34 research outputs found
Van Hove singularities in the paramagnetic phase of the Hubbard model: a DMFT study
Using the dynamical mean-field theory (DMFT) we study the paramagnetic phase
of the Hubbard model with the density of states (DOS) corresponding to the
three-dimensional cubic lattice and the two-dimensional square lattice, as well
as a DOS with inverse square root singularity. We show that the electron
correlations rapidly smooth out the square-root van Hove singularities (kinks)
in the spectral function for the 3D lattice and that the Mott metal-insulator
transition (MIT) as well as the magnetic-field-induced MIT differ only little
from the well-known results for the Bethe lattice. The consequences of the
logarithmic singularity in the DOS for the 2D lattice are more dramatic. At
half filling, the divergence pinned at the Fermi level is not washed out, only
its integrated weight decreases as the interaction is increased. While the Mott
transition is still of the usual kind, the magnetic-field-induced MIT falls
into a different universality class as there is no field-induced localization
of quasiparticles. In the case of a power-law singularity in the DOS at the
Fermi level, the power-law singularity persists in the presence of interaction,
albeit with a different exponent, and the effective impurity model in the DMFT
turns out to be a pseudo-gap Anderson impurity model with a hybridization
function which vanishes at the Fermi level. The system is then a generalized
Fermi liquid. At finite doping, regular Fermi liquid behavior is recovered.Comment: 7 pages, 9 figure
The Kondo effect in the presence of the Rashba spin-orbit interaction
We study the temperature scale of the Kondo screening of a magnetic impurity
which hybridizes with a two-dimensional electron gas in the presence of the
Rashba spin-orbit interaction. The problem is mapped to an effective
single-band impurity model with a hybridization function having an
inverse-square-root divergence at the bottom of the band. We study the effect
of this divergence on the Kondo screening. The problem is solved numerically
without further approximations using the numerical renormalization group
technique. We find that the Rashba interaction leads to a small variation of
the Kondo temperature (increase or decrease) which depends on the values of the
impurity parameters
Quantum phase transitions in the systems of parallel quantum dots
We study the low-temperature transport properties of the systems of parallel
quantum dots described by the N-impurity Anderson model. We calculate the
quasiparticle scattering phase shifts, spectral functions and correlations as a
function of the gate voltage for N up to 5. For any N, the conductance at the
particle-hole symmetric point is unitary. For N >= 2, a transition from
ferromagnetic to antiferromagnetic impurity spin correlations occurs at some
gate voltage. For N >= 3, there is an additional transition due to an abrupt
change in average impurity occupancy. For odd N, the conductance is
discontinuous through both quantum phase transitions, while for even N only the
magnetic transition affects the conductance. Similar effects should be
experimentally observable in the systems of quantum dots with ferromagnetic
conduction-band-mediated inter-dot exchange interactions.Comment: 5 pages, 4 figure
Spin-charge separation and simultaneous spin and charge Kondo effect
We study the spin-charge separation in a Kondo-like model for an impurity
with a spin and a charge (isospin) degree of freedom coupled to a single
conduction channel (the ``spin-charge'' Kondo model). We show that the spin and
charge Kondo effects can occur simultaneously at any coupling strength. In the
continuum (wide-band or weak coupling) limit, the Kondo screening in each
sector is independent, while at finite bandwidth and strong coupling the
lattice effects lead to a renormalization of the effective Kondo exchange
constants; nevertheless, universal spin and charge Kondo effects still occur.
We find similar behavior in the two-impurity Anderson model with positive and
negative electron-electron interaction and in the two-impurity
Anderson-Holstein model with a single phonon mode. We comment on the
applicability of such models to describe the conductance of deformable
molecules with a local magnetic moment.Comment: 13 pages, 11 figure