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Superconductivity has long been one of science’s most mysterious states of matter.  For the past 100 years, scientists have sought to understand which materials will display the unusual properties of exhibiting zero electrical resistance and expelling magnetic fields below a critical transition temperature and why.  In 2008, the intrigue intensified when a new class of superconducting iron arsenic materials was discovered.

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Structural domains in a calcium-iron-arsenic compound,
one of the parent materials of iron-arsenide supercon-
ductors.  Ames Lab physicists discovered that the
domains in iron-arsenides form into stripe-like
patterns when exposed to low temperatures.
Researchers wondered if iron-arsenide superconductors show a combination of qualities needed for use in high-tech applications. The most useful superconductors have relatively high transition temperatures, are able to carry a significant amount of current, and are inexpensive and easy to make.  But before industry can decide whether iron-arsenide superconductors can be used in technologies like MRI machines, levitation trains or lossless electrical wires, researchers must determine the materials’ basic properties.  And that’s where Ames Laboratory scientists come in.  Since 2008, Ames Lab physicists and materials scientists have made a number of significant discoveries in synthesizing, characterizing and discovering members of the iron-arsenide class of superconductors:
  • First crystals, first measurements. Ames Laboratory scientists were able to grow single crystals of the difficult-to-make rare-earth-iron-arsenic-oxygen compound soon after it was discovered to be a superconductor. Single-crystal samples are a crucial step to doing detailed research and measurements that help scientists understand how the iron-arsenides work.  With these crystals in hand, other Ames Lab researchers were able to make some of the world’s first measurements of the materials’ electronic and magnetic structure.
  • 14 hours to single crystals.  When a barium-iron-arsenic compound was found to be part of the new class of superconductors, Ames Laboratory scientists grew single crystals of the materials within 14 hours of hearing of the discovery.  “That gave Ames Laboratory the earliest access to single crystals of these materials,” says Paul Canfield, a senior physicist.  “We published the first papers, and it just opened up a whole world of research.”
  • New member of the family. Ames Laboratory scientists discovered a new member of the iron-arsenide family, a calcium-iron-arsenic compound.  The findings were a rare example of a material discovered in a single crystal form.  Ames Lab researchers then went on to map out how the compound responds to pressure and temperature.
  • Writing the rules.  As new compounds within the iron-arsenide family have been discovered, Ames Laboratory scientists have mapped out the rules for where and why the materials are superconductors.  “We’ve come up with the rules and now, based on the rules, we’re trying to develop both empirical and theoretical understanding of why these are the rules,” says Canfield.
  • Safer methods. Ames Laboratory scientists developed a new method to grow a type of iron-arsenic crystal using a solution of sodium and arsenic, without high pressure, making the process safer and more accessible to scientists.

~ by Breehan Gerleman Lucchesi

Team Effort

Many of the lab’s physicists and materials scientists have been collaborating to study the iron-arsenide superconductors.  Canfield and Sergey Bud’ko and their research team, as well as groups led by Tom Lograsso and David Johnston, have been growing single crystals and doing initial measurements, and then providing samples to others for further characterization.  Alan Goldman, Andreas Kreyssig, Robert McQueeney, and David Vaknin, along with graduate students who work with them, have made critical discoveries about the magnetism and structure using X-ray and neutron scattering.

At the same time, Ruslan Prozorov and Makariy Tanatar and their team do detailed low-temperature magnetic measurements and thermal conductivity measurements.  Adam Kaminski’s group uses angle-resolved photo emission characterization to help understand the iron-arsenide’s electronic structure.  Along the way, Bruce Harmon, Youngbin Lee, Vladimir Antropov, Vladimir Kogan and Jörg Schmalian, all theoretical physicists, have been helping to guide analysis and growth. Materials scientists Bill McCallum and Kevin Dennis have offered materials preparation expertise.

“In large part, the advances made at Ames Laboratory are due to how well our collaboration has worked,” says Goldman.  “We have such a wide range of expertise in condensed matter physics, both in theory and experiment, and in crystal preparation and characterization.  Because all those elements are present here, and because we all like working with each other, we are able to simultaneously converge on a scientific question.  That’s what’s happening with the iron-arsenide research.”

In all, Ames Lab scientists have submitted more than 125 papers on their work for publication.
“This is the payoff of having this range of expertise and collaboration spirit within the Ames Laboratory,” says Goldman.

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Ames Laboratory scientists are mapping out
the rules for where and why iron-arsenide
materials are superconductors.  The maps are
called phase diagrams.

 

Homegrown Competition

 Ames Laboratory scientists train many under-graduates, graduate students and postdoctoral associates, and they do it so well that they often create their own competition.  This is especially true in the area of iron-arsenide superconductors.

 
“Within the United States, our primary scientific competition has all been from researchers we’ve trained here,” says Canfield.  “They all learned how to do what they do here at the Ames Laboratory.”

 

 

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A co-aligned array of about 60 iron-arsenide single
crystals used for neutron scattering measurements.
Ames Laboratory physicists used X-ray and neutron
diffraction, along with several other characterization
techniques, to make some of the world’s first
measurements of the iron-arsenides’ electronic
and magnetic structure.