Made to order
Chirality is a common property of materials in nature, perhaps most readily recognized in molecules that are left- and right-handed mirror images of one another. In condensed-matter physics, crystalline electronic structures can similarly display geometrical chirality, which tends to be predetermined when the crystal lattice is formed. But in some materials, electrons can spontaneously arrange themselves to impart chirality to a structure that was not chiral. This gyrotropic ordering has been described as the quantum equivalent of cholesteric liquid crystals, but it has proved hard to observe. In this week’s issue, Nuh Gedik and his colleagues report they have induced and observed gyrotropic ordering in a transition-metal dichalcogenide — the semimetal 1T-TiSe2. The cover image offers an artist’s interpretation of the team’s method, by which circularly polarized light shone on 1T-TiSe2 while cooling it below the critical temperature results in preferential formation of one chiral domain.