9 research outputs found
Quantum Critical Behavior in Kondo Systems
This article briefly reviews three topics related to the quantum critical
behavior of certain heavy-fermion systems. First, we summarize an extended
dynamical mean-field theory for the Kondo lattice, which treats on an equal
footing the quantum fluctuations associated with the Kondo and RKKY couplings.
The dynamical mean-field equations describe an effective Kondo impurity model
with an additional coupling to vector bosons. Two types of quantum phase
transition appear to be possible within this approach---the first a
conventional spin-density-wave transition, the second driven by local physics.
For the second type of transition to be realized, the effective impurity model
must have a quantum critical point exhibiting an anomalous local spin
susceptibility. In the second part of the paper, such a critical point is shown
to occur in two variants of the Kondo impurity problem. Finally, we propose an
operational test for the existence of quantum critical behavior driven by local
physics. Neutron scattering results suggest that CeCuAu passes this
test.Comment: 6 pages, 4 eps figures, REVTeX (epsf style
Kosterlitz-Thouless Transition and Short Range Spatial Correlations in an Extended Hubbard Model
We study the competition between intersite and local correlations in a
spinless two-band extended Hubbard model by taking an alternative limit of
infinite dimensions. We find that the intersite density fluctuations suppress
the charge Kondo energy scale and lead to a Fermi liquid to non-Fermi liquid
transition for repulsive on-site density-density interactions. In the absence
of intersite interactions, this transition reduces to the known
Kosterlitz-Thouless transition. We show that a new line of non-Fermi liquid
fixed points replace those of the zero intersite interaction problem.Comment: 11 pages, 2 figure
Spatial Correlations in Dynamical Mean Field Theory
We further develop an extended dynamical mean field approach introduced
earlier. It goes beyond the standard dynamical mean field theory by
incorporating quantum fluctuations associated with intersite (RKKY-like)
interactions. This is achieved by scaling the intersite interactions to the
same power in 1/D as that for the kinetic terms. In this approach, a correlated
lattice problem is reduced to a single-impurity Anderson model with additional
self-consistent bosonic baths. Here, we formulate the approach in terms of
perturbation expansions. We show that the two-particle vertex functions are
momentum-dependent, while the single-particle self-energy remains local. In
spite of this, the approach is conserving. Finally, we also determine the form
of a momentum-dependent dynamical susceptibility; the resulting expression
relates it to the corresponding Weiss field, local correlation function and
(momentum-dependent) intersite coupling.Comment: 28 pages, REVTEX, 8 figures include
Locally critical quantum phase transitions in strongly correlated metals
When a metal undergoes a continuous quantum phase transition, non-Fermi
liquid behaviour arises near the critical point. It is standard to assume that
all low-energy degrees of freedom induced by quantum criticality are spatially
extended, corresponding to long-wavelength fluctuations of the order parameter.
However, this picture has been contradicted by recent experiments on a
prototype system: heavy fermion metals at a zero-temperature magnetic
transition. In particular, neutron scattering from CeCuAu has
revealed anomalous dynamics at atomic length scales, leading to much debate as
to the fate of the local moments in the quantum-critical regime. Here we report
our theoretical finding of a locally critical quantum phase transition in a
model of heavy fermions. The dynamics at the critical point are in agreement
with experiment. We also argue that local criticality is a phenomenon of
general relevance to strongly correlated metals, including doped Mott
insulators.Comment: 20 pages, 3 figures; extended version, to appear in Natur
From mixed valence to the Kondo lattice regime
Many heavy fermion materials are known to crossover from the Kondo lattice
regime to the mixed-valent regime or vice-versa as a function of pressure or
doping. We study this crossover theoretically by employing the periodic
Anderson model within the framework of the dynamical mean field theory. Changes
occurring in the dynamics and transport across this crossover are highlighted.
As the valence is decreased (increased) relative to the Kondo lattice regime,
the Kondo resonance broadens significantly, while the lower (upper) Hubbard
band moves closer to the Fermi level. The resistivity develops a two peak
structure in the mixed valent regime: a low temperature coherence peak and a
high temperature 'Hubbard band' peak. These two peaks merge yielding a broad
shallow maximum upon decreasing the valence further. The optical conductivity,
likewise exhibits an unusual absorption feature (shoulder) in the deep
mid-infrared region, which grows in intensity with decreasing valence. The
involvement of the Hubbard bands in dc transport, and of the effective f-level
in the optical conductivity are shown to be responsible for the anomalous
transport properties. A two-band hybridization-gap model, which neglects
incoherent effects due to many-body scattering, commonly employed to understand
the optical response in these materials is shown to be inadequate, especially
in the mixed-valent regime. Comparison of theory with experiment carried out
for (a) dc resistivities of CeRhIn5, Ce2Ni3Si5, CeFeGe3 and YbIr2Si2; (b)
pressure dependent resistivity of YbInAu2 and CeCu6; and (c) optical
conductivity measurements in YbIr2Si2 yields excellent agreement.Comment: 24 pages,12 figures,accepted in J.Phys.: Condens. Matte
Local fluctuations in quantum critical metals
We show that spatially local, yet low-energy, fluctuations can play an
essential role in the physics of strongly correlated electron systems tuned to
a quantum critical point. A detailed microscopic analysis of the Kondo lattice
model is carried out within an extended dynamical mean-field approach. The
correlation functions for the lattice model are calculated through a
self-consistent Bose-Fermi Kondo problem, in which a local moment is coupled
both to a fermionic bath and to a bosonic bath (a fluctuating magnetic field).
A renormalization-group treatment of this impurity problem--perturbative in
, where is an exponent characterizing the spectrum
of the bosonic bath--shows that competition between the two couplings can drive
the local-moment fluctuations critical. As a result, two distinct types of
quantum critical point emerge in the Kondo lattice, one being of the usual
spin-density-wave type, the other ``locally critical.'' Near the locally
critical point, the dynamical spin susceptibility exhibits scaling
with a fractional exponent. While the spin-density-wave critical point is
Gaussian, the locally critical point is an interacting fixed point at which
long-wavelength and spatially local critical modes coexist. A Ginzburg-Landau
description for the locally critical point is discussed. It is argued that
these results are robust, that local criticality provides a natural description
of the quantum critical behavior seen in a number of heavy-fermion metals, and
that this picture may also be relevant to other strongly correlated metals.Comment: 20 pages, 12 figures; typos in figure 3 and in the main text
corrected, version as publishe