Crystalline samples of CeRhIn5 from Los Alamos National Laboratory were cut into microscopic, crystalline conducting paths with a focused ion beam at MPI-CPfS. Image: MPI CPfS.
Crystalline samples of CeRhIn5 from Los Alamos National Laboratory were cut into microscopic, crystalline conducting paths with a focused ion beam at MPI-CPfS. Image: MPI CPfS.

A team of scientists from Germany and the US has detected a rare state of matter in which electrons in a superconducting crystal organize collectively. Their findings lay the groundwork for answering one of the most compelling questions in physics: how do correlated electron systems work, and are they related to one another? The scientists report their findings in a paper in Nature.

Electrons in most metals act individually, free to move in any direction through a metal to conduct electricity and heat. But in a crystal made from layered cerium, rhodium and indium (CeRhIn5), the scientists have discovered that electrons unite to flow in the same direction (a behavior called ‘breaking symmetry’) when in high magnetic fields of 30 tesla. Known as ‘electronic nematic’, this is a rare state of matter between a liquid and a crystal.

“It’s sort of like in ancient times,” explains Phillip Moll, principal investigator of this work and leader of the Physics of Microstructured Quantum Matter Group at the Max-Planck Institute for Chemical Physics of Solids (MPI-CPfS) in Germany. “People would draw maps in whatever direction best served them. But this state is like the moment when the world’s mapmakers unified to arbitrarily pick north as the orientation for all maps.”

Scientists believe that the electronic nematic state may be closely related to superconductivity, another strongly correlated electron state in which electrons flow with no resistance. Under high pressure, the cerium crystal is known to become a superconductor, but when placed in a high magnetic field, it displays this electronic nematic state. Because it exhibits both behaviors, CeRhIn5 appears uniquely positioned to reveal possible interactions between these two correlated electron phases.

“This fundamental question in materials in which the electrons interact was the starting point for my PhD thesis,” says Maja Bachmann, a doctoral student at MPI-CPfS. “Do the electrons have to decide either to pair or to all go in one direction? In other words, are superconductivity and nematicity competitive phenomena, or could the same interaction that leads to pairing also create nematicity?”

To try to answer this question, the scientists used focused ion beam (FIB) machining to fabricate a sample from a single crystal of CeRhIn5, and then conducted experiments using both pulsed and resistive magnets. Work in the DC Field Facility’s 45-tesla hybrid at the US National High Magnetic Field Laboratory (MagLab) showed that the nematic phase appears in very high fields, beginning at 30 tesla and remaining through the hybrid’s full field. The researchers wanted to understand how far this phase extended and, through experiments at the MagLab’s Pulsed Field Facility, found that at around 50 tesla, the nematicity vanishes, possibly even undergoing another exotic phase transition.

But something else happened during the pulsed experiments: the researchers noticed that they could control the direction of the electrons when they tilted the field slightly. Returning back to the DC Field Facility, the scientists were able to vary this tilt angle continuously while keeping the field steady at 45 tesla, a unique experimental parameter at the MagLab.

“One big advantage of the MagLab is that it offers all the state-of-the-art magnet technologies, and throughout a project, the magnet type can be changed easily if it becomes clear that a different technology was required,” Moll said. “Really, the close technological, scientific and administrative integration of these very different but complementary high-field technologies was the key to this success, and is a major strength of the MagLab.”

Moll’s team performed additional work in the lab’s 100-tesla pulsed magnet that will be featured in a future paper. The researchers are continuing to explore how the nematic phase merges into the superconducting phase, part of an ongoing project that will involve additional MagLab experiments.

This story is adapted from material from the University of Texas at Dallas, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.