Researchers in Kyle Shen’s lab have determined that imperfections within a material’s structure, not electron count, govern superconductivity in iron selenide, a finding that challenges conventional understanding of high-temperature superconductors. Using a novel technique combining synthesis with electrical and spectroscopic probes, the team discovered iron selenide’s superconducting behavior is linked to resistance caused by defects in its crystal lattice.
“We found that in this material, that dome is driven by factors much different than what you see normally,” said postdoctoral researcher Paul Malinowski, formerly a Klarman Postdoctoral Fellow. The study, published August 20 in Proceedings of the National Academy of Sciences, offers insight into a material fundamentally different than other unconventional superconductors.
MBE & Alkali Deposition Control Iron Selenide Doping
Molecular beam epitaxy and alkali deposition enabled control over iron selenide’s electronic properties, allowing researchers to pinpoint the role of crystal lattice imperfections in its superconductivity. The team, led by Kyle Shen, bypassed traditional doping methods that require synthesizing entirely new materials with each adjustment to electron count; instead, they directly added electrons to the surface of existing iron selenide samples.
This approach avoids introducing unintended variables that could obscure the relationship between electron density and superconductivity, a challenge that has long hampered progress in the field. “But that means every time you want to change the number of electrons, you have to grow a completely new material.
Not only is that a lot of work; you’re also changing the material every time so other things might be changing that you don’t know and can’t control,” explained Paul Malinowski, a postdoctoral researcher formerly with the College of Arts and Sciences. Precisely manipulating doping levels allowed the team to discover that iron selenide’s superconducting dome is primarily governed by the degree of disorder within its crystal lattice.
Unlike many other high-temperature superconductors where electron count dictates performance, imperfections in the arrangement of iron and selenium atoms appear to be the dominant factor in iron selenide. “It’s not driven by how many electrons you’re adding in, but rather, it’s driven by the obstacles the electrons are hitting – how perfect or imperfect is the crystal lattice?” Malinowski stated.
This finding challenges conventional understanding, as disorder is typically considered a secondary influence on superconductivity; Shen noted, “Disorder is there but it’s generally believed to be a secondary or tertiary player, playing a minor role.” The comprehensive nature of this new technique distinguishes it from previous attempts to characterize iron selenide’s behavior.
The researchers were able to synthesize samples and simultaneously control doping levels using MBE and alkali deposition, providing a detailed view of the material’s properties. “The novel thing here is we can do it all at once in a very comprehensive way,” Malinowski said.
It’s an important new result that could give a lot of insight into high-temperature superconductivity.
Shen, the James A. Weeks Professor of Physical Sciences and Stephen H. Weiss Presidential Fellow (A&S) and director of the Laboratory of Atomic and Solid State Physics
Crystal Lattice Disorder Defines FeSe Superconductivity
Data from a novel experimental approach revealed that physical disorder, not electron count, controls the superconducting dome in iron selenide, challenging established understandings of high-temperature superconductivity. Researchers demonstrated this control by simultaneously manipulating both doping levels and crystal structure, a feat previously difficult to achieve.
“That turned out to be essential for figuring out what’s going on,” explained Kyle Shen, the James A. Weeks Professor of Physical Sciences and Stephen H. Weiss Presidential Fellow, emphasizing the importance of the technique for unraveling the material’s behavior. Postdoctoral researcher Paul Malinowski, first author of the study, described how these imperfections create obstacles for electron flow, directly impacting the strength of the superconducting state.
Adding electrons to the material inherently introduces disorder, a factor often considered secondary but demonstrably central in iron selenide. This prominence of disorder in controlling the superconducting dome sets iron selenide apart from other materials in the field, offering a unique opportunity to refine theoretical models of superconductivity.
It’s not driven by how many electrons you’re adding in, but rather, it’s driven by the obstacles the electrons are hitting – how perfect or imperfect is the crystal lattice?
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