The number indicates what-fold rotational symmetry they have, so the symbol labeled C2 has two-fold symmetry, for example. All cyclic shapes have a mirror image that “spins the other way. ...

The corresponding paper (Superconductivity with Two-Fold Symmetry in Topological Superconductor SrxBi2Se3) is published on Science China Physics, Mechanics & Astronomy.

Two-Fold-Symmetric Magnetoresistance in Single Crystals of Tetragonal BiCh2-Based Superconductor LaO0.5F0.5BiSSe. Journal of the Physical Society of Japan , 2019; 88 (3): 033704 DOI: 10.7566/JPSJ ...

The image has a two-fold symmetry, which is inconsistent with the bulk four-fold symmetry. (Image: Yoshiyuki Ohtsubo, Shin-ichi Kimura) Physicists have believed that metallic surface states of ...

YOU might remember learning about symmetry at school. Maybe a teacher showed you a snowflake’s six-fold symmetry and you marvelled at how it looked the same no matter how you rotated it.