FIGURE 1. Scanning electron microscopic images of NbSe₃ crystal topology.

a–c, The three types of topology, classified by the nature of their twisting (shown schematically beneath images; white ribbons represent NbSe₃ crystals, red spheres represent selenium droplets). a, Ring structure (0 twist). b, Möbius strip (1 twist). c, Figure-of-eight strip (2 twist). d, NbSe₃ fibres (white streaks) bend into rings around a selenium drop of diameter about 50 m. The NbSe₃ ribbon is spooled onto a selenium droplet by surface tension until its ends bind together. e, High-magnification image showing a twist in a ribbon of NbSe₃ crystals; spooling of the ribbon can produce a twist, as in the formation of a Möbius crystal, owing to its anisotropic elastic properties. Scale bars, 10 m.