Heavy fermion systems represent one of the paradigmatic strongly correlated states of matter. They have been used as a platform for investigating exotic behavior ranging from quantum criticality and non-Fermi liquid behavior to unconventional topological superconductivity. Heavy fermions arise from the exchange interaction between localized magnetic moments and conduction electrons that leads to the well-known Kondo effect. In a Kondo lattice, the interaction between the localized moments gives rise to a band with heavy effective mass. This intriguing phenomenology has so far only been realized in compounds containing rare-earth elements with 4f or 5f electrons. Here, we realize a designer van der Waals heterostructure where artificial heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS$_2$ heterostructure. We study the heterostructure using scanning tunneling microscopy (STM) and spectroscopy (STS) and show that depending on the stacking order of the monolayers, we can either reveal the localized magnetic moments and the associated Kondo effect, or the conduction electrons with a heavy-fermion hybridization gap. Our experiments realize an ultimately tuneable platform for future experiments probing enhanced many-body correlations, dimensional tuning of quantum criticality, and unconventional superconductivity in two-dimensional artificial heavy-fermion systems.
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