Uranium is located on the itinerant side of the 5f localized/itinerant crossover that occurs in the actinide series. Its 5f electrons experience strong Coulomb repulsion, spin–orbit coupling, covalency, and electron correlations. These comparable interactions are sensitive to crystal structure, bonding, and dimensionality, which generate a rich diversity of ground states including complex magnetism, quantum criticality, and exotic superconductivity. Two new structural motifs, U-based Kagome and 2-D Van der Waals compounds, provide an opportunity to explore and understand 5f electron behavior.
The electronic band structure of hexagonal Kagome materials is predicted to contain coexisting flat bands, van Hove singularities, and Dirac points. Materials in the hexagonal RM6X6 (166) family (R=rare-earth, M=transition metal, X=Ge, Sn) contain Kagome nets of transition metal ions and are of particular interest because their chemical tunability enables access to these interesting band structure features and to novel ground states. We successfully synthesized single crystals of UV6Sn6, UNb6Sn6, and UCr6Ge6. We probed the physical properties of these compounds with x-ray and neutron diffraction, magnetic susceptibility, electrical resistivity, thermal expansion, and heat capacity. We found a complex magnetic field-temperature phase diagram and two zero-field magnetic transitions at TN=29 K, T2=24 K for UV6Sn6 and TN=46 K, T2=42 K for UNb6Sn6. Neutron and x-ray diffraction measurements point to deviations from the ideal hexagonal P6/mmm structure, a feature often observed in the 166 family due to the large M-X cage-like voids in the structure. Our band structure calculations indicate that the Cr Kagome flat bands are close to the Fermi level in UCr6Ge6, while they are somewhat further away in UV6Sn6 and UNb6Sn6.
2-D Van der Waals materials provide the means to explore strong 5f electronic correlations, quantum criticality, and magnetism in the 2-D limit. We recently synthesized single crystals of the 2-D VdW material -UTe3, which crystallizes in the orthorhombic Cmcm structure. Here, we find 3-D bulk ferromagnetism at TC~10 K. Neutron diffraction indicates the presence of 3-D magnetic order and 2-D fluctuations. Angle-resolved photoemission measurements suggest a 2-D electronic structure. We discuss our recent efforts to investigate this material in the 2-D limit through exfoliation.
