  Kondo lattices are ideal testbeds for the exploration of heavy-fermion
quantum phases of matter. While our understanding of Kondo lattices has
traditionally relied on complex bulk f-electron systems, transition metal
dichalcogenide heterobilayers have recently emerged as simple, accessible and
tunable 2D Kondo lattice platforms where, however, their ground state remains
to be established. Here we present evidence of a coherent ground state in the
1T/1H-TaSe2 heterobilayer by means of scanning tunneling
microscopy/spectroscopy at 340 mK. Our measurements reveal the existence of two
symmetric electronic resonances around the Fermi energy, a hallmark of
coherence in the spin lattice. Spectroscopic imaging locates both resonances at
the central Ta atom of the charge density wave of the 1T phase, where the
localized magnetic moment is held. Furthermore, the evolution of the electronic
structure with the magnetic field reveals a non-linear increase of the energy
separation between the electronic resonances. Aided by ab initio and
auxiliary-fermion mean-field calculations, we demonstrate that this behavior is
inconsistent with a fully screened Kondo lattice, and suggests a ground state
with magnetic order mediated by conduction electrons. The manifestation of
magnetic coherence in TMD-based 2D Kondo lattices enables the exploration of
magnetic quantum criticality, Kondo breakdown transitions and unconventional
superconductivity in the strict two-dimensional limit.
