Physicists from three continents report the first tests to explain the unusual electronic behavior behind the world’s thinnest superconductor, a material with countless applications because it transmits electricity extremely efficiently. In this case, the superconductor is thick only one atomic layer.
The work, led by an MIT professor and a physicist at Brookhaven National Laboratory, was made possible thanks to new instruments available in only a few locations around the world. The resulting data can help guide the development of better superconductors. These in turn can transform the fields of medical diagnostics, quantum computing, and energy transportation, which all conductors use.
The working subject belongs to an exciting class of superconductors that become superconductors at temperatures of an order of magnitude higher than their conventional counterparts, making them easier to use in applications. Conventional superconductors only work at temperatures around 10 Kelvin, or -442 degrees Fahrenheit.
However, these so-called high-temperature superconductors are not yet fully understood. Their microscopic stimuli and dynamics are essential to understanding superconductivity, yet after 30 years of research, many questions are still very open, says Riccardo Comin, Assistant Professor of Physics in the 1947 MIT Career Development Class. The new job, which it was reported recently in Nature Communications, helps answer those questions.
Other work colleagues include Jonathan Pelliciari, a former MIT postdoctoral fellow who is now a physicist assistant at Brookhaven National Laboratory and lead author of this study. Other authors are Seher Karakuzu and Thomas A. Maier from Oak Ridge National Laboratory; Qi Song, Tianlun Yu, Xiaoyang Chen, Rui Peng, Qisi Wang, Jun Zhao and Donglai Feng of Fudan University; Riccardo Arpaia, Matteo Rossi and Giacomo Ghiringhelli from Politecnico di Milano (Arpaia is also affiliated with Chalmers University of Technology); Abhishek Nag, Jiemin Li, Mirian Garca-Fernndez, Andrew C. Walters and Ke-Jin Zhou from Diamond Light Source in the UK; and Steven Johnston of the University of Tennessee at Knoxville.
The worlds thinnest superconductor
In 2015, scientists discovered a new type of high-temperature superconductor: a sheet of selenide iron only a thick atomic layer capable of superconducting at 65 K. In contrast, samples most of the same superconducting material in a much lower temperature (8 K). The discovery sparked an investigative storm to decipher the secrets of the world’s thinnest superconductor, says Comin, who is also affiliated with the MITs Research Laboratory.
In a regular metal, electrons behave much like individual people dancing in a room. In a superconducting metal, electrons move in pairs, like couples in a dance. And all of these pairs are moving in unison, as if they were part of a quantum choreography, eventually leading to some sort of electronic superfluid, says Comin.
But what is the interaction, or adhesive, that holds these electron pairs together? Scientists have known for a long time that in conventional superconductors, that adhesive is derived from the motion of atoms within a material. If you see a rigid sitting at a table, it does not appear that you are doing anything, says Comin. However, many things are happening on the nanosale scale. Within this material, electrons are flying in all possible directions and atoms are being shaken; they are vibrating. In conventional superconductors, electrons use the energy stored in that atomic motion to pair.
The adhesive after electron coupling in high temperature superconductors is different. Scientists have assumed that this adhesive has to do with a property of electrons called rotation (another well-known property of electrons is their charge). Rotation can be thought of as an elemental magnet, says Pelliciari. The idea is that in a high-temperature superconductor, electrons can receive some of their energy from these spins, known as spin excitations. And that energy is the glue they use to mate.
Until now, most physicists thought it would be impossible to detect or measure the excitation of rotations in a thick material of only one atomic layer. This is the extraordinary achievement of the work reported in Nature Communications. Not only did the physicists discover the excitation of the rotations, but, among other things, they also showed that the dynamics of the rotations in the ultra-thin sample were dramatically different from those in the bulk sample. Specifically, the energy of the oscillating rotations in the ultra-thin sample was much higher by a factor of four or five than the energy of the rotations in the bulk sample.
This is the first proof of the presence of rotational excitations in an atomically thin material, says Pelliciari.
The latest art equipment
Historically, neutron scattering has been used to study magnetism. Since rotation is a fundamental property of magnetism, neutron scattering will appear to be a good probe. The problem is that neutron scattering does not work in a material that is as thick as an atomic layer, says Pelliciari.
Introduce resonant inelastic X-ray scattering (RIXS), a new technique that Pelliciari helped pioneer.
He and Comin discussed the potential for using RIXS to study the rotation dynamics of the new ultra-thin superconductor, but Comin was initially skeptical. I thought, Yes, it would be nice if we could do that, but ly it would be impossible, Comin recalls. I thought it was a real moon. As a result, when Johnny collected the first results, it was boring for me. Id kept my expectations low, so when I saw the data, I jumped into my chair.
Only a few devices in the world have advanced RIXS instruments. One, located in Diamond Light Source (MB) and led by Zhou, is where the team conducted their experiment. Another, which was still being built at the time of the experiment, is at the Brookhaven National Laboratory. The grocer is now part of the team leading the RIXS structure, known as the Beamline SIX, at the Synchrotron II National Light Source located at Brookhaven Lab.
The impact of this work is twofold, says Thorsten Schmitt, head of the Material Spectroscopy Group in Rome at the Paul Scherrer Institute in Switzerland, who was not involved in the work. Experimentally, it is an impressive demonstration of RIXS sensitivity to rotational excitations in a superconducting material just a thick atomic layer. For more, [resulting data] are expected to contribute to the sense of increasing the superconducting transition temperature in such thin superconductors. In other words, the work can lead to even better superconductors.
Valentina Bisogni, lead scientist for Beamline SIX, which was not included in the study, says understanding unconventional superconductivity is one of the major challenges facing scientists today. The recent discovery of high-temperature superconductivity in a thin mono-layer iron selenide film renewed interest in the iron selenide system, as it provides a new way to investigate the mechanisms that enable high-temperature superconductivity.
In this context, the work of Pelliciari et al. presents an illuminating, comparative study of bulk iron selenide and thin-walled iron seleniderevealing a dramatic reconfiguration of rotational stimuli, says Bisogni.
This research was supported by the U.S. Air Force Bureau of Scientific Research, the MIT-POLIMI Program (Progetto Rocca), the Swiss National Science Foundation, the US Department of Energy (DOE), the US Naval Research Bureau, the CARIPLO Foundation, and the Regione Lombardy, the Swedish Research Council, the Alfred P. Sloan Foundation and the National Natural Science Foundation of China.
This research used the resources of National Synchrotron Light Source II, a DOE Science Office user facility located at the DOEs Brookhaven Lab.
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