Xiang Li

A thin nanowire of α-Bi₄Br₄ — a compound of bismuth and bromine — conducts current not like an ordinary wire. Inside, electricity moves only along the edges, like water in a trough, not touching the bottom or sides. This behavior is called topological insulation: the interior does not conduct, but electrons glide losslessly along the surface or edges.

The crystal of perovskite iridate SrIrO₃ resembles a row of soldiers, where each iridium carries a magnetic moment. When some of the iridium 'soldiers' are replaced by tin, which has no magnetic moment, the row thins out.

A black hole cooled to nearly absolute zero resembles a musical instrument with only a limited set of possible notes. Strike it, and it will respond not with any sound, but only with a strictly defined set of frequencies.