Research identifies origin of iron selenide superconductor’s enigmatic behavior

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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 do electrons in iron selenide dance to a tune? different when moving right and left rather than forward and backward?

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 x-ray scattering (RIXS) to measure the behavior electron spins in iron selenide at high energy levels.

Spin is the property of electrons related to magnetism, and the researchers found that spins in iron selenide begin to behave in a direction-dependent manner at the same time that the material begins to exhibit direction-dependent electronic behavior , or nematicity. The team’s results have been published online in Natural Physics.

Electronic nematicity is thought 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’s Swiss Light Source are a surprise because iron selenide is the only iron-based superconductor in which nematicity occurs in the absence of a magnetic order at long-range electron spins.

“There’s something special about iron selenide,” said Rice study co-author Qimiao Si, who, like Dai, is a member of the Rice Quantum 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 discovered something really striking, namely that high-energy spin excitations are dispersive and undamped, meaning they have a well-defined relationship between energy and momentum.”

In all iron-based superconductors, iron atoms are arranged in 2D sheets which are sandwiched between 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 fields, 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 precise measurement impossible until a breakthrough in 2019 by Dai, Lu and study co-author Tong Chen. , a former graduate student from Dai’s lab who graduated in 2021.

In iron-based superconductors, twinning occurs when thin sheets of materials 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 did in 2019 because it can probe high-energy spin excitations but lacks the resolution to examine low-energy excitations.”

Despite the absence 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 spin anisotropy comparable to – if not greater than – that of the already highly anisotropic iron barium arsenide,” said Lu, a physics professor 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.

“This shows that quantum magnetic fluctuations are primarily responsible for the development of electronic nematic correlation,” said Si. strong electronic correlation effects 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).

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