Boundary quantum criticality in models of magnetic impurities coupled to bosonic baths

TL;DR

This paper studies magnetic impurity models coupled to bosonic environments, revealing a quantum phase transition between screened and unscreened phases, distinct from traditional spin-boson models, using renormalization group analysis.

Contribution

It introduces a new class of bosonic impurity models with a flow to strong coupling and identifies a quantum phase transition, differing from known spin-boson behaviors.

Findings

Existence of a stable strong-coupling phase.

Quantum phase transition at intermediate coupling.

Distinct behavior from spin-isotropic spin-boson models.

Abstract

We investigate quantum impurity problems, where a local magnetic moment is coupled to the spin density of a bosonic environment, leading to bosonic versions of the standard Kondo and Anderson impurity models. In a physical situation, these bosonic environments can correspond either to deconfined spinons in certain classes of Z_2 frustrated antiferromagnets, or to particles in a multicomponent Bose gase (in which case the spin degree of freedom is attributed to hyperfine levels). Using renormalization group techniques, we establish that our impurity models, which feature an exchange interaction analogous to Kondo impurities in Fermi liquids, allow the flow towards a stable strong-coupling state. Since the low-energy bosons live around a single point in momentum space, and there is no Fermi surface, an impurity quantum phase transition occurs at intermediate coupling, separating screened and unscreened phases. This behavior is qualitatively different from previously studied spin-isotropic variants of the spin-boson model, which display stable intermediate-coupling fixed points and no screening.