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April 06, 2010

Another Piece of the Superconducting Puzzle - Particle-Hole Symmetry Breaking in the Pseudogap State of Bi2201

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Work by the Stanford Institute for Materials and Energy Science provides a new understanding of how high-temperature superconductors work—with potential applications toward the design of new superconductors that work at or near room temperature, allowing them to be used in everything from electronics to smart grids that deliver energy with dramatically higher efficiency.

"In 2006, our group published a paper suggesting that there are two types of distinct energy gaps," He said. "This more recent work provides a conclusive argument that there are two different mechanisms involved here."



The researchers trained the X-ray beam of SLAC's Stanford Synchrotron Radiation Lightsource on a high-temperature superconductor to reveal the material's electronic structure and explore the nature of the pseudogap. They were on a hunt for evidence of the electron pairing seen in conventional superconductors, in the form of what's called "electron-hole symmetry"; if it were present in the pseudogap, then the road to designing even higher temperature superconductors would be to make the pairs dance together instead of resting dormant.

But that's not what the researchers found; under the bright X-ray beam, the high-temperature superconductor showed a clear lack of the telltale symmetry—and thus of electron pairing. This suggests that the electron pairs were not lying dormant; they were simply not there.

The researchers posit that the electrons do not pair in this temperature range and instead travel in a wave; what they observed at SSRL were crests and troughs of electron density. The electrons' tendency to travel in a density wave may compete with their efforts to pair, suggesting that scientists will need a different approach in order to create a room-temperature superconductor.

"This is a very difficult problem, but an important one to solve," said Hashimoto. "We don't yet know the details of the density wave, but by extending our studies to different materials we are now seeking to understand it."

Once researchers better understand how electrons travel in high-temperature superconductors, they can then begin trying to design materials that superconduct at even higher temperatures. So far, high-temperature superconductors have been found only through serendipity. A robust understanding of how electrons travel at high temperatures may allow researchers to design new superconductors from the ground up, pinpointing the most useful temperature range for each application.

"If we can figure out the elusive recipe for making a superconductor," said SIMES Co-deputy Director and paper co-author Tom Devereaux, "we can begin designing them for important

Nature Physics - Particle–hole symmetry breaking in the pseudogap state of Bi2201

In conventional superconductors, a gap exists in the energy absorption spectrum only below the transition temperature (Tc), corresponding to the price to pay in energy for breaking a Cooper pair of electrons and creating two excited states. In high-Tc cuprate superconductors above Tc but below a temperature T*, an energy gap called the pseudogap1 exists, and is controversially attributed either to pre-formed superconducting pairs, which would show particle–hole symmetry, or to competing phases that would typically break it. Scanning tunnelling microscopy (STM) studies suggest that the pseudogap stems from lattice translational symmetry breaking and is associated with a different characteristic spectrum for adding or removing electrons (particle–hole asymmetry). However, no signature of either energy or spatial symmetry breaking of the pseudogap has previously been observed by angle-resolved photoemission spectroscopy (ARPES). Here we report ARPES data from Bi2201, which reveal both particle–hole symmetry breaking and pronounced spectral broadening—indicative of spatial symmetry breaking without long-range order at the opening of the pseudogap. Our finding supports the STM proposal that the pseudogap state is a broken-symmetry state that is distinct from homogeneous superconductivity

13 page of supplemental information



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