Instabilities near Ultrastrong Coupling in a Microwave Optomechanical Cavity.

With artificially engineered systems, it is now possible to realize the coherent interaction rate, which can become comparable to the mode frequencies, a regime known as ultrastrong coupling (USC). We experimentally realize a cavity-electromechanical device using a superconducting waveguide cavity and a mechanical resonator. In the presence of a strong pump, the mechanical-polaritons splitting can nearly reach 81% of the mechanical frequency, overwhelming all the dissipation rates. Approaching the USC limit, the steady-state response becomes unstable. We systematically measure the boundary of the unstable response while varying the pump parameters. The unstable dynamics display rich phases, such as self-induced oscillations, period-doubling bifurcation, and period-tripling oscillations, ultimately leading to the chaotic behavior. The experimental results and their theoretical modeling suggest the importance of residual nonlinear interaction terms in the weak-dissipative regime.

Light-Induced Hypoxia in Carbon Quantum Dots and Ultrahigh Photocatalytic Efficiency.

Carbon quantum dots (CQDs) represent a class of carbon materials exhibiting photoresponse and many potential applications. Here, we present a unique property that dissolved CQDs capture large amounts of molecular oxygen from the air, the quantity of which can be controlled by light irradiation. The O2 content can be varied between a remarkable 1 wt % of the CQDs in the dark to nearly half of it under illumination, in a reversible manner. Moreover, O2 depletion enhances away from the air-solution interface as the nearby CQDs quickly regain them from the air, creating a pronounced concentration gradient in the solution. We elucidate the role of the CQD functional groups and show that excitons generated under light are responsible for their tunable adsorbed-oxygen content. Because of O2 enrichment, the photocatalytic efficiency of the CQDs toward oxidation of benzylamines in the air is the same as under oxygen flow and far higher than the existing photocatalysts. The findings should encourage the development of a new class of oxygen-enricher materials and air as a sustainable oxidant in chemical transformations.

Ab-initio investigation of preferential triangular self-formation of oxide heterostructures of monolayer [Formula: see text].

Triangular growth patterns of pristine two-dimensional (2D) transition metal dichalcogenides (TMDs) are ubiquitous in experiments. Here, we use first-principles calculations to investigate the growth of triangular shaped oxide islands upon layer-by-layer controlled oxidation in monolayer and few-layer [Formula: see text] systems. Pristine 2D TMDs with a trigonal prismatic geometry prefer the triangular growth morphology due to structural stability arising from the edge chalcogen atoms along its three sides. Our ab-initio energetics and thermodynamic study show that, since the Se atoms are more susceptible to oxygen replacement, the preferential oxidation happens along the Se zigzag lines, producing triangular islands of transition metal oxides. The thermodynamic stability arising from the preferential triangular self-formation of TMD based oxide heterostructures and their electronic properties opens a new avenue for their exploration in advanced electronic and optoelectronic devices.