Electricity flows through wires to deliver power, but it loses energy as it moves, delivering less than it started with. But that energy loss isn't a given. Scientists at Penn State have found a new way to identify types of materials known as superconductors that allow power to travel without any resistance, meaning no energy is lost.
The catch is that these superconductor materials are limited in how they can be used in everyday life, especially because superconductivity requires extreme temperatures too low for things like next-generation energy or advanced electronic devices. With the support from the "Theory of Condensed Matter" program at Basic Energy Science of Department of Energy (DOE), a team at Penn State developed a new approach to predict which materials could behave as superconductors, potentially bringing us closer to discover new superconductors at higher temperatures.
The theoretical prediction of superconductors, particularly the high temperature ones, remain elusive as it is commonly believed that the existing superconductivity theory is applicable only to low temperature superconductors, explained Zi-Kui Liu, professor of materials science and engineering at Penn State.
"The goal has always been to raise the temperature at which superconductivity persists," said Liu, who is lead author of a new study published in Superconductor Science and Technology. "But first, we need to understand exactly how superconductivity happens, and that is where our work comes in."
For decades, scientists have typically subscribed to the Bardeen-Cooper-Schrieffer (BCS) theory explaining how conventional superconductors, which operate at very low temperatures, work. The BCS theory says the ability to conduct electricity with absolutely no resistance relies on electron-phonon interactions enabling electrons pairing up - called Cooper pairs - and moving through the material in a coordinated way that avoids collisions with atoms, which means they do not lose energy as heat.
"Imagine a superhighway just for electrons," Liu explained. "If there are too many routes, electrons bump into things and lose energy. But if you create a straight tunnel for them, like the Autobahn in Germany, they can travel fast and freely without resistance."
That resistance-free electron flow is what makes superconductors so attractive for real-world applications, Liu said. Without resistance, electrons can flow further with more energy - meaning if scientists can discover new superconducting materials at higher temperatures, it could lead to long-lasting power sources, transforming how we transmit and use electricity. The DOE project aims to understand superconductivity using theoretical tools known as density functional theory (DFT) to differentiate how electrons behave in normal conductors versus in superconductors. The hypothesis is that even though DFT does not explicitly treat the formation of Cooper pairs, the electron density predicted by DFT should resemble that due to Cooper pairs, so that researchers can model how the subatomic particles may behave in a potential superconducting material.
Until now, the BCS theory based on the formation of Cooper pairs and DFT predictions based on quantum mechanics have remained separate. Liu's team found a way to connect them.
The key to the discovery, the researchers said, is a concept closely related to what is called zentropy theory. Zentropy theory combines ideas from statistical mechanics, which is the study of how large groups of particles behave, with quantum physics and modern computer modeling. Zentropy theory helps explain how the electronic structures of a material affect its properties as temperature changes, which in turn affects when it changes from a superconductor to a non-superconductor. However, zentropy theory requires understanding and prediction of the superconducting configuration of a material at zero Kelvin - the coldest temperature possible, also called absolute zero, where all motion of atoms and molecules stops. Liu's team showed that even DFT, a popular computational method not originally designed for studying superconductivity, can reveal important clues about when and how this phenomenon occurs.
This approach is especially valuable because it offers a new approach to predict whether a material is a superconductor or not, and the zentropy theory can then be used to predict the transition temperature from superconducting to non-superconducting, Liu said. The BCS theory works well only for superconductors with very low transition temperatures since the Cooper pairs are easily destroyed at high temperatures, and currently there is no theory for high temperature superconductors. Through the DFT predictions, Liu's team found that the resistant-free electron superhighway in high temperature superconductor is protected by a unique atomic structure resembling a pontoon bridge in rough water, so the superhighway can be maintained at higher temperatures predicted by the BCS theory.
The team used this method to successfully predict signs of superconductivity in materials including both conventional superconductors explainable by the BCS theory and a high temperature superconductor which is believed to be unexplainable by the BCS theory. The team further predicted the superconductivity in copper, silver and gold, which are not usually considered superconductors, probably due to their ultra-low temperatures. This new capability could help uncover new and superconducting materials at higher temperatures, according to Liu.
The researchers' next steps are two-fold: One is to apply this new method to predict the transition temperature from superconducting to non-superconducting as a function of pressure using the zentropy theory in existing high temperature superconductors, and the other is to search for new superconductors with higher transition temperatures through a comprehensive database with five millions materials that the team has been building. The team would like to identify potential candidates with the right properties for superconductivity and work with experimental scientists to test the most promising ones.
"We are not just explaining what is already known," Liu said. "We are building a framework to discover something entirely new. If successful, the approach could lead to the discovery of high-temperature superconductors that work in practical settings, potentially even at room temperature if they exist. That kind of breakthrough could have an enormous impact on modern technology and energy systems."
Shun-Li Shang, research professor of materials science and engineering at Penn State, is a co-investigator on this study.
The U.S. Department of Energy supported this research.
At Penn State, researchers are solving real problems that impact the health, safety and quality of life of people across the commonwealth, the nation and around the world.
For decades, federal support for research has fueled innovation that makes our country safer, our industries more competitive and our economy stronger. Recent federal funding cuts threaten this progress.
Learn more about the implications of federal funding cuts to our future at Research or Regress.
