Comment on "A Tale of Two Theories: Quantum Griffiths Effects in Metallic Systems" by A. H. Castro-Neto and B. A. Jones

TL;DR

This paper critiques the viability of quantum Griffiths-McCoy effects explaining non-Fermi-liquid behavior in heavy fermion metals, emphasizing the importance of spin-electron damping and coupling strength.

Contribution

It clarifies the conditions under which quantum Griffiths-McCoy effects can occur, highlighting the role of damping and coupling in metallic systems.

Findings

Quantum Griffiths-McCoy effects require weak damping of spin dynamics.

Strong carrier-spin coupling in heavy fermions suppresses Griffiths effects.

The observed non-Fermi-liquid behavior is unlikely due to Griffiths effects in these materials.

Abstract

In a recent paper Castro-Neto and Jones argue that because the observability of quantum Griffiths-McCoy effects in metals is controlled by non-universal quantities, the quantum Griffiths-McCoy scenario may be a viable explanation for the non-fermi-liquid behavior observed in heavy fermion compounds. In this Comment we point out that the important non-universal quantity is the damping of the spin dynamics by the metallic electrons; quantum Griffiths-McCoy effects occur only if this is parametrically weak relative to other scales in the problem, i.e. if the spins are decoupled from the carriers. This suggests that in heavy fermion materials, where the Kondo effect leads to a strong carrier-spin coupling, quantum Griffiths-McCoy effects are unlikely to occur.