Two-Dimensional Heavy Fermion Materials

The proposed research will create a family of layered van der Waals (vdW) heavy fermion materials that can be exfoliated to produce atomically-thin flakes, and will investigate their physical properties at the two-dimensional (2D) limit and in heterostructures. The project comprises three research thrusts: (Thrust 1) Design and synthesis of vdW heavy fermion single crystals; (Thrust 2) Study of their bulk thermodynamic and spectroscopic properties; and (Thrust 3) Creation and investigation of 2D layers and heterostructures. The proposed program is built on a new class of 2D heavy fermion materials, whose electronic properties cannot be reproduced or approximated in any other materials systems. Heavy fermion materials are intermetallic compounds with periodic lattices of localized magnetic moments that entangle with itinerant electrons to produce coherent narrow bands of electrons whose motions are strongly correlated. Such heavy fermion bands are a playground for the observation of quantum phases driven by electron interactions, including unconventional superconductivity, non-Fermi liquid behavior, quantum criticality and Kondo insulator state. Traditional heavy fermion metals are three-dimensional (3D) structures in which the effective Coulomb interactions are highly screened within even a few unit cells because of the high 3D concentration of itinerant electrons. In recent years, 2D materials have emerged as a new paradigm for changing both single-electron band structure and the effective Coulomb interactions between electrons. Confining heavy fermions in two dimensions therefore offers new and unique opportunities to understand and manipulate the balance between electron screening and magnetism. To this end, the proposed research will holistically investigate the physical properties of vdW heavy fermion materials, both as bulk single crystals and as exfoliated 2D flakes. Specifically, the team will create layered vdW heavy fermion materials by incorporating halogen atoms into the crystal structures of 3D intermetallic compounds, which will have for effect to slice their 3D lattices into vdW sheets (Thrust 1); characterize the bulk form of these vdW heavy fermion metals to understand the interplay of magnetism and Kondo screening (Thrust 2); and use mechanical exfoliation to create monolayer and few layer flakes, and assemble heterostructures with other quantum materials to induce new quantum phenomena (Thrust 3). In addition, the project will develop experimental techniques to electrically contact and precisely stack these air sensitive 2D materials into twisted bilayers and other multilayers, which will enable a wide range of new heterostructures between disparate quantum phases. This focused study of 2D heavy fermion materials will be carried out by PI Roy and co-PI Pasupathy, who have strong track records of collaboration and have complementary expertise in solid state chemistry, magnetism, scanning tunneling microscopy, spectroscopy and transport. The team will also benefit from strong existing collaborations with experts in theoretical physics, synchrotron photoemission spectroscopy and neutron diffraction. The integrated expertise in synthesis, characterization and modeling will foster a strong synergy between materials development and fundamental understanding. The knowledge gained through this interdisciplinary effort will contribute to define the next generation of 2D quantum materials, and push the ultimate physical limits of strongly correlated materials.