Schematic drawing of the surface reconstruction in Ca(CoxFe1-x)2As2. The circles indicate the As position, the gray lines indicate the surface reconstruction lines seen in the topographic images. For better visibility, we don’t include possible surface dimerization (31) in this drawing. The black rectangle marks the orthorhombic unit cell with lattice parameters as reported by Ref. 32. The inset illustrates how the angle of the surface reconstruction changes relative to the Fe-Fe lattice from the tetragonal (right) to the orthorhombic phase (left). From Ref. 32, we calculate an angle of 90.5° or 89.5°, depending on the orientation of the a- and b-axis. This angle is exaggerated in the drawing for better visibility.
An international team lead by scientists at the U.S. Department of Energy’s (DOE) Center for Emergent Superconductivity, an Energy Frontier Research Center headquartered at DOE’s Brookhaven National Laboratory, has discovered evidence for ‘electronic liquid crystal’ states within the parent compound of one type of iron-based, high-temperature (high-Tc) superconductor.
“Because these findings appear similar to what we have observed in the parent state of cuprate superconductors, it suggests this could represent a common factor in the mechanism for high-Tc superconductivity in these two otherwise very different families of materials,” said team leader Séamus Davis, Director of the Center for Emergent Superconductivity at Brookhaven and the J.D. White Distinguished Professor of Physical Sciences at Cornell University. The team of scientists describes their findings, which may help elucidate that long-sought mechanism and lead to higher-temperature superconductors.
An important breakthrough was the capability demonstrated by the team to achieve atomically flat and perfectly debris-free surfaces for these studies. Without these conditions the spectroscopic imaging STM techniques cannot be applied. But as soon as the first large-scale images of the electronic arrangements were achieved, it became clear to the team that they were onto something very different than expected.
The scientists observed static, nanoscale arrangements of electrons measuring about eight times the distance between individual iron atoms, all aligned along one crystal axis reminiscent of the way molecules spatially order in a liquid crystal display. They also found that the electrons that are free to travel through the material do so in a direction perpendicular to these aligned ‘electronic liquid crystal’ states. This indicates that the electrons carrying the current are distinct from those apparently aligned in the electronic liquid crystals.
The next step will be to see how these conditions affect the superconductivity of the material when it is transformed to a superconductor.
“Then, if we’re able to relate our observations in the iron-based superconductors to what happens in cuprate superconductors, it may help us understand the overall mechanism for high-Tc superconductivity in all of these materials. That understanding could, in turn, help us to engineer new materials with improved superconducting properties for energy applications,” Davis said.
The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging–scanning tunneling microscopy to study the electronic structure of a representative compound CaFe1.94Co0.06As2 in the “parent” state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter–iron-atom distance aFe-Fe and aligned along the crystal a axis are observed. In contrast, the delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b axis only and are consistent with a nematic 2 band with an apparent band folding having wave vector along the a axis. All these effects rotate through 90 degrees at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected.
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