Quantum Griffiths effects in metallic systems

Elementary analytical extremal statistics arguments are used to analyze the possibility of quantum Griffiths effects in nearly critical systems with overdamped dynamics, such as arise in conventional theories of metallic quantum criticality. The overdamping is found to strongly suppress quantum tunneling of rare regions, leading to superparamagnetic rather than quantum Griffiths behavior. Implications for theories of non-Fermi-liquid behavior in heavy-fermion materials are discussed.