It's been more than a quarter century since the discovery of high-temperature superconductivity in a class of copper oxide compounds, and the materials still harbor many secrets.
Among the biggest mysteries of high-temperature superconductors is the so-called pseudogap observed in cuprates doped with a suboptimal concentration of charge carriers. At temperatures well above the superconducting transition temperature Tc, underdoped cuprates exhibit changes in the character of charge carriers—somewhat similar to the gap in the electronic density of states found in the superconducting phase. Two rival explanations have been advanced. In one, the pseudogap represents the gradual onset of a precursor to superconductivity. In the other, it heralds an entirely new phase, characterized by the gain or loss of some hidden order. Several experiments in recent years have favored the latter theory. But the smoking gun, the thermodynamic signature of a phase transition, has remained elusive until now. Using resonant ultrasound spectroscopy, postdoc Arkady Shekhter, Albert Migliori, and colleagues at the National High Magnetic Field Laboratory at Los Alamos National Laboratory measured the temperature-dependent elastic stiffness of two cuprate crystals, one underdoped and one overdoped.
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