Iron selenide revealed as 'garden-variety iron-based superconductor'

Superconductor's magnetic persona unmasked
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. Credit: Jeff Fitlow/Rice University

In the pantheon of unconventional superconductors, iron selenide is a rock star. But new experiments by U.S., Chinese and European physicists have found the material's magnetic persona to be unexpectedly mundane.

Rice University physicist Pengcheng Dai, corresponding author of a study of the results published online this week in Nature Materials, offered this bottom-line assessment of 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 based on data from neutron scattering experiments performed over the past year in the U.S., 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 as the material is cooled but before it is cooled to the point of superconductivity.

"Iron selenide is completely different from all the other iron-based superconductors in several ways," 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 much more complicated structure. Iron selenide is also the only one that has no magnetic order and no parent compound."

Dozens of iron-based superconductors have been discovered since 2008. In each, the iron atoms form a 2-D sheet that's sandwiched between top and bottom sheets made up of other elements. In the case of iron selenide, the top and bottom sheets are pure selenium, but in other materials these sheets are made of two or more elements. In iron selenide and other iron-based superconductors, iron atoms in the central 2-D sheet are spaced in checkerboard fashion, exactly the same distance from one another in both the left-right direction and forward-back directions.

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

Superconductor's magnetic persona unmasked
Graduate student Tong Chen spent weeks creating samples to test in neutron scattering beams. About 20 to 30 1-millimeter squares of iron selenide had to be aligned and glued in place atop each crystal of barium iron arsenide. Credit: Jeff Fitlow/Rice University

Physicists refer to this directionally dependent behavior as anisotropy or nematicity, and while structural nematicity is known to occur in iron selenide, Dai said it has been impossible to measure the exact electronic and of the material because of a property known as twinning. Twinning occurs when layers of randomly oriented 2-D crystals are stacked. Imagine 100 baseball diamonds stacked one atop the other, with the line between home plate and second base varying 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," Dai said. "We had to detwin samples of iron selenide to see if there was nematic electronic order."

Study lead author Tong Chen, a third-year Ph.D. student in Dai's research group, solved the twinning problem by cleverly piggybacking on a 2014 study in which Dai and colleagues applied pressure to detwin crystals of barium iron arsenide. It was impossible to apply the same method to iron selenide because the crystals were 100 times smaller, so Chen glued the smaller crystals atop the larger ones, reasoning that the pressure needed to align the larger sample would also cause the layers of iron selenide to snap into alignment.

Chen spent weeks creating several samples to test in neutron scattering beams. About 20 to 30 1-millimeter squares of iron selenide had to be aligned and placed atop each crystal of barium iron arsenide. And applying each of the tiny squares was painstaking work that involved a microscope, tweezers and special, hydrogen-free glue that cost almost $1,000 per ounce.

The work paid off when Chen tested the samples and found the iron selenide was detwinned. Those tests with neutron scattering beams at Oak Ridge National Laboratory, the National Institute of Standards and Technology, the Technical University of Munich and U.K.'s Rutherford-Appleton Laboratory also showed iron selenide's electronic behavior is very 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," Dai said. "That has been a very controversial point, because iron selenide, unlike all other iron-based , does not have a 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."

Explore further

Physicists show quantum materials can be tuned for superconductivity

More information: Tong Chen et al, Anisotropic spin fluctuations in detwinned FeSe, Nature Materials (2019). DOI: 10.1038/s41563-019-0369-5
Journal information: Nature Materials

Provided by Rice University
Citation: Iron selenide revealed as 'garden-variety iron-based superconductor' (2019, May 20) retrieved 17 June 2019 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Feedback to editors

User comments

May 21, 2019
Magnetic fields destroy superconductivity at any temperature

Even if currents can flow only in particular directions, as in iron selenide
Where ever it does flows
A magnetic field instantly materialises
Resulting in this eternal problematic problem
It's not superconductivity that is the difficulty
the zero resistance returns to resistance once a current flows as a magnetic field is produced repelling the electrons to the outer circumference of the conductor
Instantly creating resistance

Superconductivity has a proviso; it is only of use at room temperature
Room temperature is not 1K, 20K or even 170K
Room temperature is 298K, a sunny 25C summers day temperature
As these Room Temperature Superconductors are expected to operate at even higher balmy days

For in these Saudi Arabian towns, the thermometer hits 50C
That is 323K, in the shade

May 21, 2019
I remember this one, you needed pressure to see the high temperature superconducting state. But now when I checked they have tested to grow it epitaxially with dramatic better atmospheric pressure results! https://en.wikipe...selenide

May 21, 2019
told you

May 21, 2019

Jan 17, 2019
in a left field approach may I add that in new age thinking : exposure to selenite crystals ( is believed ) % (can cause) an increase in telepathic ability and conversely when some psychics get stressed out , power grids can fail due to the resistance in the power lines falling causing a current overload. then the circuit breakers to click in.

May 21, 2019
ok here's another one , at a garden centre , I fell into a trance and another reality was superimposed in the surroundings @ wisely garden centre . the chi / orgone / prahna / ect was the basis for the technology , it was surreal , objects like silk were the antennas and absorbers / conductors of the chi - air carries our dreams ect - I once saw them flying through a window

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more