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?
A research team led by Xingye Lu of Beijing Normal University, Pengcheng Dai of Rice and Thorsten Schmitt of the Paul Scherrer Institute (PSI) in Switzerland used inelastic resonant X-ray scattering (RIXS) to measure the spin behavior of electrons in the iron selenide at high energy levels.
Spin is a property of electrons related to magnetism, and the researchers found that the 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 were posted online in Natural Physics.
Electron nematicity is believed to be an important ingredient in determining superconductivity in iron selenide and similar iron-based materials. Discovered in 2008, there are dozens of these iron-based superconductors. All become superconductors at very low temperatures and most show nematicity before reaching the critical temperature where superconductivity begins.
It is not known whether nematicity aids or hinders the onset of superconductivity. But the results of the high-energy spin experiments at the Swiss PSI light source are a surprise because iron selenide is the only iron-based superconductor in which nematicity occurs in the absence of an electron spin magnetic order. long range.
“There’s something special about iron selenide,” said study co-author Rice Qimiao Si, who, like Dai, is a member of the Rice Quantum Initiative. “Being nematics without long-range magnetic order provides an extra button to access the physics of iron-based superconductors. In this work, the experiment discovered something truly surprising, namely that high-energy rotational excitations are dispersive and undamped, which means that they have a well-defined relationship between energy and the amount of motion. “
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 2D iron sheets are spaced in a checkerboard pattern, at exactly the same distance from each other in the left-right and front-back directions. But when 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 diamonds like baseball fields, where the distance between home plate and second base is less than the distance between first and third base. Electronic nematicity occurs parallel to this change, taking the form of increased or decreased electrical resistance or conductivity only in the direction from home to second or first to third.
Although structural nematicity in iron selenide was known to exist, a property known as twinning made accurate measurement impossible until a breakthrough in 2019 by Dai, Lu and study co-author Tong Chen., A former graduate student. of 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 levels 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 demonstrated. to be able to untangle by squeezing. The shoulder hauling paid off when experiments showed that the iron selenide layers lined up as the iron and barium arsenide were untangled.
In the 2019 study, Dai, Chen, and Lu, another former Dai student, measured the behavior of low-energy electron spins with the inelastic scattering of neutrons. In the latest experiments, inelastic X-ray scattering revealed rotational behavior at high energy levels.
“Because the penetration depth of RIXS is just a few micrometers, the point of the RIXS beam can be shifted from iron selenide to iron barium arsenide, allowing us to clearly distinguish what goes into each,” said Dai, Professor Rice’s Sam and Helen Worden. 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, high-energy experiments revealed a very strong direction-dependent rotation behavior known as spin anisotropy.
“Remarkably, we were able to reveal spin anisotropy comparable – if not superior – to 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 characteristics are the dreams of theorists, because they directly inform theoretical understanding,” said Si, one of the two theorists in the article. The other, Rong Yu of Renmin University in Beijing, is a longtime collaborator and former postdoctoral fellow in Si at Rice’s group.
“We were able to provide a qualitative and even semiquantitative 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 demonstrates that quantum magnetic fluctuations are primarily responsible for the development of nematic electron correlation,” Si electronics said, implicating strong electron correlation effects as a cause of high-temperature superconductivity in iron-based superconductors. “
It is Harry C. and Olga K. Wiess Professor of Physics and Astronomy and Director of the Rice Center for Quantum Materials.
Other co-authors include Yu Song ’17 of Zhejiang University Rice, Wenliang Zhang, Yi Tseng, Eugenio Paris and Vladimir Strocov of PSI and Ruixian Liu, Zhen Tao and Panpan Liu of Beijing Normal.
Rice research was funded by the Department of Energy (DE-SC0012311, DE-SC0018197) and the Welch Foundation (C-1839, C-1411).