Impurity Quantum Phase Transitions

Details Impurity Quantum Phase Transitions Impurity Quantum Phase Transitions

2004-12-09

arxiv.org/abs/cond-mat/0412208v2

Phil. Mag. 86, 1807 (2006)

Physics

Condensed Matter

Statistical Mechanics

22 pages, 11 figs, review article to be published in Phil. Mag

Scientific paper

We review recent work on continuous quantum phase transitions in impurity models, both with fermionic and bosonic baths - these transitions are interesting realizations of boundary critical phenomena at zero temperature. The models with fermion bath are generalizations of the standard Kondo model, with the common feature that Kondo screening of the localized spin can be suppressed due to competing processes. The models with boson bath are related to the spin-boson model of dissipative two-level systems, where the interplay between tunneling and friction results in multiple phases. The competition inherent to all models can generate unstable fixed points associated with quantum phase transitions, where the impurity properties undergo qualitative changes. Interestingly, certain impurity transitions feature both lower-critical and upper-critical "dimensions" and allow for epsilon-type expansions. We present results for a number of observables, obtained by both analytical and numerical renormalization group techniques, and make connections to experiments.

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