Magnetic quantum phase transition in a metallic Kondo heterostructure

TL;DR

This paper investigates magnetic quantum phase transitions in a two-dimensional Heisenberg spin system embedded in a three-dimensional metal, revealing two metallic phases and the persistence of composite quasiparticles across the transition.

Contribution

It introduces a novel model combining 2D spins with 3D metallic electrons and employs sign-free quantum Monte Carlo to study quantum phase transitions in this setup.

Findings

Identification of two metallic phases as a function of Kondo coupling

Emergence of a heavy-fermion phase with screened spins at strong coupling

Presence of magnetic order with Goldstone modes at weak coupling

Abstract

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.