RESUMEN
We present an ab initio simulation of 90° ferroelastic twins that were recently observed in methylammonium lead iodide. There are two inequivalent types of 90° walls that we calculate to act as either electron or hole sinks, which leads us to propose a mechanism for enhancing charge carrier separation in photovoltaic devices. Despite separating nonpolar domains, we show these walls to have a substantial in-plane polarization of â¼6 µCâ¯cm-2, due in part to flexoelectricity. We suggest this in turn could allow for the photoferroic effect and create efficient pathways for photocurrents within the wall.
RESUMEN
Historians have found it difficult to give a general account of the early medical use of X-rays in medicine. While the rays were hailed by some as a miracle technology, their early medical application was patchy, often remaining subsidiary to traditional methods of diagnosis and treatment, and was of disputed value. In this essay, I argue that the selective appropriation of the new technology needs to be understood within the wider medical practice of the period. The argument is developed around the case of orthopedic surgery in Germany, probably the first example in which doctors quickly made X-rays indispensable as a medical tool. I show that value of X-rays in this case was contingent upon an ongoing dispute, the theory and practice of surgical intervention, and the sociology of new surgical knowledge.