517 research outputs found
Periodic Anderson model with correlated conduction electrons
We investigate a periodic Anderson model with interacting conduction
electrons which are described by a Hubbard-type interaction of strength U_c.
Within dynamical mean-field theory the total Hamiltonian is mapped onto an
impurity model, which is solved by an extended non-crossing approximation. We
consider the particle-hole symmetric case at half-filling. Similar to the case
U_c=0, the low-energy behavior of the conduction electrons at high temperatures
is essentially unaffected by the f-electrons and for small U_c a quasiparticle
peak corresponding to the Hubbard model evolves first. These quasiparticles
screen the f-moments when the temperature is reduced further, and the system
turns into an insulator with a tiny gap and flat bands. The formation of the
quasiparticle peak is impeded by increasing either U_c or the c-f
hybridization. Nevertheless almost dispersionless bands emerge at low
temperature with an increased gap, even in the case of initially insulating
host electrons. The size of the gap in the one-particle spectral density at low
temperatures provides an estimate for the low-energy scale and increases as U_c
increases.Comment: 11 pages RevTeX with 13 ps figures, accepted by PR
The boson-fermion model with on-site Coulomb repulsion between fermions
The boson-fermion model, describing a mixture of itinerant electrons
hybridizing with tightly bound electron pairs represented as hard-core bosons,
is here generalized with the inclusion of a term describing on-site Coulomb
repulsion between fermions with opposite spins. Within the general framework of
the Dynamical Mean-Field Theory, it is shown that around the symmetric limit of
the model this interaction strongly competes with the local boson-fermion
exchange mechanism, smoothly driving the system from a pseudogap phase with
poor conducting properties to a metallic regime characterized by a substantial
reduction of the fermionic density. On the other hand, if one starts from
correlated fermions described in terms of the one-band Hubbard model, the
introduction in the half-filled insulating phase of a coupling with hard-core
bosons leads to the disappearance of the correlation gap, with a consequent
smooth crossover to a metallic state.Comment: 7 pages, 6 included figures, to appear in Phys. Rev.
Interaction Effect in the Kondo Energy of the Periodic Anderson-Hubbard Model
We extend the periodic Anderson model by switching on a Hubbard for the
conduction electrons. The nearly integral valent (Kondo) limit of the
Anderson--Hubbard model is studied with the Gutzwiller variational method. The
new formula for the Kondo energy contains the -dependent chemical
potential of the Hubbard subsystem in the exponent, and the correlation-induced
band narrowing in the prefactor. Both effects tend to suppress the Kondo scale,
which can be understood to result from the blocking of hybridization (this
behaviour is the opposite of that found for Kondo--Hubbard models). At
half-filling, we find a Brinkman--Rice-type transition which leads from a
small-gap Kondo insulator to a Mott insulator.Comment: 4 pages (ReVTeX), submitted for publicatio
Generalized Heisenberg algebras and k-generalized Fibonacci numbers
It is shown how some of the recent results of de Souza et al. [1] can be
generalized to describe Hamiltonians whose eigenvalues are given as
k-generalized Fibonacci numbers. Here k is an arbitrary integer and the cases
considered by de Souza et al. corespond to k=2.Comment: 8 page
Magnetic Impurity in a Metal with Correlated Conduction Electrons: An Infinite Dimensions Approach
We consider the Hubbard model with a magnetic Anderson impurity coupled to a
lattice site. In the case of infinite dimensions, one-particle correlations of
the impurity electron are described by the effective Hamiltonian of the
two-impurity system. One of the impurities interacts with a bath of free
electrons and represents the Hubbard lattice, and the other is coupled to the
first impurity by the bare hybridization interaction. A study of the effective
two-impurity Hamiltonian in the frame of the 1/N expansion and for the case of
a weak conduction-electron interaction (small U) reveals an enhancement of the
usual exponential Kondo scale. However, an intermediate interaction (U/D = 1 -
3), treated by the variational principle, leads to the loss of the exponential
scale. The Kondo temperature T_K of the effective two-impurity system is
calculated as a function of the hybridization parameter and it is shown that
T_K decreases with an increase of U. The non-Fermi-liquid character of the
Kondo effect in the intermediate regime at the half filling is discussed.Comment: 12 pages with 8 PS figures, RevTe
Co-operative Kondo Effect in the two-channel Kondo Lattice
We discuss the possibility of a co-operative Kondo effect driven by channel
interference in a Kondo lattice where local moments are coupled to a single
Fermi sea via two orthogonal scattering channels. In this situation, the
channel quantum number is not conserved. We argue that the absence of channel
conservation causes the Kondo effect in the two channels to constructively
interfere, giving rise to a superconducting condensate of composite pairs,
formed between the local moments and the conduction electrons. Our arguments
are based on the observation that a heavy Fermi surface gives rise to zero
modes for Kondo singlets to fluctuate between screening channels of different
symmetry, producing a divergent composite pair susceptibility. Secondary
screening channels couple to these divergent fluctuations, promoting an
instability into a state with long-range composite order. We present detailed a
detailed mean-field theory for this superconducting phase, and discuss the
possible implications for heavy fermion physics.Comment: 23 double column pages. 9 fig
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