Berezinskii–Kosterlitz–Thouless (BKT) transition: https://en.wikipedia.org/wiki/Berezinskii%E2%80%93Kosterlitz%E2%80%93Thouless_transition https://en.wikipedia.

Berezinskii–Kosterlitz–Thouless (BKT) transition: https://en.wikipedia.org/wiki/Berezinskii%E2%80%93Kosterlitz%E2%80%93Thouless_transition https://en.wikipedia.org/wiki/Berezinskii%E2%80%93Kosterlitz...