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Asymmetric Scattering in Superconductor Dopants

Cobalt AtomsRecently scientists have uncovered materials that can be converted from magnetic insulators or metals into Superconductors, capable of carrying electrical current with no energy loss. It is an extremely promising concept for zero-resistance electronics, energy-storage and transmission systems. Currently, in addition to keeping the materials very cold, a major step to achieving superconductivity is to substitute a different kind of atom into some positions of the “parent” material’s crystal framework.

Up until now, scientists believed this process, called doping, merely added electrons or other charge carriers, thus rendering the electronic environment more conducive to the formation of electron pairs that could move with no energy loss if the material is held at a certain chilly temperature.

But new studies of an iron-based superconductor scientistsat Brookhaven National Laboratory and Cornell University suggest that the story is a bit more complicated. The research shows that doping, in addition to adding electrons, significantly changes the atomic-scale electronic structure of the parent material, with important consequences for the behavior of the current-carrying electrons.

Elongated Impurity States

“The key observation—that dopant atoms introduce elongated impurity states which scatter electrons in the material in an asymmetric way—helps explain most of the unusual properties,” said the study’s lead author, J.C. Séamus Davis, director of the Center for Emergent Superconductivity at Brookhaven Lab and the J.G. White Distinguished Professor of Physical Sciences at Cornell University. “Our findings provide a new starting point for theorists trying to grapple with how these materials work, and could potentially point to new ways to design superconductors with improved properties.”

A method developed by Davis called spectroscopic imaging scanning tunneling microscopy was used by researchers to visualize the electronic properties around individual superconductor dopant atoms in the parent material, and concurrently to monitor how electrons scatter around these dopants, in this case, cobalt.

Previous studies demonstrate that certain electronic properties of the non-superconducting “parent” material had a strong directional dependence. For instance, electrons were able to move more easily in one direction through the crystal than in a perpendicular direction. But in those studies, the signal of a strong directional dependence only appeared when the scientists put the dopants into the material, and got stronger the more dopants they added.

“But the emergence of directional dependence of electronic properties as more dopants are added suggests that the strong directionality is a result of the dopants, not an intrinsic property of the material,” Davis said. “We decided to test this idea by directly imaging what each dopant atom does to the nearby atomic-level electronic structure in these materials.”

The latest paper reports two exceptionally clear results.

1. At each cobalt dopant atom, there is an elongated impurity state, a quantum mechanical state bound to the cobalt atom, that aligns in a particular direction, the same for each cobalt atom, relative to the overall crystal.
2. These oblong, aligned impurity states scatter the current-carrying electrons away from the impurity state in an asymmetric way—similar to the way ripples of water would propagate asymmetrically outward from an elongated stick thrown into a pond, rather than forming the circular pattern produced by a pebble.

“These direct observational findings explain most of the outstanding mysteries about how the electrical current moves through these materials—for example, with greater ease perpendicular to the direction you would expect based solely on the characteristics of the parent material,” Davis said. “The results show that the dopants actually do dramatic things to the electronic structure of the parent material.”

More Useful Materials

“It’s possible that what we’ve found could be similar to an effect dopants had on early semiconductors,” Davis said. “Early versions of these materials, though useful, had nowhere near the performance as those developed after the 1970s, when scientists at Bell Labs figured out a way to move the dopant atoms far away from the electrons so they wouldn’t mess up the electronic structure.” The advance made possible all the microelectronics we now use every day, including cell phones, he said.

“If we find out the dopant atoms are doing something we don’t want in the iron and even copper superconductors, maybe we can find a way to move them away from the active electrons to make more useful materials.”

Reference:

Anisotropic impurity states, quasiparticle scattering and nematic transport in underdoped Ca(Fe1−xCox)2As2
M. P. Allan, T-M. Chuang, F. Massee, Yang Xie, Ni Ni, S. L. Bud’ko, G. S. Boebinger, Q. Wang, D. S. Dessau, P. C. Canfield, M. S. Golden & J. C. Davis
Nature Physics
(2013) doi:10.1038/nphys2544