153 research outputs found
Critical quasiparticles in single-impurity and lattice Kondo models
Quantum criticality in systems of local moments interacting with itinerant
electrons has become an important and diverse field of research. Here we review
recent results which concern (a) quantum phase transitions in single-impurity
Kondo and Anderson models and (b) quantum phase transitions in heavy-fermion
lattice models which involve critical quasiparticles. For (a) the focus will be
on impurity models with a pseudogapped host density of states and their
applications, e.g., in graphene and other Dirac materials, while (b) is devoted
to strong-coupling behavior near antiferromagnetic quantum phase transitions,
with potential applications in a variety of heavy-fermion metals.Comment: 18 pages, 4 figs, mini-review. arXiv admin note: text overlap with
arXiv:1208.311
Interference of quantum critical excitations and soft diffusive modes in a disordered antiferromagnetic metal
We study the temperature-dependent quantum correction to conductivity due to
the interplay of spin density fluctuations and weak disorder for a
two-dimensional metal near an antiferromagnetic (AFM) quantum critical point.
AFM spin density fluctuations carry large momenta around the ordering vector
and, at lowest order of the spin-fermion coupling, only scatter
electrons between "hot spots" of the Fermi surface which are connected by
. Earlier, it was seen that the quantum interference between AFM
spin density fluctuations and soft diffusive modes of the disordered metal is
suppressed, a consequence of the large-momentum scattering. The suppression of
this interference results in a non-singular temperature dependence of the
corresponding interaction correction to conductivity. However, at higher order
of the spin-fermion coupling, electrons on the entire Fermi surface can be
scattered successively by two spin density fluctuations and, in total, suffer a
small momentum transfer. This higher-order process can be described by
composite modes which carry small momenta. We show that the interference
between formally subleading composite modes and diffusive modes generates
singular interaction corrections which ultimately dominate over the
non-singular first-order correction at low temperatures. We derive an effective
low-energy theory from the spin-fermion model which includes the
above-mentioned higher-order process implicitly and show that for weak
spin-fermion coupling the small-momentum transfer is mediated by a composite
propagator. Employing the conventional diagrammatic approach to impurity
scattering, we find the correction for
temperatures above an exponentially small crossover scale.Comment: 13 pages, 7 figures. Published versio
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