Despite its seemingly unique characteristics, iron selenide has been revealed to be a "garden-variety" iron-based superconductor, with fundamental physics no different from other such materials.

Researchers from Germany, the United Kingdom, and the United States investigated the superconductivity of iron selenide (β-FeSe).

In a scientific first, the team successfully measured the dynamic magnetic properties of β-FeSe crystals, which have undergone structural shifts after being cooled right before the point of superconductivity.

Properties Of Iron Selenide

Prof. Pengcheng Dai, a physicist at Rice University and one of the authors of the study, explained what makes β-FeSe different from other iron-based superconductors.

"It has the simplest structure, being composed of only two elements," Dai said.

"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."

Of the dozens of iron-based superconductors discovered by scientists since 2008, each have been shown to have a 2-D sheet made of iron atoms that are sandwiched between two other sheets consisting of other elements.

While most other superconductors have top and bottom sheets made of two or more other elements, the sheets for β-FeSe are made solely of pure selenium.

The iron atoms in the 2-D sheet of β-FeSe and other iron-based superconductors are arranged in a checkerboard pattern, where the atoms are spaced equally from each other in the forward-back and left-right directions.

When these materials are cooled, they typically undergo moderate shifts in their structure. The iron atoms tend to form oblong rhombuses instead of squares.

These can be compared to the diamonds in a baseball field, where the home plate is a lot closer to the second base compared to the distance between the first and third bases.

The shift in iron atoms caused iron-based superconductors to show directionally dependent behavior, such as having increased electrical conductivity or resistance but only between the home-to-second or first-to-third bases. This behavior is known as nematicity or anisotropy.

Iron selenide has shown structural nematicity in the past. However, the researchers said it has been impossible to get an exact measurement of its electronic and magnetic order because of a property called twinning.

Twinning occurs when several layers of randomly oriented 2-D crystals are placed on top of each other. It is like having 100 baseball diamonds stacked, with the distance between the home plate and second base varying for each diamond.

Dai said even if a twinned sample has a directionally dependent electronic order, it cannot be measured because the differences in distances tend to average out. This leaves researchers to measure a net effect of zero.

To solve this challenge, Dai and his colleagues first had to detwin β-FeSe samples to find out if a nematic electronic order is present.

Detwinning Iron Selenide Samples

The researchers built on the findings of another study conducted in 2014. In this earlier research, Dai and his team applied pressure to detwin barium iron arsenide crystals.

For iron selenide samples, lead author Tong Chen had to glue the material's smaller crystals on top of larger ones because the pressure necessary to align them would cause β-FeSe layers to fall into place as well.

Chen created several samples of β-FeSe that tested using neutron scattering beams. He then aligned and placed 20 to 30 1-millimeter squares of the material on top of each barium iron arsenide crystals. He used a microscope, a pair of tweezers, and specially made glue that did not contain hydrogen.

The researchers successfully detwinned the β-FeSe samples. They then subjected the material to neutron scattering beams in different laboratories around the world.

The team found that the electronic behavior of iron selenide is quite similar to that of other iron-based superconductors.

Dai said the key conclusion is that iron selenide's magnetic correlations are highly anisotropic, much like many other iron superconductors.

The point has been controversial in the past since β-FeSe does not come from a parent compound that manifests an antiferromagnetic order unlike other superconductors. This led many to believe β-FeSe's superconductivity arose through a manner quite different from others.

Dai pointed out that this was not the case, as suggested by their findings. He said there is no need for an entirely new method to understand β-FeSe's properties.

The findings of the international study are featured in the journal Nature Materials.

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