9,783 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
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
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
Dynamics and transport properties of heavy fermions: theory
The paramagnetic phase of heavy fermion systems is investigated, using a
non-perturbative local moment approach to the asymmetric periodic Anderson
model within the framework of dynamical mean field theory. The natural focus is
on the strong coupling Kondo-lattice regime wherein single-particle spectra,
scattering rates, dc transport and optics are found to exhibit w/w_L,T/w_L
scaling in terms of a single underlying low-energy coherence scale w_L.
Dynamics/transport on all relevant (w,T)-scales are encompassed, from the
low-energy behaviour characteristic of the lattice coherent Fermi liquid,
through incoherent effective single-impurity physics likewise found to arise in
the universal scaling regime, to non-universal high-energy scales; and which
description in turn enables viable quantitative comparison to experiment.Comment: 27 pages, 12 figure
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
Determination of Higgs-boson couplings at the LHC
We investigate the determination of Higgs boson couplings to gauge bosons and
fermions at the LHC from data on Higgs boson production and decay. We
demonstrate that very mild theoretical assumptions, which are valid in general
multi-Higgs doublet models, are sufficient to allow the extraction of absolute
values of the couplings rather than just ratios of the couplings. For Higgs
masses below 200 GeV we find accuracies of 10-40% for the Higgs couplings and
the total Higgs boson width after several years of LHC running. The sensitivity
of the Higgs coupling measurements to deviations from the Standard Model
predictions is studied for an MSSM scenario.Comment: 9 pages, contribution to the proceedings of the XXXIXth Rencontres de
Moriond, La Thuile, March 200
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
Finite temperature dynamics of the Anderson model
The recently introduced local moment approach (LMA) is extended to encompass
single-particle dynamics and transport properties of the Anderson impurity
model at finite-temperature, T. While applicable to arbitrary interaction
strengths, primary emphasis is given to the strongly correlated Kondo regime
(characterized by the T=0 Kondo scale ). In particular the
resultant universal scaling behaviour of the single-particle spectrum
D(\omega; T) \equiv F(\frac{\w}{\omega_{\rm K}}; \frac{T}{\omega_{\rm K}})
within the LMA is obtained in closed form; leading to an analytical description
of the thermal destruction of the Kondo resonance on all energy scales.
Transport properties follow directly from a knowledge of . The -dependence of the resulting resistivity , which is
found to agree rather well with numerical renormalization group calculations,
is shown to be asymptotically exact at high temperatures; to concur well with
the Hamann approximation for the s-d model down to ,
and to cross over smoothly to the Fermi liquid form in the low-temperature limit. The underlying
approach, while naturally approximate, is moreover applicable to a broad range
of quantum impurity and related models
Single-particle dynamics of the Anderson model: a local moment approach
A non-perturbative local moment approach to single-particle dynamics of the
general asymmetric Anderson impurity model is developed. The approach
encompasses all energy scales and interaction strengths. It captures thereby
strong coupling Kondo behaviour, including the resultant universal scaling
behaviour of the single-particle spectrum; as well as the mixed valent and
essentially perturbative empty orbital regimes. The underlying approach is
physically transparent and innately simple, and as such is capable of practical
extension to lattice-based models within the framework of dynamical mean-field
theory.Comment: 26 pages, 9 figure
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