Hybridizing collective spin excitations and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics and can also have potential applications to quantum information. Here we report an experimental study of cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet (YIG) sphere at both cryogenic and room temperatures. We observe for the first time a strong coupling of the same cavity mode to both a ferromagnetic-resonance (FMR) mode and a magnetostatic (MS) mode near FMR in the quantum limit. This is achieved at a temperature~22 mK, where the average microwave photon number in the cavity is less than one. At room temperature, we also observe strong coupling of the cavity mode to the FMR mode in the same YIG sphere and find a slight increase of the damping rate of the FMR mode. These observations reveal the extraordinary robustness of the FMR mode against temperature. However, the MS mode becomes unobservable at room temperature in the measured transmission spectrum of the microwave cavity containing the YIG sphere. Our numerical simulations show that this is due to a drastic increase of the damping rate of the MS mode.

Co 3 O 4 is an attractive earth-abundant catalyst for CO oxidation, and its high catalytic activity has been attributed to Co 3+ cations surrounded by Co 2+ ions. Hence, the majority of efforts for enhancing the activity of Co 3 O 4 have been focused on exposing more Co 3+ cations on the surface. Herein, we enhance the catalytic activity of Co 3 O 4 by replacing the Co 2+ ions in the lattice with Cu 2+ . Polycrystalline Co 3 O 4 nanowires for which Co 2+ is substituted with Cu 2+ are synthesized using a modified hydrothermal method. The Cusubstituted Co 3 O 4 _Cux polycrystalline nanowires exhibit much higher catalytic activity for CO oxidation than pure Co 3 O 4 polycrystalline nanowires and catalytic activity similar to those single crystalline Co 3 O 4 nanobelts with predominantly exposed most active {110} planes. Our computational simulations reveal that Cu 2+ substitution for Co 2+ is preferred over Co 3+ both in the Co 3 O 4 bulk and at the surface. The presence of Cu dopants changes the CO adsorption on the Co 3+ surface sites only slightly, but the oxygen vacancy is more favorably formed in the bonding of Co 3+ −O−Cu 2+ than in Co 3+ −O−Co 2+ . This study provides a general approach for rational optimization of nanostructured metal oxide catalysts by substituting inactive cations near the active sites and thereby increasing the overall activity of the exposed surfaces. ■ INTRODUCTIONCarbon monoxide (CO) emission from transportation and industrial activities is harmful to both human health and the environment. Currently, CO emission is effectively reduced, mainly through catalytic oxidation over catalysts. 1−4 The most active catalysts for CO oxidation are noble metals, but they are expensive and are of limited supply. Co 3 O 4 has emerged as an attractive alternative catalyst for CO oxidation because of its optimal CO adsorption strength, low barrier for CO reaction with lattice O, and excellent redox capacity. 1,5−8 A breakthrough on Co 3 O 4 for catalytic CO oxidation showing that Co 3 O 4 nanorods with predominantly exposed {110} planes exhibit a much higher catalytic activity for CO oxidation and larger resistance to deactivation by water than Co 3 O 4 nanoparticles was reported by Xie et al. 9 The high catalytic activity of Co 3 O 4 {110} planes is attributed to its higher concentration of Co 3+ cations (correspondingly fewer Co 2+ cations) than other crystal planes, since only Co 3+ cations surrounded by Co 2+ ions are active for catalytic oxidation of CO. 10,11 Subsequently, a number of Co 3 O 4 nanostructures, ranging from nanobelts, nanospheres, nanocubes, and nanotubes to nanowires, have been synthesized with the purpose of preferentially exposing Co 3+ cations. 11−14 Nevertheless, regardless of the morphology of the Co 3 O 4 nanostructures, even the highly active Co 3 O 4 {110} planes still contain Co 2+ cations, which have been assumed to be inactive for catalytic oxidation of CO, 9−11 and ultimately limits the catalytic activity of Co 3 O 4 for CO oxidation. Therefore, substituting Co 2+ with...

A three-dimensional conductive sponge made of electrospun nanofibers can be used in the design of a highly sensitive tactile pressure sensor.

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