Electrons are normally among the lightest fundamental particles, but in so-called “heavy fermion” materials, they move as if they were hundreds of times more massive. This unusual heaviness occurs because of strong interactions between conducting electrons and localized magnetic moments in the material, and it is thought to play an important role in the behaviour of high-temperature or “unconventional” superconductors.
Researchers in the US, Sweden, Spain and Germany have now synthesized a new two-dimensional heavy fermion material from a layered intermetallic crystal made of cerium, silicon and iodine (CeSiI). The new material could give scientists fresh opportunities to study the interactions that give rise to poorly-understood behaviour such as unconventional superconductivity and related quantum phenomena.
“Typically, these heavy fermion materials are intermetallic structures with strong bonding in three dimensions, but it has been known for some time that making these materials more two dimensional can help to promote the unconventional superconductivity that appears in some heavy fermion compounds,” explains Xavier Roy, a chemist at Columbia University in the US who led the new study. “We have identified heavy fermions in the van der Waals layered material CeSiI, which contains strong bonding in two dimensions but is only weakly held together in the third.”
Conduction electrons couple strongly to local magnetic moments
The researchers chose to study CeSiI, which was first synthesized in 1998, after searching crystallographic databases for materials that might host these strong interactions (known as Kondo interactions). In particular, they aimed to combine three key elements: cerium atoms, which provide a local magnetic moment; metallic conductivity, which ensures the presence of charge carriers; and a van der Waals layered structure that would allow them to exfoliate (peel off) thin layers of the material just a few atoms thick. These individual layers can then be twisted and strained, or stacked on top of other materials, to change the material’s properties.
To make CeSiI, the researchers combined cerium metal, silicon and cerium iodide and heated the ensemble to high temperature. This procedure, which they detail in Nature, generates hexagonal platelets of the desired material. “Just as we hoped, we find that the conduction electrons couple strongly to the local magnetic moments on the Ce atoms, which results in the enhanced effective mass and antiferromagnetic order at low temperature,” explains Victoria Posey, a PhD student in Roy’s lab who synthesized the material.
Using scanning tunnelling microscopy measurements performed in Abhay Pasupathy’s lab at Columbia, the researchers found that the material’s spectrum is characteristic of heavy fermions. They backed up these results with photoemission spectroscopy measurements at the Brookhaven National Laboratory, electron transport measurements at Harvard University and magnetic measurements at the National High Magnetic Field Laboratory in Florida. They also worked with a group of theorists at Columbia, the Flatiron Institute, the Max Planck Institute in Germany, Sweden’s Uppsala University and two institutions in San Sebastián, Spain to develop a theoretical framework to explain their observations.
Unconventional superconductor is even odder than expected
Team member Michael Ziebel explains that the result was possible, in part, because of a collective effort by Columbia, Brookhaven and the Flatiron Institute to engineer new properties in 2D materials. “One major challenge we had to overcome was the air sensitivity of the material, which meant we had to develop new ways to handle samples in our lab,” Ziebel says. “More broadly, establishing the presence of heavy fermions themselves can be quite challenging – there’s no ‘smoking gun’ measurement.”
The researchers now plan to substitute different atoms into the cerium, silicon or iodine sites in CeSiI to try to suppress its magnetic order and induce new electronic ground states. Then, by exfoliating the material to different thicknesses, they aim to study the effects of dimensionality on these compounds. “In parallel, we are applying the techniques we used in this work to systematically alter the properties of CeSiI at the 2D limit, something that will, hopefully, induce new quantum phenomena arising from the combination of strong electronic interactions and low dimensionality,” says Roy.
