8,008 research outputs found
Single-particle dynamics of the Anderson model: a two-self-energy description within the numerical renormalization group approach
Single-particle dynamics of the Anderson impurity model are studied using
both the numerical renormalization group (NRG) method and the local moment
approach (LMA). It is shown that a 'two-self-energy' description of dynamics
inherent to the LMA, as well as a conventional 'single-self-energy'
description, arise within NRG; each yielding correctly the same local
single-particle spectrum. Explicit NRG results are obtained for the broken
symmetry spectral constituents arising in a two-self-energy description, and
the total spectrum. These are also compared to analytical results obtained from
the LMA as implemented in practice. Very good agreement between the two is
found, essentially on all relevant energy scales from the high-energy Hubbard
satellites to the low-energy Kondo resonance.Comment: 12 pages, 6 figure
Magnetic properties of the Anderson model: a local moment approach
We develop a local moment approach to static properties of the symmetric
Anderson model in the presence of a magnetic field, focussing in particular on
the strong coupling Kondo regime. The approach is innately simple and
physically transparent; but is found to give good agreement, for essentially
all field strengths, with exact results for the Wilson ratio, impurity
magnetization, spin susceptibility and related properties.Comment: 7 pages, 3 postscript figues. Latex 2e using the epl.cls Europhysics
Letters macro packag
Spectral scaling and quantum critical behaviour in the pseudogap Anderson model
The pseudogap Anderson impurity model provides a classic example of an
essentially local quantum phase transition. Here we study its single-particle
dynamics in the vicinity of the symmetric quantum critical point (QCP)
separating generalized Fermi liquid and local moment phases, via the local
moment approach. Both phases are shown to be characterized by a low-energy
scale that vanishes at the QCP; and the universal scaling spectra, on all
energy scales, are obtained analytically. The spectrum precisely at the QCP is
also obtained; its form showing clearly the non-Fermi liquid, interacting
nature of the fixed point.Comment: 7 pages, 2 figure
Kondo insulators in the periodic Anderson model: a local moment approach
The symmetric periodic Anderson model is well known to capture the essential
physics of Kondo insulator materials. Within the framework of dynamical
mean-field theory, we develop a local moment approach to its single-particle
dynamics in the paramagnetic phase. The approach is intrinsically
non-perturbative, encompasses all energy scales and interaction strengths, and
satisfies the low-energy dictates of Fermi liquid theory. It captures in
particular the strong coupling behaviour and exponentially small quasiparticle
scales characteristic of the Kondo lattice regime, as well as simple
perturbative behaviour in weak coupling. Particular emphasis is naturally given
to strong coupling dynamics, where the resultant clean separation of energy
scales enables the scaling behaviour of single-particle spectra to be obtained.Comment: 15 pages, 10 postscript figures, accepted for publication in EPJ B;
HyperTex disable
Dynamics of capacitively coupled double quantum dots
We consider a double dot system of equivalent, capacitively coupled
semiconducting quantum dots, each coupled to its own lead, in a regime where
there are two electrons on the double dot. Employing the numerical
renormalization group, we focus here on single-particle dynamics and the
zero-bias conductance, considering in particular the rich range of behaviour
arising as the interdot coupling is progressively increased through the strong
coupling (SC) phase, from the spin-Kondo regime, across the SU(4) point to the
charge-Kondo regime; and then towards and through the quantum phase transition
to a charge-ordered (CO) phase. We first consider the two-self-energy
description required to describe the broken symmetry CO phase, and implications
thereof for the non-Fermi liquid nature of this phase. Numerical results for
single-particle dynamics on all frequency scales are then considered, with
particular emphasis on universality and scaling of low-energy dynamics
throughout the SC phase. The role of symmetry breaking perturbations is also
briefly discussed.Comment: 14 pages, 6 figure
Quantum phase transition in capacitively coupled double quantum dots
We investigate two equivalent, capacitively coupled semiconducting quantum
dots, each coupled to its own lead, in a regime where there are two electrons
on the double dot. With increasing interdot coupling a rich range of behavior
is uncovered: first a crossover from spin- to charge-Kondo physics, via an
intermediate SU(4) state with entangled spin and charge degrees of freedom;
followed by a quantum phase transition of Kosterlitz-Thouless type to a
non-Fermi liquid `charge-ordered' phase with finite residual entropy and
anomalous transport properties. Physical arguments and numerical
renormalization group methods are employed to obtain a detailed understanding
of the problem.Comment: 4 pages, 3 figure
A spin-dependent local moment approach to the Anderson impurity model
We present an extension of the local moment approach to the Anderson impurity
model with spin-dependent hybridization. By employing the two-self-energy
description, as originally proposed by Logan and co-workers, we applied the
symmetry restoration condition for the case with spin-dependent hybridization.
Self-consistent ground states were determined through variational minimization
of the ground state energy. The results obtained with our spin-dependent local
moment approach applied to a quantum dot system coupled to ferromagnetic leads
are in good agreement with those obtained from previous work using numerical
renormalization group calculations
BCS - BEC crossover at T=0: A Dynamical Mean Field Theory Approach
We study the T=0 crossover from the BCS superconductivity to Bose-Einstein
condensation in the attractive Hubbard Model within dynamical mean field
theory(DMFT) in order to examine the validity of Hartree-Fock-Bogoliubov (HFB)
mean field theory, usually used to describe this crossover, and to explore
physics beyond it. Quantum fluctuations are incorporated using iterated
perturbation theory as the DMFT impurity solver. We find that these
fluctuations lead to large quantitative effects in the intermediate coupling
regime leading to a reduction of both the superconducting order parameter and
the energy gap relative to the HFB results. A qualitative change is found in
the single-electron spectral function, which now shows incoherent spectral
weight for energies larger than three times the gap, in addition to the usual
Bogoliubov quasiparticle peaks.Comment: 11 pages,12 figures, Published versio
Magnetic field effects in few-level quantum dots: theory, and application to experiment
We examine several effects of an applied magnetic field on Anderson-type
models for both single- and two-level quantum dots, and make direct comparison
between numerical renormalization group (NRG) calculations and recent
conductance measurements. On the theoretical side the focus is on
magnetization, single-particle dynamics and zero-bias conductance, with
emphasis on the universality arising in strongly correlated regimes; including
a method to obtain the scaling behavior of field-induced Kondo resonance shifts
over a very wide field range. NRG is also used to interpret recent experiments
on spin-1/2 and spin-1 quantum dots in a magnetic field, which we argue do not
wholly probe universal regimes of behavior; and the calculations are shown to
yield good qualitative agreement with essentially all features seen in
experiment. The results capture in particular the observed field-dependence of
the Kondo conductance peak in a spin-1/2 dot, with quantitative deviations from
experiment occurring at fields in excess of 5 T, indicating the eventual
inadequacy of using the equilibrium single-particle spectrum to calculate the
conductance at finite bias.Comment: 15 pages, 12 figures. Version as published in PR
- …