Kondo Lattice Semimetallic Behavior in NpTe_1.75 Single Crystals

XRD, magnetic susceptibility, magnetization, heat capacity, and resistivity results are reported for single crystals of NpTe_1.75 that were synthesized using iodine vapor transport. XRD and EDS measurements show that these crystals form in the layered tetragonal UAs₂-type structure, with vacancies on the Te site. Magnetic susceptibility measurements reveal anisotropic paramagnetism, where Curie Weiss fits to the data yield a μ_eff of 3.6 μ_B/Np that is consistent with expectations for Np⁴⁺. Gradual deviation from this behavior is seen at low temperatures, which may be due to hybridization between f- and conduction electrons or crystal electric field effects. Heat capacity measurements reveal enhanced mass Fermi liquid behavior at low temperatures, as well as evidence for a possible phase transition at T = 1.5 K. Together with electrical resistivity measurements that show semimetallic behavior, these results indicate that hybridization between the f- and conduction electron states dominates the physics of NpTe_1.75, leading to a hybridization gap semimetal.