Fundamentally strong
The Dutch physicist Heike Kamerlingh Onnes discovered more than a century ago that elemental mercury can transmit electricity without energy loss, making it the first known superconductor.
The catch was that it had to be very cold—near absolute zero— and while scientists have since discovered or created higher-temperature superconductors, they have a long way yet to go. Even now, the highest-temperature superconducting wire must be cooled well below minus 200 degrees Fahrenheit to do its thing, so its use is not widespread.
Better superconductors could revolutionize how we store and distribute electric power, but we have only a tenuous understanding of what makes superconductors behave the way they do.
Superconductors fall into the category of quantum materials. Whether it’s a high-temperature superconductor or a simple magnet, a quantum material behaves as it does because of what’s going on at the level of atoms and their electrons. In addition, it displays its special behavior only below a specific temperature.
Why do quantum behaviors appear only below certain temperatures, and how can these transition temperatures be controlled? These are the questions at hand.
Investigating Quantum Behavior
“Nearly every quantum material was found serendipitously,” explained ORNL chemist Athena Safa Sefat. “At room temperature they mostly don’t do anything interesting.
“What are the collective chemical, electronic, and spin phenomena at atomic scales that cause superconductivity below a particular temperature in these materials? That’s what we’re trying to understand.”
Sefat and her colleagues at ORNL bolster their experimental results on materials with theoretical modeling. And while superconductors hold enormous promise—because of their ability to transmit very large amounts of electricity without loss, creating exceptionally strong magnetic fields—the team’s efforts go beyond the practical to a fundamental exploration of the nature of quantum materials and the functionality of matter.
“This is what fundamental science is trying to understand,” she said. “What is it about the local chemical, electronic, and spin structures—as well as other factors—that combine to make a material superconducting below a critical temperature in a bulk sample?”
A collaborative lab
The work of Sefat and her colleagues belongs to a long tradition of fundamental research at ORNL. Born of the government’s drive during World War II to create nuclear weapons, ORNL has grown to become the DOE Office of Science’s largest and most diverse laboratory.
Indeed, of 24 core capabilities recognized by the Office of Science—skills such as applied mathematics and advanced instrumentation as well as many areas of physics, chemistry, biology, engineering, earth sciences and computing—ORNL lays claim to 23, the most of any DOE science lab. Thomas Zacharia, the lab’s deputy for science and technology, says ORNL’s diversity makes it uniquely able to tackle science’s most difficult challenges.
“I think the laboratory is fundamentally distinguished by team science,” he said, “asking more of the bigger questions that require a team of people from different fields of science, different expertise, different capabilities.”
This diversity can be seen in the range of accomplishments coming from ORNL researchers. A team of scientists from the Chemical Sciences Division and the lab’s Spallation Neutron Source recently announced they had discovered a new state of water molecules—neither solid, liquid nor gas—under extreme confinement (see here). Elsewhere at the lab researchers are using their expertise in computational science to promote cancer research by mining data from cancer reports to reveal promising approaches that might have been overlooked (see here).
Perhaps most visibly, ORNL was a key player in the collaboration that discovered tennessine, an element with atomic number 117 (see here).
“That is truly fundamental science,” Zacharia said. “It’s not every day that you change the periodic table, and the work that was done here is going to change the periodic table in every textbook in the world.”
Unprecedented scientific tools
ORNL’s collaborative environment is distinguished not only by the talent and diversity of its staff but also by the tools at their disposal, unique facilities that allow researchers to interrogate matter at a level that would otherwise be impossible.
ORNL hosts a number of DOE Office of Science user facilities. These include two leading neutron science facilities, the Spallation Neutron Source and the High Flux Isotope Reactor (which was instrumental in the discovery of tennessine); the Center for Nanophase Materials Sciences, which supports nanoscience with broad capabilities in synthesis, characterization, microscopy and theory; and the Oak Ridge Leadership Computing Facility, which is home to the country’s most powerful supercomputer, Titan.
“These are unique, world-leading scientific facilities,” Zacharia said. “They require not only a team of scientists to operate but also a team approach to tackle the fundamental questions that require these types of facilities, because each of these facilities allows us to probe matter or ask questions at a deeper level.”
The lab’s collaborative approach—and its advanced tools—can be seen in the work of Sefat and her collaborators in exploring superconductivity.
Sefat herself, a solid-state chemist working in a materials group, uses insights from her team to create custom crystals in atomic configurations that may be promising. While they don’t know for certain how superconductivity comes about, they do have clues; for instance, high-temperature superconductors all start with antiferromagnets (materials whose atoms have magnetic directions opposite those of their neighbors), are similarly layered, and have a small amount of chemical doping that creates some level of disorder.
Because of her expertise Sefat is able to take a material and replace atoms at specific sites throughout its structure. Then she does the first tests of a material herself, measuring bulk properties such as its resistance, its response to a magnetic field, changes in its crystal structure, and its transition temperature.
Colleagues at ORNL’s Center for Nanophase Materials Sciences get a nanoscale look at the material’s electronic structure through scanning tunneling spectroscopy. Materials also are subjected to neutron analyses at the lab’s two neutron facilities, which provide information on spin structures, ordering types and magnetic excitations.
Meanwhile these experimental results are supported by models done by theoreticians at the lab.
“Once you synthesize the crystal, you can study properties and see whether your hypotheses have worked or not,” Sefat explained. “That’s a loop that goes around: What did I change? What property has it manifested? Then you go back, make a change, and take another look at the properties and theoretical explanations.”
Ultimately the team’s work is likely to help make a class of very valuable materials—superconductors—even more so, but it will also help us answer essential questions. For instance, how and why does temperature play such a crucial role in the way matter behaves?
“This is fundamental science, where we aim to understand the causes of a material’s quantum behavior by probing it near its transition temperature, Sefat said.”
The importance of temperature is important in our everyday lives as well, she noted.
“As your warm things up, some transform into a liquid while others stay solid, so temperature really matters—as well as other parameters such as applied magnetic field and pressure. In my research we’re really trying to understand what’s determining the transition point between what it is and what it becomes. Understanding such causes through fundamental science, we can potentially design improved materials to function specifically at desired temperatures.”
Of course the nature of superconductors and other quantum materials is only one of the mysteries waiting to be revealed, these explorations.
“I think this laboratory is poised to do even more remarkable things,” Zacharia said, pointing to efforts such as the development of exascale supercomputers that perform more than a billion billion calculations each second, or quantum computers that use the behavior of individual atoms to solve especially difficult problems, or new technologies for providing nuclear energy.
“I see this laboratory emerging as a vanguard of scientific institutions, not only in this country but globally," Zacharia said. "My vision for this laboratory is that it be the place where the best minds will want to come to be part of this enterprise.”
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