. 

We study superfluid drag in the two-component Bose-Hubbard model with infinitely strong repulsive interac- tions. In this system, all transport is mediated by the motion of empty sites, or “holes,” and it is hard to move one component without moving the other. We demonstrate, with a combination of analytic and numeric techniques, that the motion of holes leads to strong dissipationless coupling between currents in the two components. This behavior is attributable to polaronic correlations that emerge in the presence of spin currents, which can be observed in experiments. We derive a closed-form expression for the coupling on various lattices in arbitrary spatial dimensions, which we verify through numerical simulations on two-dimensional lattices. 

Abstract The spatial and temporal control of material properties at a distance has yielded many unique innovations including photo-patterning, 3D-printing, and architected material design. To date, most of these innovations have relied on light, heat, sound, or electric current as stimuli for controlling the material properties. Here, we demonstrate that an electric field can induce chemical reactions and subsequent polymerization in composites via piezoelectrically-mediated transduction. The response to an electric field rather than through direct contact with an electrode is mediated by a nanoparticle transducer, i.e., piezoelectric ZnO, which mediates reactions between thiol and alkene monomers, resulting in tunable moduli as a function of voltage, time, and the frequency of the applied AC power. The reactivity of the mixture and the modulus of a naïve material containing these elements can be programmed based on the distribution of the electric field strength. This programmability results in multi-stiffness gels. Additionally, the system can be adjusted for the formation of an electro-adhesive. This simple and generalizable design opens avenues for facile application in adaptive damping and variable-rigidity materials, adhesive, soft robotics, and potentially tissue engineering.