New evidence from studies of Bi2201 (crystal structure inset) along the temperature range shown in green strongly supports the idea that the pseudogap is in fact a distinct phase of matter that persists into the superconducting phase. If so, the T* phase transition must terminate in a quantum critical point (Xc) at zero temperature. Credit: Ruihua He; Berkeley Lab.
Investigators in the field of high-temperature superconductors have been stumped for some time about what is occurring between when the temperature of a material drops to the point (T*) where electrons begin to form Cooper pairs and the critical temperature (Tc) for full superconductivity. Heretofore, this odd transitional region has been dubbed a “pseudogap,” but now a collaborative research project has revealed that three different tests suggest the pseudogap is actually a distinct phase.
The collaboration included scientists from the Lawrence Berkeley National Laboratory, the University of California at Berkeley, Stanford University and the SLAC National Accelerator Lab and their results have just been published in Science (doi:10.1126/science.1198415).
Led by Zhi-Xun Shen, director of the Stanford Institute for Materials and Energy Science at SLAC and a professor of physics at Stanford University, the group focused only on Pb-Bi2201 (a lead bismuth strontium lanthanum copper oxide) because of the materials relatively wide range between T* and Tc.
Previous research supported two separate theories about the odd pseudogap: One theory is that it is just a range of gradual transition to superconductivity, and the other is that it is a state of material distinct from both superconductivity and normal “metallicity” with a quantum critical point.
“Promising as the ‘quantum critical’ paradigm is for explaining a wide range of exotic materials, high-Tc superconductivity in cuprates has stubbornly refused to fit the mold. For 20 years, the cuprates managed to conceal any evidence of a phase-transition line where the quantum critical point is supposed to be found,” says Joseph Orenstein in a news release from the Berkeley Lab. Orenstein works in the lab’s Materials Sciences Division and is a professor of physics at UC Berkeley, whose group conducted one of the research team’s three experiments.
As is always the case in these kinds of situations, the question becomes, so what?
According to the release, the hope is that once scientists can wrap their thinking around the concept of a quantum critical point (Xc), new routes to superconductivity can be found. “This is a paradigm shift in the way we understand high-temperature superconductivity,” says Ruihua He, lead author with Makoto Hashimoto. “The involvement of an additional phase, once fully understood, might open up new possibilities for achieving superconductivity at even higher temperatures in these materials.” These two worked with Shen at SIMES and also worked at Stanford’s Department of Applied Physics and Berkeley Lab’s Advanced Light Source.
One of the tests they conducted involved angle-resolved photoemission spectroscopy to track the kinetic energy and momentum of the emitted electrons over a temperature range.
In another test, investigators measured changes in rotations of the plane of polarization light reflected from the same Pb-Bi2201 sample under a zero magnetic field (magneto-optical Kerr effects). The rotations are proportional to the net magnetization of the sample at different temperatures.
Orenstein’s group performed the third test, a study of time-resolved reflectivity of the Pb-Bi2201 sample.
None of these tests were particularly novel — except that this time they were conducted on the same material and all yielded results consistent with what they expected if there indeed is a phase transition at the pseudogap phase boundary at T*.
Looking ahead, members of the group hope to exploit their discovery that the electronic states dominating the pseudogap phase do not include electron Cooper pairs found in a superconducting phase, yet seem to influence the motion of Cooper pairs in a way previously overlooked.
“Instead of pairing up, the electrons in the pseudogap phase organize themselves in some very different way,” says He. “We currently don’t know what exactly it is, and we don’t know whether it helps superconductivity or hurts it. But we know the direction to take to move forward.”
On the SLAC website, He outlines a plan, saying, “First to-do: uncover the nature of the pseudogap order. Second to-do: determine whether the pseudogap order is friend or foe to superconductivity. Third to-do: find a way to promote the pseudogap order if it’s a friend and suppress it if it’s a foe.”
In the SLAC story, Shen also confidently notes, “Our findings point to management and control of this other phase as the correct path toward optimizing these novel superconductors for energy applications, as well as searching for new superconductors.”