Excitons have fundamental impacts on optoelectronic properties of semiconductors. Halide perovskites, with long carrier lifetimes and ionic crystal structures, may support highly mobile excitons because the dipolar nature of excitons suppresses phonon scattering. Inspired by recent experimental progress, we perform device modeling to rigorously analyze exciton formation and transport in methylammonium lead triiodide under local photoexcitation by using a finite element method. Mobile excitons, coexisting with free carriers, can dominate photocurrent generation at low temperatures. The simulation results are in excellent agreement with the experimentally observed strong temperature and gate dependence of carrier diffusion. This work signifies that efficient exciton transport can substantially influence charge transport in the family of perovskite materials.
Experimental detection of multiple stable magnetic configurational states in isolated square permalloy particles of side length on the order of 200 nm is reported. The magnetic states are characterized using the anisotropic magnetoresistance via four-terminal resistance measurements of individual particles, and results are corroborated with micromagnetic simulations. The particles tend to relax into a ground state U-shaped “buckle” configuration at larger sizes and for an applied field swept parallel to the particle's edge, but assume an S-shaped configuration at smaller sizes and for slight variations in the applied field angle. The occurrence of this metastable state at room temperature indicates that typical models characterizing such particles in terms of energy landscapes or local effective fields may not be sufficient to accurately describe systems at this scale.
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