First discovered in 1975, heavy-fermion materials often have atypical correlated phases, such as multiple superconducting states (see Physics Today, November 2021, page 19). Their electronic behavior arises from quasiparticles of large effective mass, anywhere from 50 to 1000 electron masses, which in turn arise from the hybridization of conduction electrons and those in the 4f and 5f orbitals of their rare-earth or actinide ions. Exploring heavy fermions’ novel physics requires working with exotic materials—for example, uranium ditelluride—that offer limited ways to adjust their properties.

Peter Liljeroth, his graduate student Viliam Vaňo, and their colleagues at Aalto University in Finland are now the first to observe heavy-fermion behavior in a material without rare-earth or actinide elements. Their two-layer stack of tantalum disulfide is easy to make, handle, and tweak and will offer a method to explore the full range of heavy-fermion physics.

The researchers stumbled on the heavy-fermion physics by accident. They were originally growing TaS₂ to investigate whether it hosts a quantum spin liquid. (For more on spin liquids, see the article by Takashi Imai and Young Lee, Physics Today, August 2016, page 30.) Their growth process, which uses molecular beam epitaxy, produces islands of monolayer 1T-TaS₂ and 1H-TaS₂—two possible crystal structures for the three-atom-thick layer. It also produces bilayer islands of 1T-TaS₂ stacked on 1H-TaS₂ and vice versa. Monolayer 1T-TaS₂ was what the researchers were after, but on a whim, they decided to also characterize the two heterostructure stacks with scanning tunneling microscopy (shown in the image) and spectroscopy.

The Liljeroth group was surprised to see a dip in the tunneling spectrum of the 1H-TaS₂ on 1T-TaS₂ heterostructure. A material could have a drop in its conductance for a few reasons, including the Coulomb repulsion between quasiparticles. After testing how the spectral dip responded to temperature, magnetic fields, and lateral size of the heterostructure, Liljeroth and his colleagues ruled out all the explanations except one- The heterostructure was behaving like a heavy-fermion material.

The two layers provide the necessary ingredients to form artificial heavy fermions. The 1T-TaS₂ layer is in a charge-density-wave state, which results in atoms clustering into a Star of David pattern. Each unit cell hosts a magnetic moment, from one unbonded orbital at the center, which serves the role filled by f electrons in heavy-fermion materials. And metallic 1H-TaS₂ adds conduction electrons, which hybridize with the magnetic moments through what’s known as Kondo exchange coupling.

Unlike conventional heavy-fermion materials, the heterostructure offers tunability in, for example, the relative angle of the layers and applied electric gates. The researchers plan to eventually chart the heterostructure’s whole phase diagram, which should include unconventional superconductivity among other exotic phases observed so far only in rare-earth and actinide compounds. Heavy-fermion superconductivity is of interest in part because it may be topological, in that the behavior arises from the connectedness of the band structure rather than from its symmetry. (V. Vaňo et al., Nature 599, 582, 2021.)