A long-sought phase transition in superconducting cuprates observed via resonant ultrasound spectroscopy

Abstract

Among the biggest mysteries of high-temperature superconductors is the so-called pseudogap—somewhat similar to the gap in the electronic density of states found in the superconducting phase, but occurring at a different temperature. The pseudogap may represent either the gradual onset of a precursor to superconductivity or an entirely new phase, characterized by the gain or loss of some hidden order. Several experiments in recent years have favored the latter, but the smoking gun, the thermodynamic signature of a pseudogap phase transition, had not been observed. Using resonant ultrasound spectroscopy, we measured the temperature-dependent elastic stiffness of two cuprate superconducting crystals, one underdoped and one overdoped and found a break in slope at a doping-dependent temperature T*. 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* < Tc so 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 (Nature 498, 75 (2013).