  We consider a two-dimensional quantum spin system described by a Heisenberg
model that is embedded in a three-dimensional metal. The two systems couple via
an antiferromagnetic Kondo interaction. In such a setup, the ground state
generically remains metallic down to the lowest temperatures and allows us to
study magnetic quantum phase transitions in metallic environments. From the
symmetry point of view, translation symmetry is present in two out of three
lattice directions such that crystal momentum is only partially conserved.
Importantly, the construction provides a route to study, with
negative-sign-free auxiliary-field quantum Monte Carlo methods, the physics of
local moments in metallic environments. Our large-scale numerical simulations
show that as a function of the Kondo coupling, the system has two metallic
phases. In the limit of strong Kondo coupling, a paramagnetic heavy-fermion
phase emerges. Here, the spin degree of freedom is screened by means of the
formation of a composite quasiparticle that participates in the Luttinger
count. At weak Kondo coupling, magnetic order is present. This phase is
characterized by Landau-damped Goldstone modes. Furthermore, the aforementioned
composite quasiparticle remains intact across the quantum phase transition.
