HOUSTON – (May 20, 2022) – Quantum physicists at Rice University are part of an international team that has answered a puzzling question at the forefront of iron-based superconductor research: why electrons in selenide do they dance to a different tune as they move? right and left rather than forwards and backwards?
A research team led by Xingye Lu at Beijing Normal University, Pengcheng Dai at Rice and Thorsten Schmitt at the Paul Scherrer Institute (PSI) in Switzerland used resonant inelastic scattering of X-rays (RIXS) to measure the behavior of electron spins in iron selenide at high energy levels.
Spin is the property of electrons related to magnetism, and the researchers found that iron selenide spins begin to behave in a direction-dependent manner at the same time that the material begins to exhibit direction-dependent electronic behavior, Where nematicity. The team’s results were published online this week in Natural Physics.
Electronic nematicity is believed to be an important ingredient in bringing about superconductivity in iron selenide and similar iron-based materials. Discovered in 2008, these iron-based superconductors number in the dozens. All become superconductors at very cold temperatures and most exhibit nematicity before reaching the critical temperature where superconductivity begins.
It is not known whether nematicity helps or hinders the onset of superconductivity. But the results of high-energy spin experiments at PSI Swiss Light Source are a surprise because iron selenide is the only iron-based superconductor in which nematicity occurs in the absence of long-range magnetic ordering of electron spins.
“There’s something special about iron selenide,” study co-author Rice said. Qimiao Siwho, like Dai, is a member of the Quantum Rice Initiative. “Being nematic without long-range magnetic order provides an extra button to access the physics of iron-based superconductors. In this work, the experiment revealed something really striking, namely that high-energy spin excitations are dispersive and undamped, which means that they have a well-defined relationship between energy and the amount of movement.
In all iron-based superconductors, the iron atoms are arranged in 2D sheets which are sandwiched between the top and bottom sheets of other elements, selenium in the case of iron selenide. The atoms in the 2D iron sheets are spaced in a checkerboard pattern, exactly the same distance from each other in the left-right and front-back directions. But as the materials are cooled near the point of superconductivity, the iron sheets undergo a slight structural change. Instead of exact squares, the atoms form oblong diamond shapes like baseball diamonds, where the distance between home plate and second base is shorter than the distance between first and third base. Electronic nematicity occurs alongside this change, taking the form of increased or decreased electrical resistance or conductivity only in the direction from home to second or from first to third.
While structural nematicity was known to exist in iron selenide, a property known as twinning made it impossible to accurately measure until a breakthrough in 2019 by Dai, Lu and study co-author Tong Chen, a former graduate student in Dai’s lab who graduated in 2021.
In iron-based superconductors, twinning occurs when thin sheets of material are stacked together and the iron layers in the sheets are misaligned. Imagine 100 baseball diamonds stacked on top of each other, with the line between home plate and second base pointing in a random direction in each layer. To accurately measure nematicity, all layers had to be aligned.
Iron selenide is a soft material that deforms easily, but Chen painstakingly bonded dozens of layers of soft crystals to a harder iron-based superconductor, iron barium arsenide, which Dai’s lab had. previously shown that it could detangle by squeezing. Piggybacking paid off when experiments showed that the layers of iron selenide aligned when iron and barium arsenide were disentangled.
In the 2019 study, Dai, Chen and Lu, another former student of Dai, measured the behavior of low-energy electron spins with inelastic neutron scattering. In the latest experiments, inelastic X-ray scattering revealed spin behavior at high energy levels.
“Since the penetration depth of RIXS is only a few micrometers, the spot of the RIXS beam can be moved from iron selenide to iron barium arsenide, allowing us to clearly distinguish what is goes into everyone,” said Dai, Professor Sam and Helen Worden of Rice. professor of physics and astronomy. “RIXS is complementary to the experiments we performed in 2019 because it can probe high-energy spin excitations but lacks the resolution to examine low-energy excitations.”
Despite the lack of magnetic order, the high-energy experiments revealed a very strong direction-dependent spin behavior known as spin anisotropy.
“Extraordinarily, we were able to reveal a spin anisotropy comparable to – if not greater than – that of the already highly anisotropic iron barium arsenide,” said Lu, professor of physics at Beijing Normal. “This spin anisotropy decreases with increasing temperature and disappears around the nematic transition temperature – the temperature at which the material ceases to be in an electronic nematic state.”
The researchers said the results indicate that nematicity in iron selenide is driven by quantum spin excitations.
“These features are theorists’ dreams, because they directly inform theoretical understanding,” said Si, one of the two theorists in the paper. The other, Rong Yu of Renmin University in Beijing, is a longtime collaborator and former postdoctoral researcher in Si’s group at Rice.
“We were able to provide a qualitative and even semi-quantitative understanding of the observed spin excitation spectrum based on a theoretical model of quantum magnetism that Rong Yu and I advanced several years ago for iron selenide,” Si said. didn’t think possible before.
“This shows that quantum magnetic fluctuations are primarily responsible for the development of electronic nematic correlation,” said Si. electronics involving strong electron correlation effects such as causing high temperature superconductivity in iron-based superconductors.
Si is Harry C. and Olga K. Wiess Professor of Physics and Astronomy and Director of the Rice Center for Quantum Materials.
Additional co-authors include Rice alumnus Yu Song ’17 of Zhejiang University, Wenliang Zhang, Yi Tseng, Eugenio Paris and Vladimir Strocov of PSI and Ruixian Liu, Zhen Tao and Panpan Liu of Beijing Normal.
The research at Rice was funded by the Department of Energy (DE-SC0012311, DE-SC0018197) and the Welch Foundation (C-1839, C-1411).
- Peer-reviewed article
“Spin Excitation Anisotropy in the Nematic State of Demaculated FeSe”, Nature Physics
- Image downloads
CAPTION: Resonant inelastic X-ray scattering experiments at the Paul Scherrer Institute in Switzerland have revealed high-energy electron spin correlations in iron selenide crystals that had transitioned to a nematic electronic state. (Image: Beijing Normal University/Qi Tang and Xingye Lu)
CAPTION: Quantum physicists Pengcheng Dai (left) and Qimiao Si outside Rice’s Brockman Hall for Physics in November 2021. (Photo by Jeff Fitlow/Rice University)
CAPTION: Former Rice graduate student Tong Chen ’21 ‘untwinned’ iron selenide crystals in 2019. Several dozen fragile sheets of iron selenide had to be painstakingly aligned, stacked and glued to another superconductor based on iron, barium iron arsenide. (Photo by Jeff Fitlow/Rice University)
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