Current Runs Along Secret Paths express

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In the crystal CsV3Sb5-xSnx, which becomes a superconductor at low temperatures, current does not flow as a uniform stream but along narrow, winding paths. A scanning microscope with a sensitive magnetic field sensor helped reveal these paths: it picks up the faintest currents, much like a sonar probes the seafloor. The picture resembles a network of streams cutting through rock: some regions carry current, others remain blocked.

These current channels emerge exactly at the temperature where the material loses its resistance. From then on, they lead their own life: they do not vanish upon warming, nor do they blur even when the current is increased. Their behavior is nonlinear — current grows not smoothly but in jumps. This reveals that the channels act as a chain of Josephson junctions (places where supercurrent leaks through a thin barrier). Such 'streamlet' transport naturally explains long-standing puzzles: why the material exhibits a spontaneous diode effect (current flows more easily in one direction) and where the internal Josephson junctions come from.

The experiment provides the first spatial map of how supercurrent flows in kagome superconductors. Previously, the internal structure could only be inferred from bulk electrical measurements, like guessing the bottom topography from ripples on the water. Now it is clear that the material spontaneously fragments into conducting filaments, and this fragmentation is enhanced by impurities. This approach paves the way for local studies of other systems where different types of order coexist and compete.