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UC Santa Barbara researchers (see story below) were able to link Einstein’s general theory of relativity to a totally different area of physics and hope the tools will one day shed light on new types of superconductors. Credit: Jorge Santos.

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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.”

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A new story in Science reports that an international team of researchers have been able to turn a non-superconducting form of copper oxide into a superconductor using a strong laser burst.

The team, working in Germany, Japan and the U.K., says it hopes its discovery might open a new path to high temperature superconductivity.

“We have used light to turn a normal insulator into a superconductor,” says Andrea Cavalleri, in a news release from Oxford University.

The group says it used a femtosecond pulses of a mid-infrared laser to transform non-superconducting (above 5 K) La_1.675Eu_0.2Sr_0.125CuO₄ (LESCO_1/8) — a strip-ordered compound — into a transient superconductor. The material was at a  base temperature under 20 K superconductor, and displayed superconductivity for a fraction of a second before returning to its normal state.

“We have shown that the non-superconducting state and the superconducting one are not that different in these materials, in that it takes only a millionth of a millionth of a second to make the electrons ’synch up’ and superconduct,” says Cavalleri. “This must mean that they were essentially already synched in the non-superconductor, but something was preventing them from sliding around with zero resistance. The precisely tuned laser light removes the frustration, unlocking the superconductivity.”

Cavalleri, a professor in the Department of Physics at Oxford and the Max Planck Department for Structural Dynamics, Hamburg, continues. “That’s already exciting in terms of what it tells us about this class of materials. But the question now is can we take a material to a much higher temperature and make it a superconductor?”

Cavalleri’s group also included researchers from the Department of Advanced Materials Science, University of Tokyo, and the RIKEN Advanced Science Institute, Japan.

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Physicists at Brookhaven National Laboratory have identified a single layer responsible for one such material’s ability to become superconducting. The technique, described in the Oct. 30, 2009, issue of Science, could be used to engineer ultrathin films with “tunable” superconductivity for higher-efficiency electronic devices.

The thinner the material (and the higher its transition temperature to a superconductor), the greater its potential for applications where the superconductivity can be controlled by an external electric field. “This type of control is difficult to achieve with thicker films, because an electric field does not penetrate into metals more than a nanometer or so,” explains Brookhaven physicist and the group leader Ivan Bozovic.

To explore the limits of thinness, Bozovic’s group synthesized a series of films based on the high-temperature superconducting cuprates — materials that carry current with no energy loss when cooled below a certain transition temperature. Since zinc is known to suppress the superconductivity in these materials, the scientists systematically substituted a small amount of zinc into each of the copper-oxide layers. Any layer where the zinc’s presence had a suppressing effect would be clearly identified as essential to superconductivity in the film.

This discovery opens a path toward the fabrication of electronic devices with modulated, or tunable, superconducting properties which can be controlled by electric or magnetic fields.

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Superconducting signature

For a while now, some scientists have thought that conditions necessary for superconductivity at higher temperatures exist. Now, a paper published in Science adds some fuel to their argument. It concerns the work of a group of U.S. and Japanese researchers – sponsored by a set of U.S. and Japanese government agencies – doing observations of low-temperature superconducting materials who say the spectroscopic signature of the materials seems to indicate that some superconductivity properties continue as the temperature increases.

“Our measurements give the most definitive spectroscopic evidence that the material we studied is a superconductor, even above the transition temperature, but one without the quantum phase coherence required for current to flow with no resistance,” said team leader Seamus Davis, a physicist at the Brookhaven National Lab and Cornell University, who led the research team. “The spectroscopic ‘fingerprint’ confirms that, at these higher temperatures, electrons are pairing up as they must in a superconductor, but for some reason they are not co-operating coherently to carry current.”

Recently there has been interest in the high-temperature possibilities of copper-oxide superconductors containing bismuth, strontium and calcium (BSCCO). Previously, Davis’ group was able to assemble a detailed spectroscopic signature containing all the quantum mechanical details of that superconducting state. Once this was established, they made spectroscopic observations of the cuprate material as it warmed above the 37 K transition temperature.

BNL scientists are now able to grow large BSCCO crystals. Credit: BNL

“We found that the characteristic signature passes unchanged from the superconducting state into the parent state - up to temperatures of at least 55 K, or 1.5 times the transition temperature,” Davis said. “We know of no explanation for why this fingerprint should remain other than that it represents the phase-incoherent superconducting state which has been proposed to exist based on other kinds of measurements.”

The group’s next step is to try to get a handle on why the cooperation between electron pairs breaks down. Their plan is to start tinkering with the doping of the copper-oxide planes in the layered material and measure the strength of quantum phase fluctuations.

Also, another group of BNL researchers has been on a similar mission, but studying how the magnetic properties of BSCCO change as temperatures are increased above normal superconducting ranges. One hurdle for this group has been creating BSCCO crystals large enough for observation, but as the accompanying picture shows, that the size problem has been solved.

“Many theorists believe that magnetism is important for high-temperature superconductivity, although they don’t agree on how it is important,” said Brookhaven physicist John Tranquada, who led the research team.

“The calculations based on the material’s electronic properties — which change dramatically as the material is cooled and transitions from its electrically resistive state to become a superconductor — predicted there would be a similar large change in magnetic characteristics below the transition temperature,” said Brookhaven physicist Guangyong Xu.

“But our direct measurements of the magnetic properties showed surprisingly little change. This implies that the model the theorists have been using to describe these magnetic properties is incomplete. It could be that the magnetism somehow drives the electronic structure, rather than the other way around — or that something underlying both magnetism and electronic structure influences both but in different ways,” Xu said.

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