Physicists at the University of Miami may have discovered what has been preventing the steady progression of higher critical temperatures (T _c) in superconducting systems. In a theoretical physics article published in the May issue of Europhysics Letters (DOI: 10.1209/0295-5075/98/47011), Josef Ashkenazi and Neil F. Johnson from the University of Miami show that it is the quantum critical regime, in which different symmetry-breaking instability states are combined, which makes the suppression of superconductivity by such states diminish or disappear. The emergence of instabilities has been blocking progress to superconductivity at high temperatures. Such instabilities occur also in the cuprate system, where they are known as “striped” states; within the quantum critical zone, striped states containing different orientations and magnetic phases are combined.

“What we realized is that the existence of this quantum critical regime does take a combination of these different states,” Ashkenazi said. “When these different states are combined, then the suppression of superconductivity by the instability goes away, or at least diminishes a lot.”

Proposed phase diagram for hole-doped cuprates, under changing temperature and doping level p. Solid and dashed lines represent phase and gradual transitions, respectively. Dotted lines represent gradual transitions in the case that pairing is suppressed down to T = 0. AF stands for antiferromagnetic, SG for spin glass, and SC for superconducting. Image credit: Josef Ashkenazi and Neil F. Johnson, University of Miami.

So how are broken symmetry states combined? There has been a lot of discussion about a type of “glue” that would hold paired charge carriers together, in particular the two electrons in a Cooper pair. In Johnson and Ashkenazi’s auxiliary Bose condensates (ABC) theory, a strong glue arises in the cuprates in a quantum critical regime while, by contrast, the system moves outside this regime at low and high levels of doping. At low doping levels the glue becomes inactive because of a broken symmetry instability state that blocks superconductivity, while at high doping levels the glue “fades away,” the researchers said.

“The reason that superconductivity could not advance above low T _c in the past was not because there were no strong coupling constants,” Ashkenazi said. Strong coupling due to phonons, and non-phononic magnetic excitations, or other excitations, do exist. “But the system could not progress to very high T _c because of the competing symmetry-breaking instabilities, which would kill it.”

The researchers suggested a phase diagram by plotting the absolute temperature T versus the hole-doping concentration p for the cuprate system. If pairing is suppressed down to T = 0, a quantum critical point p _c defines the starting point for a quantum phase transition. In the absence of pairing, the quantum critical regime starts at this point and includes the area between the dotted lines in the figure, representing extensions of the Fermi liquid phase and the pseudogap phase of the system. The occurrence of pairing extends the range of quantum criticality into the superconductivity and pseudogap regimes.

As far as what this discovery might mean to experimentalists, Ashkenazi said, “I think this work is an important breakthrough toward a more focused way to give instruction to experimentalists to look for where to go to find high T _c superconductors and, hopefully, maybe even room-temperature superconductors. In general you have to look close to phase transitions—in particular, the proximity of a metal–insulator Mott transition is a good place to look for higher T _c materials—but our work sets the stage for more work to be done.”