Iron selenide revealed as a "garden-based iron superconductor"

In the pantheon of unconventional superconductors, iron selenide is a rock star. But new experiments conducted by American, Chinese and European physicists revealed that the magnetic character of this material was surprisingly commonplace.

Pengcheng Dai, physicist at Rice University, corresponding author of a results study published online this week in Nature's materials, proposed this fundamental evaluation of iron selenide: "It is a superconductor based on iron-type garden.The fundamental physics of superconductivity is similar to that found in all other superconductors based on iron."

This conclusion is based on data from neutron scattering experiments conducted last year in the United States, Germany and the United Kingdom. The experiments produced the first measurements of the dynamic magnetic properties of iron selenide crystals that had undergone a characteristic structural shift that occurs when the material is cooled, but before it is cooled to the point of superconductivity.

"Iron selenide is completely different from all other iron-based superconductors," said Dai, a professor of physics and astronomy at Rice and a member of Rice's Center for Quantum Materials (RCQM). "It has the simplest structure, being composed of only two elements.All the others have at least three elements and a much more complicated structure.The iron selenide is also the only one not to have magnetic order and of parent compound. "

Dozens of iron – based superconductors have been discovered since 2008. In each of them, the iron atoms form a 2D sheet sandwiched between upper and lower leaves composed of other elements. In the case of iron selenide, the upper and lower leaves are pure selenium, but in other materials these leaves consist of two or more elements. In iron selenide and other iron-based superconductors, the iron atoms of the central 2D sheet are spaced in a checkerboard pattern at the same distance from each other in the left-right and front-back directions.

When the materials cool, they undergo a slight structural change. Instead of exact squares, the iron atoms form oblong lozenges. These are like baseball diamonds, where the distance between the base plate and the second base is shorter than the distance between the first and third base. And this change between the iron atoms causes the iron-based superconductors to exhibit direction-dependent behavior, such as increased electrical resistance or conductivity, only in the home-by-second or first-to-third direction.

Physicists describe this behavior as depending on the direction of anisotropy or nematicity and, while it is known that a structural nematicity occurs in iron selenide, Dai explained that it It was impossible to measure the exact electronic and magnetic order of the material due to a property called pairing. Matching occurs when randomly oriented 2D crystal layers are stacked. Imagine 100 superimposed baseball diamonds, the line between the marble and the second base varying randomly for each.

"Even if there is a direction-dependent electronic order in a paired sample, you can not measure it because these differences are average and you end up measuring a net effect of zero," Dai said. "We had to assay iron selenide samples to see if there was a nematic electronic order."

The lead author of the study, Tong Chen, a third-year doctorate A student from Dai's research group solved the twinning problem by cleverly basing himself on a study conducted in 2014 in which Dai and his colleagues colleagues applied pressure to separate the barium iron arsenide crystals. It was impossible to apply the same method to iron selenide because the crystals were 100 times smaller. Chen therefore stuck the smaller crystals on top of the larger ones, reasoning that the pressure needed to align the larger sample would also snap the iron selenide layers. in alignment.

Chen spent weeks creating multiple test samples in neutron scattering beams. About 20 to 30 squares of one millimeter of iron selenide had to be aligned and placed on top of each barium iron arsenide crystal. And the application of each of the tiny squares was a tedious job that involved a microscope, tweezers and a special glue without hydrogen that cost nearly $ 1,000 an ounce.

The work paid off when Chen analyzed the samples and found that iron selenide was mixed. Neutron beam tests carried out by the Oak Ridge National Laboratory, the National Institute of Standards and Technology, the Technical University of Munich and the Rutherford-Appleton Laboratory of the United Kingdom have also shown that the Electronic behavior of iron selenide is very similar to that of other iron superconductors.

"The key finding is that the magnetic correlations associated with superconductivity in iron selenide are highly anisotropic, just like in other iron-based superconductors," said Dai. "This has been a very controversial point because iron selenide, unlike all other iron-based superconductors, has no parent compound with antiferromagnetic order, which has led some to believe that the Superconductivity appeared in a completely different form that arises in these others.Our results suggest that this is not the case.You do not need an entirely new method to understand it. "