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TT: Tiefe Temperaturen

TT 16: Posters Correlated Electrons, Measuring Devices, Cryotechnique

TT 16.22: Poster

Samstag, 5. März 2005, 11:00–16:30, Poster TU C

Phase Transitions in the Pseudogap Anderson and Kondo models — •Lars Fritz, Marijana Kircan, and Matthias Vojta — Institut für Theorie der Kondensierten Materie, Universität Karlsruhe,76128 Karlsruhe

The Pseudogap Kondo problem, describing quantum impurities coupled to fermionic quasiparticles with a pseudogap density of states, ρ(ω)∝ |ω|^r, shows a rich zero-temperature phase diagram, with different screened and free moment phases and associated transitions. We analyze both the particle-hole symmetric and asymmetric cases using renormalization group techniques. In the vicinity of r=0, which plays the role of a lower critical dimension, an expansion in the Kondo coupling is appropriate. In contrast, r=1 is the upper-critical dimension in the absence of particle-hole symmetry, and here insight can be gained using an expansion in the hybridization strength of the Anderson model. As a by-product, we show that the particle-hole symmetric strong coupling fixed point for r<1 is described by a resonant level model, and corresponds to an intermediate-coupling fixed point in the renormalization group language. Interestingly, the value r=1/2 plays the role of a second lower-critical dimension in the particle-hole symmetric case, and there we can make progress by a novel expansion performed around a resonant level model. The different expansions allow a complete description of all critical fixed points of the models and can be used to compute a variety of properties near criticality, describing universal local-moment fluctuations at these impurity quantum phase transitions.