  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.
