Crossed Andreev reflection is a non-local transport phenomenon that creates and detects Cooper pair correlations between distant locations. It is also the basis of Cooper pair splitting to generate remote entanglement. Although crossed Andreev reflection has been extensively studied in semiconductors proximity-coupled to a superconductor, observing it in a topological insulator has been very difficult. Here we report the observation of this effect in a proximitized topological insulator nanowire. We perform local and non-local conductance spectroscopy on mesoscopic devices in which superconducting niobium and metallic contacts are connected to a bulk-insulating nanowire. In our local conductance measurements we detect a hard gap and the appearance of Andreev bound states that can reach zero bias. We also occasionally observe a negative non-local conductance when sweeping the chemical potential, providing evidence of crossed Andreev reflection. This signal is detected even over length scales much longer than the expected superconducting coherence length of either niobium or the proximitized nanowire. We suggest that this long-range effect is due to the intricate role of disorder in proximitized nanowires.
During the early development of multi-cellular animals, cells self-organize to set up the body axes such as the primary head-to-tail axis. Several signalling pathways are known to control body axis formation. Here we show that tissue mechanics also plays an important role. We focus on the emergence of a primary axis in initially spherical aggregates of mouse embryonic stem cells, which mirrors events in the development of the early mouse embryo. These aggregates break rotational symmetry by establishing domains of different expression profiles, for example, of the transcription factor T/Brachyury and the adhesion molecule E-cadherin. By combining quantitative microscopy and physical modelling, we identify large-scale tissue flows with a recirculating component that contribute substantially to the symmetry breaking. We show that the recirculating flows are—akin to Marangoni flows—driven by a difference in tissue surface tensions, whose existence we further confirm using aggregate fusion experiments. Our work highlights that body axis formation is not only driven by biochemical processes but can also be amplified by tissue flows. We expect that this type of amplification may operate in many other organoid and in vivo systems.
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