materials that can conduct electricity without energy loss - are crucial for next-generation high-efficiency, ultrafast electronics. However, most superconductors share a critical limitation: they lose their superconducting properties in strong magnetic fields. In contrast, a class of superconductors containing heavy elements can sustain an unusual type of superconductivity in magnetic fields beyond the conventional limit. Now, new research has demonstrated that this limitation can be overcome by sandwiching atomically thin films of a lightweight element called gallium between two other materials to engineer quantum interactions at the interfaces between the layers.
A paper describing the research, led by an interdisciplinary team at Penn State's U.S. National Science Foundation (NSF)-funded Materials Research Science and Engineering Center (MRSEC) for Nanoscale Science, was published today (April 13) in the journal Nature Materials. The team showed that when just three atomic layers of gallium are layered between graphene and a silicon carbide substrate, the resulting structure maintains superconductivity in magnetic fields that are parallel to the surface of the material, or in-plane, well above the expected limit.
"This discovery highlights the strength of collaborative, cross-disciplinary research fostered by the Penn State MRSEC," said Cui-Zu Chang, professor of physics at Penn State Eberly College of Science and leader of the research team. "By bringing together expertise in materials synthesis, quantum transport and theoretical modeling, we were able to uncover a phenomenon that would have been difficult to realize within a single research group."
At everyday temperatures - from the coldest winter day to the height of summer - most materials exhibit some electrical resistance. This means that electrical current does not flow freely - imagine water flowing through a narrowing pipe, its flow is restricted and some energy is lost as heat. At extremely low temperatures, approaching absolute zero, some materials - known as superconductors - allow current to flow without any resistance.
"In a superconductor, negatively charged electrons, which would usually repel each other, form pairs - called Cooper pairs - that can move through the material together without resistance," Chang said. "But strong magnetic fields - stronger than what is known as the 'Pauli paramagnetic limit' - can break these Cooper pairs apart."
This breaking of Cooper pairs by strong magnetic fields occurs for most materials, resulting in the loss of superconductivity, the researchers explained.
"However, when electrons move through a material, particularly in compounds containing heavy elements, their spins - a fundamental property governing their quantum behavior - can interact with their motion through a mechanism known as spin-orbit coupling," said Chao-Xing Liu, professor of physics at Penn State and a lead author of the paper. "When such materials become superconducting, this interaction can lead to unconventional superconducting states."
One prominent example is Ising-type superconductivity, where the spins of the electrons are locked perpendicular to the plane of the crystal. This locking protects the electron pairs from magnetic fields, allowing superconductivity to persist even beyond the Pauli paramagnetic limit, the researchers explained.
"Ising-type superconductivity had only been seen in materials that include heavy elements, which inherently have strong spin-orbit coupling - the property that leads to locking the electron spin," Chang said. "In our sandwich structure with a lighter element, we expected the superconductivity to weaken as the magnetic field increased, but instead, it persisted beyond the usual limit of conventional superconductors."
The sandwich consisted of a bottom layer of silicon carbide that provides a substrate to grow the three-atomic-layer gallium film - the sandwich filling - and a top layer of graphene - an atomically thin form of carbon - that protects the gallium from air exposure, preventing oxidation and contamination.
"The interfaces between these three materials create a unique quantum environment, opening the door to emergent and potentially novel quantum phenomena," Chang said. "In this case, it allowed the lightweight element gallium to maintain its superconductivity in in-plane magnetic fields more than three times the Pauli limit."
The results, the researchers said, demonstrate that Ising-type superconductivity is not limited to heavy-element superconductors but can emerge from interface effects in lightweight element superconductors.
"This work establishes a widely applicable design strategy for realizing unconventional superconductivity in light-element superconductors," Chang said. "We aim to extend this strategy to other light-element metals, including indium and tin, on suitable substrates, to establish a broader family of unconventional superconductors engineered using interfacial effects."
The research relied on a close collaboration between experimental and theoretical research groups across different disciplines within the Penn State MRSEC, the team said. Joshua A. Robinson, professor of materials science and engineering, of chemistry and of physics, synthesized the ultrathin gallium films. The Chang group in the physics department carried out electrical transport measurements to investigate the films' properties. The Liu group in the physics department led the theoretical efforts for the project. Vincent H. Crespi, distinguished professor of physics, of materials science and engineering and of chemistry, also contributed to interpreting the results and uncovering the underlying mechanism.
"This is a clear example of how the MRSEC environment accelerates discovery," Chang said. "By integrating materials synthesis, advanced characterization and theory simulations and modeling within the Penn State MRSEC, we can design and realize new quantum states of matter."
In addition to Chang, Liu, Robinson and Crespi, the research team at Penn State included postdoctoral researchers Hemian Yi, Chengye Dong, Bing Xia, Bomin Zhang and Hongtao Rong; graduate students Yunzhe Liu, Zi-Jie Yan, Zihao Wang, Lingjie Zhou, Stephen Paolini, Xiaoda Liu, Annie G. Wang, Saswata Mandal, Kaijie Yang and Benjamin N. Katz. This project is primarily supported by the Penn State NSF-funded MRSEC for Nanoscale Science, which the team credited for enabling the close collaboration across disciplines that made this discovery possible. A complete list of authors and funders can be found in the paper.
