Laser ARPES Helps Explain Superconductivity

A new ultrafast laser angle-resolved photoemission spectroscopy (ARPES) technique may soon help scientists realize some of the biggest obstacles to the electronic states of high-temperatures superconductors so that they may one day put these energy-saving metals to practical use.

Superconductivity, in which electric current flows without resistance, promises significant energy savings, but for everyday applications such as low-voltage electric grids with no transmission losses or superefficienct motors and generators, conventional superconductivity can’t do the job. Superconductors must be maintained at temperatures a few degrees above absolute zero, which is difficult and expensive.

For wider uses, higher-temperature superconductors that can function well above absolute zero will need to be created. Yet known high-temperature (high-Tc) superconductors are complex materials whose electronic structures, despite decades of work, are still far from clear.

Part of the momentum map of Bi2212 derived from ultrafast laser ARPES shows that, after initial excitation by a pump probe, the speed with which quasiparticles recombine into Cooper pairs greatly depends on their position in momentum space. (Only one of the four corners of the Fermi surface momentum map is shown, as insets in left panels.) Near the central nodes, the quasiparticles recombine very slowly. Far from the nodes, they recombine quickly. (Image: Lanzara Group, Lawrence Berkeley National Laboratory and UC Berkeley)

Now scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley, have used a powerful new tool to attack some of the biggest obstacles to understanding the electronic states of high-temperature superconductors — the ultrafast laser ARPRES.

“What we’ve done with ultrafast laser ARPES is to start with a high-Tc superconductor called Bi2212 and cool it to well below the critical temperature where it becomes superconducting,” said Christoper Smallwood, first author of the paper.

The researchers fired an infrared laser pulse at the sample, temporarily cracking open some of the Cooper pairs — pairs of electrons that form correlated charge carriers that barely interact with their crystalline surroundings — into their constituent parts, called quasiparticles. As these states decayed, recombining back into Cooper pairs, the researchers used ARPES to measure their changing energy and momentum.

“The relaxation process takes just a few trillionths of a second from start to finish, and in the end, we were able to assemble and watch an extremely slow-motion movie of Cooper-pair formation, which showed that the quasiparticles tend to recombine faster or slower, depending both on their momentum and on the intensity of the pump pulse,” Smallwood said. “It’s an exciting development, because these trends may be directly connected to the mechanism holding Cooper pairs together.”

A Cooper pair has less energy than two independent electrons, leaving an energy gap between the sea of Cooper pairs and the usual lowest energy of the charge carriers in the material. Maps of this superconducting gap can be calculated, or, remarkably, they can be drawn directly by the charge carriers themselves.

In ARPES experiments, the electron’s momenta and angles that are knocked loose by a sufficiently energetic beam of light are used to map out the material’s momentum space on a flat detector screen. The momentum-space map shows the material’s band structure, the energy levels accessible to its charge carriers.

“We’re stuck with 5.9 electron-volt photon energy, and we can’t tune it much, like we could at the ALS,” Smallwood said. “But by happenstance, this energy is great for looking at high-Tc superconductors, and the low photon energy gives us better momentum resolution.”

Most high-Tc superconductors, including Bi2212, resemble cuprate ceramics, rich in copper and oxygen. The superconducting gap is uniform for almost all conventional metal superconductors, but in the cuprates, it varies greatly. For some momenta the gap is large, but at four special points in momentum space, it drops all the way to zero. The existence of such “nodes” in the gap is a distinguishing characteristic of cuprate high-Tc superconductors.

“This is where ultrafast laser ARPES, which is only about five years old, really comes into play to give us results not accessible by other means,” he said. “The laser we use is a titanium-sapphire laser that can emit femtosecond-scale pulses.”

Christopher Smallwood, left, led the ARPES work, and James Hinton led the optical work. Inset, top: Alessandra Lanzara led the collaboration with Joseph Orenstein and Dung-Hai Lee. All are with Berkeley Lab’s Materials Sciences Division and UC Berkeley’s Department of Physics. (Image: Lawrence Berkeley National Laboratory)

The same beam pulse that creates the infrared pump pulse is split to form the more energetic ultraviolet probe pulse, by passing part of it through frequency-doubling crystals. A motorized mirror can be used to adjust with femtosecond precision the time delay between pump and probe. The tiny sample can be tilted to any desired angle, which determines which part of the band structure is being examined by ARPES.

The research team determined the relation between the initial excitation energy, the quasiparticles’ position in momentum space, and how quickly the quasiparticles decay. Greater initial excitation energy gives faster recombination into Cooper pairs, but so does crystal momentum far from the nodes. Quasiparticles with momentum that places them near the nodes on the Fermi surface decay very slowly.

When additional ultrafast all-optical techniques, using infrared for both pump and probe pulses, were applied to the same sample, the results were in good agreement with ARPES.

“It’s exciting that now we are able to measure these components of recombination distinctly and see what each contributes,” Smallwood said. “It gives us a new handle on ways to assess some of the candidate ideas about how Cooper pairs form, such as the suggestion that the energy and momenta of quasiparticles far from a node may resonate with waves of spin density or charge density to form Cooper pairs. We’ve shown the way to measure this and other ideas to see if they play a significant role in the transition to high-temperature superconductivity.”

The research appeared in the June 1 issue of Science.

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