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Physicists Reveal Magnetic Properties of Iron-Based Superconductors Through Neutron Scattering Experiments

Iron selenide is a superhero in the realm of uncommon superconductors. However, new experiments conducted by the U.S., European, and Chinese physicists have revealed that the magnetic properties of the material are astonishingly ordinary.

Tong Chen, a Rice PhD student “detwinned” iron selenide crystals by gluing them atop much larger crystals of barium iron arsenide. Using a 2014 method developed at Rice, the larger crystals are placed under pressure and detwinned, causing the smaller iron selenide crystals to also snap into alignment. (Image credit: Jeff Fitlow/Rice University)

Pengcheng Dai, a Rice University physicist who is the corresponding author of a study, the results of which have been published online this week in Nature Materials, came up with this bottom-line evaluation of iron selenide: “It’s a garden-variety iron-based superconductor. The fundamental physics of superconductivity are similar to what we find in all the other iron-based superconductors.”

That conclusion is dependent on data obtained from neutron scattering experiments carried out in the past year in the United States, Germany, and the United Kingdom. The experiments offered the first-ever measurements of the dynamic magnetic persona of iron selenide crystals that had experienced a characteristic structural shift that takes place when the material is cooled but prior to being cooled to the point of superconductivity.

Iron selenide is completely different from all the other iron-based superconductors in several ways. It has the simplest structure, being composed of only two elements. All the others have at least three elements and much more complicated structure. Iron selenide is also the only one that has no magnetic order and no parent compound.

Pengcheng Dai, Professor of Physics and Astronomy, Rice University

Dai is also a member of Rice’s Center for Quantum Materials (RCQM).

From 2008, a number of iron-based superconductors have been unearthed. The iron atoms in each of these superconductors form a 2D sheet that is sandwiched between top and bottom sheets formed of other elements. When it comes to iron selenide, the top and bottom sheets are made of pure selenium; however, in the case of other materials, these sheets are formed of two or more elements. Iron atoms in the central 2D sheet of iron selenide and other iron-based superconductors are spaced in a checkerboard fashion, precisely the same distance from each another in the left-right direction and the forward-backward direction.

When the materials are cooled, they experience a negligible structural shift. The iron atoms form oblong rhombus shapes rather than exact squares. These are similar to baseball diamonds, in which the distance between home plate and second base is shorter compared to the distance between the first and third base. Moreover, this variation between iron atoms makes the iron-based superconductors to display directionally dependent behavior, such as increased electrical conductivity or resistance only in the home-to-second or first-to-third direction.

Physicists call this directionally dependent behavior as nematicity or anisotropy. Moreover, although it is well known that structural nematicity occurs in iron selenide, according to Dai, it has not been possible to measure the precise magnetic and electronic order of the material due to a property called twinning. Twinning arises when randomly oriented 2D crystal layers are stacked. Consider stacking 100 baseball diamonds one on top of the other, where the line between second base and home plate varies randomly for each.

Even if there is directionally dependent electronic order in a twinned sample, you cannot measure it because those differences average out and you wind up measuring a net effect of zero. We had to detwin samples of iron selenide to see if there was nematic electronic order.

Pengcheng Dai, Professor of Physics and Astronomy, Rice University

Tong Chen, who is the lead author of the study and a third-year PhD student in Dai’s research group, came up with the solution for the twinning problem by judiciously learning from a 2014 research in which Dai and team detwinned barium iron arsenide crystals through the application of pressure.

It was not possible to use the same technique for iron selenide since the crystals were 100 times smaller. Therefore, Chen stuck the smaller crystals on top of the larger ones, considering that the pressure required to align the larger sample would also make the iron selenide layers to snap into alignment.

After spending several weeks, Chen created a number of samples to test in neutron scattering beams. Nearly 20 to 30 squares of iron selenide measuring 1 mm in size had to be aligned and positioned on top of each barium iron arsenide crystal. Moreover, it was difficult to apply each of the tiny squares using a microscope, tweezers, and unique, hydrogen-free glue that cost nearly $1000 per ounce.

The work was rewarding when Chen investigated the samples and discovered that the iron selenide was detwinned. The tests with neutron scattering beams at Oak Ridge National Laboratory, the National Institute of Standards and Technology, the Technical University of Munich, and United Kingdom’s Rutherford-Appleton Laboratory also revealed that the electronic behavior of iron selenide is quite similar to that of other iron superconductors.

The key conclusion is that the magnetic correlations that are associated with superconductivity in iron selenide are highly anisotropic, just as they are in other iron superconductors. That has been a very controversial point, because iron selenide, unlike all other iron-based superconductors, does not have a parent compound that exhibits antiferromagnetic order, which has led some to suggest that superconductivity arose in iron selenide in a completely different way than it arises in these others. Our results suggest that is not the case. You don’t need an entirely new method to understand it.

Pengcheng Dai, Professor of Physics and Astronomy, Rice University

Additional co-authors include Rui Zhang and Yu Li, both of Rice; Youzhe Chen of Johns Hopkins University; Andreas Kreisel of the University of Leipzig; Xingye Lu and Yan Rong, both of Beijing Normal University; Astrid Schneidewind of the Jülich Center for Neutron Sciences; Yiming Qiu of the National Institute of Standards and Technology; Jitae Park of the Technical University of Munich; Toby Perring and Ross Stewart, both of the Rutherford-Appleton Laboratory; Huibo Cao of Oak Ridge National Laboratory; Yuan Wei of the Chinese Academy of Sciences; Brian Andersen of the University of Copenhagen; P.J. Hirschfeld of the University of Florida; and Collin Broholm of both Johns Hopkins University and the National Institute of Standards and Technology.

The Department of Energy, the Welch Foundation, the National Natural Science Foundation of China, the Carlsberg Foundation, the National Institute of Standards and Technology, and the National Science Foundation supported the study.

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