Evidence Accumulates for Unusual Behavior in Underdoped High‐$Tc$ Superconductors

As you cool a conventional superconductor, it acts as a metal until it reaches the critical temperature $Tc,$ below which it rapidly goes superconducting. But some of the high‐temperature, copper oxide materials—in particular, the underdoped ones, whose densities of charge carriers are below that which gives the highest $Tc$—seem to be preparing for the transition to a superconducting state well before they reach $Tc.$ Experiment after experiment on underdoped materials at temperatures above $Tc$ has revealed some kind of crossover or change in behavior, which some feel is related to the eventual onset of superconductivity at lower temperatures. When the temperature does reach $Tc,$ the material goes superconducting, but some of its properties exhibit relatively little additional discontinuity in behavior.