A long-sought phase transition in superconducting cuprates

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 T _c, 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. That stiffness, a fundamental thermodynamic property, exhibited a break in slope at a doping-dependent temperature T*. (The team’s two measurements are indicated by the red dots in the figure.) For the underdoped cuprate, T* coincides with the onset of the pseudogap and with earlier neutron-scattering measurements of the appearance of magnetic order (blue squares). Crucially, for the overdoped cuprate, T* < T _c. The researchers conclude that extrapolating to higher doping where T* = 0 will yield a quantum critical point , which may be key to understanding the mechanism of high-temperature superconductivity. (A. Shekhter et al., Nature 498, 75, 2013 .)—Johanna Miller