Learning about High‐$Tc$ Superconductors from Their Imperfections

In the old nursery rhyme, little Jack Horner stuck in his thumb and pulled out a plum. That's more or less what experimenters from the University of California, Berkeley, and the University of Tokyo did recently: They stuck a zinc atom into the copper site of a high‐$Tc$ superconductor and extracted a plum of an image using a specially developed scanning tunneling microscope (STM). That image provided dramatic visual confirmation of the d‐wave nature of the electron pairing state in such materials (see the cover of this issue).